INDI Control with Direct Lift for a Tilt Rotor UAV

Abstract

The benefits of adding direct lift control capabilities to a high aspect ratio, low wing load, tilt rotor UAV within an Incremental Nonlinear Dynamic Inversion framework are presented in this paper. The primary motivation is the need for some gust alleviation scheme to mitigate the large sensitivity to turbulence of light weight, low wing loaded aircraft, such as small tilt rotor UAVs, designed to operate at low speeds. The highly nonlinear dynamic model of the tilt rotor aircraft was developed using the component buildup method associated with the lifting line theory. The effector redundancy was managed through a control allocation module based on the daisy chaining strategy. Classic aerodynamic effectors are used in forward flight, whereas thrust vectoring is required in hover. During transition phases aerodynamic effectors are preferred over thrust vectoring, which intervenes upon saturation of aerodynamic effectors or when they become ineffective at low speed. Since dynamic inversion exhibits poor performance in turbulence due to its intrinsic aggressive use of controls, direct lift is proposed in this paper for gust alleviation, improving performance and reducing overall control effort. Direct lift has been achieved adopting full span flaperons: symmetric deflection generates lift whereas differential deflection produces roll. Direct lift control has the benefit of providing direct, thus fast control of the vertical degree of freedom, whereas conventional elevators try to minimize vertical speed fluctuations through pitch attitude changes, resulting in considerable lags. Another desirable feature of direct lift control for tilt rotor application is the closer commonality between the forward and hover flight regimes, by providing direct control of the vertical motion in forward flight, as provided by thrust vectoring in hover. A comparison was done between similar control law schemes with and without direct lift. Simulations proved the effectiveness of the approach in improving reference tracking in presence of turbulence and reducing control effort.