Anhedral tip blades for tiltrotor aircraft
A tiltrotor aircraft having a fixed wing and tilting rotors has a rotor blade with a shaped tip portion that provides improved hover performance. The shaped tip portion preferably has a terminal anhedral of at least 20° with respect to its stacking line, and the blade has an overall twist from root to tip of at least 20°, and a thickness ratio between 19% and 30% at a radial station of 10%. These features advantageously conspire to provide a hover figure of merit of at least 0.84 and a cruise propulsive efficiency of at least 0.85. A controller preferably limits the rotor speed in sustained airplane-mode forward flight cruise of at most 40% of a hover maximum rotor speed, and alternatively or additionally limits a rotor edgewise advance ratio to at most 0.20.
1. A rotorcraft comprising:
a fixed wing of sufficient size to allow wing-borne flight;
the wing carrying a first rotor on a first side of the rotorcraft and a second rotor on an opposite side of the aircraft;
the rotors configured to tilt from a predominately horizontal position to a predominantly vertical position; and
the first rotor with a first blade having a stacking line, with the first blade having a shaped tip portion with a terminal anhedral of at least 33° and a terminal sweep of at least 20° with respect to the stacking line, wherein the rotor is oriented for edgewise flow with an advance ratio of at least 0.05 during at least a portion of a flight.
2. The rotorcraft of claim 1 wherein the first blade has a terminal anhedral of at least 40° with respect to the stacking line.
3. The rotorcraft of claim 1 wherein the first blade has an overall twist from root to tip of at least 20°.
4. The rotorcraft of claim 1 wherein each of first and second rotors is a hingeless rotor.
5. The rotorcraft of claim 1 wherein the first blade has a thickness to chord ratio between 19% and 30% at a radial station of 10% of a blade radius, and wherein the rotor is oriented for axial flow with a sectional Mach number of at least 0.55 during at least a portion of a flight.
6. The rotorcraft of claim 1 wherein at an edgewise advance ratio of 0.35, each of first and second rotors has a power consumption at least 5% greater than a corresponding rotors without the shaped tip portion.