Improving performance of flying wing UAV with propeller thrust involved trimming the pitching moment

For static stable fixed wing aircraft, there are contradictory requirements from the flight performance and the longitudinal stability. This is true especially for flying wing aircraft. The flight performance is deteriorated in order to maintain the static stability of the aircraft. To solve this problem, a new aerodynamics configuration which involves the propeller thrust to balance the pitching moment was proposed. The optimal design of the flying wing hand-launched mini-UAV is discussed as the example. The performances and dimensions of three configurations, namely the one with reflex airfoils, the one with cambered airfoil and the one involving the propeller thrust to balance the pitching moment, are compared. For the configuration with propeller thrust involved to trim the aircraft, two approaches are adopted. One is shifting the propeller thrust line well below the center of gravity. The other is using vectored thrust obtained by tilting the propeller disk. The aerodynamics coefficients are computed by CMARC, which is based on panel method. Frictional drag is evaluated by boundary layer analysis and empirical formulation correction. The maximum lift coefficient is obtained by strip theory and is corrected by wind tunnel tests. The wind tunnel experiments were performed to validate the optimal design. Good agreements between the computation results and the experimental data were obtained. The electric propulsion system including propeller and motor is designed optimally. The propeller is designed as the maximum cruise efficiency through vortex theory and several motors are tested in order to match propeller well. Then the propulsion system efficiency is involved into the conceptual design at different phases of mission. The genetic algorithm is implemented to optimize the performance of the UAV and the sensitivity analysis is performed. From the analysis, it is found that with the help of propeller thrust, the amount of wash-out and sweepback can be reduced and the maximum lift coefficient can be increased. As a result, the wing loading can be increased and hence the wing area can be reduced. This leads to larger aspect ratio if the wingspan is kept as a constant. Combined with few elevon deflections and reduced wash-out, the lift to drag ratio is improved during cruise and loitering. Hence the endurance of the aircraft can be extended.