Full-scale Experimental Validation of Dynamic, Centrifugally Powered, Pneumatic Actuators for Active Rotor Blade Surfaces

A novel actuation approach for active rotor blade surfaces is introduced. The approach utilizes the rotor centripetal acceleration to generate an internal, on-blade pressure differential that can be used for active surface actuation. The advantages of this approach include high force and displacement output with extremely little added blade weight and low electrical power requirements. An analysis of the available spanwise pressure differential is presented that has been previously experimentally validated in a full-scale whirl test of a K-MAX rotor. The development of a scaled pneumatic actuator designed to fit within a K-MAX rotor blade is then detailed. The actuator consists of miniature, three-way piezoelectric valves that control air flow into and out of actuating diaphragms. The experimental results of a full-scale whirl test are then reported. The rotational actuator is capable of following various multi-frequency command signals with frequency components up to 30 Hz, and the results suggest that actuator performance and response time could be improved with increased valve flow. A state space model of a pneumatic actuator and trailing edge flap system is then developed that captures the instantaneous air flow through the valves into the diaphragms, as well as the flap dynamics. Upon model validation using the experimental whirl test results, an analytical feasibility study is then conducted to determine the valve and diaphragm geometry requirements to power a trailing edge for both higher harmonic vibration control and primary flight control for a light-medium sized helicopter. This study assumes that the actuator must provide at least ±5° at 30 Hz for vibration control and ±20° at 6 Hz for primary control. The results of this study suggest that the actuation approach can provide sufficient torque and angular displacement at the required bandwidths provided there is adequate valve performance and a minimum pneumatic supply line diameter. Using the Darcy-Weisbach formula for flow through pipes, a minimum diameter for the supply line diameter to maintain a sufficient pressure differential is estimated given the volumetric air flow through the actuator. The effects of forward flight and varying altitude on the available pressure differential is investigated, where the azimuthal RMS pressure differential is little changed in forward flight versus hover and the available pressure differential decreases nearly in proportion to the decreasing air density with altitude. Because aerodynamic forces on active surfaces also decrease in proportion to air density, a pneumatic actuator can therefore be designed to be operable in all flight regimes. Finally, a next generation actuator design is presented that will enable high frequency, high displacement actuator performance.