Review of designs and flight control techniques of hybrid and convertible VTOL UAVs

This paper focusses on the attitude stabilization of a mini tail-sitter aircraft, considering aerodynamic effects. The main characteristic of this vehicle is that it operates in either the hover mode for launch and recovery, or the horizontal mode during cruise. The dynamic model is obtained using the Euler–Lagrange formulation, and aerodynamic effects are obtained by studying the propeller effects. A nonlinear saturated Proportional-Integral-Derivative (SPID) control with compensation of aerodynamic moments is proposed in order to achieve the asymptotic stabilization of the vehicle in horizontal mode. In addition, a homemade inertial measurement unit (HIMU) is built for operating the complete operational range of the vehicle (including vertical and horizontal modes). Finally, simulation results are presented for validating the control law, and practical results are obtained in real-time during the flight.

Attitude Stabilization with Real-time Experiments of a Tail-sitter Aircraft in Horizontal Flight

The paper presents automatic control of an aircraft in the longitudinal channel during automatic landing. There are two crucial components of the system presented in the paper: a vision system and an automatic landing system. The vision system processes pictures of dedicated on-ground signs which appear to an on-board video camera to determine a glide path. Image processing algorithms used by the system were implemented into an embedded system and tested under laboratory conditions according to the hardware-in-the-loop method. An output from the vision system was used as one of the input signals to an automatic landing system. The major components are control algorithms based on the fuzzy logic expert system. They were created to imitate pilot actions while landing the aircraft. Both systems were connected with one another for cooperation and to control an aircraft model in a simulation environment. Selected results of tests presenting control efficiency and precision are shown in the final section of the paper.

Fixed Wing Aircraft Automatic Landing with the Use of a Dedicated Ground Sign System

The main drawback of unmanned aerial systems (UAS) is that almost their entire field of application is autonomous in terms of energy. Flights beyond 50 min are nearly impossible when using conventional energy storage systems (lithium-ion polymer or lithium-ion batteries). Several commercial products have been developed using hybrid systems (H-UAS). Although the improvement they have provided is undeniable, H-UAS in the present market are strongly limited by their low thrust vs. weight ratio, which is caused by limited electrical power generation and a non-optimal energy conversion with relatively low efficiencies. This paper reviews these systems to show the preliminary results of a prototype of hybrid generator which state-of-the-art electronics as well as a new approach using a supercapacitor (SC) array are used to save fuel, increase the thrust vs. weight ratio, optimize losses during conversion and prevent the overheating of the internal combustion unit (ICU). Whereas current generators mostly operate with the ICU at a constant speed, delivering maximum power, the presented prototype includes a throttle control system, and the engine works with a variable regime according to the power demand. Thus, fuel consumption is reduced, as well as heating and wear. The lifespan of the engine is also increased, and the time between maintenance operations is lengthened. The designed system provides almost twice the power of the hybrid current generators. The reduction in the RPM regime of the engine is achieved by means of a supercapacitor array that provides the necessary energy to keep the DC output power constant during the engine acceleration when the flight envelope experiences a perturbation or a sudden manoeuvre is performed by the pilot. To obtain maximum efficiency, the diode rectifiers and conventional converters used in the reviewed products are replaced by synchronous converters and rectifiers. The whole system is controlled by means of a FPGA where a specific control loop has been implemented for every device: ICU’s throttle, DC bus converter, charge and discharge of the SC’s array, cooling and monitoring of temperature for the cylinders heads, and on-line transmission, by means of a XBEE™ module, of all the monitored data to the flight ground station.

/pdf/preliminary-results-of-a-hybrid-thermoelectric-propulsion-4y9ybbb99b.pdf

Preliminary Results of a Hybrid Thermoelectric Propulsion System for a Multirotor UAS with Active Rectifying, Electronic Throttle Control and Supercapacitors

Quadrotors with rotating and extendable arms, are a new type of transformable drones, which are appeared more recently. According to the assigned paths or missions, they can adapt their variable geometries by folding or extending their arms using some mechanical mechanisms. These latter are based on motors and servomotors distributed in different ways on the quadrotor prototype. The goal of this manuscript is to design and apply a control strategy based on Gains Scheduling using a conventional Proportional Integral Derivative (PID) controller, where its variable gains are well determined for each aerial morphology. This strategy takes into account the variable dynamic of the proposed structure and calculates the quadrotor geometric changes.

PID Gain Scheduling for 3D Trajectory Tracking of a Quadrotor with Rotating and Extendable Arms

Hybrid Unmanned Aerial Vehicles (H-UAVs) are currently a very interesting field of research in the modern scientific community due to their ability to perform Vertical Take-Off and Landing (VTOL) and Conventional Take-Off and Landing (CTOL). This paper focuses on the Dual Tilt-wing UAV, a vehicle capable of performing both flight modes (VTOL and CTOL). The UAV complete dynamic model is obtained using the Newton–Euler formulation, which includes aerodynamic effects, as the drag and lift forces of the wings, which are a function of airstream generated by the rotors, the cruise speed, tilt-wing angle and angle of attack. The airstream velocity generated by the rotors is studied in a test bench. The projected area on the UAV wing that is affected by the airstream generated by the rotors is specified and 3D aerodynamic analysis is performed for this region. In addition, aerodynamic coefficients of the UAV in VTOL mode are calculated by using Computational Fluid Dynamics method (CFD) and are embedded into the nonlinear dynamic model. To validate the complete dynamic model, PD controllers are adopted for altitude and attitude control of the vehicle in VTOL mode, the controllers are simulated and implemented in the vehicle for indoor and outdoor flight experiments.

https://hal.archives-ouvertes.fr/hal-03137616/document

Development, Modeling and Control of a Dual Tilt-Wing UAV in Vertical Flight

A hybrid unmanned autonomous vehicle (UAV) combines the high-speed cruise capability of a fixed-wing airplane, with the hovering flight, vertical take-off and landing capabilities of a helicopter. The change between both flight modes refers to as a transition flight process. This paper presents the flight dynamics during the transition stage of a hybrid UAV of Dual Tilt-Wing type. A non-linear mathematical model is developed that includes the UAV complete dynamics, which presents significant changes in aerodynamic characteristics, especially during the transition flight. These changes are analyzed and characterized by flight tests and CFD simulations. The adaptation to the variation of aerodynamic characteristics caused by tilt angle changes of transition system is verified, so a PD controller for altitude is implemented.

Transition Flight Dynamics of a Dual Tilt-Wing UAV *

This paper presents the pitching moment stability analysis and adjustment for tilt-wing unmanned aerial vehicle (UAV) in hovering flight and vertical take-off and landing (VTOL). The tilt-wing UAVs combine the features of fixed-wing UAVs with VTOL capabilities by tilting the wing and rotor system for the transition between flight modes. Due to its hybrid configuration, the altitude control study, position and attitude, poses a major problem during the entire flight for a given mission. Namely, in the first flight phase the pitching moment could present instabilities with different disturbances, such as wind, that are difficult to compensate. In this paper this problem is analyzed and possible solutions for the stabilization are presented and evaluated in a test bench. These solutions were implemented in new test prototypes demonstrating satisfactory results in outdoor flight tests.

Pitching moment analysis and adjustment for tilt-wing UAV in VTOL mode

In this paper, we present the use of reverse-engineering like techniques to analyse the proposal of a Dual-System Unmanned Aerial Vehicle. Such vehicles are capable of performing vertical flight with the use of a multi-rotor system and then transition into horizontal flight without modifying their structure. The main features and dimensions are captured. A prototype was built and then modeled using Computer-Aided Design (CAD) software. Aerodynamic characteristics for the model were obtained using accurate computational analysis tools, namely XFLR5, Xfoil and ANSYS Fluent®. Simulation results for the airfoil, the wing and the complete airframe are presented and compared. Results were studied and adapted for the longitudinal model of the UAV by means of the stability derivatives. The study serves as good basis to implement the aerodynamic data in the mathematical model of the platform and perform future control simulations. Besides, it lays the foundation for future research and improvement on the knowledge of the characteristics and capabilities of this type of aircraft.

Luz M. Sanchez-Rivera

Papers

Development, Modeling and Control of a Dual Tilt-Wing UAV in Vertical Flight

Transition Flight Dynamics of a Dual Tilt-Wing UAV *

Pitching moment analysis and adjustment for tilt-wing UAV in VTOL mode

Aerodynamic Analysis for the Mathematical Model of a Dual-System UAV