Flight Simulation and Control of a Helicopter Undergoing Rotor Span Morphing

This study focuses on the flight simulation and control of a helicopter undergoing rotor span morphing. A model-following dynamic inversion controller with inner and outer loop Control Laws (CLAWS) is implemented, and radius change is introduced as a feedforward component to the inner loop CLAWS. Closed-loop poles associated with the low-frequency aircraft modes are observed to be robust to change in rotor span, eliminating the need for model updates due to span morphing during the dynamic inversion process. The error compensators in the CLAWS use PID control for roll and pitch attitude, PI control for yaw rate and lateral and longitudinal ground speed, but require PII control for vertical speed to avoid altitude loss observed with only PI control, during span morphing. Simulations are based on a span-morphing variant of a UH-60A Black Hawk helicopter at 18,300 lbs gross-weight and 40 kts cruise. From a baseline rotor radius of 26.8 ft, retraction to 22.8 ft, as well as extension to 31.5 ft is considered, nominally over a 60 sec duration. The controller is observed to regulate the longitudinal, lateral and vertical ground speeds well over the duration of the span morphing. Further, the controller is observed to maintain its effectivess in regulating the ground speeds when the span morphing duration is reduced to 30 sec.