Abstract:
This paper describes the system identification methodology and the identified flight dynamics model of a cyclocopter micro air vehicle (MAV) in forward flight. The cyclocoper utilizes two cycloidal rotors (cyclorotors), a novel horizontal-axis propulsion system with rotating wings. The forward flight control strategy employs independent rpm control of all three rotors and thrust vectoring of two cyclorotors. Unlike a conventional helicopter, a cyclocopter is propelled in forward flight purely by thrust vectoring. This allows the vehicle to maintain a steady, level attitude in forward flight. Even though such a strategy could facilitate power-efficient forward flight, it is accompanied by a strong yaw-roll cross coupling, which is in addition to the inherent gyroscopic coupling that is also observed in hover. To understand these couplings and characterize the bare airframe dynamics, a 6-DOF flight dynamics model of the cyclocopter was extracted using time domain system identification techniques. The existence of a control derivative in the longitudinal translation mode showed that thrust vectoring by phasing the cyclic pitch commands forward speed. After a phasing input, the vehicle achieves steady state within two seconds. The model was able to validate the existence of the inherent roll-yaw coupling in forward flight, which was identified by contributions of roll-yaw coupling stability and control derivatives. The gyroscopic coupling is caused by unbalanced angular momentum and the controls coupling arises from increased propulsive forces at high phase angles. Decoupling methods involve simultaneously mixing roll and yaw inputs in the controller. With onboard feedback stabilization and roll-yaw mixing strategy, the cyclocopter is able to achieve decoupled dynamics in forward flight.