Implications of Propeller-Wing Interactions on the Control of Aerodynamic-Surface-Free Tilt-Rotor Quad-Planes

Quad-planes are a type of vehicle which combine the hovering capability of quadcopters and the forward flight efficiency of winged aircraft. Flight tests conducted on a dual-axis tilting-rotor quad-plane, designed to fly without aerodynamic surfaces, observed that the quad-plane suffered from insufficient roll authority during fast, forward flight. It was hypothesized that the propellers located in front of the wing are less efficient in generating a rolling moment due to potential propeller-wing interactions. Wind tunnel tests, performed at TU Delft's Open Jet Facility, confirmed a two- to fourfold reduction in roll moment generation from propellers mounted in front of the wing at similar levels of tilt as their rear counterparts. To address the mismatch in actuator effectiveness shown by the wind tunnel experiment, the effect of the propeller-wing interactions was incorporated into the aero-propulsive model of the quad-plane by means of a global polynomial, the structure of which was found using multivariate orthogonal function modelling. This augmented aero-propulsive model was then integrated into the sequential quadratic programming based control allocation algorithm used by the quad-plane. New flight tests demonstrated that, by including the propeller-wing interactions in the control allocation, the vehicle is capable of tracking a figure 8 maneuver without aerodynamic surfaces, and without compromising tracking performance.