X-57 High-Lift Propeller Control Schedule Development

The NASA X-57 distributed electric propulsion flight demonstrator uses a high-lift propeller system to maintain low-speed capability with a highly loaded, cruise-efficient wing. Previous research showed that the control scheme for the high-lift propellers was a crucial factor to enable an adequate balance between stabilized glideslope control and appropriate lift margin during the approach-to-landing phase of flight. This paper expands the high-lift propeller control considerations to include all phases of low-speed flight, including preflight, taxi, takeoff, initial climb, approach, and landing. Two control modes, termed airspeed and fixed mode, are developed in this paper. The airspeed mode varies propeller torque based on equivalent airspeed to account for different phases of ground and flight operations. The fixed mode operates the high-lift propeller system at a fixed rotational speed to account for failures in the air data system, preflight checkout, and high-performance takeoff operations. Modeling of the high-lift propellers within the X-57 low-speed flight envelope shows that (1) the structural rotational speed limit of the propeller blades limits their effectiveness at higher altitudes and airspeeds, (2) the speed setting of the fixed mode is set by the maximum torque of the airspeed mode, and (3) minimizing the potential for windmilling in non-standard conditions sets the torque schedule at higher speeds. Additionally, the fixed mode is shown to provide an increased climb rate during normal takeoff operations.