Development of control strategies and flight testing of a twin-cyclocopter in forward flight
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This paper describes the control strategies, avionics system development, and wind tunnel testing that enabled the first successful stable forward flight of a cycloidal-rotor aircraft (cyclocopter) purely using thrust vectoring. The present 550-gram vehicle has a hybrid configuration utilizing two optimized cycloidal rotors and a conventional propeller which counteracts the pitching moment generated by the cyclorotors and also provides pitch control. Stable free flight is achieved through independent rotational speed control of the three rotors and thrust vectoring of the cyclorotors through onboard closed-loop control. Since the cyclorotors spin in the same direction, there is a net angular momentum that induces a strong gyroscopic coupling between the roll and yaw degrees of freedom. The gyroscopic couplings are eliminated by implementing a careful mixing of roll and yaw inputs onboard. Unlike a conventional helicopter, a cyclocopter is propelled in forward flight purely by vectored thrust from the cyclorotors. Even though such a strategy could facilitate efficient, high-speed steady level forward flight, it is accompanied by a strong yaw-roll control cross couplings, which is in addition to the inherent gyroscopic coupling. To understand these challenges, a flight dynamics model of the vehicle derived from first principles clearly demonstrates that these couplings arise because the resultant force vector is tilted forward during forward flight. The control mixing obtained from the simulation showed good agreement with the values obtained experimentally from the wind tunnel. This mixing ratio between roll and yaw control formed the basis of the control strategy for forward flight along with aggressive closed-loop control implemented on a custom-built processor sensor board. The wind tunnel studies showed that mixing ratio was a function of phasing of blade cyclic pitch especially at high phase angles. Therefore, these mixing ratios needed to be actively changed during flight depending on phase angle. Based on these studies, steady level forward flight up to 5 m/s was successfully achieved. Copyright 2014 by the American Helicopter Society International, Inc. All rights reserved.
