Modelling and Control of a Convertible VTOL Aircraft J. Escareño, S. Salazar and R. Lozano. Abstract— The aim of this paper is to present the complete model of an unmanned convertible aerial vehicle in hover mode. This vehicle is capable of performing either in hover or in forward flight. A nonlinear control strategy is presented to stabilize the aircraft in hovering mode. An embedded low-cost pilot is described as well the experimental results of a hover flight using a prototype built in the laboratory. Index Terms— Aircraft control algorithm, VTOL, Nonlinear control, amplitude bounded input, Embbeded architecture. I. INTRODUCTION Unmanned aerial vehicles (UAVs) are used in a wide variety of missions, like surveillance and reconnaissance, performed in different scenarios or environments. Therefore the task nature requires an appropriate flight profile vehicle in order to deal satisfactorily with the scenario’s drawbacks. For example, bridge inspection looking for structure’s cracks requires a vehicle with vertical take-off, hovering and low- speed flight capabilities, then rotary-wing aircraft fits per- fectly to that type of task. On the other hand, missions that require long-distance flights and relatively high forward velocity like forest fire detection or desert reconnaissance, usually use fixed-wing vehicles. However missions which involve the two scenarios described above require a vehicle which combines rotorcraft and fixed wing aircraft capabili- ties. We developed a convertible plane prototype capable of per- forming vertical take-off, hover and forward flight [see figure 1]. This type of convertible planes require specific control algorithms for handling take-off, hover, forward flight as well as the transition between those modes of operation. The aerodynamic configuration of the prototype we built is sim- ilar to the Boeing commercial aircraft Heliwing. There exist very few publications concerning the control of convertible planes. One of the few contributions in this area is given in [7] where H. Stone presented the control of the T-wing aircraft, which is a twin-engine tailsitter UAV that uses an LQR algorithm applied to the linearized hover dynamics. The main contribution of this paper is to provide a complete dynamical model of the convertible plane in hover mode. The nonlinear dynamical model is derived via the Newton- Euler formulation including the modelling of the gyroscopic effect and the adverse drag torque. In view of the physical limitations of the actuators, we propose a nonlinear control Heudiasyc-UTC UMR 6599 Centre de Recherches de Royallieu B.P. 20529 60205 Compiegne France Tel.: + 33 (0)3 44 23 44 23 ; fax: +33 (0)3 44 23 44 77 Corresponding author rlozano@hds.utc.fr juan.escareno@hds.utc.fr sergio@hds.utc.fr Vertical take off Forward flight Hover flight Vertical landing fixed-wing dynamics rotay-wing dynamics Transition Transition Hover flight Hover flight Fig. 1. UAV flightpath algorithm satisfying the constraints on the inputs ampli- tude. We developed a specific embedded microcontroller architecture platform to be able to implement the control algorithm on-board. We present real-time experiments of the convertible plane in autonomous hover. The paper is organized as follows: Section II presents the complete dynamical model of the convertible VTOL. A control algorithm convergence analysis is presented in section III. Stabilization of the attitude and position of the aircraft is shown in simulation in section IV. The embedded architecture composed of the microcontroller, the sensors and filters is described in section V. The real-time application results of the stabilization of the VTOL in hover is presented in section VI. Finally some concluding remarks are given in section VII. II. DYNAMIC MODEL In this section we present the complete model of the convertible VTOL using a Newton-Euler formulation. The pitch, roll and yaw torques required for controlling the flying vehicle in hover are obtained from the speed difference between the two rotors and the control surfaces (aileron, elevon). The altitude of the vehicle is regulated by increasing or decreasing the propeller thrust. The roll torque is obtained from the difference of the rotors’ angular velocities. Since the control surfaces are submerged in the propeller slip- stream (prop-wash), the aerodynamic forces are generated with the elevon and ailerons deflection to provide the pitch and yaw motion respectively [see figure 2]. Let I ={i I x ,j I y ,k I z } denote the right hand inertial frame. Let B={i B x ,j B y ,k B z } denote the rigid-body frame with origin at the gravity center. Let the vector q =(ξ,η) T denote the generalized coordinates where ξ =(x, y, z) T ∈ℜ 3 denotes the translation coordinates relative to the frame I , and η =(ψ,θ,φ) T ∈ℜ 3 describes the vehicle orientation Proceedings of the 45th IEEE Conference on Decision & Control Manchester Grand Hyatt Hotel San Diego, CA, USA, December 13-15, 2006 WeA02.6 1-4244-0171-2/06/$20.00 ©2006 IEEE. 69