Servo-Flap Rotor Design for High Density Altitude Operation

It is a unique and challenging task to design a servo-flap main rotor control system that is operational at high density altitudes. At such elevated altitudes, both the main rotor blade and the servo-flap aerodynamics strongly affect helicopter operation. The changes in flap lift alter the aerodynamic pitching moment with respect to the main rotor blade, which results in an interesting blade feathering motion and its required control input. The servo-flap rotor design does not require any pitch link control spring rate. In addition, the blade torsional stiffness and tennis racket moment due to centrifugal force, are almost un-changed by the high altitude air density. The helicopter’s high altitude operation can be achieved through a unique combination of rotor design, servo-flap control, and installed engine power. The servo-flap rotor has a unique ability that allows flap adjustment every time the pilot alters the operating aircraft density altitude by 5,000 feet. This allows the aircraft to retain the same flight characteristics at any altitude that it had at sea level. This unique feature makes the servo-flap rotor one of the best candidates for high altitude flight operation. A new and improved design feature is proposed in this paper. The high altitude actuator control system provides a steady null offset that is simultaneously applied to all flaps. The new actuator signal is introduced in parallel with the basic servo-flap control system. The new hardware is designed to be used within the present control system. This design opens up the autorotation flap control range without impacting servo-flap flight control operation and safety.