Aerospace
http://www.mdpi.com/journal/aerospace
Latest open access articles published in Aerospace at http://www.mdpi.com/journal/aerospace<![CDATA[Aerospace, Vol. 4, Pages 33: Aerodynamic Testing of Helicopter Side Intake Retrofit Modifications]]>
http://www.mdpi.com/2226-4310/4/3/33
Aerodynamic characteristics of helicopter engine side air intakes are investigated. The experimental data set is obtained by wind tunnel testing of a full-scale helicopter fuselage section model. For the simulation of realistic engine operation, engine mass flow rates are reproduced. Five-hole pressure probe data of the aerodynamic interface plane as well as local surface pressure distributions are compared for different geometries and operation conditions. Previous investigations indicate that unshielded, sideways-facing air intakes yield lowest distortion and highest total pressure levels in low speed conditions. In fast forward flight condition, however, forward-facing intake shapes are more beneficial. On this basis, the current research assesses the optimization potential of retrofit modifications such as a rear spoiler (small scoop) and an intake guide vane. Two optimal configurations of retrofit modifications are identified, combining benefits of the various basic intake and plenum chamber shapes.Aerospace2017-06-2343Article10.3390/aerospace4030033332226-43102017-06-23doi: 10.3390/aerospace4030033Florian KnothChristian Breitsamter<![CDATA[Aerospace, Vol. 4, Pages 32: Aerial Target Tracking Algorithm Based on Faster R-CNN Combined with Frame Differencing]]>
http://www.mdpi.com/2226-4310/4/2/32
We propose a robust approach to detecting and tracking moving objects for a naval unmanned aircraft system (UAS) landing on an aircraft carrier. The frame difference algorithm follows a simple principle to achieve real-time tracking, whereas Faster Region-Convolutional Neural Network (R-CNN) performs highly precise detection and tracking characteristics. We thus combine Faster R-CNN with the frame difference method, which is demonstrated to exhibit robust and real-time detection and tracking performance. In our UAS landing experiments, two cameras placed on both sides of the runway are used to capture the moving UAS. When the UAS is captured, the joint algorithm uses frame difference to detect the moving target (UAS). As soon as the Faster R-CNN algorithm accurately detects the UAS, the detection priority is given to Faster R-CNN. In this manner, we also perform motion segmentation and object detection in the presence of changes in the environment, such as illumination variation or “walking persons”. By combining the 2 algorithms we can accurately detect and track objects with a tracking accuracy rate of up to 99% and a frame per second of up to 40 Hz. Thus, a solid foundation is laid for subsequent landing guidance.Aerospace2017-06-2042Article10.3390/aerospace4020032322226-43102017-06-20doi: 10.3390/aerospace4020032Yurong YangHuajun GongXinhua WangPeng Sun<![CDATA[Aerospace, Vol. 4, Pages 31: Nonlinear Model Predictive Control for Unmanned Aerial Vehicles]]>
http://www.mdpi.com/2226-4310/4/2/31
This paper discusses the derivation and implementation of a nonlinear model predictive control law for tracking reference trajectories and constrained control of a quadrotor platform. The approach uses the state-dependent coefficient form to capture the system nonlinearities into a pseudo-linear system matrix. The state-dependent coefficient form is derived following a rigorous analysis of aerial vehicle dynamics that systematically accounts for the peculiarities of such systems. The same state-dependent coefficient form is exploited for obtaining a nonlinear equivalent of the model predictive control. The nonlinear model predictive control law is derived by first transforming the continuous system into a sampled-data form and and then using a sequential quadratic programming solver while accounting for input, output and state constraints. The boundedness of the tracking errors using the sampled-data implementation is shown explicitly. The performance of the nonlinear controller is illustrated through representative simulations showing the tracking of several aggressive reference trajectories with and without disturbances.Aerospace2017-06-1742Article10.3390/aerospace4020031312226-43102017-06-17doi: 10.3390/aerospace4020031Pengkai RuKamesh Subbarao<![CDATA[Aerospace, Vol. 4, Pages 30: Multi-Mode Electric Actuator Dynamic Modelling for Missile Fin Control]]>
http://www.mdpi.com/2226-4310/4/2/30
Linear first/second order fin direct current (DC) actuator model approximations for missile applications are currently limited to angular position and angular velocity state variables. Furthermore, existing literature with detailed DC motor models is decoupled from the application of interest: tail controller missile lateral acceleration (LATAX) performance. This paper aims to integrate a generic DC fin actuator model with dual-mode feedforward and feedback control for tail-controlled missiles in conjunction with the autopilot system design. Moreover, the characteristics of the actuator torque information in relation to the aerodynamic fin loading for given missile trim velocities are also provided. The novelty of this paper is the integration of the missile LATAX autopilot states and actuator states including the motor torque, position and angular velocity. The advantage of such an approach is the parametric analysis and suitability of the fin actuator in relation to the missile lateral acceleration dynamic behaviour.Aerospace2017-06-1442Article10.3390/aerospace4020030302226-43102017-06-14doi: 10.3390/aerospace4020030Bhimashankar GuravJohn EconomouAlistair SaddingtonKevin Knowles<![CDATA[Aerospace, Vol. 4, Pages 29: An Efficiently Parallelized High-Order Aeroacoustics Solver Using a Characteristic-Based Multi-Block Interface Treatment and Optimized Compact Finite Differencing]]>
http://www.mdpi.com/2226-4310/4/2/29
This paper presents the development of a fourth-order finite difference computational aeroacoustics solver. The solver works with a structured multi-block grid domain strategy, and it has been parallelized efficiently by using an interface treatment based on the method of characteristics. More importantly, it extends the characteristic boundary condition developments of previous researchers by introducing a characteristic-based treatment at the multi-block interfaces. In addition, most characteristic methods do not satisfy Pfaff’s condition, which is a requirement for any mathematical relation to be valid. A mathematically-consistent and valid method is used in this work to derive the characteristic interface conditions. Furthermore, a robust and efficient approach for the matching of turbulence quantities at the multi-block interfaces is developed. Finally, the implementation of grid metric relations to minimise grid-induced errors has been adopted. The code was validated against a number of benchmark cases, which demonstrated its accuracy and robustness across a range of problem types.Aerospace2017-05-2842Article10.3390/aerospace4020029292226-43102017-05-28doi: 10.3390/aerospace4020029Bidur KhanalAlistair SaddingtonKevin Knowles<![CDATA[Aerospace, Vol. 4, Pages 27: Stochastic Trajectory Generation Using Particle Swarm Optimization for Quadrotor Unmanned Aerial Vehicles (UAVs)]]>
http://www.mdpi.com/2226-4310/4/2/27
The aim of this paper is to provide a realistic stochastic trajectory generation method for unmanned aerial vehicles that offers a tool for the emulation of trajectories in typical flight scenarios. Three scenarios are defined in this paper. The trajectories for these scenarios are implemented with quintic B-splines that grant smoothness in the second-order derivatives of Euler angles and accelerations. In order to tune the parameters of the quintic B-spline in the search space, a multi-objective optimization method called particle swarm optimization (PSO) is used. The proposed technique satisfies the constraints imposed by the configuration of the unmanned aerial vehicle (UAV). Further particular constraints can be introduced such as: obstacle avoidance, speed limitation, and actuator torque limitations due to the practical feasibility of the trajectories. Finally, the standard rapidly-exploring random tree (RRT*) algorithm, the standard (A*) algorithm and the genetic algorithm (GA) are simulated to make a comparison with the proposed algorithm in terms of execution time and effectiveness in finding the minimum length trajectory.Aerospace2017-05-0842Article10.3390/aerospace4020027272226-43102017-05-08doi: 10.3390/aerospace4020027Babak SalamatAndrea Tonello<![CDATA[Aerospace, Vol. 4, Pages 28: Good Code Sets from Complementary Pairs via Discrete Frequency Chips]]>
http://www.mdpi.com/2226-4310/4/2/28
It is shown that replacing the sinusoidal chip in Golay complementary code pairs by special classes of waveforms that satisfy two conditions, symmetry/anti-symmetry and quazi-orthogonality in the convolution sense, renders the complementary codes immune to frequency selective fading and also allows for concatenating them in time using one frequency band/channel. This results in a zero-sidelobe region around the mainlobe and an adjacent region of small cross-correlation sidelobes. The symmetry/anti-symmetry property results in the zero-sidelobe region on either side of the mainlobe, while quasi-orthogonality of the two chips keeps the adjacent region of cross-correlations small. Such codes are constructed using discrete frequency-coding waveforms (DFCW) based on linear frequency modulation (LFM) and piecewise LFM (PLFM) waveforms as chips for the complementary code pair, as they satisfy both the symmetry/anti-symmetry and quasi-orthogonality conditions. It is also shown that changing the slopes/chirp rates of the DFCW waveforms (based on LFM and PLFM waveforms) used as chips with the same complementary code pair results in good code sets with a zero-sidelobe region. It is also shown that a second good code set with a zero-sidelobe region could be constructed from the mates of the complementary code pair, while using the same DFCW waveforms as their chips. The cross-correlation between the two sets is shown to contain a zero-sidelobe region and an adjacent region of small cross-correlation sidelobes. Thus, the two sets are quasi-orthogonal and could be combined to form a good code set with twice the number of codes without affecting their cross-correlation properties. Or a better good code set with the same number codes could be constructed by choosing the best candidates form the two sets. Such code sets find utility in multiple input-multiple output (MIMO) radar applications.Aerospace2017-05-0742Article10.3390/aerospace4020028282226-43102017-05-07doi: 10.3390/aerospace4020028Ravi KadlimattiAdly T. Fam<![CDATA[Aerospace, Vol. 4, Pages 26: Development of Hollow Cathodes for Space Electric Propulsion at Sitael]]>
http://www.mdpi.com/2226-4310/4/2/26
Hollow cathodes are electron sources used for the gas ionization and the beam neutralization in both ion and Hall effect thrusters (HETs). A reduction of power and propellant consumption from the cathode is particularly needed in small satellite applications, where power and mass budgets are inherently limited. Concurrently, the interest in high-power HETs is increasingly fostered for a number of space applications, including final positioning and station-keeping of Geostationary Earth Orbit (GEO) satellites, spacecraft transfers from Low Earth Orbit (LEO) to GEO, and deep-space exploration missions. As such, several hollow cathodes have been developed and tested at Sitael, each conceived for a specific power class of thrusters. A numerical model was used during the cathode design to define the geometry, in accordance with the thruster unit specifications in terms of discharge current, mass flow rate, and lifetime. Lanthanum hexaboride (LaB6) hollow cathodes were successfully developed for HETs with discharge power ranging from 100 W to 20 kW. Experimental campaigns were carried out in both stand-alone and coupled configurations, to verify the operation of the cathodes and validate the numerical model. The comparison between experimental and theoretical results are presented, offering a sound framework to drive the design of future hollow cathodes.Aerospace2017-05-0642Article10.3390/aerospace4020026262226-43102017-05-06doi: 10.3390/aerospace4020026Daniela PedriniTommaso MisuriFabrizio PaganucciMariano Andrenucci<![CDATA[Aerospace, Vol. 4, Pages 25: Assessment of CFD Capability for Hypersonic Shock Wave Laminar Boundary Layer Interactions]]>
http://www.mdpi.com/2226-4310/4/2/25
The goal of this study is to assess CFD capability for the prediction of shock wave laminar boundary layer interactions at hypersonic velocities. More specifically, the flow field over a double-cone configuration is simulated using both perfect gas and non-equilibrium Navier–Stokes models. Computations are compared with recent experimental data obtained from measurements conducted in the LENS XX (Large Energy National Shock Expansion Tunnel Version 2) at the Calspan University of Buffalo Research Center (CUBRC). Four separate cases of freestream conditions are simulated to examine the models for a range of stagnation enthalpies from 5.44 MJ/kg to 21.77 MJ/kg and Mach numbers from 10.9 to 12.82.Aerospace2017-04-2542Article10.3390/aerospace4020025252226-43102017-04-25doi: 10.3390/aerospace4020025Mehrnaz Rouhi YoussefiDoyle Knight<![CDATA[Aerospace, Vol. 4, Pages 24: Effects of Wake Shapes on High-Lift System Aerodynamic Predictions]]>
http://www.mdpi.com/2226-4310/4/2/24
High-lift devices are commonly modelled using potential flow methods at the conceptual design stage. Often, these analyses require the use of prescribed wake shapes in order to avoid numerical stability issues. The wake type used, however, has an impact on the absolute aerodynamic load predictions, which is why, in general, these methods are used to assess performance changes due to configuration variations. Therefore, a study was completed that compared the predicted aerodynamic performance changes of such variations of high-lift configurations using different wake types. Lift and induced drag results are compared with the results that were obtained using relaxed wakes and various prescribed wake shapes. Specific attention is given to predictions of performance changes due to changes in geometry. It was found that models with wakes that are prescribed below the freestream direction yield the best results when investigating performance changes due to flap deflections and flap-span changes. The effect of flap-gap sizes is best evaluated using a fully-relaxed model. The numerically most stable approach of wakes that are prescribed leaving the trailing edge upwards seems to be least reliable in predicting performance changes.Aerospace2017-04-1942Article10.3390/aerospace4020024242226-43102017-04-19doi: 10.3390/aerospace4020024William BissonnetteGötz Bramesfeld<![CDATA[Aerospace, Vol. 4, Pages 23: Direct Entry Minimal Path UAV Loitering Path Planning]]>
http://www.mdpi.com/2226-4310/4/2/23
Fixed Wing Unmanned Aerial Vehicles (UAVs) performing Intelligence, Surveillance and Reconnaissance (ISR) typically fly over Areas of Interest (AOIs) to collect sensor data of the ground from the air. If needed, the traditional method of extending sensor collection time is to loiter or turn circularly around the center of an AOI. Current Autopilot systems on small UAVs can be limited in their feature set and typically follow a waypoint chain system that allows for loitering, but requires that the center of the AOI to be traversed which may produce unwanted turns outside of the AOI before entering the loiter. An investigation was performed to compare the current loitering techniques against two novel smart loitering methods. The first method investigated, Tangential Loitering Path Planner (TLPP), utilized paths tangential to the AOIs to enter and exit efficiently, eliminating unnecessary turns outside of the AOI. The second method, Least Distance Loitering Path Planner (LDLPP), utilized four unique flight maneuvers that reduce transit distances while eliminating unnecessary turns outside of the AOI present in the TLPP method. Simulation results concluded that the Smart Loitering Methods provide better AOI coverage during six mission scenarios. It was also determined that the LDLPP method spends less time in transit between AOIs. The reduction in required transit time could be used for surveying additional AOIs.Aerospace2017-04-1842Article10.3390/aerospace4020023232226-43102017-04-18doi: 10.3390/aerospace4020023Jay WilhelmGarrett ClemGina Eberhart<![CDATA[Aerospace, Vol. 4, Pages 22: Vortex Lattice Simulations of Attached and Separated Flows around Flapping Wings]]>
http://www.mdpi.com/2226-4310/4/2/22
Flapping flight is an increasingly popular area of research, with applications to micro-unmanned air vehicles and animal flight biomechanics. Fast, but accurate methods for predicting the aerodynamic loads acting on flapping wings are of interest for designing such aircraft and optimizing thrust production. In this work, the unsteady vortex lattice method is used in conjunction with three load estimation techniques in order to predict the aerodynamic lift and drag time histories produced by flapping rectangular wings. The load estimation approaches are the Katz, Joukowski and simplified Leishman–Beddoes techniques. The simulations’ predictions are compared to experimental measurements from wind tunnel tests of a flapping and pitching wing. Three types of kinematics are investigated, pitch-leading, pure flapping and pitch lagging. It is found that pitch-leading tests can be simulated quite accurately using either the Katz or Joukowski approaches as no measurable flow separation occurs. For the pure flapping tests, the Katz and Joukowski techniques are accurate as long as the static pitch angle is greater than zero. For zero or negative static pitch angles, these methods underestimate the amplitude of the drag. The Leishman–Beddoes approach yields better drag amplitudes, but can introduce a constant negative drag offset. Finally, for the pitch-lagging tests the Leishman–Beddoes technique is again more representative of the experimental results, as long as flow separation is not too extensive. Considering the complexity of the phenomena involved, in the vast majority of cases, the lift time history is predicted with reasonable accuracy. The drag (or thrust) time history is more challenging.Aerospace2017-04-1842Article10.3390/aerospace4020022222226-43102017-04-18doi: 10.3390/aerospace4020022Thomas LambertNorizham Abdul RazakGrigorios Dimitriadis<![CDATA[Aerospace, Vol. 4, Pages 21: Modeling Aerodynamics, Including Dynamic Stall, for Comprehensive Analysis of Helicopter Rotors]]>
http://www.mdpi.com/2226-4310/4/2/21
To fulfill the objective of a predictive tool for rotorcraft, comprehensive analysis (CA) needs to be capable of providing both accurate and time-efficient predictions of rotor air loads and structural loads. The more recent methodology based on comprehensive analysis coupled with high-fidelity computational fluid dynamics (CFD) has shown improved predictions of air loads, but it has not the strength of computational efficiency and the versatility of stand-alone CA. The present article is concerned with modeling aerodynamics about helicopter rotors for CA. The aerodynamics about rotors are very complex, encompassing subsonic to transonic flow with unsteady, stalled behavior and 3D effects. CA treats aerodynamics as separated into local and global flows. Semi-empirical models of dynamic stall were created in the 1970s–1990s for modeling unsteady local aerodynamics, including stalled flow. Most of them fail to provide good predictions of experimental results and also suffer problems of numerical convergence. The main effort in this study is about modeling local aerodynamics based on the revised “ONERA–Hopf bifurcation model”. It is implemented in the comprehensive analysis code of ONERA according to a scheme that ensures numerical convergence. The experimental results obtained in the Wind Tunnel S1 of Modane (France) in 1991 on the Rotor 7A are considered for validation of the analysis under three flight test conditions: high-speed test, high-thrust tests with light stall and deep stall, respectively. There is a reasonable agreement between the predictions of CA with experimental results. The distinct features of the stall model are the modeling of the boundary-layer effects and the vortex-shedding phenomenon.Aerospace2017-04-1442Article10.3390/aerospace4020021212226-43102017-04-14doi: 10.3390/aerospace4020021Khiem Truong<![CDATA[Aerospace, Vol. 4, Pages 20: Perpetual Solar-Powered Flight across Regions around the World for a Year-Long Operation]]>
http://www.mdpi.com/2226-4310/4/2/20
This study aims to promote the conventional solar-powered unmanned aerial vehicle (UAV) to be used as a satellite known as a pseudo-satellite (pseudolite). The applications of UAV as a satellite are still in the initial stages because these proposed UAVs are required to fly for long hours at a specified altitude. Any solar-powered system requires extensive mission operation planning to ensure sufficient power to sustain a level flight. This study simulates the optimal UAV configurations at various global locations, and determines the feasibility of a solar-powered UAV to sustain a continuous mission. This study is divided into two different phases. An all-year operation of the average UAV (AVUAV) is simulated in Phase One and is designed specifically for each of 12 cities, namely, Ottawa, Honolulu, Quito, Tahiti, Brasilia, London, Riyadh, Tokyo, Kuala Lumpur, Accra, Port Louis, and Suva. Phase Two is a simulation of a solar-powered UAV design model known as 1UAV, applicable to any city around the world for a year-long flight. The findings state that a single UAV design is sufficient to operate continuously around the world if its detailed mission path planning has been defined.Aerospace2017-04-1142Technical Note10.3390/aerospace4020020202226-43102017-04-11doi: 10.3390/aerospace4020020Parvathy RajendranMuhammad MasralHairuniza Kutty<![CDATA[Aerospace, Vol. 4, Pages 19: Flight Dynamics and Control Using Folding Wingtips: An Experimental Study]]>
http://www.mdpi.com/2226-4310/4/2/19
This paper presents an experimental investigation on using FOLDing wingtips sERving as cONtrol effectorS (FOLDERONS) for a mini Unmanned Aerial Vehicle (UAV). A representative off-the-shelf mini-UAV with a conventional configuration was selected. The main theme of this paper is to utilise FOLDERONS as a control effector (mainly in roll) to augment the control authority of conventional control surfaces. Furthermore, the impact of actuation rate on the effectiveness of FOLDERONS is assessed. The paper describes the preliminary and detailed design and sizing of the morphing wing. In addition, the manufacturing of the wing system and its integration with the UAV are addressed. Wind-tunnel testing in the RJ Mitchell wind-tunnel at the University of Southampton was performed. Both static (straight and sideslip) and dynamic (straight flight) tests are conducted at a range of airspeeds and Angles Of Attack (AOAs). The impact of folding wingtips on the lateral and directional stability is analysed. The main finding of this paper is that FOLDERONS are effective (especially at large dynamic pressure and AOAs) in controlling the lateral and directional stability. Finally, this study shows that FOLDERONS cannot fully replace conventional ailerons especially at low dynamic pressures, and their strong dependence on the AOA makes them prone to a roll reversal phenomena when the wing (and FOLDERONS) is operating at negative AOAs.Aerospace2017-03-2642Article10.3390/aerospace4020019192226-43102017-03-26doi: 10.3390/aerospace4020019Josh MillsRafic Ajaj<![CDATA[Aerospace, Vol. 4, Pages 18: Spray Characteristics of Alternative Aviation Fuel Blends]]>
http://www.mdpi.com/2226-4310/4/2/18
The compatibility of spray characteristics of alternative fuel blends, in relation to currently used Jet A-1 fuel, has been assessed experimentally. Tested blends were selected based on a narrow cut of paraffins, mixed with appropriately selected aromatics and naphthenes. Relevant physical properties including the density, viscosity, and surface tension were estimated first. The jet spray was produced using a single fluid, generic nozzle at operating pressures 5–11 bars. The atomization characteristics were assessed through measurements of droplet velocity field and droplet size, using phase Doppler anemometry. The physical properties varied within 10% of the reference fuel values. The spray results indicate that all tested blends produced similar atomized jets and droplet sizes, although observed differences may influence the implementation of combustion schemes which require precise control of the flow pattern.Aerospace2017-03-2342Article10.3390/aerospace4020018182226-43102017-03-23doi: 10.3390/aerospace4020018Andreas VourosAlexandros VourosThrassos Panidis<![CDATA[Aerospace, Vol. 4, Pages 17: Design and Performance of Modular 3-D Printed Solid-Propellant Rocket Airframes]]>
http://www.mdpi.com/2226-4310/4/2/17
Solid-propellant rockets are used for many applications, including military technology, scientific research, entertainment, and aerospace education. This study explores a novel method for design modularization of the rocket airframes, utilizing additive manufacturing (AM) technology. The new method replaces the use of standard part subsystems with complex multi-function parts to improve customization, design flexibility, performance, and reliability. To test the effectiveness of the process, two experiments were performed on several unique designs: (1) ANSYS CFX® simulation to measure the drag coefficients, the pressure fields, and the streamlines during representative flights and (2) fabrication and launch of the developed designs to test their flight performance and consistency. Altitude and 3-axis stability was measured during the eight flights via an onboard instrument package. Data from both experiments demonstrated that the designs were effective, but varied widely in their performance; the sources of the performance differences and errors were documented and analyzed. The modularization process reduced the number of parts dramatically, while retaining good performance and reliability. The specific benefits and caveats of using extrusion-based 3-D printing to produce airframe components are also demonstrated.Aerospace2017-03-2342Article10.3390/aerospace4020017172226-43102017-03-23doi: 10.3390/aerospace4020017Rachel HernandezHarpreet SinghSherri MessimerAlbert Patterson<![CDATA[Aerospace, Vol. 4, Pages 16: An Improved Electron Pre-Sheath Model for TSS-1R Current Enhancement Computations]]>
http://www.mdpi.com/2226-4310/4/1/16
This report presents improvements of investigations on the Tethered Satellite System (TSS)-1R electron current enhancement due to magnetic limited collections. New analytical expressions are obtained for the potential and temperature changes across the pre-sheath. The mathematical treatments in this work are more rigorous than one past approach. More experimental measurements collected in the ionosphere during the TSS-1R mission are adopted for validations. The relations developed in this work offer two bounding curves for these data points quite successfully; the average of these two curves is close to the curve-fitting results for the measurements; and an average of 2.95 times larger than the Parker-Murphy theory is revealed. The results indicate that including the pre-sheath analysis is important to compute the electron current enhancement due to magnetic limitations.Aerospace2017-03-1641Article10.3390/aerospace4010016162226-43102017-03-16doi: 10.3390/aerospace4010016Chunpei Cai<![CDATA[Aerospace, Vol. 4, Pages 15: Chirp Signals and Noisy Waveforms for Solid-State Surveillance Radars]]>
http://www.mdpi.com/2226-4310/4/1/15
Since the advent of “pulse compression” radar, the “chirp” signal (Linear Frequency Modulation, LFM) has been one of the most widely used radar waveforms. It is well known that, by changing its modulation into a Non-Linear Frequency Modulation (NLFM), better performance in terms of Peak-to-Sidelobes Ratio (PSLR) can be achieved to mitigate the masking effect of nearby targets and to increase the useful dynamic range. Adding an appropriate amplitude modulation, as occurs in Hybrid-NLFM (HNLFM), the PSLR can reach very low values (e.g., PSLR &lt; −60 dB), comparable to the two-way antenna sidelobes in azimuth. On the other hand, modern solid-state power amplifier technology, using low-power modules, requires them to be combined at the Radio Frequency (RF) stage in order to achieve the desired transmitted power. Noise Radar Technology (NRT) represents a valid alternative to deterministic waveforms. It makes use of pseudo-random waveforms—realizations of a noise process. The higher its time-bandwidth (or BT) product, the higher the (statistical) PSLR. With practical BT values, the achievable PSLR using pure random noise is generally not sufficient. Therefore, the generated pseudorandom waveforms can be “tailored” (TPW: Tailored Pseudorandom Waveforms) at will through suitable algorithms in order to achieve the desired sidelobe level, even only in a limited range interval, as shown in this work. Moreover, the needed high BT, i.e., the higher time duration T having fixed the bandwidth B, matches well with the low power solid-state amplifiers of Noise Radar. Focusing the interest on (civil) surveillance radar applications, such as ATC (Air Traffic Control) and marine radar, this paper proposes a general review of the two classes of waveforms, i.e., HNLFM and TPW.Aerospace2017-03-1441Article10.3390/aerospace4010015152226-43102017-03-14doi: 10.3390/aerospace4010015Gaspare GalatiGabriele PavanFrancesco De Palo<![CDATA[Aerospace, Vol. 4, Pages 14: Optimization of Heat Exchangers for Intercooled Recuperated Aero Engines]]>
http://www.mdpi.com/2226-4310/4/1/14
In the framework of the European research project LEMCOTEC, a section was devoted to the further optimization of the recuperation system of the Intercooled Recuperated Aero engine (IRA engine) concept, of MTU Aero Engines AG. This concept is based on an advanced thermodynamic cycle combining both intercooling and recuperation. The present work is focused only on the recuperation process. This is carried out through a system of heat exchangers mounted inside the hot-gas exhaust nozzle, providing fuel economy and reduced pollutant emissions. The optimization of the recuperation system was performed using computational fluid dynamics (CFD) computations, experimental measurements and thermodynamic cycle analysis for a wide range of engine operating conditions. A customized numerical tool was developed based on an advanced porosity model approach. The heat exchangers were modeled as porous media of predefined heat transfer and pressure loss behaviour and could also incorporate major and critical heat exchanger design decisions in the CFD computations. The optimization resulted in two completely new innovative heat exchanger concepts, named as CORN (COnical Recuperative Nozzle) and STARTREC (STraight AnnulaR Thermal RECuperator), which provided significant benefits in terms of fuel consumption, pollutants emission and weight reduction compared to more conventional heat exchanger designs, thus proving that further optimization potential for this technology exists.Aerospace2017-03-1341Article10.3390/aerospace4010014142226-43102017-03-13doi: 10.3390/aerospace4010014Dimitrios MisirlisZinon VlahostergiosMichael FlourosChristina SalpingidouStefan DonnerhackApostolos GoulasKyros Yakinthos<![CDATA[Aerospace, Vol. 4, Pages 13: CFD Study of a New Annular Lift Fan Configuration with High Lift Efficiency]]>
http://www.mdpi.com/2226-4310/4/1/13
A new annular lift fan configuration that has very high lift efficiency is explored by using a numerical scheme. The inlet lip radius and diffuser angle are maximized by semicircle duct walls and the location of the lift fan is moved from the throat to the diffuser area to maximize the diffusion effect of the ducted fan. The improved lift fan achieves the figure of merit of 0.772 and the power loading of 9.03 lbs/hp without ground effect, very close to the theoretical limit. Under the ground effect, the figure of merit reaches 0.822 with the power loading of 9.62 lbs/hp. The improved lift efficiency deteriorates the transition characteristics with higher momentum drag and pitching moment. However, with the aid of jet thrusts directly providing part of the lift during transition, the peak of momentum drag and pitching moment can be lowered. A total thrust to weight ratio of 0.7 is enough for all of the requirements in transition and in hover and for the maximum speed of 0.75 Mach in cruise flight.Aerospace2017-03-0341Article10.3390/aerospace4010013132226-43102017-03-03doi: 10.3390/aerospace4010013Yun JiangBo Zhang<![CDATA[Aerospace, Vol. 4, Pages 11: Sense and Avoid Airborne Radar Implementations on a Low-Cost Weather Radar Platform]]>
http://www.mdpi.com/2226-4310/4/1/11
Traditionally, multi-mission applications in airborne radar are implemented through very expensive phased array architectures. The emerging applications from civilian surveillance, on the other hand, prefer low-cost and low-SWaP (space, weight and power) systems. This study introduces asoftware-basedsolutionthatintendstouselow-costhardwareandadvancedalgorithms/processing backend to meet the remote sensing goals for multi-mission applications. The low-cost airborne radar platform from Garmin International is used as a representative example of the system platform. The focus of this study is the optimal operating mode, data quality and algorithm development in cases of all-weather sense and avoid (SAA) applications. The main challenges for the solution are the resolution limitation due to the small aperture size, limitations from the ﬁeld-of-view (FOV) and the scan speed from mechanical scanning. We show that the basic operational needs can be satisﬁed with software processing through various algorithms. The concept and progress of polarimetric airborne radar for dual-function operations at X-band Generation 1 (PARADOX1) based on the platform are also discussed.Aerospace2017-03-0141Article10.3390/aerospace4010011112226-43102017-03-01doi: 10.3390/aerospace4010011Ramesh NepalYan ZhangWilliam Blake<![CDATA[Aerospace, Vol. 4, Pages 12: Effects of Varied Shear Correction on the Thermal Vibration of Functionally-Graded Material Shells in an Unsteady Supersonic Flow]]>
http://www.mdpi.com/2226-4310/4/1/12
A model is presented for functionally-graded material (FGM), thick, circular cylindrical shells under an unsteady supersonic flow, following first-order shear deformation theory (FSDT) with varied shear correction coefficients. Some interesting vibration results of the dynamics are calculated by using the generalized differential quadrature (GDQ) method. The varied shear correction coefficients are usually functions of FGM total thickness, power law index, and environment temperature. Two parametric effects of the environmental temperature and FGM power law index on the thermal stress and center deflection are also presented. The novelty of the paper is that the maximum flutter value of the center deflection amplitude can be predicted and occurs at a high frequency of applied heat flux for a supersonic air flow.Aerospace2017-03-0141Article10.3390/aerospace4010012122226-43102017-03-01doi: 10.3390/aerospace4010012Chih Hong<![CDATA[Aerospace, Vol. 4, Pages 10: 3D CFD Simulation and Experimental Validation of Small APC Slow Flyer Propeller Blade]]>
http://www.mdpi.com/2226-4310/4/1/10
The current work presents the numerical prediction method to determine small-scale propeller performance. The study is implemented using the commercially available computational fluid dynamics (CFD) solver, FLUENT. Numerical results are compared with the available experimental data for an advanced precision composites (APC) Slow Flyer propeller blade to determine the discrepancy of the thrust coefficient, power coefficient, and efficiencies. The study utilized unstructured tetrahedron meshing throughout the analysis, with a standard k-ω turbulence model. The Multiple Reference Frame model was also used to consider the rotation of the propeller toward its local reference frame at 3008 revolutions per minute (RPM). Results show reliable thrust coefficient, power coefficient, and efficiency data for the case of low advance ratio and an advance ratio less than the negative thrust conditions.Aerospace2017-02-2541Technical Note10.3390/aerospace4010010102226-43102017-02-25doi: 10.3390/aerospace4010010Hairuniza KuttyParvathy Rajendran<![CDATA[Aerospace, Vol. 4, Pages 9: Control of Triple-Shock Configurations and Vortex Structures Forming in High Speed Flows of Gaseous Media past an AD Body under the Action of External Energy Sources]]>
http://www.mdpi.com/2226-4310/4/1/9
The problem of supersonic streamlining of an aerodynamic (AD) body, “a plate blunted by a cylinder”, by a flow with the freestream Mach number M = 4 containing an external energy source has been studied, taking into account physicochemical transformations. The results of the effect of the ratio of specific heats γ changing in the range from 1.1 to 1.4 on the dynamics of triple-shock configurations and vortex-contact structures are presented for the interaction of an energy source with the bow shock wave. The energy source is modeled via the heated rarefied layer (filament). The angles in the triple-shock configurations, the stagnation pressure, together with the frontal drag force, have been studied dependent on the specific heats ratio γ, the characteristics of the energy source, and also on the angle of the incident shock. Vortex-contact structures have been researched for the Mach numbers 7, 8, 9, as well as the generation of the Richtmyer-Meshkov instability accompanying the formation of a triple-shock configuration. The results show a strong influence of the specific heats ratio of the gas medium and the parameters of the energy source on the triple-shock configuration and aerodynamic characteristics of the body. This conclusion can be useful for aerospace applications in the area of the design of nozzles, intakes, and high speed flying vehicles. Additionally, the results show the possibility of flow control in the atmospheres of other planets using external energy deposition.Aerospace2017-02-2041Article10.3390/aerospace401000992226-43102017-02-20doi: 10.3390/aerospace4010009Olga AzarovaLudmila Gvozdeva<![CDATA[Aerospace, Vol. 4, Pages 8: On Multirate GARK Schemes with Adaptive Micro Step Sizes for Fluid–Structure Interaction: Order Conditions and Preservation of the Geometric Conservation Law]]>
http://www.mdpi.com/2226-4310/4/1/8
The application of partitioned schemes to ﬂuid–structure interaction (FSI) allows the use of already developed solvers specifically designed for the efficient solution of the corresponding subproblems. In this work, we propose and describe a loosely coupled partitioned scheme based on the recently introduced generalized-structure additively partitioned Runge-Kutta (GARK) framework. The resulting scheme combines implicit-explicit (IMEX) and multirate approaches while coupling of the subproblems is realized both on the level of the discrete time steps and at the level of interior Runge-Kutta stages. Specifically, we allow for varying micro step sizes for the fluid subproblem and therefore extend the multirate GARK framework based on constant micro steps. Furthermore, we derive the order conditions for this extension allowing for coupled time integration schemes of up to third order and discuss specific choices of the Runge-Kutta coefficients complying with the geometric conservation law. Finally, numerical experiments are carried out for uniform flow on a moving grid as well as the classical FSI test case of a moving piston.Aerospace2017-02-1441Article10.3390/aerospace401000882226-43102017-02-14doi: 10.3390/aerospace4010008Sascha Bremicker-TrübelhornSigrun Ortleb<![CDATA[Aerospace, Vol. 4, Pages 7: Electromagnetic Simulation and Alignment of Dual-Polarized Array Antennas in Multi-Mission Phased Array Radars]]>
http://www.mdpi.com/2226-4310/4/1/7
Electromagnetic (EM) simulation of dual-polarized antennas is necessary for precise initial alignments, calibration and performance predictions of multi-function phased array radar systems. To achieve the required flexibility and scalability, a novel Finite-Difference Time-Domain (FDTD) solution is developed for rectangular, cylindrical and non-orthogonal coordinate systems to simulate various types of array antenna manifolds. Scalable array pattern predictions and beam generations are obtained by combining the FDTD simulation solutions with the Near-Field (NF) chamber measurements. The effectiveness and accuracy of this approach are validated by comparing different simulations and comparing simulations with measurements.Aerospace2017-02-1041Article10.3390/aerospace401000772226-43102017-02-10doi: 10.3390/aerospace4010007Sudantha PereraYan ZhangDusan ZrnicRichard Doviak<![CDATA[Aerospace, Vol. 4, Pages 6: Parametric Approach to Assessing Performance of High-Lift Device Active Flow Control Architectures]]>
http://www.mdpi.com/2226-4310/4/1/6
Active Flow Control is at present an area of considerable research, with multiple potential aircraft applications. While the majority of research has focused on the performance of the actuators themselves, a system-level perspective is necessary to assess the viability of proposed solutions. This paper demonstrates such an approach, in which major system components are sized based on system flow and redundancy considerations, with the impacts linked directly to the mission performance of the aircraft. Considering the case of a large twin-aisle aircraft, four distinct active flow control architectures that facilitate the simplification of the high-lift mechanism are investigated using the demonstrated approach. The analysis indicates a very strong influence of system total mass flow requirement on architecture performance, both for a typical mission and also over the entire payload-range envelope of the aircraft.Aerospace2017-02-1041Article10.3390/aerospace401000662226-43102017-02-10doi: 10.3390/aerospace4010006Yu CaiZhenyu GaoImon ChakrabortySimon BricenoDimitri Mavris<![CDATA[Aerospace, Vol. 4, Pages 5: A New Gaskinetic Model to Analyze Background Flow Effects on Weak Gaseous Jet Flows from Electric Propulsion Devices]]>
http://www.mdpi.com/2226-4310/4/1/5
Recent work on studying rarefied background and jet flow interactions is reported. A new gaskinetic method is developed to investigate two closely related problems. The first problem is how a collisionless background flow can affect a highly rarefied jet flow. The rarefied jet and background flow conditions are assumed available and described with seven parameters. Gaskinetic theories are applied and formulas are obtained for the mixture properties. Simulations are performed to validate these expressions, and excellent agreement is obtained. The second problem is to recover the collisionless background and jet flow parameters with limited measurements. A group of linearized equations are derived for the flowfield properties. The solving process includes initial estimations on the seven parameters, followed with iterations. Numerical tests are performed and the results indicate the procedure is accurate and efficient. The new method and expressions can reduce the amount of experimental work and numerical simulations to analyze facility effects. Parameter studies with particle simulations may require several months; however, the new methods may require minutes. These methods can be used to quantify and predict jet performance, vacuum chamber designs and optimization. Applications may be for many societies using vacuum conditions.Aerospace2017-01-2741Article10.3390/aerospace401000552226-43102017-01-27doi: 10.3390/aerospace4010005Chunpei Cai<![CDATA[Aerospace, Vol. 4, Pages 4: SMA-Based System for Environmental Sensors Released from an Unmanned Aerial Vehicle]]>
http://www.mdpi.com/2226-4310/4/1/4
In the work at hand, a shape memory alloy (SMA)-based system is presented. The system, conceived for releasing environmental sensors from ground or small unmanned aerial vehicles, UAV (often named UAS, unmanned aerial system), is made of a door, integrated into the bottom of the fuselage, a device distributor, operated by a couple of antagonistic SMA springs, and a kinematic chain, to synchronize the deployment operation with the system movement. On the basis of the specifications (weight, available space, energy supply, sensors size, etc.), the system design was addressed. After having identified the main system characteristics, a representative mock-up was manufactured, featuring the bottom part of the reference fuselage. Functionality tests were performed to prove the system capability to release the sensors; a detailed characterization was finally carried out, mainly finalized at correlating the kinematic chain displacement with the SMA spring temperature and the supplied electrical power. A comparison between theoretical predictions and experimental outcomes showed good agreement.Aerospace2017-01-2441Article10.3390/aerospace401000442226-43102017-01-24doi: 10.3390/aerospace4010004Lorenzo PelloneSalvatore AmeduriNunzia FavaloroAntonio Concilio<![CDATA[Aerospace, Vol. 4, Pages 3: Trajectory Tracking of a Tri-Rotor Aerial Vehicle Using an MRAC-Based Robust Hybrid Control Algorithm]]>
http://www.mdpi.com/2226-4310/4/1/3
In this paper, a novel Model Reference Adaptive Control (MRAC)-based hybrid control algorithm is presented for the trajectory tracking of a tri-rotor Unmanned Aerial Vehicle (UAV). The mathematical model of the tri-rotor is based on the Newton–Euler formula, whereas the MRAC-based hybrid controller consists of Fuzzy Proportional Integral Derivative (F-PID) and Fuzzy Proportional Derivative (F-PD) controllers. MRAC is used as the main controller for the dynamics, while the parameters of the adaptive controller are fine-tuned by the F-PD controller for the altitude control subsystem and the F-PID controller for the attitude control subsystem of the UAV. The stability of the system is ensured and proven by Lyapunov stability analysis. The proposed control algorithm is tested and verified using computer simulations for the trajectory tracking of the desired path as an input. The effectiveness of our proposed algorithm is compared with F-PID and the Fuzzy Logic Controller (FLC). Our proposed controller exhibits much less steady state error, quick error convergence in the presence of disturbance or noise, and model uncertainties.Aerospace2017-01-1941Article10.3390/aerospace401000332226-43102017-01-19doi: 10.3390/aerospace4010003Zain AliDaobo WangMuhammad AamirSuhaib Masroor<![CDATA[Aerospace, Vol. 4, Pages 2: Manganese and Zinc Spinel Ferrites Blended with Multi-Walled Carbon Nanotubes as Microwave Absorbing Materials]]>
http://www.mdpi.com/2226-4310/4/1/2
Magnetic and dielectric materials can be blended to enhance absorption properties at microwave frequencies, although the materials may have relatively weak attenuation capabilities by themselves. The specific goal of this work is to enhance microwave absorption properties of materials with interesting dielectric behavior by blending them with magnetic materials based on transition metals. The synthesized Mn1−xZnxFe2O4 (x = 0.0 and 1.0) spinel ferrite nanoparticles (MZF NPs) were blended with commercial multi-walled carbon nanotubes (MWCNTs) in various proportions with a binder matrix of paraffin. This simple and efficient process did not cause a significant variation in the energy states of MWCNTs. MZF NPs were synthesized with a citric acid assisted sol–gel method. Their electromagnetic characteristics and microwave absorption properties were investigated. These properties were derived from the microwave scattering parameters measured via the transmission line technique by using a vector network analyzer (VNA) in conjunction with an X band waveguide system. The return loss (RL) values of the samples were obtained from the electromagnetic constitutive parameters (permittivity and permeability). The results indicate that the minimum RL value and the bandwidth change significantly with the amount of ferrite material in the blend. These results encourage further development of MWCNTs blended with ferrite nanoparticles for broadband microwave applications.Aerospace2017-01-1441Article10.3390/aerospace401000222226-43102017-01-14doi: 10.3390/aerospace4010002Ahmet TeberKadir CilTurgut YilmazBusra EraslanDilara UysalGokce SurucuAbdul BaykalRajeev Bansal<![CDATA[Aerospace, Vol. 4, Pages 1: Acknowledgement to Reviewers of Aerospace in 2016]]>
http://www.mdpi.com/2226-4310/4/1/1
The editors of Aerospace would like to express their sincere gratitude to the following reviewers for assessing manuscripts in 2016.[...]Aerospace2017-01-1041Editorial10.3390/aerospace401000112226-43102017-01-10doi: 10.3390/aerospace4010001 Aerospace Editorial Office<![CDATA[Aerospace, Vol. 3, Pages 46: Correction: Iemma, U. Theoretical and Numerical Modeling of Acoustic Metamaterials for Aeroacoustic Applications. Aerospace 2016, 3, 15]]>
http://www.mdpi.com/2226-4310/3/4/46
The author regrets that this paper [1] contains a typographical error in Equation (1) [...]Aerospace2016-12-2234Correction10.3390/aerospace3040046462226-43102016-12-22doi: 10.3390/aerospace3040046Umberto Iemma<![CDATA[Aerospace, Vol. 3, Pages 45: A Two-Temperature Open-Source CFD Model for Hypersonic Reacting Flows, Part Two: Multi-Dimensional Analysis †]]>
http://www.mdpi.com/2226-4310/3/4/45
hy2Foam is a newly-coded open-source two-temperature computational fluid dynamics (CFD) solver that has previously been validated for zero-dimensional test cases. It aims at (1) giving open-source access to a state-of-the-art hypersonic CFD solver to students and researchers; and (2) providing a foundation for a future hybrid CFD-DSMC (direct simulation Monte Carlo) code within the OpenFOAM framework. This paper focuses on the multi-dimensional verification of hy2Foam and firstly describes the different models implemented. In conjunction with employing the coupled vibration-dissociation-vibration (CVDV) chemistry–vibration model, novel use is made of the quantum-kinetic (QK) rates in a CFD solver. hy2Foam has been shown to produce results in good agreement with previously published data for a Mach 11 nitrogen flow over a blunted cone and with the dsmcFoam code for a Mach 20 cylinder flow for a binary reacting mixture. This latter case scenario provides a useful basis for other codes to compare against.Aerospace2016-12-1434Article10.3390/aerospace3040045452226-43102016-12-14doi: 10.3390/aerospace3040045Vincent CasseauDaniel EspinozaThomas ScanlonRichard Brown<![CDATA[Aerospace, Vol. 3, Pages 43: Gaskinetic Modeling on Dilute Gaseous Plume Impingement Flows]]>
http://www.mdpi.com/2226-4310/3/4/43
This paper briefly reviews recent work on gaseous plume impingement flows. As the major part of this paper, also included are new comprehensive studies on high-speed, collisionless, gaseous, circular jet impinging on a three-dimensional, inclined, diffuse or specular flat plate. Gaskinetic theories are adopted to study the problems, and several crucial geometry-location and velocity-direction relations are used. The final complete results include impingement surface properties such as pressure, shear stress, and heat flux. From these surface properties, averaged coefficients of pressure, friction, heat flux, moment over the entire flat plate, and the distance from the moment center to the flat plate center are obtained. The final results include accurate integrations involving the geometry and specific speed ratios, inclination angle, and the temperature ratio. Several numerical simulations with the direct simulation Monte Carlo method validate these analytical results, and the results are essentially identical. The gaskinetic method and processes are heuristic and can be used to investigate other external high Knudsen (Kn) number impingement flow problems, including the flow field and surface properties for a high Knudsen number jet from an exit and flat plate of arbitrary shapes. The results are expected to find many engineering applications, especially in aerospace and space engineering.Aerospace2016-12-0934Article10.3390/aerospace3040043432226-43102016-12-09doi: 10.3390/aerospace3040043Chunpei Cai<![CDATA[Aerospace, Vol. 3, Pages 44: Continuation Methods for Nonlinear Flutter]]>
http://www.mdpi.com/2226-4310/3/4/44
Continuation methods are presented that are capable of treating frequency domain flutter equations, including multiple nonlinearities represented by describing functions. A small problem demonstrates how a series of continuation processes can find all limit-cycle oscillations within a specified region with a reasonable degree of confidence. Curves of the limit-cycle amplitude variation with velocity, indicating regions of stability and instability with colors, give a compact view of the nonlinear behavior throughout the flight regime. A continuation technique for reducing limit-cycle amplitudes by adjusting various system parameters is presented. These processes are economical enough to be a routine part of aircraft design and certification.Aerospace2016-12-0934Article10.3390/aerospace3040044442226-43102016-12-09doi: 10.3390/aerospace3040044Edward Meyer<![CDATA[Aerospace, Vol. 3, Pages 42: Analysis of Pilot-Induced-Oscillation and Pilot Vehicle System Stability Using UAS Flight Experiments]]>
http://www.mdpi.com/2226-4310/3/4/42
This paper reports the results of a Pilot-Induced Oscillation (PIO) and human pilot control characterization study performed using flight data collected with a Remotely Controlled (R/C) unmanned research aircraft. The study was carried out on the longitudinal axis of the aircraft. Several existing Category 1 and Category 2 PIO criteria developed for manned aircraft are first surveyed and their effectiveness for predicting the PIO susceptibility for the R/C unmanned aircraft is evaluated using several flight experiments. It was found that the Bandwidth/Pitch rate overshoot and open loop onset point (OLOP) criteria prediction results matched flight test observations. However, other criteria failed to provide accurate prediction results. To further characterize the human pilot control behavior during these experiments, a quasi-linear pilot model is used. The parameters of the pilot model estimated using data obtained from flight tests are then used to obtain information about the stability of the Pilot Vehicle System (PVS) for Category 1 PIOs occurred during straight and level flights. The batch estimation technique used to estimate the parameters of the quasi-linear pilot model failed to completely capture the compatibility nature of the human pilot. The estimation results however provided valuable insights into the frequency characteristics of the human pilot commands. Additionally, stability analysis of the Category 2 PIOs for elevator actuator rate limiting is carried out using simulations and the results are compared with actual flight results.Aerospace2016-11-2934Article10.3390/aerospace3040042422226-43102016-11-29doi: 10.3390/aerospace3040042Tanmay MandalYu Gu<![CDATA[Aerospace, Vol. 3, Pages 41: On the Importance of Morphing Deformation Scheduling for Actuation Force and Energy]]>
http://www.mdpi.com/2226-4310/3/4/41
Morphing aircraft offer superior properties as compared to non-morphing aircraft. They can achieve this by adapting their shape depending on the requirements of various conflicting flight conditions. These shape changes are often associated with large deformations and strains, and hence dedicated morphing concepts are developed to carry out the required changes in shape. Such intricate mechanisms are often heavy, which reduces, or even completely cancels, the performance increase of the morphing aircraft. Part of this weight penalty is determined by the required actuators and associated batteries, which are mainly driven by the required actuation force and energy. Two underexposed influences on the actuation force and energy are the flight condition at which morphing should take place and the order of the morphing manoeuvres, also called morphing scheduling. This paper aims at highlighting the importance of both influences by using a small Unmanned Aerial Vehicle (UAV) with different morphing mechanisms as an example. The results in this paper are generated using a morphing aircraft analysis and design code that was developed at the Delft University of Technology. The importance of the flight condition and a proper morphing schedule is demonstrated by investigating the required actuation forces for various flight conditions and morphing sequences. More importantly, the results show that there is not necessarily one optimal flight condition or morphing schedule and a tradeoff needs to be made.Aerospace2016-11-2534Article10.3390/aerospace3040041412226-43102016-11-25doi: 10.3390/aerospace3040041Roeland De BreukerNoud Werter<![CDATA[Aerospace, Vol. 3, Pages 40: Recent Advances in Aeroacoustics]]>
http://www.mdpi.com/2226-4310/3/4/40
Acoustics is one of the oldest examples of applied research, long before the term was even coined: [...]Aerospace2016-11-2334Editorial10.3390/aerospace3040040402226-43102016-11-23doi: 10.3390/aerospace3040040Luís Campos<![CDATA[Aerospace, Vol. 3, Pages 38: Climate-Compatible Air Transport System—Climate Impact Mitigation Potential for Actual and Future Aircraft]]>
http://www.mdpi.com/2226-4310/3/4/38
Aviation guarantees mobility, but its emissions also contribute considerably to climate change. Therefore, climate impact mitigation strategies have to be developed based on comprehensive assessments of the different impacting factors. We quantify the climate impact mitigation potential and related costs resulting from changes in aircraft operations and design using a multi-disciplinary model workflow. We first analyze the climate impact mitigation potential and cash operating cost changes of altered cruise altitudes and speeds for all flights globally operated by the Airbus A330-200 fleet in the year 2006. We find that this globally can lead to a 42% reduction in temperature response at a 10% cash operating cost increase. Based on this analysis, new design criteria are derived for future aircraft that are optimized for cruise conditions with reduced climate impact. The newly-optimized aircraft is re-assessed with the developed model workflow. We obtain additional climate mitigation potential with small to moderate cash operating cost changes due to the aircraft design changes of, e.g., a 32% and 54% temperature response reduction for a 0% and 10% cash operating cost increase. Hence, replacing the entire A330-200 fleet by this redesigned aircraft ( M a c r = 0.72 and initial cruise altitude (ICA) = 8000 m) could reduce the climate impact by 32% without an increase of cash operating cost.Aerospace2016-11-1734Article10.3390/aerospace3040038382226-43102016-11-17doi: 10.3390/aerospace3040038Katrin DahlmannAlexander KochFlorian LinkeBenjamin LührsVolker GreweTom OttenDoreen SeiderVolker GollnickUlrich Schumann<![CDATA[Aerospace, Vol. 3, Pages 39: Effect of Leading-Edge Slats at Low Reynolds Numbers]]>
http://www.mdpi.com/2226-4310/3/4/39
One of the most commonly implemented devices for stall control on wings and airfoils is a leading-edge slat. While functioning of slats at high Reynolds number is well documented, this is not the case at the low Reynolds numbers common for small unmanned aerial vehicles. Consequently, a low-speed wind tunnel investigation was undertaken to elucidate the performance of a slat at Re = 250,000. Force balance measurements accompanied by surface flow visualization images are presented. The slat extension and rotation was varied and documented. The results indicate that for small slat extensions, slat rotation is deleterious to performance, but is required for larger slat extensions for effective lift augmentation. Deployment of the slat was accompanied by a significant drag penalty due to premature localized flow separation.Aerospace2016-11-1734Communication10.3390/aerospace3040039392226-43102016-11-17doi: 10.3390/aerospace3040039Lance TraubMashaan Kaula<![CDATA[Aerospace, Vol. 3, Pages 37: Results of Long-Duration Simulation of Distant Retrograde Orbits]]>
http://www.mdpi.com/2226-4310/3/4/37
Distant Retrograde Orbits in the Earth–Moon system are gaining in popularity as stable “parking” orbits for various conceptual missions. To investigate the stability of potential Distant Retrograde Orbits, simulations were executed, with propagation running over a thirty-year period. Initial conditions for the vehicle state were limited such that the position and velocity vectors were in the Earth–Moon orbital plane, with the velocity oriented such that it would produce retrograde motion about Moon. The resulting trajectories were investigated for stability in an environment that included the eccentric motion of Moon, non-spherical gravity of Earth and Moon, gravitational perturbations from Sun, Jupiter, and Venus, and the effects of radiation pressure. The results indicate that stability may be enhanced at certain resonant states within the Earth–Moon system.Aerospace2016-11-0834Article10.3390/aerospace3040037372226-43102016-11-08doi: 10.3390/aerospace3040037Gary Turner<![CDATA[Aerospace, Vol. 3, Pages 35: Numerical Investigation of Effect of Parameters on Hovering Efficiency of an Annular Lift Fan Aircraft]]>
http://www.mdpi.com/2226-4310/3/4/35
The effects of various parameters on the hovering performance of an annular lift fan aircraft are investigated by using numerical scheme. The pitch angle, thickness, aspect ratio (chord length), number of blades, and radius of duct inlet lip are explored to optimize the figure of merit. The annular lift fan is also compared with a conventional circular lift fan of the same features with the same disc loading and similar geometry. The simulation results show that the pitch angle of 27°, the thickness of 4% chord length, the aspect ratio of 3.5~4.0, 32 blades, and the radius of inlet lip of 4.7% generate the maximum figure of merit of 0.733. The optimized configuration can be used for further studies of the annular lift fan aircraft.Aerospace2016-10-1934Article10.3390/aerospace3040035352226-43102016-10-19doi: 10.3390/aerospace3040035Yun JiangBo Zhang<![CDATA[Aerospace, Vol. 3, Pages 36: Wing Tip Drag Reduction at Nominal Take-Off Mach Number: An Approach to Local Active Flow Control with a Highly Robust Actuator System]]>
http://www.mdpi.com/2226-4310/3/4/36
This paper discusses wind tunnel test results aimed at advancing active flow control technology to increase the aerodynamic efficiency of an aircraft during take-off. A model of the outer section of a representative civil airliner wing was equipped with two-stage fluidic actuators between the slat edge and wing tip, where mechanical high-lift devices fail to integrate. The experiments were conducted at a nominal take-off Mach number of M = 0.2. At this incidence velocity, separation on the wing section, accompanied by increased drag, is triggered by the strong slat edge vortex at high angles of attack. On the basis of global force measurements and local static pressure data, the effect of pulsed blowing on the complex flow is evaluated, considering various momentum coefficients and spanwise distributions of the actuation effort. It is shown that through local intensification of forcing, a momentum coefficient of less than c μ = 0.6 % suffices to offset the stall by 2.4°, increase the maximum lift by more than 10% and reduce the drag by 37% compared to the uncontrolled flow.Aerospace2016-10-1934Article10.3390/aerospace3040036362226-43102016-10-19doi: 10.3390/aerospace3040036Matthias BauerThomas GrundWolfgang NitscheVlad Ciobaca<![CDATA[Aerospace, Vol. 3, Pages 34: A Two-Temperature Open-Source CFD Model for Hypersonic Reacting Flows, Part One: Zero-Dimensional Analysis]]>
http://www.mdpi.com/2226-4310/3/4/34
A two-temperature CFD (computational fluid dynamics) solver is a prerequisite to any spacecraft re-entry numerical study that aims at producing results with a satisfactory level of accuracy within realistic timescales. In this respect, a new two-temperature CFD solver, hy2Foam, has been developed within the framework of the open-source CFD platform OpenFOAM for the prediction of hypersonic reacting flows. This solver makes the distinct juncture between the trans-rotational and multiple vibrational-electronic temperatures. hy2Foam has the capability to model vibrational-translational and vibrational-vibrational energy exchanges in an eleven-species air mixture. It makes use of either the Park TTv model or the coupled vibration-dissociation-vibration (CVDV) model to handle chemistry-vibration coupling and it can simulate flows with or without electronic energy. Verification of the code for various zero-dimensional adiabatic heat baths of progressive complexity has been carried out. hy2Foam has been shown to produce results in good agreement with those given by the CFD code LeMANS (The Michigan Aerothermodynamic Navier-Stokes solver) and previously published data. A comparison is also performed with the open-source DSMC (direct simulation Monte Carlo) code dsmcFoam. It has been demonstrated that the use of the CVDV model and rates derived from Quantum-Kinetic theory promote a satisfactory consistency between the CFD and DSMC chemistry modules.Aerospace2016-10-1834Article10.3390/aerospace3040034342226-43102016-10-18doi: 10.3390/aerospace3040034Vincent CasseauRodrigo PalhariniThomas ScanlonRichard Brown<![CDATA[Aerospace, Vol. 3, Pages 33: Linearized Euler Equations for the Determination of Scattering Matrices for Orifice and Perforated Plate Configurations in the High Mach Number Regime]]>
http://www.mdpi.com/2226-4310/3/4/33
The interaction of a plane acoustic wave and a sheared flow is numerically investigated for simple orifice and perforated plate configurations in an isolated, non-resonant environment for Mach numbers up to choked conditions in the holes. Analytical derivations found in the literature are not valid in this regime due to restrictions to low Mach numbers and incompressible conditions. To allow for a systematic and detailed parameter study, a low-cost hybrid Computational Fluid Dynamic/Computational Aeroacoustic (CFD/CAA) methodology is used. For the CFD simulations, a standard k–ϵ Reynolds-Averaged Navier–Stokes (RANS) model is employed, while the CAA simulations are based on frequency space transformed linearized Euler equations (LEE), which are discretized in a stabilized Finite Element method. Simulation times in the order of seconds per frequency allow for a detailed parameter study. From the application of the Multi Microphone Method together with the two-source location procedure, acoustic scattering matrices are calculated and compared to experimental findings showing very good agreement. The scattering properties are presented in the form of scattering matrices for a frequency range of 500–1500 Hz.Aerospace2016-10-1734Article10.3390/aerospace3040033332226-43102016-10-17doi: 10.3390/aerospace3040033Moritz SchulzeMichael WagnerThomas Sattelmayer<![CDATA[Aerospace, Vol. 3, Pages 32: An Empirical Study of Overlapping Rotor Interference for a Small Unmanned Aircraft Propulsion System]]>
http://www.mdpi.com/2226-4310/3/4/32
The majority of research into full-sized helicopter overlapping propulsion systems involves co-axial setups (fully overlapped). Partially overlapping rotor setups (tandem, multirotor) have received less attention, and empirical data produced over the years is limited. The increase in demand for compact small unmanned aircraft has exposed the need for empirical investigations of overlapping propulsion systems at a small scale (Reynolds Number &lt; 250,000). Rotor-to-rotor interference at the static state in various overlapping propulsion system configurations was empirically measured using off the shelf T-Motor 16 inch × 5.4 inch rotors. A purpose-built test rig was manufactured allowing various overlapping rotor configurations to be tested. First, single rotor data was gathered, then performance measurements were taken at different thrust and tip speeds on a range of overlap configurations. The studies were conducted in a system torque balance mode. Overlapping rotor performance was compared to an isolated dual rotor propulsion system revealing interference factors which were compared to the momentum theory. Tests revealed that in the co-axial torque-balanced propulsion system the upper rotor outperforms the lower rotor at axial separation ratios between 0.05 and 0.85. Additionally, in the same region, thrust sharing between the two rotors changed by 21%; the upper rotor produced more thrust than the lower rotor at all times. Peak performance was recorded as a 22% efficiency loss when the axial separation ratio was greater than 0.25. The performance of a co-axial torque-balanced system reached a 27% efficiency loss when the axial separation ratio was equal to 0.05. The co-axial system swirl recovery effect was recorded to have a 4% efficiency gain in the axial separation ratio region between 0.05 and 0.85. The smallest efficiency loss (3%) was recorded when the rotor separation ratio was between 0.95 and 1 (axial separation ratio was kept at 0.05). Tests conducted at a rotor separation ratio of 0.85 showed that the efficiency loss decreased when the axial separation ratio was greater than 0.25. The lower rotor outperformed the upper rotor in the rotor separation ratio region from 0.95 to 1 (axial separation ratio was kept at 0.05) at an overall system thrust of 8 N, and matched the upper rotor performance at the tested overall thrust of 15 N.Aerospace2016-10-1034Article10.3390/aerospace3040032322226-43102016-10-10doi: 10.3390/aerospace3040032Mantas BrazinskasStephen PriorJames Scanlan<![CDATA[Aerospace, Vol. 3, Pages 31: Comparison of Power Requirements: Flapping vs. Fixed Wing Vehicles]]>
http://www.mdpi.com/2226-4310/3/4/31
The power required by flapping and fixed wing vehicles in level flight is determined and compared. Based on a new modelling approach, the effects of flapping on the induced drag in flapping wing vehicles are mathematically described. It is shown that flapping causes a significant increase in the induced drag when compared with a non-flapping, fixed wing vehicle. There are two effects for that induced drag increase; one is due to tilting of the lift vector caused by flapping the wings and the other results from changes in the amount of the lift vector during flapping. The induced drag increase yields a significant contribution to the power required by flapping wing vehicles. Furthermore, the power characteristics of fixed wing vehicles are dealt with. It is shown that, for this vehicle type, the propeller efficiency plays a major role. This is because there are considerable differences in the propeller efficiency when taking the size of vehicles into account. Comparing flapping and fixed wing vehicles, the conditions are shown where flapping wing vehicles have a lower power demand and where fixed wing vehicles are superior regarding the required power. There is a tendency such that fixed wing vehicles have an advantage in the case of larger size vehicles and flapping wing vehicles have an advantage in the case of smaller size ones.Aerospace2016-09-2834Article10.3390/aerospace3040031312226-43102016-09-28doi: 10.3390/aerospace3040031Gottfried Sachs<![CDATA[Aerospace, Vol. 3, Pages 30: Numerical Study of Transition of an Annular Lift Fan Aircraft]]>
http://www.mdpi.com/2226-4310/3/4/30
The present study aimed at studying the transition of annular lift fan aircraft through computational fluid dynamics (CFD) simulations. The oscillations of lift and drag, the optimization for the figure of merit, and the characteristics of drag, yawing, rolling and pitching moments in transition are studied. The results show that a two-stage upper and lower fan lift system can generate oscillations of lift and drag in transition, while a single-stage inner and outer fan lift system can eliminate the oscillations. The characteristics of momentum drag of the single-stage fans in transition are similar to that of the two-stage fans, but with the peak of drag lowered from 0.63 to 0.4 of the aircraft weight. The strategy to start transition from a negative angle of attack −21° further reduces the peak of drag to 0.29 of the weight. The strategy also reduces the peak of pitching torque, which needs upward extra thrusts of 0.39 of the weight to eliminate. The peak of rolling moment in transition needs differential upward thrusts of 0.04 of the weight to eliminate. The requirements for extra thrusts in transition lead to a total thrust–weight ratio of 0.7, which makes the aircraft more efficient for high speed cruise flight (higher than 0.7 Ma).Aerospace2016-09-2334Article10.3390/aerospace3040030302226-43102016-09-23doi: 10.3390/aerospace3040030Yun JiangBo Zhang<![CDATA[Aerospace, Vol. 3, Pages 29: Improved Separation of Tone and Broadband Noise Components from Open Rotor Acoustic Data]]>
http://www.mdpi.com/2226-4310/3/3/29
The term “open rotor” refers to unducted counter-rotating dual rotors or propellers used for propulsion. The noise generated by an open rotor is very complicated and requires special techniques for its analysis. The determination of its tone and broadband components is vital for properly assessing the noise control parameters and also for validating open rotor noise prediction codes. The data analysis technique developed by Sree for processing raw acoustic data of open rotors has been modified to yield much better results of tone and broadband separation particularly for the case when the two rotor speeds are approximately the same. The modified algorithm is found to eliminate most or all of the “spikes” previously observed in the broadband spectra computed from the original algorithm. A full description of the modified algorithm and examples of improved results from its application are presented in this paper.Aerospace2016-09-2033Article10.3390/aerospace3030029292226-43102016-09-20doi: 10.3390/aerospace3030029Dave SreeDavid Stephens<![CDATA[Aerospace, Vol. 3, Pages 28: An Implementation of Real-Time Phased Array Radar Fundamental Functions on a DSP-Focused, High-Performance, Embedded Computing Platform]]>
http://www.mdpi.com/2226-4310/3/3/28
This paper investigates the feasibility of a backend design for real-time, multiple-channel processing digital phased array system, particularly for high-performance embedded computing platforms constructed of general purpose digital signal processors. First, we obtained the lab-scale backend performance benchmark from simulating beamforming, pulse compression, and Doppler filtering based on a Micro Telecom Computing Architecture (MTCA) chassis using the Serial RapidIO protocol in backplane communication. Next, a field-scale demonstrator of a multifunctional phased array radar is emulated by using the similar configuration. Interestingly, the performance of a barebones design is compared to that of emerging tools that systematically take advantage of parallelism and multicore capabilities, including the Open Computing Language.Aerospace2016-09-0933Article10.3390/aerospace3030028282226-43102016-09-09doi: 10.3390/aerospace3030028Xining YuYan ZhangAnkit PatelAllen ZahraiMark Weber<![CDATA[Aerospace, Vol. 3, Pages 27: Aerodynamic Modeling of NREL 5-MW Wind Turbine for Nonlinear Control System Design: A Case Study Based on Real-Time Nonlinear Receding Horizon Control]]>
http://www.mdpi.com/2226-4310/3/3/27
The work presented in this paper has two major aspects: (i) investigation of a simple, yet efficient model of the NREL (National Renewable Energy Laboratory) 5-MW reference wind turbine; (ii) nonlinear control system development through a real-time nonlinear receding horizon control methodology with application to wind turbine control dynamics. In this paper, the results of our simple wind turbine model and a real-time nonlinear control system implementation are shown in comparison with conventional control methods. For this purpose, the wind turbine control problem is converted into an optimization problem and is directly solved by the nonlinear backwards sweep Riccati method to generate the control protocol, which results in a non-iterative algorithm. One main contribution of this paper is that we provide evidence through simulations, that such an advanced control strategy can be used for real-time control of wind turbine dynamics. Examples are provided to validate and demonstrate the effectiveness of the presented scheme.Aerospace2016-08-3033Article10.3390/aerospace3030027272226-43102016-08-30doi: 10.3390/aerospace3030027Pedro GalvaniFei SunKamran Turkoglu<![CDATA[Aerospace, Vol. 3, Pages 26: Optimization of a Human-Powered Aircraft Using Fluid–Structure Interaction Simulations]]>
http://www.mdpi.com/2226-4310/3/3/26
The special type of aircrafts in which the human power of the pilot is sufficient to take off and sustain flight are known as Human-Powered Aircrafts (HPAs). To explore the peculiarities of these aircrafts, the aerodynamic performance of an existing design is evaluated first, using both the vortex lattice method and computational fluid dynamics. In a second step, it is attempted to design and optimize a new HPA capable of winning the Kremer International Marathon Competition. The design will be special in that it allows one to include a second pilot on board the aircraft. As the structural deflection of the wing is found to be a key aspect during design, fluid–structure interaction simulations are performed and included in the optimization procedure. To assess the feasibility of winning the competition, the physical performance of candidate pilots is measured and compared with the predicted required power.Aerospace2016-08-2633Article10.3390/aerospace3030026262226-43102016-08-26doi: 10.3390/aerospace3030026Bob VanderhoydonckGilberto SantoJan VierendeelsJoris Degroote<![CDATA[Aerospace, Vol. 3, Pages 25: Effect of the Backward-Facing Step Location on the Aerodynamics of a Morphing Wing]]>
http://www.mdpi.com/2226-4310/3/3/25
Over the last decade, aircraft morphing technology has drawn a lot of attention in the aerospace community, because it is likely to improve the aerodynamic performance and the versatility of aircraft at different flight regimes. With the fast paced advancements in this field, a parallel stream of research is studying different materials and designs to develop reliable morphing skins. A promising candidate for a viable morphing skin is the sliding skin, where two or more rigid surfaces remain in contact and slide against each other during morphing. The overlapping between each two panels create a backward-facing step on the airfoil surface which has a critical effect on the aerodynamics of the wing. This paper presents a numerical study of the effect of employing a backward-facing step on the suction side of a National Advisory Committee for Aeronautics (NACA) 2412 airfoil at a high Reynolds number of 5.9 × 106. The effects of the step location on the lift coefficient, drag coefficient and critical angle of attack are studied to find a favorable location for the step along the chord-wise direction. Results showed that employing a step on the suction side of the NACA 2412 airfoil can adversely affect the aforementioned aerodynamic properties. A drop of 21.1% in value of the lift coefficient and an increase of 120.8% in the drag coefficient were observed in case of a step located at 25% of the chord length. However, these effects are mitigated by shifting the step location towards the trailing edge. Introducing a step on the airfoil caused the airfoil’s thickness to change, which in turn has affected the transition point of the viscous boundary layer from laminar to turbulent. The location of the step, prior or post the transition point, has a noteworthy effect on the pressure and shear stress distribution, and consequently on the values of the lift and drag coefficients.Aerospace2016-08-1133Article10.3390/aerospace3030025252226-43102016-08-11doi: 10.3390/aerospace3030025Fadi MishrikyPaul Walsh<![CDATA[Aerospace, Vol. 3, Pages 24: Comparison of the Average Lift Coefficient ͞CL and Normalized Lift ͞ηL for Evaluating Hovering and Forward Flapping Flight]]>
http://www.mdpi.com/2226-4310/3/3/24
The capability of flapping wings to generate lift is currently evaluated by using the lift coefficient C ¯ L , a dimensionless number that is derived from the basal equation that calculates the steady-state lift coefficient CL for fixed wings. In contrast to its simple and direct application to fixed wings, the equation for C ¯ L requires prior knowledge of the flow field along the wing span, which results in two integrations: along the wing span and over time. This paper proposes an alternate average normalized lift η ¯ L that is easy to apply to hovering and forward flapping flight, does not require prior knowledge of the flow field, does not resort to calculus for its solution, and its lineage is close to the basal equation for steady state CL. Furthermore, the average normalized lift η ¯ L converges to the legacy CL as the flapping frequency is reduced to zero (gliding flight). Its ease of use is illustrated by applying the average normalized lift η ¯ L to the hovering and translating flapping flight of bumblebees. This application of the normalized lift is compared to the same application using two widely-accepted legacy average lift coefficients: the first C ¯ L as defined by Dudley and Ellington, and the second lift coefficient by Weis-Fogh. Furthermore, it is shown that the average normalized lift η ¯ L has a physical meaning: that of the ratio of work exerted by the flapping wings onto the surrounding flow field and the kinetic energy available at the aerodynamic surfaces during the generation of lift. The working equation for the average normalized lift η ¯ L is derived and is presented as a function of Strouhal number, St.Aerospace2016-07-2933Article10.3390/aerospace3030024242226-43102016-07-29doi: 10.3390/aerospace3030024Phillip Burgers<![CDATA[Aerospace, Vol. 3, Pages 23: Analysis of Kinematics of Flapping Wing UAV Using OptiTrack Systems]]>
http://www.mdpi.com/2226-4310/3/3/23
An analysis of the kinematics of a flapping membrane wing using experimental kinematic data is presented. This motion capture technique tracks the positon of the retroreflective marker(s) placed on the left wing of a 1.3-m-wingspan ornithopter. The time-varying three-dimensional data of the wing kinematics were recorded for a single frequency. The wing shape data was then plotted on a two-dimensional plane to understand the wing dynamic behaviour of an ornithopter. Specifically, the wing tip path, leading edge bending, wing membrane shape, local twist, stroke angle and wing velocity were analyzed. As the three characteristic angles can be expressed in the Fourier series as a function of time, the kinematics of the wing can be computationally generated for the aerodynamic study of flapping flight through the Fourier coefficients presented. Analysis of the ornithopter wing showed how the ornithopter closely mimics the flight motions of birds despite several physical limitations.Aerospace2016-07-2633Article10.3390/aerospace3030023232226-43102016-07-26doi: 10.3390/aerospace3030023Matthew RongfaTeppatat PantuphagSutthiphong Srigrarom<![CDATA[Aerospace, Vol. 3, Pages 22: Exploring Civil Drone Accidents and Incidents to Help Prevent Potential Air Disasters]]>
http://www.mdpi.com/2226-4310/3/3/22
A recent alleged “drone” collision with a British Airways Airbus A320 at Heathrow Airport highlighted the need to understand civil Remotely Piloted Aircraft Systems (RPAS) accidents and incidents (events). This understanding will facilitate improvements in safety by ensuring efforts are focused to reduce the greatest risks. One hundred and fifty two RPAS events were analyzed. The data was collected from a 10-year period (2006 to 2015). Results show that, in contrast to commercial air transportation (CAT), RPAS events have a significantly different distribution when categorized by occurrence type, phase of flight, and safety issue. Specifically, it was found that RPAS operations are more likely to experience (1) loss of control in-flight, (2) events during takeoff and in cruise, and (3) equipment problems. It was shown that technology issues, not human factors, are the key contributor in RPAS events. This is a significant finding, as it is contrary to the industry view which has held for the past quarter of a century that human factors are the key contributor (which is still the case for CAT). Regulators should therefore look at technologies and not focus solely on operators.Aerospace2016-07-2233Communication10.3390/aerospace3030022222226-43102016-07-22doi: 10.3390/aerospace3030022Graham WildJohn MurrayGlenn Baxter<![CDATA[Aerospace, Vol. 3, Pages 21: Bio-Inspired Principles Applied to the Guidance, Navigation and Control of UAS]]>
http://www.mdpi.com/2226-4310/3/3/21
This review describes a number of biologically inspired principles that have been applied to the visual guidance, navigation and control of Unmanned Aerial System (UAS). The current limitations of UAS systems are outlined, such as the over-reliance on GPS, the requirement for more self-reliant systems and the need for UAS to have a greater understanding of their environment. It is evident that insects, even with their small brains and limited intelligence, have overcome many of the shortcomings of the current state of the art in autonomous aerial guidance. This has motivated research into bio-inspired systems and algorithms, specifically vision-based navigation, situational awareness and guidance.Aerospace2016-07-2033Review10.3390/aerospace3030021212226-43102016-07-20doi: 10.3390/aerospace3030021Reuben StrydomAymeric DenuelleMandyam Srinivasan<![CDATA[Aerospace, Vol. 3, Pages 20: Hydrodynamic Performance of Aquatic Flapping: Efficiency of Underwater Flight in the Manta]]>
http://www.mdpi.com/2226-4310/3/3/20
The manta is the largest marine organism to swim by dorsoventral oscillation (flapping) of the pectoral fins. The manta has been considered to swim with a high efficiency stroke, but this assertion has not been previously examined. The oscillatory swimming strokes of the manta were examined by detailing the kinematics of the pectoral fin movements swimming over a range of speeds and by analyzing simulations based on computational fluid dynamic potential flow and viscous models. These analyses showed that the fin movements are asymmetrical up- and downstrokes with both spanwise and chordwise waves interposed into the flapping motions. These motions produce complex three-dimensional flow patterns. The net thrust for propulsion was produced from the distal half of the fins. The vortex flow pattern and high propulsive efficiency of 89% were associated with Strouhal numbers within the optimal range (0.2–0.4) for rays swimming at routine and high speeds. Analysis of the swimming pattern of the manta provided a baseline for creation of a bio-inspired underwater vehicle, MantaBot.Aerospace2016-07-1133Article10.3390/aerospace3030020202226-43102016-07-11doi: 10.3390/aerospace3030020Frank FishChristian SchreiberKeith MooredGeng LiuHaibo DongHilary Bart-Smith<![CDATA[Aerospace, Vol. 3, Pages 19: The Efficiency of a Hybrid Flapping Wing Structure—A Theoretical Model Experimentally Verified]]>
http://www.mdpi.com/2226-4310/3/3/19
To propel a lightweight structure, a hybrid wing structure was designed; the wing’s geometry resembled a rotor blade, and its flexibility resembled an insect’s flapping wing. The wing was designed to be flexible in twist and spanwise rigid, thus maintaining the aeroelastic advantages of a flexible wing. The use of a relatively “thick” airfoil enabled the achievement of higher strength to weight ratio by increasing the wing’s moment of inertia. The optimal design was based on a simplified quasi-steady inviscid mathematical model that approximately resembles the aerodynamic and inertial behavior of the flapping wing. A flapping mechanism that imitates the insects’ flapping pattern was designed and manufactured, and a set of experiments for various parameters was performed. The simplified analytical model was updated according to the tests results, compensating for the viscid increase of drag and decrease of lift, that were neglected in the simplified calculations. The propelling efficiency of the hovering wing at various design parameters was calculated using the updated model. It was further validated by testing a smaller wing flapping at a higher frequency. Good and consistent test results were obtained in line with the updated model, yielding a simple, yet accurate tool, for flapping wings design.Aerospace2016-07-0533Article10.3390/aerospace3030019192226-43102016-07-05doi: 10.3390/aerospace3030019Yuval KerenHaim AbramovichRimon Arieli<![CDATA[Aerospace, Vol. 3, Pages 18: Large Scale Applications Using FBG Sensors: Determination of In-Flight Loads and Shape of a Composite Aircraft Wing]]>
http://www.mdpi.com/2226-4310/3/3/18
Technological advances have enabled the development of a number of optical fiber sensing methods over the last few years. The most prevalent optical technique involves the use of fiber Bragg grating (FBG) sensors. These small, lightweight sensors have many attributes that enable their use for a number of measurement applications. Although much literature is available regarding the use of FBGs for laboratory level testing, few publications in the public domain exist of their use at the operational level. Therefore, this paper gives an overview of the implementation of FBG sensors for large scale structures and applications. For demonstration, a case study is presented in which FBGs were used to determine the deflected wing shape and the out-of-plane loads of a 5.5-m carbon-composite wing of an ultralight aerial vehicle. The in-plane strains from the 780 FBG sensors were used to obtain the out-of-plane loads as well as the wing shape at various load levels. The calculated out-of-plane displacements and loads were within 4.2% of the measured data. This study demonstrates a practical method in which direct measurements are used to obtain critical parameters from the high distribution of FBG sensors. This procedure can be used to obtain information for structural health monitoring applications to quantify healthy vs. unhealthy structures.Aerospace2016-06-2333Article10.3390/aerospace3030018182226-43102016-06-23doi: 10.3390/aerospace3030018Matthew NicolasRani SullivanW. Richards<![CDATA[Aerospace, Vol. 3, Pages 17: On a Non-Symmetric Eigenvalue Problem Governing Interior Structural–Acoustic Vibrations]]>
http://www.mdpi.com/2226-4310/3/2/17
Small amplitude vibrations of a structure completely filled with a fluid are considered. Describing the structure by displacements and the fluid by its pressure field, the free vibrations are governed by a non-self-adjoint eigenvalue problem. This survey reports on a framework for taking advantage of the structure of the non-symmetric eigenvalue problem allowing for a variational characterization of its eigenvalues. Structure-preserving iterative projection methods of the the Arnoldi and of the Jacobi–Davidson type and an automated multi-level sub-structuring method are reviewed. The reliability and efficiency of the methods are demonstrated by a numerical example.Aerospace2016-06-1732Article10.3390/aerospace3020017172226-43102016-06-17doi: 10.3390/aerospace3020017Heinrich Voss<![CDATA[Aerospace, Vol. 3, Pages 16: A Shape Memory Alloy Application for Compact Unmanned Aerial Vehicles]]>
http://www.mdpi.com/2226-4310/3/2/16
Shape memory alloys materials, SMA, offer several advantages that designers can rely on such as the possibility of transmitting large forces and deformations, compactness, and the intrinsic capability to absorb loads. Their use as monolithic actuators, moreover, can lead to potential simplifications of the system, through a reduction of number of parts and the removal of many free play gaps among mechanics. For these reasons, technological aerospace research is focusing on this kind of technology more and more, even though fatigue life, performance degradation, and other issues are still open. In the work at hand, landing gear for unmanned aerial vehicles, UAV, is presented, integrated with shape memory alloys springs as actuation devices. A conceptual prototype has been realized to verify the system ability in satisfying specs, in terms of deployment and retraction capability. Starting from the proposed device working principle and the main design parameters identification, the design phase is faced, setting those parameters to meet weight, deployment angle, energy consumption, and available room requirements. Then, system modeling and performance prediction is performed and finally a correlation between numerical and experimental results is presented.Aerospace2016-05-3132Article10.3390/aerospace3020016162226-43102016-05-31doi: 10.3390/aerospace3020016Salvatore AmeduriAntonio ConcilioNunzia FavaloroLorenzo Pellone<![CDATA[Aerospace, Vol. 3, Pages 15: Theoretical and Numerical Modeling of Acoustic Metamaterials for Aeroacoustic Applications]]>
http://www.mdpi.com/2226-4310/3/2/15
The advent, during the first decade of the 21st century, of the concept of acoustic metamaterial has disclosed an incredible potential of development for breakthrough technologies. Unfortunately, the extension of the same concepts to aeroacoustics has turned out to be not a trivial task, because of the different structure of the governing equations, characterized by the presence of the background aerodynamic convection. Some of the approaches recently introduced to circumvent the problem are biased by a fundamental assumption that makes the actual realization of devices extremely unlikely: the metamaterial should guarantee an adapted background aerodynamic convection in order to modify suitably the acoustic field and obtain the desired effect, thus implying the porosity of the cloaking device. In the present paper, we propose an interpretation of the metamaterial design that removes this unlikely assumption, focusing on the identification of an aerodynamically-impermeable metamaterial capable of reproducing the surface impedance profile required to achieve the desired scattering abatement. The attention is focused on a moving obstacle impinged by an acoustic perturbation induced by a co-moving source. The problem is written in a frame of reference rigidly connected to the moving object to couple the convective wave equation in the hosting medium with the inertially-anisotropic wave operator within the cloak. The problem is recast in an integral form and numerically solved through a boundary-field element method. The matching of the local wave vector is used to derive a convective design of the metamaterial applicable to the specific problem analyzed. Preliminary numerical results obtained under the simplifying assumption of a uniform aerodynamic flow reveal a considerable enhancement of the masking capability of the convected design. The numerical method developed shows a remarkable computational efficiency, completing a simulation of the entire field in a few minutes on mid-end workstations. The results are re-interpreted in term of boundary impedance, assuming a locally-reacting behavior of the outer boundary of the cloaking layer. The formulation is currently being extended to the analysis of arbitrarily complex external flows in order to remove the limitation of the background uniform stream in the host.Aerospace2016-05-2632Article10.3390/aerospace3020015152226-43102016-05-26doi: 10.3390/aerospace3020015Umberto Iemma<![CDATA[Aerospace, Vol. 3, Pages 14: Fixed-Wing UAV Attitude Estimation Using Single Antenna GPS Signal Strength Measurements]]>
http://www.mdpi.com/2226-4310/3/2/14
This article considers a novel approach to using global positioning system (GPS) signal strength readings and estimated velocity vector for estimating the attitude of a small fixed-wing unmanned aerial vehicle (UAV). This approach has the benefit being able to estimate full position, velocity and attitude states of a UAV using only the data from a single GPS receiver and antenna. Two different approaches for utilizing GPS signal strength within measurement updates for UAV attitude in a nonlinear Kalman filter are discussed and assessed using recorded UAV flight data. Comparisons of UAV pitch and roll estimates against measurements from a high-grade mechanical gyroscope are used to show that approximately 5° error with respect to both mean and standard-deviation on both axes is achievable.Aerospace2016-05-1332Article10.3390/aerospace3020014142226-43102016-05-13doi: 10.3390/aerospace3020014Jason GrossYu GuMatthew Rhudy<![CDATA[Aerospace, Vol. 3, Pages 13: Suppression of Low-Frequency Shock Oscillations over Boundary Layers by Repetitive Laser Pulse Energy Deposition]]>
http://www.mdpi.com/2226-4310/3/2/13
The effect of repetitive energy deposition on low Strouhal number oscillations of the shock wave induced by boundary-layer interaction over a cylinder-flare model was studied. The fluctuation of the energy deposition frequency was induced in the flow, because the bubble generated by the energy deposition flowed downstream along the surface repeatedly. The region before the bubble size was affected by the energy deposition directly, so the fluctuation frequency was equal to the energy deposition frequency. However, the flare shock behavior at a position farther from the surface than the bubble size was also affected strongly by the energy deposition. For low-frequency unsteadiness and the effect of energy deposition on its unsteadiness, two categories have been observed. In the relatively small flare angle case, the flare shock was oscillated owing to the fluctuation induced by the boundary-layer interaction at the shock foot, and its oscillation occurred at 2.1 kHz with a small amplitude. The amplitude of this oscillation was decreased by highly repetitive energy depositions, and its amplitude could not be detected at a highly repetitive energy deposition. In the longer cylinder section case, the region of the shock-wave interaction was widened, and the amplitude of the flare shock oscillation was increased. In this case, the amplitude drastically decreased because of energy deposition.Aerospace2016-04-2732Article10.3390/aerospace3020013132226-43102016-04-27doi: 10.3390/aerospace3020013Akira IwakawaTatsuro ShodaHoang PhamTakahiro TambaAkihiro Sasoh<![CDATA[Aerospace, Vol. 3, Pages 12: Experimental Aeroelastic Models Design and Wind Tunnel Testing for Correlation with New Theory]]>
http://www.mdpi.com/2226-4310/3/2/12
Several examples of experimental model designs, wind tunnel tests and correlation with new theory are presented in this paper. The goal is not only to evaluate a new theory, new computational method or new aeroelastic phonomenon, but also to provide new insights into nonlinear aeroelastic phenomena, flutter, limit cycle oscillation (LCO) and gust response.Aerospace2016-04-1432Article10.3390/aerospace3020012122226-43102016-04-14doi: 10.3390/aerospace3020012Deman TangEarl Dowell<![CDATA[Aerospace, Vol. 3, Pages 11: A Six Degrees of Freedom Dynamic Wire-Driven Traverse]]>
http://www.mdpi.com/2226-4310/3/2/11
A novel support mechanism for a wind tunnel model is designed, built, and demonstrated on an aerodynamic platform undergoing dynamic maneuvers, tested with periodic motions up to 20 Hz. The platform is supported by a 6-DOF (six degrees of freedom) traverse that utilizes eight thin wires, each mounted to a servo motor with an in-line load cell to accurately monitor or control the platform motion and force responses. The system is designed such that simultaneous control of the servo motors effects motion within ±50 mm translations, ±15° pitch, ±9° yaw, and ±8° roll at lower frequencies. The traverse tracks a desired trajectory and resolves the induced forces on the platform at 1 kHz. The effected motion of the platform is measured at 0.6 kHz with a motion capture system, which utilizes six near-infrared (NIR) cameras for full spatial and temporal resolution of the platform motion, which is used for feedback control. The traverse allows different platform model geometries to be tested, and the present work demonstrates its capabilities on an axisymmetric bluff body. Programmable timed outputs are synchronized relative to the model motion and can be used for triggering external systems and processes. In the present study, particle image velocimetry (PIV) is used to characterize the realized wakes of the platform undergoing canonical motions that are effected by this new wind tunnel traverse.Aerospace2016-04-1432Article10.3390/aerospace3020011112226-43102016-04-14doi: 10.3390/aerospace3020011Thomas LambertBojan VukasinovicAri Glezer<![CDATA[Aerospace, Vol. 3, Pages 10: Vertical Wind Tunnel for Prediction of Rocket Flight Dynamics]]>
http://www.mdpi.com/2226-4310/3/2/10
A customized vertical wind tunnel has been built by the University of Canterbury Rocketry group (UC Rocketry). This wind tunnel has been critical for the success of UC Rocketry as it allows the optimization of avionics and control systems before flight. This paper outlines the construction of the wind tunnel and includes an analysis of flow quality including swirl. A minimal modelling methodology for roll dynamics is developed that can extrapolate wind tunnel behavior at low wind speeds to much higher velocities encountered during flight. The models were shown to capture the roll flight dynamics in two rocket launches with mean roll angle errors varying from 0.26° to 1.5° across the flight data. The identified model parameters showed consistent and predictable variations over both wind tunnel tests and flight, including canard–fin interaction behavior. These results demonstrate that the vertical wind tunnel is an important tool for the modelling and control of sounding rockets.Aerospace2016-03-2932Article10.3390/aerospace3020010102226-43102016-03-29doi: 10.3390/aerospace3020010Hoani BrysonHans SültropGeorge BuchananChristopher HannMalcolm SnowdonAvinash RaoAdam SleeKieran FanningDavid WrightJason McVicarBrett ClarkGraeme HarrisXiao Chen<![CDATA[Aerospace, Vol. 3, Pages 9: Semi-Empirical Prediction of Airfoil Hysteresis]]>
http://www.mdpi.com/2226-4310/3/2/9
A semi-empirical method is presented to estimate the angular excursion and the lift loss associated with static hysteresis on an airfoil. Wind tunnel data of various airfoils is used to define and validate the methodology. The resulting equation provides a relationship between the size of the hysteresis loop and characteristics of the airfoil. Comparisons of the equation with experiment show encouraging agreement both in terms of the magnitude of the lift loss and the extent of the loop.Aerospace2016-03-2432Communication10.3390/aerospace302000992226-43102016-03-24doi: 10.3390/aerospace3020009Lance Traub<![CDATA[Aerospace, Vol. 3, Pages 8: Guidance, Navigation and Control of Unmanned Airships under Time-Varying Wind for Extended Surveillance]]>
http://www.mdpi.com/2226-4310/3/1/8
This paper deals with the control of lighter-than-air vehicles, more specifically the design of an integrated guidance, navigation and control (GNC) scheme that is capable of navigating an airship through a series of constant-altitude, planar waypoints. Two guidance schemes are introduced, a track-specific guidance law and a proportional navigation guidance law, that provide the required signals to the corresponding controllers based on the airship position relative to a target waypoint. A novel implementation of the extended Kalman filter, namely the scheduled extended Kalman filter, estimates the required states and wind speed to enhance the performance of the track-specific guidance law in the presence of time-varying wind. The performance of the GNC system is tested using a high fidelity nonlinear dynamic simulation for a variety of flying conditions. Representative results illustrate the performance of the integrated system for chosen flight conditions.Aerospace2016-02-1731Article10.3390/aerospace301000882226-43102016-02-17doi: 10.3390/aerospace3010008Ghassan AtmehKamesh Subbarao<![CDATA[Aerospace, Vol. 3, Pages 7: Validation of a Discontinuous Galerkin Implementation of the Time-Domain Linearized Navier–Stokes Equations for Aeroacoustics]]>
http://www.mdpi.com/2226-4310/3/1/7
The propagation of small perturbations in complex geometries can involve hydrodynamic-acoustic interactions, coupling acoustic waves and vortical modes. A propagation model, based on the linearized Navier–Stokes equations, is proposed. It includes the mechanism responsible for the generation of vorticity associated with the hydrodynamic modes. The linearized Navier–Stokes equations are discretized in space using a discontinuous Galerkin formulation for unstructured grids. Explicit time integration and non-reflecting boundary conditions are described. The linearized Navier–Stokes (LNS) model is applied to two test cases. The first one is the time-harmonic source line in an incompressible inviscid two-dimensional mean shear flow in an infinite domain. It is shown that the proposed model is able to capture the trailing vorticity field developing behind the mass source and to represent the redistribution of the vorticity. The second test case deals with the analysis of the acoustic propagation of an incoming perturbation inside a circular duct with a sudden area expansion in the presence of a mean flow and the evaluation of its scattering matrix. The computed coefficients of the scattering matrix are compared to experimental data for three different Mach numbers of the mean flow, M0 = 0.08, 0.19 and 0.29. The good agreement with the experimental data shows that the proposed method is suitable for characterizing the acoustic behavior of this kind of network.Aerospace2016-02-1731Article10.3390/aerospace301000772226-43102016-02-17doi: 10.3390/aerospace3010007Renzo Arina<![CDATA[Aerospace, Vol. 3, Pages 6: Turbulence Effects on Modified State Observer-Based Adaptive Control: Black Kite Micro Aerial Vehicle]]>
http://www.mdpi.com/2226-4310/3/1/6
This paper presents the implementation of a modified state observer-based adaptive dynamic inverse controller for the Black Kite micro aerial vehicle. The pitch and velocity adaptations are computed by the modified state observer in the presence of turbulence to simulate atmospheric conditions. This state observer uses the estimation error to generate the adaptations and, hence, is more robust than model reference adaptive controllers which use modeling or tracking error. In prior work, a traditional proportional-integral-derivative control law was tested in simulation for its adaptive capability in the longitudinal dynamics of the Black Kite micro aerial vehicle. This controller tracks the altitude and velocity commands during normal conditions, but fails in the presence of both parameter uncertainties and system failures. The modified state observer-based adaptations, along with the proportional-integral-derivative controller enables tracking despite these conditions. To simulate flight of the micro aerial vehicle with turbulence, a Dryden turbulence model is included. The turbulence levels used are based on the absolute load factor experienced by the aircraft. The length scale was set to 2.0 meters with a turbulence intensity of 5.0 m/s that generates a moderate turbulence. Simulation results for various flight conditions show that the modified state observer-based adaptations were able to adapt to the uncertainties and the controller tracks the commanded altitude and velocity. The summary of results for all of the simulated test cases and the response plots of various states for typical flight cases are presented.Aerospace2016-02-0531Article10.3390/aerospace301000662226-43102016-02-05doi: 10.3390/aerospace3010006Venkatasubramani S. R. PappuJames SteckGuruganesh Ramamurthi<![CDATA[Aerospace, Vol. 3, Pages 5: Target Tracking in 3-D Using Estimation Based Nonlinear Control Laws for UAVs]]>
http://www.mdpi.com/2226-4310/3/1/5
This paper presents an estimation based backstepping like control law design for an Unmanned Aerial Vehicle (UAV) to track a moving target in 3-D space. A ground-based sensor or an onboard seeker antenna provides range, azimuth angle, and elevation angle measurements to a chaser UAV that implements an extended Kalman filter (EKF) to estimate the full state of the target. A nonlinear controller then utilizes this estimated target state and the chaser’s state to provide speed, flight path, and course/heading angle commands to the chaser UAV. Tracking performance with respect to measurement uncertainty is evaluated for three cases: (1) stationary white noise; (2) stationary colored noise and (3) non-stationary (range correlated) white noise. Furthermore, in an effort to improve tracking performance, the measurement model is made more realistic by taking into consideration range-dependent uncertainties in the measurements, i.e., as the chaser closes in on the target, measurement uncertainties are reduced in the EKF, thus providing the UAV with more accurate control commands. Simulation results for these cases are shown to illustrate target state estimation and trajectory tracking performance.Aerospace2016-02-0131Article10.3390/aerospace301000552226-43102016-02-01doi: 10.3390/aerospace3010005Mousumi AhmedKamesh Subbarao<![CDATA[Aerospace, Vol. 3, Pages 4: The Effect of the Phase Angle between the Forewing and Hindwing on the Aerodynamic Performance of a Dragonfly-Type Ornithopter]]>
http://www.mdpi.com/2226-4310/3/1/4
Dragonflies achieve agile maneuverability by flapping four wings independently. Different phase angles between the flapping forewing and hindwing have been observed during various flight modes. The aerodynamic performance depends on phase angle control, as exemplified by an artificial flying ornithopter. Here, we present a dragonfly-like ornithopter whose phase angle was designed to vary according to the phase lag between the slider-cranks of the forewing and hindwing. Two microelectromechanical systems (MEMS) differential pressure sensors were attached to the center of both forewing and hindwing to evaluate the aerodynamic performance during flapping motions when the phase angle was changed. By varying the phase angle in both the tethered condition and free-flight, the performance of the forewing remained approximately constant, whereas that of the hindwing exhibited obvious variations; the maximum average value was two-fold higher than the minimum. The experimental results suggest that simple phase angle changes enable a flying ornithopter to control flight force balance without complex changes in the wing kinematics.Aerospace2016-01-2531Article10.3390/aerospace301000442226-43102016-01-25doi: 10.3390/aerospace3010004Hidetoshi TakahashiAlice ConcordelJamie PaikIsao Shimoyama<![CDATA[Aerospace, Vol. 3, Pages 3: Acknowledgement to Reviewers of Aerospace in 2015]]>
http://www.mdpi.com/2226-4310/3/1/3
The editors of Aerospace would like to express their sincere gratitude to the following reviewers for assessing manuscripts in 2015. [...]Aerospace2016-01-2131Editorial10.3390/aerospace301000332226-43102016-01-21doi: 10.3390/aerospace3010003 Aerospace Editorial Office<![CDATA[Aerospace, Vol. 3, Pages 2: Analysis of Radio Frequency Blackout for a Blunt-Body Capsule in Atmospheric Reentry Missions]]>
http://www.mdpi.com/2226-4310/3/1/2
A numerical analysis of electromagnetic waves around the atmospheric reentry demonstrator (ARD) of the European Space Agency (ESA) in an atmospheric reentry mission was conducted. During the ARD mission, which involves a 70% scaled-down configuration capsule of the Apollo command module, radio frequency blackout and strong plasma attenuation of radio waves in communications with data relay satellites and air planes were observed. The electromagnetic interference was caused by highly dense plasma derived from a strong shock wave generated in front of the capsule because of orbital speed during reentry. In this study, the physical properties of the plasma flow in the shock layer and wake region of the ESA ARD were obtained using a computational fluid dynamics technique. Then, electromagnetic waves were expressed using a frequency-dependent finite-difference time-domain method using the plasma properties. The analysis model was validated based on experimental flight data. A comparison of the measured and predicted results showed good agreement. The distribution of charged particles around the ESA ARD and the complicated behavior of electromagnetic waves, with attenuation and reflection, are clarified in detail. It is suggested that the analysis model could be an effective tool for investigating radio frequency blackout and plasma attenuation in radio wave communication.Aerospace2016-01-0631Article10.3390/aerospace301000222226-43102016-01-06doi: 10.3390/aerospace3010002Yusuke TakahashiReo NakasatoNobuyuki Oshima<![CDATA[Aerospace, Vol. 3, Pages 1: Multiple-Scenario Unmanned Aerial System Control: A Systems Engineering Approach and Review of Existing Control Methods]]>
http://www.mdpi.com/2226-4310/3/1/1
The use of unmanned aerial systems (UASs) in both the public and military environments is predicted to grow significantly. As the demand for UASs grows, the availability of more robust and capable vehicles that can perform multiple mission types will be needed. In the public sector, the demand will grow for UASs to be used for agriculture, forestry, and search and rescue missions. Militaries continue to demand more UAS capabilities for diverse operations around the world. Significant research has been performed and continues to progress in the areas of autonomous UAS control. A majority of the work focuses on subsets of UAS control: path planning, autonomy, small UAS controls, and sensors. Minimal work exists on a system-level problem of multiple-scenario UAS control for integrated systems. This paper provides a high-level modular system architecture definition that is modifiable across platform types and mission requirements. A review of the current research and employment of UAS capabilities is provided to evaluate the state of the capabilities required to enable the proposed architecture.Aerospace2016-01-0431Review10.3390/aerospace301000112226-43102016-01-04doi: 10.3390/aerospace3010001Christopher EatonEdwin ChongAnthony Maciejewski<![CDATA[Aerospace, Vol. 2, Pages 637-672: C0 Layerwise Model with Fixed Degrees of Freedom and Variable In- and Out-of-Plane Kinematics by Strain Energy Updating Technique]]>
http://www.mdpi.com/2226-4310/2/4/637
Physically based zigzag models have the merit of giving accurate stress predictions for laminates and sandwiches keeping fixed the functional degrees of freedom, though at the expense of the introduction of their derivatives. In the present paper, a technique that enables deleting these derivatives is developed. The objective is finding a priori corrections of displacements, which make the energy of the model with all the derivatives neglected equivalent to that of its initial counterpart model containing all the derivatives. Numerical applications show that this technique can obtain accurate results, even for strongly asymmetrical lay-ups, keeping low the computational cost.Aerospace2015-11-1724Article10.3390/aerospace20406376376722226-43102015-11-17doi: 10.3390/aerospace2040637Ugo IcardiFederico Sola<![CDATA[Aerospace, Vol. 2, Pages 627-636: Acoustic Radiation by 3D Vortex Rings in Air]]>
http://www.mdpi.com/2226-4310/2/4/627
Acoustic radiation emitted by three-dimensional (3D) vortex rings in air has been investigated on the basis of the unsteady Navier–Stokes equations. Power series expansions of the unknown functions with respect to the initial vorticity which is supposed to be small are used. In such a manner the system of the Navier–Stokes equations is reduced to a parabolic system with constant coefficients at high derivatives. The initial value problem is as follows. The vorticity is defined inside a toroid at t = 0. Other gas parameters are assumed to be constant throughout the whole space at t = 0. The solution is expressed by multiple integrals which are evaluated with the aid of the Korobov grids. Density oscillations are analyzed. The results show that the frequency band depends on the initial size of the vortex ring and its helicity. The presented data may be applied to the study of a flow in a wake region behind an aerodynamic body.Aerospace2015-11-0624Article10.3390/aerospace20406276276362226-43102015-11-06doi: 10.3390/aerospace2040627Fedor ShugaevDmitri CherkasovOxana Solenaya<![CDATA[Aerospace, Vol. 2, Pages 602-626: Optimization of Variable Stiffness Laminates and Sandwiches Undergoing Impulsive Dynamic Loading]]>
http://www.mdpi.com/2226-4310/2/4/602
This paper, which deals with variable stiffness composites, is aimed at showing the effects of optimization on the response characteristics and stress fields of these materials. A new optimization technique that has recently been developed is used to find spatially variable distributions of stiffness properties at any point, which minimize the interlaminar stresses without significant stiffness loss. After solving the Euler–Lagrange equations obtained by the strain energy extremization with varying the stiffness properties, curvilinear paths of fibres are found in closed form that modify natural frequencies, improve dynamic response and aid in recovery of critical interlaminar stresses. In the current version of the optimization technique, a more realistic description of the optimized shear coefficients is provided in order to accurately describe local effects. As a structural model, a zig-zag model with variable through-the-thickness kinematics is adopted, which is able to adapt itself to variations in solutions, thus providing accurate results from constitutive equations. This model is adopted because an accurate description of strain energy is mandatory for an effective application of the optimization procedure proposed. The numerical results show that the optimization procedure effectively recovers the stress concentrations while simultaneously improving the dynamic response of laminates and sandwiches.Aerospace2015-10-2324Article10.3390/aerospace20406026026262226-43102015-10-23doi: 10.3390/aerospace2040602Ugo IcardiFederico Sola<![CDATA[Aerospace, Vol. 2, Pages 581-601: East–West GEO Satellite Station-Keeping with Degraded Thruster Response]]>
http://www.mdpi.com/2226-4310/2/4/581
The higher harmonic terms of Earth’s gravitational potential slowly modify the nominal longitude of geostationary Earth orbit (GEO) satellites, while the third-body presence (Moon and Sun) mainly affects their latitude. For this reason, GEO satellites periodically need to perform station-keeping maneuvers, namely, east–west and north–south maneuvers to compensate for longitudinal and latitudinal variations, respectively. During the operational lifetime of GEO satellites, the thrusters’ response when commanded to perform these maneuvers slowly departs from the original nominal impulsive behavior. This paper addresses the practical problem of how to perform reliable east–west station-keeping maneuvers when thruster response is degraded. The need for contingency intervention from ground-based satellite operators is reduced by breaking apart the scheduled automatic station-keeping maneuvers into smaller maneuvers. Orbital alignment and attitude are tracked on-board during and in between sub-maneuvers, and any off nominal variations are corrected for with subsequent maneuvers. These corrections are particularly important near the end of the lifetime of GEO satellites, where thruster response is farthest from nominal performance.Aerospace2015-09-2924Article10.3390/aerospace20405815816012226-43102015-09-29doi: 10.3390/aerospace2040581Stoian BorissovYunhe WuDaniele Mortari<![CDATA[Aerospace, Vol. 2, Pages 555-580: CFD Study of an Annular-Ducted Fan Lift System for VTOL Aircraft]]>
http://www.mdpi.com/2226-4310/2/4/555
The present study aimed at assessing a novel annular-ducted fan lift system for VTOL aircraft through computational fluid dynamics (CFD) simulations. The power and lift efficiency of the lift fan system in hover mode, the lift and drag in transition mode, the drag and flight speed of the aircraft in cruise mode and the pneumatic coupling of the tip turbine and jet exhaust were studied. The results show that the annular-ducted fan lift system can have higher lift efficiency compared to the rotor of the Apache helicopter; the smooth transition from vertical takeoff to cruise flight needs some extra forward thrust to overcome a low peak of drag; the aircraft with the lift fan system enclosed during cruise flight theoretically may fly faster than helicopters and tiltrotors based on aerodynamic drag prediction, due to the elimination of rotor drag and compressibility effects on the rotor blade tips; and pneumatic coupling of the tip turbine and jet exhaust of a 300 m/s velocity can provide enough moment to spin the lift fan. The CFD results provide insight for future experimental study of the annular-ducted lift fan VTOL aircraft.Aerospace2015-09-2924Article10.3390/aerospace20405555555802226-43102015-09-29doi: 10.3390/aerospace2040555Yun JiangBo ZhangTao Huang<![CDATA[Aerospace, Vol. 2, Pages 524-554: Performance Comparison between Optimised Camber and Span for a Morphing Wing]]>
http://www.mdpi.com/2226-4310/2/3/524
Morphing technology offers a strategy to modify the wing geometry, and the wing planform and cross-sectional parameters can be optimised to the flight conditions. This paper presents an investigation into the effect of span and camber morphing on the mission performance of a 25-kg UAV, with a straight, rectangular, unswept wing. The wing is optimised over two velocities for various fixed wing and morphing wing strategies, where the objective is to maximise aerodynamic efficiency or range. The investigation analyses the effect of the low and high speed velocity selected, the weighting of the low and high velocity on the computation of the mission parameter, the maximum allowable span retraction and the weight penalty on the mission performance. Models that represent the adaptive aspect ratio (AdAR) span morphing concept and the fish bone active camber (FishBAC) camber morphing concept are used to investigate the effect on the wing parameters. The results indicate that generally morphing for both span and camber, the aerodynamic efficiency is maximised for a 30%–70% to 40%–60% weighting between the low and high speed flight conditions, respectively. The span morphing strategy with optimised fixed camber at the root can deliver up to 25% improvement in the aerodynamic efficiency over a fixed camber and span, for an allowable 50% retraction with a velocity range of 50–115 kph. Reducing the allowable retraction to 25% reduces the improvement to 8%–10% for a 50%–50% mission weighting. Camber morphing offers a maximum of 4.5% improvement approximately for a velocity range of 50–90 kph. Improvements in the efficiency achieved through camber morphing are more sensitive to the velocity range in the mission, generally decreasing rapidly by reducing or increasing the velocity range, where span morphing appears more robust for an increase in velocity range beyond the optimum. However, where span morphing requires considerable modification to the planform, the camber change required for optimum performance is only a 5% trailing edge tip deflection relative to cross-sectional chord length. Span morphing, at the optimal mission velocity range, with 25% allowable retraction, can allow up to a 12% increase in mass before no performance advantage is observed, where the camber morphing only allows up to 3%. This provides the designer with a mass budget that must be achieved for morphing to be viable to increase the mission performance.Aerospace2015-09-0823Article10.3390/aerospace20305245245542226-43102015-09-08doi: 10.3390/aerospace2030524Christopher BeaverstockBenjamin WoodsJames FinchamMichael Friswell<![CDATA[Aerospace, Vol. 2, Pages 505-523: The Flight of Birds and Other Animals]]>
http://www.mdpi.com/2226-4310/2/3/505
Methods of observing birds in flight now include training them to fly under known conditions in wind tunnels, and fitting free-flying birds with data loggers, that are either retrieved or read remotely via satellite links. The performance that comes to light depends on the known limitations of the materials from which they are made, and the conditions in which the birds live. Bird glide polars can be obtained by training birds to glide in a tilting wind tunnel. Translating these curves to power required from the flight muscles in level flight requires drag coefficients to be measured, which unfortunately does not work with bird bodies, because the flow is always fully detached. The drag of bodies in level flight can be determined by observing wingbeat frequency, and shows CD values around 0.08 in small birds, down to 0.06 in small waders specialised for efficient migration. Lift coefficients are up to 1.6 in gliding, or 1.8 for short, temporary glides. In-flight measurements can be used to calculate power curves for birds in level flight, and this has been applied to migrating geese in detail. These typically achieve lift:drag ratios around 15, including allowances for stops, as against 19 for continuous powered flight. The same calculations, applied to Pacific Black-tailed Godwits which start with fat fractions up to 0.55 at departure, show that such birds not only cross the Pacific to New Zealand, but have enough fuel in hand to reach the South Pole if that were necessary. This performance depends on the “dual fuel” arrangements of these migrants, whereby they use fat as their main fuel, and supplement this by extra fuel from burning the engine (flight muscles), as less power is needed later in the flight. The accuracy of these power curves has never been checked, although provision for stopping the bird, and making these checks at regular intervals during a simulated flight was built into the original design of the Lund wind tunnel. The Flight programme, which does these comparisons, also had provision for including contributions due to extracting energy from the atmosphere (soaring), or intermittent bounding flight in small birds (Passerines). It has been known for some time that the feathered surface allows the bird to delay or reverse detachment of the boundary layer, although exactly how this works remains a mystery, which might have practical applications. The bird wing was in use in past times, when birds were still competing with pterosaurs, although these had less efficient wings. The birds that survived the extinction that killed the pterosaurs and dinosaurs have (today) an automatic spherical navigator, which enables them to cross the Pacific and find New Zealand on the other side. Bats have never had such a device, and pterosaurs probably did not either. Animals, when seen from a zoological point of view, are adapted to whatever problems they had to deal with in earlier times.Aerospace2015-09-0123Review10.3390/aerospace20305055055232226-43102015-09-01doi: 10.3390/aerospace2030505Colin Pennycuick<![CDATA[Aerospace, Vol. 2, Pages 482-504: Decamber Morphing Concepts by Using a Hybrid Trailing Edge Control Surface]]>
http://www.mdpi.com/2226-4310/2/3/482
The idea of morphing is drawing extensive attention in aerospace technologies. Several different approaches like span, camber, twist, and sweep are finding applications. In this work, the concept of a trailing edge control surface which is capable of performing decamber morphing is explained. The upper and lower parts of the control surface undergo different chordwise elongations and the difference between these displacements gives rise to either camber or decamber morphing. The necessary force is achieved by the help of servo actuators. During the design, the structural analyses were done to determine the best viable options for the number of servo actuators, the location of the servo actuators, and the material properties used in the control surface. The control surface was designed of aluminum, composite and compliant materials hence was called a hybrid one. The structural analyses were conducted by using ANSYS® Workbench v14.0 package program. After finding the best viable design, which was made for in vacuo condition, the proposed design was also verified under the simulated aerodynamic loading. The aerodynamic loads were obtained from CFD analyses which were done with SU2 V3.2.3 open-source flow solver.Aerospace2015-07-2823Article10.3390/aerospace20304824825042226-43102015-07-28doi: 10.3390/aerospace2030482Yavuz Yamanİlhan TunçözYosheph YangPınar ArslanUğur KalkanHarun TıraşErcan GürsesMelin ŞahinSerkan Özgen<![CDATA[Aerospace, Vol. 2, Pages 461-481: Turbulence Modeling of Flows with Extensive Crossflow Separation]]>
http://www.mdpi.com/2226-4310/2/3/461
The reasons for the difficulty in simulating accurately strong 3-D shock wave/turbulent boundary layer interactions (SBLIs) and high-alpha flows with classical turbulence models are investigated. These flows are characterized by the appearance of strong crossflow separation. In view of recent additional evidence, a previously published flow analysis, which attributes the poor performance of classical turbulence models to the observed laminarization of the separation domain, is reexamined. According to this analysis, the longitudinal vortices into which the separated boundary layer rolls up in this type of separated flow, transfer external inviscid air into the part of the separation adjacent to the wall, decreasing its turbulence. It is demonstrated that linear models based on the Boussinesq equation provide solutions of moderate accuracy, while non-linear ones and others that consider the particular structure of the flow are more efficient. Published and new Reynolds Averaged Navier–Stokes (RANS) simulations are reviewed, as well as results from a recent Large Eddy Simulation (LES) study, which indicate that in calculations characterized by sufficient accuracy the turbulent kinetic energy of the reverse flow inside the separation vortices is very low, i.e., the flow is almost laminar there.Aerospace2015-07-1423Review10.3390/aerospace20304614614812226-43102015-07-14doi: 10.3390/aerospace2030461Argyris Panaras<![CDATA[Aerospace, Vol. 2, Pages 435-460: Position Estimation Using the Image Derivative]]>
http://www.mdpi.com/2226-4310/2/3/435
This article describes an image processing algorithm to identify the size and shape of a spherical reflecting celestial body prominently depicted in images taken from a spacecraft with an optical camera, with the purpose of estimating the relative distance between target and observer in magnitude and direction. The approach is based on the fact that in such images, the pixels belonging to the target’s hard edge have the highest values of the image derivative; therefore, they are easily recognizable when the image is processed with a gradient filter. Eventual extraneous points polluting the dataset (outliers) are eliminated by two methods applied in sequence. The target center and radius are estimated by non-linear least squares using circular sigmoid functions. The proposed image processing has been applied to real and synthetic Moon images. An error analysis is also performed to determine the performance of the proposed method.Aerospace2015-07-0323Article10.3390/aerospace20304354354602226-43102015-07-03doi: 10.3390/aerospace2030435Daniele MortariFrancesco de DilectisRenato Zanetti<![CDATA[Aerospace, Vol. 2, Pages 423-434: Graphene/Epoxy Coating as Multifunctional Material for Aircraft Structures]]>
http://www.mdpi.com/2226-4310/2/3/423
Recently, the use of graphene as a conductive nanofiller in the preparation of inorganic/polymer nanocomposites has attracted increasing interest in the aerospace field. The reason for this is the possibility of overcoming problems strictly connected to the aircraft structures, such as electrical conductivity and thus lightning strike protection. In addition, graphene is an ideal candidate to enhance the anti-corrosion properties of the resin, since it absorbs most of the light and provides hydrophobicity for repelling water. An important aspect of these multifunctional materials is that all these improvements can be realized even at very low filler loadings in the polymer matrix. In this work, graphene nanoflakes were incorporated into a water-based epoxy resin, and then the hybrid coating was applied to Al 2024-T3 samples. The addition of graphene considerably improved some physical properties of the hybrid coating as demonstrated by Electrochemical Impedance Spectroscopy (EIS) analysis, ameliorating anti-corrosion performances of raw material. DSC measurements and Cross-cut Test showed that graphene did not affect the curing process or the adhesion properties. Moreover, an increment of water contact angle was displayed.Aerospace2015-06-3023Article10.3390/aerospace20304234234342226-43102015-06-30doi: 10.3390/aerospace2030423Tullio MonettaAnnalisa AcquestaFrancesco Bellucci<![CDATA[Aerospace, Vol. 2, Pages 392-422: Unmanned Aerial ad Hoc Networks: Simulation-Based Evaluation of Entity Mobility Models’ Impact on Routing Performance]]>
http://www.mdpi.com/2226-4310/2/3/392
An unmanned aerial ad hoc network (UAANET) is a special type of mobile ad hoc network (MANET). For these networks, researchers rely mostly on simulations to evaluate their proposed networking protocols. Hence, it is of great importance that the simulation environment of a UAANET replicates as much as possible the reality of UAVs. One major component of that environment is the movement pattern of the UAVs. This means that the mobility model used in simulations has to be thoroughly understood in terms of its impact on the performance of the network. In this paper, we investigate how mobility models affect the performance of UAANET in simulations in order to come up with conclusions/recommendations that provide a benchmark for future UAANET simulations. To that end, we first propose a few metrics to evaluate the mobility models. Then, we present five random entity mobility models that allow nodes to move almost freely and independently from one another and evaluate four carefully-chosen MANET/UAANET routing protocols: ad hoc on-demand distance vector (AODV), optimized link state routing (OLSR), reactive-geographic hybrid routing (RGR) and geographic routing protocol (GRP). In addition, flooding is also evaluated. The results show a wide variation of the protocol performance over different mobility models. These performance differences can be explained by the mobility model characteristics, and we discuss these effects. The results of our analysis show that: (i) the enhanced Gauss–Markov (EGM) mobility model is best suited for UAANET; (ii) OLSR, a table-driven proactive routing protocol, and GRP, a position-based geographic protocol, are the protocols most sensitive to the change of mobility models; (iii) RGR, a reactive-geographic hybrid routing protocol, is best suited for UAANET.Aerospace2015-06-3023Article10.3390/aerospace20303923924222226-43102015-06-30doi: 10.3390/aerospace2030392Jean-Daniel Medjo Me BiomoThomas KunzMarc St-HilaireYifeng Zhou<![CDATA[Aerospace, Vol. 2, Pages 376-391: Transmit Energy Efficiency of Two Cognitive Radar Platforms for Target Identification]]>
http://www.mdpi.com/2226-4310/2/3/376
Cognitive radar (CRr) is a recent radar paradigm that can potentially help drive aerospace innovation forward. Two specific platforms of cognitive radar used for target identification are discussed. One uses sequential hypothesis testing (SHT) in the receiver processing and is referred to as SHT-CRr and the other one uses maximum a posteriori (MAP) and is referred to as MAP-CRr. Our main goal in this article is to make a practical comparison between SHT-CRr and MAP-CRr platforms in terms of transmission energy efficiency. Since the performance metric for the SHT-CRr is the average number of illuminations (ANI) and the performance metric for MAP-CRr is the percentage of correct decisions (\(P_{cd}\)), a direct comparison between the platforms is difficult to perform. In this work, we introduce a useful procedure that involves a metric called total transmit energy (TTE) given a fixed \(P_{cd}\) as a metric to measure the transmit energy efficiency of both platforms. Lower TTE means that the platform is more efficient in achieving a desired \(P_{cd}\). To facilitate a robust comparison, a transmit-adaptive waveform that consistently outperforms the pulsed waveform in terms of both \(P_{cd}\) and ANI is needed. We show that a certain adaptive waveform called the probability weighted energy signal-to-noise ratio-based (PWE-SNR) waveform outperforms the pulsed wideband waveform (i.e., flat frequency response) in terms of ANI and \(P_{cd}\) for all ranges of transmit waveform energy. We also note that the \(P_{cd}\) performance of SHT-CRr can be drastically different from the probability threshold (i.e., the probability value that is used to stop radar illumination for the purposes of classification), which is critically important for CRr system designers to realize. Indeed, this fact turns out to be key in accomplishing our goal to compare SHT-CRr and MAP-CRr in terms of transmit energy efficiency.Aerospace2015-06-2623Article10.3390/aerospace20303763763912226-43102015-06-26doi: 10.3390/aerospace2030376Ric RomeroEmmanouil Mourtzakis<![CDATA[Aerospace, Vol. 2, Pages 353-375: Shock Wave Attenuation Using Foam Obstacles: Does Geometry Matter?]]>
http://www.mdpi.com/2226-4310/2/2/353
A shock wave impact study on open and closed cell foam obstacles was completed to assess attenuation effects with respect to different front face geometries of the foam obstacles. Five different types of geometries were investigated, while keeping the mass of the foam obstacle constant. The front face, i.e., the side where the incident shock wave impacts, were cut in geometries with one, two, three or four convergent shapes, and the results were compared to a foam block with a flat front face. Results were obtained by pressure sensors located upstream and downstream of the foam obstacle, in addition to high-speed schlieren photography. Results from the experiments show no significant difference between the five geometries, nor the two types of foam.Aerospace2015-06-1522Article10.3390/aerospace20203533533752226-43102015-06-15doi: 10.3390/aerospace2020353Hongjoo JeonJonathan GrossSarah EstabrookStylianos KoumlisQian WanGauri KhanolkarXingtian TaoDavid MenschingEdward LesnickVeronica Eliasson<![CDATA[Aerospace, Vol. 2, Pages 325-352: A Comparison of Closed-Loop Performance of Multirotor Configurations Using Non-Linear Dynamic Inversion Control]]>
http://www.mdpi.com/2226-4310/2/2/325
Multirotor is the umbrella term for the family of unmanned aircraft, which include the quadrotor, hexarotor and other vertical take-off and landing (VTOL) aircraft that employ multiple main rotors for lift and control. Development and testing of novel multirotor designs has been aided by the proliferation of 3D printing and inexpensive flight controllers and components. Different multirotor configurations exhibit specific strengths, while presenting unique challenges with regards to design and control. This article highlights the primary differences between three multirotor platforms: a quadrotor; a fully-actuated hexarotor; and an octorotor. Each platform is modelled and then controlled using non-linear dynamic inversion. The differences in dynamics, control and performance are then discussed.Aerospace2015-06-0522Article10.3390/aerospace20203253253522226-43102015-06-05doi: 10.3390/aerospace2020325Murray IrelandAldo VargasDavid Anderson<![CDATA[Aerospace, Vol. 2, Pages 312-324: Development of UAS Design Based on Wideband Antenna Architecture]]>
http://www.mdpi.com/2226-4310/2/2/312
An Unmanned Aerial System (UAS) has been developed which is based on an aerodynamically functionalized planar wideband antenna. The antenna utilizes a planar circular dipole metallization scheme. The aerodynamic structure implements a planform similar to the Nutball flier, a hobbyist flight architecture. The resulting codesign achieved a large impedance bandwidth defined by a voltage standing wave ratio (VSWR) less than 2 from 100 MHz to over 2 GHz and omnidirectional dipole-like radiation patterns at the lower frequency region and more directional patterns at higher frequencies.Aerospace2015-06-0422Communication10.3390/aerospace20203123123242226-43102015-06-04doi: 10.3390/aerospace2020312Franklin DrummondGregory Huff<![CDATA[Aerospace, Vol. 2, Pages 279-311: Interaction between Crosswind and Aviation-Fuel Fire Engulfing a Full-Scale Composite-Type Aircraft: A Numerical Study]]>
http://www.mdpi.com/2226-4310/2/2/279
This numerical study focuses on the fire phenomenology associated with the presence of a composite-type aircraft immersed, at one particular location and orientation, within a large aviation-fuel fire in a moving fluid medium. An extension of the eddy dissipation concept is incorporated, allowing one to investigate the roles of the wind speed and its direction on the fire growth, heat flux distribution and smoke products, such as carbon monoxide and soot. The predicted flame shape compares well with the measurements for an intermediate-scale fire. The outcome of the study is interesting, and the interaction model between turbulence and combustion is indeed adequate. The prediction indicates that interaction between the large object and fire environment combined with the influence of wind conditions dramatically affects the continuous flame shape. The increase of the wind speed results in an alteration of the distribution of the incident heat fluxes to the engulfed fuselage skin for a case where the fire and fuselage are of comparable size. The highest heat flux occurs on the windward side of the fuselage for the low and medium winds, but on the leeward side of the fuselage for the high wind. The peak in heat flux to the medium or high wind is almost equal in magnitude, but about a factor four increase of that to the low wind.Aerospace2015-05-2822Article10.3390/aerospace20202792793112226-43102015-05-28doi: 10.3390/aerospace2020279Hui WangGuo Da Wang<![CDATA[Aerospace, Vol. 2, Pages 235-278: Ornithopter Type Flapping Wings for Autonomous Micro Air Vehicles]]>
http://www.mdpi.com/2226-4310/2/2/235
In this paper, an ornithopter prototype that mimics the flapping motion of bird flight is developed, and the lift and thrust generation characteristics of different wing designs are evaluated. This project focused on the spar arrangement and material used for the wings that could achieves improved performance. Various lift and thrust measurement techniques are explored and evaluated. Various wings of insects and birds were evaluated to understand how these natural flyers with flapping wings are able to produce sufficient lift to fly. The differences in the flapping aerodynamics were also detailed. Experiments on different wing designs and materials were conducted and a paramount wing was built for a test flight. The first prototype has a length of 46.5 cm, wing span of 88 cm, and weighs 161 g. A mechanism which produced a flapping motion was fabricated and designed to create flapping flight. The flapping flight was produced by using a single motor and a flexible and light wing structure. A force balance made of load cell was then designed to measure the thrust and lift force of the ornithopter. Three sets of wings varying flexibility were fabricated, therefore lift and thrust measurements were acquired from each different set of wings. The lift will be measured in ten cycles computing the average lift and frequency in three different speeds or frequencies (slow, medium and fast). The thrust measurement was measure likewise but in two cycles only. Several observations were made regarding the behavior of flexible flapping wings that should aid in the design of future flexible flapping wing vehicles. The wings angle or phase characteristic were analyze too and studied. The final ornithopter prototype weighs only 160 g, has a wing span of 88.5 cm, that could flap at a maximum flapping frequency of 3.869 Hz, and produce a maximum thrust and lift of about 0.719 and 0.264 N respectively. Next, we proposed resonance type flapping wing utilizes the near resonance phenomenon of a two-degree of freedom elastic system, that is, the wing is supported by the springs for flapping and feathering motions. Being oscillated close to the resonance frequency of the system, only by the torque in flapping motion, the amplitude gained is a few times higher than that of normal case. The first prototype was made from acrylic using a laser cutting machine. The wings were made up of carbon rods and kite material Ripstop. First test showed that the wings were too heavy for the mechanism to work. The third prototype was a smaller single gear crank design which was fabricated using a 3D printer. Initial test proved that the second prototype could withstand the high frequency flapping and near resonance amplitude as designed. With remote control, the third prototype was able to take off, climb, cruise and land in flapping mode successfully.Aerospace2015-05-1322Article10.3390/aerospace20202352352782226-43102015-05-13doi: 10.3390/aerospace2020235Sutthiphong SrigraromWoei-Leong Chan<![CDATA[Aerospace, Vol. 2, Pages 222-234: Trajectory Management of the Unmanned Aircraft System (UAS) in Emergency Situation]]>
http://www.mdpi.com/2226-4310/2/2/222
Unmanned aircraft must be characterized by a level of safety, similar to that of manned aircraft, when performing flights over densely populated areas. Dangerous situations or emergencies are frequently connected with the necessity to change the profiles and parameters of a flight as well as the flight plans. The aim of this work is to present the methods used to determine an Unmanned Aircraft System’s (UAS) flight profile after a dangerous situation or emergency occurs. The analysis was limited to the possibility of an engine system emergency and further flight continuing along a trajectory of which the shape depends on the type of the emergency. The suggested method also enables the determination of an optimal flying trajectory, based on the territory of a special protection zone (for example, large populated areas), in the case of an emergency that would disable continuation of the performed task. The method used in this work allows researchers, in a simplified way, to solve a variation task using the Ritz–Galerkin method, consisting of an approximate solution of the boundary value problem to determine the optimal flight path. The worked out method can become an element of the on-board system supporting UAS flight control.Aerospace2015-05-0422Article10.3390/aerospace20202222222342226-43102015-05-04doi: 10.3390/aerospace2020222Andrzej Majka<![CDATA[Aerospace, Vol. 2, Pages 203-221: Numerical Study of Shock Wave Attenuation in Two-Dimensional Ducts Using Solid Obstacles: How to Utilize Shock Focusing Techniques to Attenuate Shock Waves]]>
http://www.mdpi.com/2226-4310/2/2/203
Research on shock wave mitigation in channels has been a topic of much attention in the shock wave community. One approach to attenuate an incident shock wave is to use obstacles of various geometries arranged in different patterns. This work is inspired by the study from Chaudhuri et al. (2013), in which cylinders, squares and triangles placed in staggered and non-staggered subsequent columns were used to attenuate a planar incident shock wave. Here, we present numerical simulations using a different obstacle pattern. Instead of using a matrix of obstacles, an arrangement of square or cylindrical obstacles placed along a logarithmic spiral curve is investigated, which is motivated by our previous work on shock focusing using logarithmic spirals. Results show that obstacles placed along a logarithmic spiral can delay both the transmitted and the reflected shock wave. For different incident shock Mach numbers, away from the logarithmic spiral design Mach number, this shape is effective to either delay the transmitted or the reflected shock wave. Results also confirm that the degree of attenuation depends on the obstacle shape, effective flow area and obstacle arrangement, much like other obstacle configurations.Aerospace2015-04-3022Review10.3390/aerospace20202032032212226-43102015-04-30doi: 10.3390/aerospace2020203Qian WanVeronica Eliasson<![CDATA[Aerospace, Vol. 2, Pages 189-202: Unmanned Aerial Systems (UAS) Research Opportunities]]>
http://www.mdpi.com/2226-4310/2/2/189
The aerospace community is planning for growth in Unmanned Aerial Systems (UAS) funding and research opportunities. The premise that UAS will revolutionize aerospace appears to be unfolding based on current trends. There is also an anticipation of an increasing number of new platforms and research investment, which is likely but must be analysed carefully to determine where the opportunities lie. This paper draws on the state of technology, history and systems engineering. We explore what aspects of UAS will be the result of aerospace science advances and what aspects will be incremental engineering and systems integration. It becomes apparent that, for academia, the largest opportunities may exist in small and micro UAS domain due to the novelty of aerospace engineering on a small scale.Aerospace2015-04-2722Review10.3390/aerospace20201891892022226-43102015-04-27doi: 10.3390/aerospace2020189Javaan Chahl