Hover Dynamics & Flight Control of a UAM-Scale Quadcopter with Hybrid RPM & Collective Pitch Control

Hover trim and dynamic analyses were performed on a UAM-scale quadcopter with both variable rotor speed and variable collective blade pitch. The bare-airframe dynamics were first considered at three different hover trim points, where power consumption is increased to improve authority. The control and stability derivatives were examined at each trim point and an increase in base RPM caused increased authority for pitch inputs (and decreased authority for RPM inputs) in thrust-dominated axes. Explicit model following control laws wre then optimized using CONDUIT® to meet ADS-33E-PRF handling qualities specifications. Design margin optimization was then performed on each axis. Heave and yaw responses of the linearized system were examined for the three trim points with either RPM or pitch control. It was found that pitch-control outperformed RPM-control in heave, while the opposite was true for yaw. Hybrid control mixing was considered using a complementary filter, so that it uses pitch actuators for short-term responses, and RPM for trim. Effects of changes in motor time constant and complementary filter cutoff frequency were examined. The benefits of hybrid control were demonstrated through simulations that involved transition between trim points. Hybrid control required lower maximum power during thrust-driven maneuvers by allowing the aircraft to accelerate using pitch actuators, and recovers the original stall margin by using the rotor speed to re-trim. For a drop-off of 176 lbs of payload, hybrid control provided 5.0% lower trim power than pitch control with the reduced weight. Hybrid control also allowed a 3.9% reduction in power compared to pitch control at a flight speed of 30 kts.