Design of Servo-Flap Helicopter Rotor Blades with Forward Swept Wingtips

Helicopters with performance enhancing aft swept blade wingtips are used for both military and commercial purposes. Up until now, there are no research projects studying helicopter blades with forward swept wingtips. A forward swept blade wingtip generates significant amounts of nose-up aerodynamic pitching moments with respect to the blade feathering axis. For a conventional pitch-horn rotor, this type of control system cannot effectively produce large enough nose-down pitching moments to counteract the nose-up pitching moments generated by the swept portion of the blade. The main goal of this paper is to generate ample nose-down pitching moments via servo-flap aerodynamic action to overcome the swept blade's nose-up pitching moments while maintaining an adequate control safety margin for flight. These nosedown pitching moments depend on (i) the size of the servo-flap, (ii) the distance from the servo-flap pitch axis to blade feathering axis, and (iii) the servo-flap location with respect to the blade. There are three additional reasons for selecting a servo-flap rotor control as the primary control system when designing the blade with forward swept wingtips. (i) The overall blade chordwise center of gravity (c.g.) moves forward due to the forward swept wingtip, resulting in a lighter blade with less ballast weight. (ii) The required aerodynamic downloads on the flap is reduced in flight due to positive blade local angle of attacks generated by the blade's nose-up pitching moments. Positive servo-flap aerodynamics on the flap unloads the main rotor blade required lift and improves the blade flight performance. (iii) The feedback control system incorporated into the existing production blades is used to reduce the flap control requirements and to provide stability for the rotor system in flight. Several numerical examples are investigated to verify the potential blade design with forward swept effect.