Optimal Operation of Rotor-Wing Assemblies on eVTOL Aircraft

eVTOL aircraft with tilting rotors and fixed wings for cruise lift have control redundancy leading to the possibility of optimal operation over the range of airspeeds. In this study, a single rotor-wing unit is considered to examine how a combination of rotor RPM, rotor root pitch, rotor cant, and wing angle of attack can minimize the power requirement, while ensuring that the rotor-wing unit provides the necessary lifting and propulsive forces. Analysis is conducted for both a UAV-scale rotor-wing unit producing 5 lbs lift as well as a manned-UAM scale unit producing 550 lbs lift. For both cases, the power requirements are highest in hover, reduce rapidly as airspeed increases, and then increase slowly at speeds greater than maximum endurance speed. For minimum power operation, the rotors are oriented mostly up at low speed with a relatively low root pitch setting of 25-30 deg, and fully forward (operating as axial propellers) at speeds greater than maximum endurance speed, with the root pitch increasing significantly to account for axial flow through the rotor. At speeds right before the rotor starts tilting downward, the wing lift-share is ~25%, increasing to ~83% at maximum endurance speed, and 100% at higher cruise speeds. If the rotor solidity is low (similar to a UAV-scale propeller, and not high as seen on manned-UAM scale eVTOL rotors), it is feasible to use a fixed pitch rotor set at optimal low-speed values. For such low-solidity fixed-pitch rotors, the power penalty at cruising speeds may be acceptable, but the rotors must operate in a tilted position, rather than oriented fully forward like axial propellers.