Micro UAVs with Fixed Wings: Design, Technological Solutions, and Tests

Abstract

Considering the advantages of using expanded polystyrene (EPS) reinforced with adhesive tape made of glass fibers, this paper presents a design and technological solution for a functional drone of class C1, meaning a maximum take-off mass of 900 g, and tests validating the use of EPS for small UAVs under flight conditions. The selected profile was MH-49, which had a maximum chord thickness of 10.5%. This profile demonstrated a much lower coefficient of pitching moment than that of the NACA 63215 profile, giving this flying-wing UAV superior governability. This airfoil implies a geometry with greater attenuation of the trailing edge, and the design favors the placement of stress concentrators towards the trailing edge. Due to the use of fiberglass tape reinforcement technology, it is possible to address this profile, implying improved aerodynamic performance. The use of EPS in the disposable UAV industry may bring significant benefits, contributing to the development of high-performance, versatile, and low-cost UAVs, suitable for a wide range of tactical and logistic applications. This study presents the design, the fabrication, and testing of this drone, highlighting the advantages and possible challenges of the proposed solution.

1. About Small Drones

Next-generation drones represent a revolution in aviation technology, offering advanced capabilities and functionality that have transformed the way we interact with the world around us. They are the perfect combination of advanced technology and innovative materials [1,2,3,4].

The main features of new-generation drones are as follows [5,6]:

• Advanced autonomy: new-generation drones are equipped with high-precision navigation and control systems, allowing them to perform autonomous flights and perform complex tasks without human intervention;
• Payload capabilities: modern drones are capable of carrying various types of payloads, such as high-resolution cameras and video cameras, specialized sensors for environmental monitoring, and delivery equipment;
• Advanced aerodynamic performance: the aerodynamic design of next-generation drones is optimized to maximize flight efficiency and provide superior stability and maneuverability;
• Connectivity and communication: modern drones are equipped with advanced communication technologies, allowing them to stay connected and transmit data to operators and other devices in real time.

Small drones, also known as unmanned aerial vehicles (UAVs), are compact aircraft operated that can be remotely or autonomously. These have a wide range of applications in many industries and can be used in surveillance and monitoring [7,8], photography, agriculture, delivery services, disaster management, and military use. Their versatility, adaptability, and increasing affordability have made them valuable tools for both commercial and personal use.

A SWOTT analysis of small drones is formulated in Figure 1.

Drones are extensively used for surveillance, with applications in security, disaster management, and environmental monitoring. Drones that are equipped with sensors can monitor crop health, apply pesticides, and assess soil conditions, improving yield and resource use. Delivery companies use drones for faster, more efficient delivery, particularly in difficult-to-access areas. They offer dynamic perspectives for photography and cinematography [9], often replacing expensive crane shots or helicopters. Drones are essential for rapid searches in difficult terrain or following natural disasters. Small drones can observe wildlife with minimal disturbance and gather climate data in hard-to-reach areas [4].

Small drones require materials that are lightweight but strong, allowing for better maneuverability and longer flight times. Here, we present some common materials used in their construction [10,11].

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Composites with carbon fibers are lightweight, strong, durable, and resistant to corrosion; they offer high strength-to-weight ratios, making them ideal for drone frames; they also absorb vibrations, enhancing flight stability; but they are expensive and brittle when subjected to significant impacts [12].

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Polymers (e.g., ABS, polypropylene, etc.), blends, and composites containing polymers are inexpensive, lightweight, and easy to mold. Plastics are cost-effective and easy to manufacture. They are flexible and resilient, allowing drones to handle minor impacts without damage. Plastic is not as strong as other materials like carbon fiber, and it is less durable in harsh environmental conditions.

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Aluminum alloys are lightweight, corrosion-resistant, and affordable; they are easy to shape, and their strength-to-weight ratio make them good materials for the production of drone parts such as motor mounts or arms. However, they are weaker than carbon fibers and less impact-resistant.

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Foams (expanded polypropylene—EPP—and so on) are lightweight, impact-absorbing, and inexpensive; foams are commonly used in hobbyist drones due to their low cost and excellent impact resistance; they are useful for applications where weight savings are critical; however, they have short life expectancy and are more prone to wear over time than harder materials.

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Magnesium alloys are lightweight and high-strength, displaying excellent heat dissipation properties. Magnesium alloys offer the strength of metals like aluminum but at a lower weight. They are also better at dissipating heat from motors and electronics, but they are more expensive and can corrode in certain conditions.

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Graphene-based materials are ultra-lightweight, very strong, and highly conductive: while not widely used in drones yet, graphene holds immense potential for use in future drone technology due to its strength and conductivity, making them ideal for the production of structural parts and batteries. However, they are still in the experimental phase and expensive to produce in large quantities.

Small drones have become indispensable across many industries due to their versatility, ease of use, and low cost. However, they face challenges like battery life, weather sensitivity, and regulatory hurdles. The choice of materials plays a critical role in the performance and durability of small drones, with lighter and stronger materials allowing for better flight characteristics and operational capabilities. As drone technology evolves, especially with the use of advanced materials like graphene, their utility and application scope will only continue to expand.

Given the success of the Australian company SYPAQ regarding drones made from waxed cardboard acquired from Corvi [13], drones that demonstrated tactical capabilities on the Ukrainian front, it is clear that the field UAVs is a pioneering one. It involves multidisciplinary knowledge, and it has both a lot in common with traditional aircraft design. However, it displays significant differences in terms aerodynamics, designs models, and especially the materials used.

The innovative aspects of this design consist of a set unique of characteristics. These include the use of a profile MH-49 to achieve a low Reynolds number; the use of a low-density material for the drone, with reinforcement based on an adhesive band made of glass fibers; and the use of the usual components (available on the market) for propellers, the engine, and command rods.

A search on ScienceDirect for extruded polystyrene only returns several papers with these keywords, including reference [14], which mentions the use of extruded polystyrene in 2015 to build a drone with a Coanda design, weighing 1.6 kg (heavier than the design presented here), and a drone made of extruded polystyrene and balsa with a weight of 1.46 kg. A search for the keywords “drone, UAV, expanded polystyrene, fixed wing” returned only 5 results that were not related to UAV design or fabrication. These articles consisted of general comments on the use of low-cost, low-weight drones in monitoring processes. On Google, information on the use of expanded polypropylene drones is given in [15] and a recent design is presented in [16]. This is presented without mentioning the profile or technological aspects, but there is a fly test and a virtual wind tunnel test. A fixed-wing drone made of composite material is presented in [17], emphasis placed on fabrication and a set of light components.

The design proposed in this paper is based on obtaining the drone’s body using a block made of extruded polystyrene, with individual cuts for each wing and the body between. This fabrication is not sophisticated, and the time taken to produce the drone body is shortened.

This paper presents the steps involved in the design, simulation, fabrication, and testing of a scaled demonstrator model, highlighting the advantages and possible challenges of the proposed solution, the objective of the designed drone being to achieve low mass, maneuverability, and the hand launch of the fixed-wing drone.

2. Arguments in Selecting the MH Wing Profile Selection

Unlike NACA profiles [18,19,20,21,22], which are the most used airfoils in aviation, the MH (Martin Hepperle) aerodynamic airfoil series is designed for specific applications, aimed at low speed and therefore a low Reynolds number (up to 300,000) [23,24,25,26,27]. Thus, these MH airfoils are used for the construction of propellers, gliders, UAVs, and small aircraft, which need high aerodynamic efficiency at subsonic, incompressible speeds. Also, the use of these series of airfoils results in the achievement of an increased lift-to-drag ratio, unlike the properties provided by NACA airfoils, as shown in

Table 1. Profile properties.

Type of the Profile	MH (Martin Hepperle)	NACA
Designer	Martin Hepperle	NACA (National Advisory Committee for Aeronautics)
Aplications	UAVs, gliders, propellers	Aviation. General applications.
Performance	Lift-to-drag ratio better than NACA profiles to achieve low Reynolds numbers	Better aerodynamic coefficients for high Reynolds numbers
Complexity	Complex profile, obtained by computational optimization	Simpler shapes, easy to fabricate
Reynolds range	Optimized profiles to achieve low and medium Reynolds numbers (until 500,000)	Applicable for any Reynolds number

.

A comparison between the geometry of NACA airfoil (NACA 63215) and an MH airfoil (MH-49) is shown in Figure 2.

In Figure 3, the blue curves represent the aerodynamic curves of the MH-49 airfoil, whereas the red curves stand for aerodynamic polars for NACA 63215 airfoil. The values of aerodynamic coefficients are obtained using XFLR5 software, version 6.61, which is suitable for analyzing aerodynamic performances of airfoils can detect low Renolds numbers [28]. Moreover, for angles of attack ranging from 1° to 10°, the difference between the two airfoils’ lift coefficient is significant (Figure 3a). Figure 3b depicts the variation in drag coefficient as a function of angle of attack. For angles of attack greater than 4°, the MH-49 airfoil’s drag coefficient tends to be greater than that of NACA 63215 airfoil, but there are no major changes between the two analyzed airfoils. Figure 3c shows the behavior of the lift-to-drag ratio as a function of the angle of attack. Due to the fact that there are not significant differences between the airfoils’ drag coefficient, and there are notable changes in airfoils’ lift coefficient, the lift-to-drag ratio will have a noteworthy difference between the two analyzed airfoils, leading to a better lift-to-drag ratio for MH-49 than that of NACA 63215 for angles of attack ranging from 1° to 10°. It can also be observed in Figure 3d that the moment coefficient of MH-49 is negative for angles of attack greater than 2°. Furthermore, the values of the moment coefficient in the case of MH-49 are lower than those obtained for NACA 63215 for angles of attack greater than 2°, leading to better pitch stability. These aspects result in the MH-49 airfoil being suitable to operate in low-speed applications and achieving low Reynolds numbers. It is therefore recommended to use this class of airfoil for flying-wing UAV and glider construction.

Figure 2. The geometry of MH 49 profile and NACA 63215 profile, calculated based on [18,29].

Figure 2. The geometry of MH 49 profile and NACA 63215 profile, calculated based on [18,29].

3. Fabrication of the Drone

3.1. Material Considerations

The aeronautics industry has traditionally been oriented towards the use of high-performance technologies that offer good strength-to-weight ratios, reliability, and maintainability in service. However, in the current context of the development of single-use UAVs, which have low production costs, it becomes necessary to explore alternative materials, materials that are low cost but have sufficient qualities to produce high-performance aircraft.

Analyzing the specific requirements for UAVs and departing from the idea of using high-quality materials in the field of aviation, but also taking into account the notable results achieved in the utilization of cardboard as a material for UAVs, we performed a comparative analysis of cardboard with EPS. This material offers multiple advantages, making it a viable alternative material for use in the construction of single-use UAVs.

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Low production costs: EPS is a low-production-cost material with a similar price to cardboard.

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It can be obtained by in-mold expansion, which facilitates high-volume production and low-specific-cost technology, as validated in the packaging industry for household appliances and in any other sensitive products requiring high-protection packaging.

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Superior aerodynamic capabilities: its production involves processing, which allows the production of wings with complex geometries, e.g., arrow-shaped wings and/or wings with variable airfoils in span, thus ensuring a significantly better aerodynamic performance than a straight wing that is processed from cardboard in the Corvi project.

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The possibility of creating high-performance airfoils with accentuated tapers and complex shapes towards the trailing edge, which can significantly increase the diversity of UAV types.

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EPS is stable over time and independent of humidity conditions and can be stored for a long time without noticeable changes in mechanical properties, offering more stability and durability.

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UV resistance and operational without restrictions in humid, rainy, tropical or coastal conditions.

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EPS has acceptable mechanical properties.

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It provides a high degree of protection for on-board radio electronic equipment.

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It allows fiberglass reinforcement, reinforcing critical areas such as the leading edge and trailing edge of the wing, with reinforcements necessary for structural protection, especially stress relief in critical areas, such as those relating to control surfaces.

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Low radar detectability: EPS is transparent to radar waves, which makes UAVs constructed of this material extremely difficult to detect using radar. This offers a significant tactical advantage by reducing the radar signature, making the drones very difficult to detect.

Although the waxed cardboard used by SYPAQ to make Corvi has the advantage of low cost and ease of processing, there are certain limitations that can be overcome by using EPS.

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Large-scale batch production: EPS allows the finished product to be obtained by in-mold expansion, which is much more efficient for high-volume than multiple board processing operations, which can increase costs and the complexity of production.

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Geometric complexity: EPS allows for complex geometries and advanced aerodynamic airfoils, providing greater flexibility in the design of UAVs with superior aerodynamic performance, essential characteristics for UAVs with superior flight envelope performance.

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Durability and resistance to environmental conditions: EPS is unaffected by moisture and is UV-resistant, making it suitable for use in a variety of environmental conditions, including humid or tropical environments; the paperboard, even when waxed, can be susceptible to damage in humid conditions, especially when in storage conditions for considerable periods in humid environments.

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The protection of equipment: the mechanical properties of EPS provide better protection for on-board radio-electronic equipment, with an ability to absorb shock and vibration, and allow the processing of complex shaped cavities that can accurately support on-board equipment.

The comparison of these two materials incorporates qualitative aspects and the values of mechanical properties. The possibility of fabricating curved surfaces from cardboard is limited, and this material is very sensitive to humidity. These aspects are in the favor of the use of EPS as curved profiles can be hot-wire-cut with enough accuracy and the humidity does not drastically affect the material.

As for mechanical properties, corrugated cardboard is an anisotropic material, with the stress limit varying from 60 MPa in the machine direction to 20 MPa in the cross direction, depending on temperature and relative humidity [30]. Also, EPS has a large range for the stress limit and the Young modulus, spanning from several MPa [31] to higher values, depending on temperatures [32]. As for EPS, the overview on this material in terms of grade [33] shows ranges for its tensile strength (47–51 MPa), elongation at break (5–13.4%), elasticity modulus (0.006–2.65 GPa), and an isotropic character [34].

3.2. The Drone Components

The design of the drone is elaborated as follows, taking [35,36] into account. The drone components are given in Figure 4.

At this stage of the drone project, the optimal position for mounting the servomechanisms used to actuate the ailerons was determined, taking into account multiple technical factors. The thickness of the aerodynamic profile in the mounting area was evaluated both in terms of dimensional constraints, to maintain aerodynamic performance, and in terms of mechanical strength, as this area has to withstand the mechanical loads transmitted by the servomechanism to the control surfaces with minimal deformations.

Another important aspect in choosing the position was optimizing the kinematic assembly of the control system in order to minimize the length of the control rods and to reduce the risk of their buckling. The control rods were made of 2 mm diameter steel, a material selected for its optimal balance between cost, market availability, strength, and weight. Although positioning the servomechanisms toward the middle of the half-plane would have been preferable to limit the torsion exerted on the control surfaces, constraints imposed by the reduced thickness of the aerodynamic profile and the low strength of the material (EPS) led to their placement near the middle area of the half-planes.

This mounting configuration was preferred despite the need for the aileron to handle a greater torsional moment. The aileron was made of bamboo wood, a material that has demonstrated excellent torsional load-bearing capacity, thus ensuring low torsional elasticity.

Following simulations and tests, the final position was selected, providing the kinematic assembly with the necessary rigidity to avoid oscillations caused by aerodynamic acting forces on the aileron surfaces, guaranteeing the desired performance and stability.

4. Finite Element Analysis of the Body Structure Under Static Loading

4.1. The Model of the Drone

FEM uses an integral model of the phenomenon under study. This is applied to small discrete regions of the continuous structure, called finite elements, connected at points called nodes. These elements should reconstruct the real structure as faithfully as possible and allow numerical convergence to the exact solution as the finite elements become smaller [37,38,39].

This is a simplified model aimed at identifying the stress concentration zones during static loading for the purpose of adding reinforcement bands to these zones. The adhesive band is added while taking into consideration the FEM result for equivalent stress distributions during quasi-static loading. The FEM analysis of the reinforcement strip will be a subject for a more detailed approach, introducing the cohesive zone model and details for engine and battery locations.

The geometry of the model started from the wing profile, namely MH-49, shown in Figure 5. We entered the coordinates of the points. After joining the points with the Curve3D command, the airfoil was generated. Using the dimensions shown in Figure 6, the wing model was created.

4.2. Model Meshing and Material Constitutive Model

A fine grid was generated with an element size of 5 mm, in resulting 71,190 nodes and 58,250 elements (Figure 7).

To carry out the analysis, constant external pressures of 2.5 MPa and 5 MPa was applied to the upper surface of the model. The wing model was simplified, as flaps, battery, and engine slots were not included in this model.

The simplifying assumptions for this model are as follows:

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Isothermal regime: this is assumed because no traces of thermal damage are observed on the drone as it is made of a thermoset material and because the mechanical properties change little with temperature up to the point of material damage;

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Perfectly smooth body: the stress and strain state study showed very low values of roughness parameters because the granular nature of polystyrene was not taken into account;

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Elasto-plastic model for polystyrene: selecting from the literature survey on tests and models for plastics, bilinear–isotropic modeling with hardening is opted for based on experimental data provided by some researchers, with the equivalent plastic strain at break added as the failure criteria [40];

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The longitudinal median surface is considered to be fixed in the simulation (Figure 8 and Figure 9).

The material constitutive model is an isotropic bilinear hardening model, as shown by the values in

Table 2. Mechanical properties for expanded polystyrene (EPS) [34].

Property	Value
Density [kg m3]	600
Young modulus [MPa]	2650
Poisson ratio	0.3
Isotropic bilinear hardening model
Yield limit [MPa]	50
Tangent modulus [MPa]	450
Failure criterion
Equivalent plastic strain at break	0.01

. The failure criterion was based on the equivalent plastic strain.

4.3. Simulation Results

Each image shown in Figure 8 and Figure 9 has its own color scale for the equivalent stresses. The distribution of equivalent stresses for the time 2.5 × 10⁻⁴ s is shown in Figure 8 and that for the time at t = 5 × 10⁻⁴ s is shown in Figure 9 (as the final moment of the simulation). As observed, the maximum stresses occur at the trailing edge of the airfoil.

Analyzing Figure 8 and Figure 9, we see that there is no risk of plastic deformation at the nominated pressures (2.5 MPa and 5 MPa) because the maximum equivalent stresses are kept below the yield strength of the material. However, to avoid damage to the airfoil at pressures above 2.5 MPa, it is recommended to reinforce the material. These reinforcements can include additional reinforcement or design modifications.

Expanded polystyrene is prone to failure and fracturing (fragmentation), indicating a high risk of structural failure in areas of maximum equivalent stress. The maximum stresses are concentrated towards the trailing edge and control surfaces, which are the areas with the thinnest thickness.

The results of this analysis show the need for fiberglass tape reinforcement of the trailing edge and for special attention to be paid to the design and construction of the control surfaces.

As a result, in the practical realization of this drone, a stage was dedicated to the reinforcement of the trailing edge (with reinforced adhesive tape) and to the mounting of the control surfaces (ailerons) with adhesive tape, which acts as a joint and also reinforces the trailing edge.

5. Drone Fabrication

This paper presented a design of a fixed-wing drone with a body made of expanded polystyrene (EPS). It displayed the following characteristics: wingspan, 1.2 m; aspect ratio, 4.44; maximum take-of weight (MTOW), 0.9 kg; payload weight, 130–250 g; maximum endurance, >20 min; propulsion type, brushless direct current (BLDC) electric motor; battery type, Li-Ion (3S). It had a 3.2-amp hour range. The designers have the intention of improving this flying system and adding an autonomous flight system.

The stages of the fabrication are presented in Figure 10, and images of components are shown in Figure 11, where (a) shows a cut wing and (b) shows that the design of the propulsion system of the UAV was focused on low costs and component availability. Figure 12 presents the drone, with all components shown, and the reinforcements with glass fiber adhesive bands are shown.

One of the main objectives of this project was to significantly reduce the final product’s cost without compromising the performance and the safety of the propulsion system. In addition to cost optimization, special attention was paid to the selection of materials based on criteria such as availability in sufficient quantities and at low enough prices to facilitate mass production under favorable economic conditions. This section details the selection of the motor, propeller testing, and the choice of the battery pack, emphasizing the technical solutions adopted to keep costs extremely low while ensuring the long-term availability of critical components for the C1-class UAV.

The main purpose of reinforcing the leading edge is to protect the material’s integrity in case of inherent micro-impacts that occur during flight and especially during landing procedures. In this stage, we reinforced the wing’s leading edge, as shown in Figure 12. As a result of this operation, the buckling resistance of the structure increased significantly.

To minimize costs and ensure market availability, a wooden mount was created for the motor. Wood was chosen not only for its extremely low cost, but also because it is a material readily available in large quantities. This versatility provided an efficient solution for distributing the forces induced by the motor over a wide area of the UAV body, thus helping to keep the overall weight low and making integration into the UAV structure easier.

At this stage of the drone project, the optimal position for mounting the servomechanisms used to actuate the ailerons was determined, taking into account multiple technical factors. In order to maintain aerodynamic performance, the thickness of the aerodynamic profile in the mounting area was evaluated in terms of both dimensional constraints and in terms of mechanical strength. We carried out these assessments because this area has to withstand the mechanical loads transmitted by the servomechanism to the control surfaces with minimal deformations.

Another important aspect in choosing position was optimizing the kinematic assembly of the control system in order to minimize the length of the control rods and to reduce the risk of buckling. The control rods were made of 2 mm diameter steel, a material selected for its optimal balance between cost, market availability, strength, and weight. Although positioning the servomechanisms toward the middle of the wing left-side and right-side would have been preferable in order to limit the torsion exerted on the control surfaces, constraints imposed by the reduced thickness of the aerodynamic profile and the low strength of the material (EPS) led to their placement near the middle area of the wing’s left side and right side.

This mounting configuration was preferred despite the need for the aileron to handle a greater torsional moment. The aileron is made of bamboo wood, a material that has demonstrated excellent torsional load-bearing capacity, thus ensuring low torsional elasticity.

Following multiple simulations and tests, the final position was selected, providing the kinematic assembly with the necessary rigidity to avoid oscillations caused by aerodynamic forces on the aileron surfaces, thus guaranteeing the desired performance and stability.

The chosen motor, a brushless outrunner model A2212, represents a cost-effective solution in terms of both price and availability. It is manufactured by a well-known Chinese company capable of providing motors at competitive prices and in large quantities. With a mass of only 50 g and a maximum efficiency of 80%, the motor offers an excellent balance between performance, price, and accessibility, being available in various versions that allow for technical adjustments based on the UAV’s flight requirements [41].

A brushless outrunner motor was selected. This type of motor is characterized by the rotation of the outer part, which allows for efficient cooling, which is essential for continuous operation in the direct airflow of the propeller. This efficient cooling helps prevent overheating and maintains performance parameters throughout the flight, even under demanding operating conditions.

Propeller testing and selection was performed after consulting [42]. Propellers with diameters ranging from 4 inches to 8 inches and pitches ranging from 2.5 inches to 6 inches were tested (as given in Figure 13). The tested propellers, including models like GWS 4x2.5 and APC 8x4, were selected in order to evaluate motor performance in various flight conditions, ensuring optimal compatibility with the motor and achieving the proposed speed and the necessary static thrust for the imposed take-off conditions. The wide market availability and price of these propeller models were also important criteria in their selection.

Regarding battery pack selection, in order to power the propulsion system, we chose a Li-ion battery pack made of LG 18650 cells of the INR18650MH1 model. Besides its low cost, one of the great advantages of this type of battery is its wide availability on the market. These cells are found in numerous everyday devices such as laptops, electric scooters, drills, and other battery-powered tools. This versatility not only helped to keep costs low, but also ensured easy access to these cells in large quantities, which is useful if there is necessary to build alarge number of UAVs in a short time.

With a capacity of 3200 mAh and a maximum continuous discharge current of 10 A, these cells provide adequate autonomy for the C1-class UAV, with a maximum take-off weight of 900 g, while also supporting the performance of the propulsion system.

Shown below is a table (

Table 3. Propellers (green color represents the selected one for the adopted design).

	2600 Kv
Propellers	A2208 8t	3 s
7x3.5	838	gf	Transonic regime
6x5 L
6x6 APC	600	gf	Critical T regulator and motor
6x3GWS	830	gf	Sightly heated
7x6 TF
5x5 APC	430	gf	N/A
6x4 MA L			Not tested
8x4 APC C-2			Not tested
8x4 MA L			Not tested
9x6 APC e			Not tested
GWS 11X4,5			Not tested

gf—abbreviation for gram force.

) with the static tensile values obtained for the various tested propellers (Figure 13).

Thus, through innovative component selection solutions and efficient cost optimization, the UAV meets performance requirements. This can be replicated on a large scale with a positive economic impact due to the long-term availability of all critical components.

6. Results of Fly Tests

6.1. Flight Tests

Two test flights were performed to verify the practical aeronautical performance and structural reliability of the drone.

The first flight was performed to check the drone’s hand take-off capability, control efficiency, aerodynamic stability, and landing capability. The flight program consisted of the following phases: hand launch (Figure 14 left); climb to 30 m (AGL—Above Ground Level); level flight at constant altitude with flight control stability and efficiency checks (Figure 14 right); 180° turns alternately, going right at one end and left at the other end; descent to 5 m (Figure 14 right); landing.

The second flight was performed to test the strength structure of the wing under overload conditions. The flight program had the following phases: take-off from hand; climb on constant slope to a height of 30 m; level flight; 180° right turn; level flight; full 360° looping (Figure 15 left looping ascent, Figure 15 right looping descent); level flight; tight loops; landing.

In the second test flight, during the execution of the tight loops, the drone was subjected to particularly intense stress at the loop exit point (Figure 15 left), during the structural aerodynamic loads, and at the maximum reached speed values. The decrease in the aerodynamic effort of the propeller blades reached negative values due to the increased speed dictated by the aircraft’s evolution at this point. This facilitated the overloading of the engine, which in turn induced a transonic velocity regime on the blade tip, a regime to be avoided for that type of propeller, and led to engine separation. The drone’s stability and structural strength were demonstrated by its continued flight, response to commands, and safe landing (Figure 15 right).

As part of the UAV testing program, two flights were performed to evaluate the performance and structure of the aircraft. This report details the two flights and analyzes the incident that occurred during the second flight.

Weather conditions were as follows: partially cloudy sky, temperature: 34 °C, wind N-E between 15 km/h and 20 km/h, and wind gusts.

6.2. Flight Test 1

The objective of test 1 was to check several issues: take-off capability; control efficiency; stability; landing capability.

The flight program consisted of a hand launch, a climb on a constant slope to a 30 m altitude (AGL—Above Ground Level), level flight at a constant altitude, and an assessment of the stability and efficiency of flight controls. At the end of the straight portions, 180° turns are executed, alternately right at one end and left at the other end. This is followed by steady descent to an altitude of 5 m, and then landing.

Table 4. Results and observations of flight test 1.

Evolution	Mark	Observations	Fly Safety	Corrective Measures
Take-off	10	No altitude loss
Command efficiency	8	Slightly lagged commands	Not affecting
Stability		No oscillations
Landing		No shock

presents the results and observations of flight test 1.

The flight was without incident, confirming the UAV’s basic performance under normal operating conditions. Transitions were attempted under different speed regimes, including limiting speed conditions, where the aircraft demonstrated the ability to remain steerable within reasonable limits without engaging in the uncommanded dive evolutions necessary to regain speed.

6.3. Flight Test 2

The second flight had the objective of testing wing strength structure under overload conditions. Results and observations for flight test 2 are given in

Table 5. Results and observations for flight test 2.

Evolution	Mark	Observations	Fly Safety	Corrective Measures
Looping 1	10	N/A
Looping 2	10	N/A
Looping 3	6	Propeller breakage and motor dismantling	Affecting	Flight ending
Looping 4

.

The flight program included take-off, a steady climb to a height of 30 m, level flight, a 180° right turn, level flight, a full 360° loop, level flight, the resumption of the flight program, ad tight loops.

The investigation of the terrain over which the incident occurred revealed the following.

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The two propeller blades were found to be approximately 50 m apart around the incident area.

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The engine was not found.

Visual investigation of the UAV revealed the following:

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A section of the engine mount was found missing.

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The motor and propeller assembly were found missing.

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No structural integrity issues were found in the areas in the vicinity of the propeller, which indicates a lack of blade-to-body impact.

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The wing structure remained intact except for the detachment of the fiberglass reinforcement strips, which intersected the recessed area of the detached engine–engine mount section.

The analysis of the incident pointed out the following findings:

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The blade tip speed regime verification table prepared as part of the project indicated that the propulsion assembly used—consisting of the A2206 2600 Kv engine, equipped with a GWS 7x3.5 propeller and powered by a 3-cell Li-ion battery—is susceptible to transonic speed at the blade tip.

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During the last evolution, the engine was ran at full throttle, including during the descent section.

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It is known that both the structural aerodynamic efforts and the airspeed reach their maximum values at the exit point of the looping, which corresponds exactly to the time at which the incident was observed.

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The decrease in aerodynamic effort on the propeller blades can reach negative values due to the increased speed dictated by the aircraft’s evolution at this point, which facilitates engine overload, which in turn induces the blade tip to become transonic.

The probable cause of this was that the decrease in aerodynamic downforce caused the engine speed to increase above the operating limits for normal flight conditions, allowing the blade tip to enter transonic conditions.

This led to the detachment of one blade, which unbalanced the engine assembly, resulting in the immediate breakage of the engine mount and, subsequently, the detachment of the second blade due to the mechanical shock produced by the sudden change in the rotational plane.

6.4. Conclusions of These Two Flight Tests

Pilot ownership of the test program, and strict adherence to it during test flights, is required.

It is necessary to revise the propeller configuration. Lower rpm propeller assemblies should be used, or smaller diameter propellers should be used as they are less susceptible to transonic blade tip regimes.

It is necessary to fit a telemetry device on-board the UAV to control flight parameters so that they do not go outside the restrictions set in the test program.

7. Conclusions

The demonstration could be improved by future research and development directions. It could be suited for mass production. The production of UAVs could become more productive by expanding the EPS in the mold to ensure low price and uniformity.

New versions of this design could increase carrying capacity. The aerodynamic performance could also be improved with the help of a more refined finite element analysis method and simulations and the objective would be to minimize the influence of mechanical limitations imposed by material properties.

The use of such a UAV in meteorological missions can bring the advantage of a very good recovery rate of the equipment used for data acquisition, without the risk of high costs in the case of its loss in areas of strong storm fronts, which could compromise its flight capability (it is profitable to send it ten times and lose it only once).

This drone could help during disaster intervention; due to its extremely low price, this type of UAV can be used in high-risk missions, such as data acquisition in disaster areas, massive forest fires, or chemically or nuclear contaminated areas, where the degree of contamination of classical equipment makes that equipment unsuitable for reuse (decontamination costs may exceed the cost of the equipment).

The drone is suitable for the transportation of medicines: the UAV can carry a payload of minimum 130 g and maximum 250 g without any additional modifications, making it ideal for delivering medicines or other light-weight goods to disaster or hard-to-reach areas in emergency situations. Its main advantage is its ability to take off without special conditions and its low cost, which makes it suitable for no-return missions, allowing it to cover twice the distance compared to similar systems.

There is a significant potential in terms of using EPS in UAV construction, with applicability in high-risk areas with limited accessibility. This study highlights that affordable materials and advanced processing technologies, such as EPS and in-mold expansion, can revolutionize the field of UAVs, paving the way towards more efficient and economical UAVs, with varied and wide-ranging applications in the future.

The main objective of this project, which is to develop a functional UAV at an extremely low cost, was achieved by selecting accessible and widely available materials and components, such as the wooden mount for brushless motors produced by manufacturers. This allows the drone to be constructed at a competitive price. The choice of drone components not only helped to keep costs low, but also ensured reliable and accessible supplies for mass production.