Inclusion of the Rotor Speed Degree of Freedom for Substantial Increases In Advanced Rotorcraft Performance and Safety

Fixed wing aircraft have six rigid degrees of freedom (3 translational and 3 rotational). Helicopters and autogyros became practical when Juan de la Cerva added the flapping degree and rotor disk tilt as an additional of freedom in the 1930s. This addition allowed rigid rotor helicopters to fly in forward flight without tilting over due to the unsymmetrical lift on the advancing and retreating blades. Rotor speed, while originally controlled manually by pilots in early helicopters and autogyros, is now held approximately constant by use of an engine governor. There have been several flight demonstrations over the past few years that have illustrated the potential of using the rotor speed degree of freedom for substantial increases in rotorcraft performance and safety. One example is the Frontier Systems (now Boeing) A-160 Hummingbird VTOL UAV which demonstrated the potential for up to five time increases in rotorcraft endurance (from ~4 hrs to 20 hrs) by slowing the main rotor speed to allow rotor blade sections to operate at their best L/Ds. Another example is the Georgia Tech GTMax Autonomous VTOL UAV which demonstrated fault tolerant control by reconfiguring to rotor speed control when loss of collective or tail rotor control. The third example is the CarterCopter Gyroplane which has slowed the rotor speed to obtain an advance ratio of almost one, with potential for obtaining much higher advance ratios. This paper will review these breakthroughs and discuss the concept of rotor speed control for advanced rotorcraft, specifically for a reaction drive slowed rotor/compound (SR/C) “gyrodyne”.