*Article* **Toward Smart Air Mobility: Control System Design and Experimental Validation for an Unmanned Light Helicopter**

**Emanuele Luigi de Angelis1,\* , Fabrizio Giulietti <sup>1</sup> , Gianluca Rossetti 2, Matteo Turci <sup>2</sup> and Chiara Albertazzi <sup>3</sup>**


**Abstract:** Light helicopters are used for a variety of applications, attracting users from private and public market segments because of their agility and convenient storage capabilities. However, most light helicopters on the market today are designed and manufactured with technologies dating back to the 1980s, with safety issues to be addressed by advanced design methods, more powerful engines, and innovative solutions. In this regard, the DISRUPT (Development of an innovative and safe ultralight, two-seater turbine helicopter) project, led by Curti Aerospace Division (Italy) and co-funded by the EU H2020 program, is a state-of-the-art concept for a novel ultralight helicopter equipped with a ballistic parachute. In order to validate the first parachute ejection in a safe scenario, a dronization process was selected as a viable solution to be performed in collaboration with the University of Bologna. In the present paper, the steps followed to transform the helicopter into an unmanned vehicle are detailed according to the model-based design approach, with particular focus on mathematical modeling, control system design, and experimental validation. Obtained results demonstrate the feasibility of using a civil helicopter first as a remotely-piloted vehicle and then as a highly-automated personal transportation system in the framework of smart and sustainable air mobility.

**Keywords:** urban air mobility; helicopter; parachute; model-based design; control system; flight testing

#### **1. Introduction**

The interest in Urban Air Mobility (UAM) had a step increase over the last few years [1]. On the one hand, the slow growth rate of ground infrastructure led to critical traffic congestion in urban areas. On the other hand, the increasing demand for moving people and payloads further and faster drove the attention of the research community and stakeholders toward the exploitation of the vertical dimension [2]. For example, Amazon and Google pioneered the testing of urban parcel delivery by means of multirotor aircraft [3,4]. In such a way, they paved the way for a wide range of studies on highlyautomated low-altitude vehicles as an alternative means of transportation, where "the regular Joe" is capable of performing a mission without having the skills of a licensed pilot [5–7]. In this respect, two early attempts that investigated concepts of operation and technologies for a new personal transportation system based on both an aerial platform and a ground infrastructure were, respectively, PPlane (2009–2013) and myCopter (2011– 2014), projects funded by the European Commission under the 7th Framework Program (FP7) [8,9].

By taking advantage of consolidated experience in conventional aviation, high reliability of onboard systems, and rapid improvement of electrical propulsion performance, manufacturers and transport stakeholders (such as Airbus, Volocopter, and Uber) investigated concepts for personal air transportation systems. With the aim of playing a lead

**Citation:** de Angelis, E.L.; Giulietti, F.; Rossetti, G.; Turci, M.; Albertazzi, C. Toward Smart Air Mobility: Control System Design and Experimental Validation for an Unmanned Light Helicopter. *Drones* **2023**, *7*, 288. https://doi.org/ 10.3390/drones7050288

Academic Editor: Pablo Rodríguez-Gonzálvez

Received: 20 March 2023 Revised: 19 April 2023 Accepted: 21 April 2023 Published: 25 April 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

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role in this new raising market, they considered electric platforms with Vertical Take-Off and Landing (VTOL) capabilities as key elements for the next generation of controlled airspace [10,11].

Among all the above-mentioned projects and applications, it is acknowledged that a cost-effective solution to sustainable Urban Air Mobility and Delivery (UAMD) is represented by the use of small/light aircraft, where onboard flight control systems, supported by Air Traffic Management (ATM) technology, will provide safe navigation in dynamic scenarios and weather conditions in the presence of other sky users [12]. Transforming a conventional aircraft (both fixed and rotary-wing) into a Remotely–Piloted Aerial System (RPAS) may represent a successful strategy for different reasons. First of all, available light/ultralight conventional aircraft have already passed through several design, test, and certification steps with the aim of fulfilling reliability, performance, and flying quality requirements [13]. Moreover, reversible control chains can be easily replaced by Electro-Mechanical Actuators (EMA), controlled by dedicated onboard avionics. Starting from this design bias, researchers can thus focus on the design and experimental validation of all other technologies allowing for UAMD (including Guidance, Navigation, and Control (GNC) systems, telemetry, communication, and ATM devices) in addition to ground handling facilities. In this respect, thanks to their compact size and peculiar VTOL configuration, civil ultralight helicopters represent suitable test-beds for performing the transition toward a highly-automated personal transportation system.

By focusing on the very recent past, examples of the transition of conventional helicopters into RPASs can be dated back to 2004, when the Unmanned Little Bird demonstrator, derived by Boeing from a civil MD 530F, made its first autonomous flight (with a safety pilot). In particular, a pre-programmed 20-min armed intelligence, surveillance, and reconnaissance mission was performed around the United States Army's Yuma Proving Ground facility [14]. In 2006, Northrop Grumman introduced the MQ–8 Fire Scout unmanned helicopter family, obtained from Schweizer 333 and Bell 407, designed to provide reconnaissance, situational awareness, aerial fire, and precision targeting support for ground, air, and sea forces [15]. In 2008, an unmanned, highly-automated version of the Kaman K-MAX helicopter took its maiden flight, with the aim of operating in combat scenarios as well as in civilian situations involving chemical, biological, or radiological hazards [16]. Later on, Eurocopter launched a series of flights for a new rotary-wing solution designed to expand the mission capabilities of Eurocopter helicopters [17]. The Optionally–Piloted Vehicle (OPV) program, based on the EC145 helicopter platform (now Airbus Helicopters H145), was revealed during a demonstration flight: after an automatic takeoff, an EC145 flew a circuit via pre-programmed waypoints and performed a mid-route hover to deploy a load from the external sling. The EC145 continued on a return route segment representing a typical observation mission, followed by an automatic landing. Finally, Sikorsky demonstrated its OPV Matrix Technology on a modified S–76B helicopter called the Sikorsky Autonomy Research Aircraft (SARA). Since 2013, the program has made progress with more than 300 h of autonomous flight with the aim of improving decision-aiding for manned operations, while enabling both unmanned and reduced-crew operations [18].

This paper presents the results of a research work performed within DISRUPT (2016– 2018), a collaborative project co-funded by the EU within the H2020 program and led by Curti Aerospace Division. Specifically, DISRUPT proposed a new light rotorcraft configuration, the two-seater Curti Zefhir helicopter, that features a turbine engine and an emergency ballistic parachute to respectively enhance flight performance and increase passenger safety (see Figure 1). PBS Velká Bíteš manufactures the turboshaft engine, derated from 160 to 105 kW of maximum continuous power. While ballistic parachutes have been certified on some fixed-wing aircraft, such as Cirrus light airplanes, their installation on helicopters is a challenging proposition due to the overhead presence of rotating blades. Contained in a non-rotating pod above the main rotor, the parachute solution proposed by Curti and Junkers ProFly thus becomes a backup for conditions where autorotation cannot be performed, such as (a) flight control failure or loss of maneuverability, (b) flying over an

area where emergency landing cannot be safely performed, or (c) flight conditions that prevent restoring rotor rotation speed [19].

**Figure 1.** Zefhir helicopter (courtesy of Curti Aerospace Division).

Although the main objectives of DISRUPT were not strictly related to the main topics of UAM, the need for a remotely-piloted configuration arose immediately; since the experimental validation of the parachute system with the full-scale helicopter was one of the main expected results, the transition toward an unmanned configuration became a mandatory activity to perform the ejection test without a human pilot on board. A crucial but challenging step of the process was the design of a stabilization system, intended as a flexible and reliable software/hardware solution allowing the pilot to manage the ejection task while reducing the workload required by control action. Helicopters generally show nonlinear, complex dynamics that might manifest some unstable flight characteristics in limited zones of the flight envelope. In the particular case of a radio-controlled rotorcraft, without the direct perception of linear accelerations and attitude motion, the remote piloting of a helicopter is indeed an extremely hazardous task [20,21]. Hence, an Automatic Flight Control System (AFCS) was designed, tested, and implemented, allowing the pilot to safely control the aircraft in terms of desired attitude.

The main goal of the paper is to present for the first time a detailed description of all the phases allowing the successful transition of a conventional light helicopter into a RPAS while investigating the validity of a rescue system in the framework of future UAM applications. According to the Model-Based Design (MBD) philosophy, (1) mission requirements are listed and (2) system architecture is defined. Furthermore, (3) an accurate 6DOF nonlinear model is implemented in the Matlab/Simulink environment, which includes helicopter subsystems, environmental effects, and sensor and actuator behavior. (4) The mathematical model is validated and refined by using flight data collected during an identification campaign. (5) After the analysis of open loop dynamic modes, (6) an attitude control system allowing the remote pilot to easily control the aircraft is designed, implemented, and validated by means of both (7) Hardware–In–the–Loop (HIL) techniques and (8) flight tests.

The paper is structured as follows. Section 2 addresses the outline of mission requirements and the selection of system components. The entire simulation model, the trim and stability analysis, and the model validation procedure are presented in Sections 3 and 4, respectively. Control system design, implementation, and HIL validation are described in Section 5. Experimental results validating the AFCS performance and reporting the

parachute recovery mission are finally summarized in Section 6. A section of concluding remarks ends this paper.

The successful outcome of the ejection test and the interest that has arisen in several journals and broadcast media prove the relevance of the research activity presented in this paper [22,23]. Zefhir is currently the only civil helicopter equipped with a ballistic parachute. Indeed, such a test has never been filmed or documented in the entire history of aerospace technology. However, due to the highly-classified nature of the data involved in the early stages of aircraft development, a detailed description of helicopter features and both numerical and experimental results is omitted in the present framework. The focus of the analysis is thus placed on the description of methodological aspects, with particular attention to both numerical and experimental validations, supported by results available in the literature. Furthermore, the comparison between experimental data and the results of simulations is possibly characterized in terms of relative errors, while the description of the technological setup is circumscribed to functional aspects. The uniqueness of the experiment and the absence of strict performance requirements finally vindicate the limits posed by the novelty of the proposed control approach. In this respect, the necessity to rapidly design a safe single-case ejection test necessarily restricts the degree of experimentation, driving the MBD workflow to focus on long-standing results in the field of PID control. Although the latter does not guarantee optimality, it takes advantage of (1) a reduced number of involved parameters; (2) simple implementation and low computational cost; (3) the possibility to perform dedicated flight tests aiming at characterizing the closed-loop dynamic behavior one axis at a time while evaluating the effects of single gain contribution; and (4) an intuitive sizing procedure, suitable for collaboration with the candidate pilot to pursue a set of prescribed handling qualities. Alternative control techniques, such as robust nonlinear and adaptive control that involve the stabilization of vehicle speed components, are currently under experimental validation by the authors, provided small-scale rotorcraft are adopted as test beds in the direction of safe, scalable, and high-performance air mobility and delivery scenarios [24].

#### **2. Mission Requirements and System Architecture**
