Performance of Variable Pitch Propeller for Longitudinal Control in an Agile Fixed-Wing UAV

This paper carries out a simulation based study on the performance of longitudinal control in an agile fixed-wing UAV with variable pitch propeller (VPP). Transition between cruise level flight at angle of attack (AOA)= 4°, and harrier level flight at AOA = 50° is implemented, on a variable pitch propeller design, in which the pitch of the propeller is actuated to change thrust, at constant rotations per minute (RPM). In a conventional design, the thrust is changed by varying RPM which is actuated by a Brush-less DC (BLDC)motor whereas in a VPP design, the propeller pitch is actuated by a servo motor which has faster response compared to a BLDC motor used in small UAVs. Hence the thrust change is achieved faster using variable pitch actuation. A longitudinal model of an agile fixed-wing UAV which includes propwash, wing downwash, flat plate post-stall aerodynamics and actuator constraints (first order dynamics, saturation and rate limit) is used in the work. The model of a variable pitch propeller is developed from the data of a particular propeller motor combination obtained from a software named QPROP. The longitudinal control used is a backstepping based nonlinear control. The simulation results are compared with that of a conventional design in which the same flight regime with the nonlinear control is implemented. The advantages of VPP based design are observed to be faster settling of velocity, AOA and pitch rate during transitions, very small altitude variation and reduction in maximum pitch rate during cruise to harrier transition. However, an increase in maximum pitch rate during harrier to cruise transition is a disadvantage. But overall, the simulation results shows that VPP design performs better than a conventional design for the above mentioned flight regime and indicates that a VPP based design of fixed-wing agile UAVs would be better for control in agile and high AOA maneuvers like perching, transition to hover and aggressive maneuvers. However, flight testing is essential to conclude this fully.