Design, Build, & Analysis of a Multirotor Vehicle with Cyclic-Collective Pitch Control
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Design, Build, & Analysis of a Multirotor Vehicle with Cyclic-Collective Pitch Control

  • Author(s): Sledge, Colin Alexander
  • Advisor(s): Taha, Haithem E
  • et al.
Creative Commons 'BY-ND' version 4.0 license
Abstract

Multirotor vehicles are a common type of unmanned aerial vehicle typically consisting of fixed-pitch propellers, each attached directly to a single motor and each motor attached to a frame appendage. This study explores an alternative configuration where each rotor has actuators that can modulate blade pitch in both a collective and cyclic manner, in addition to varying the rotor RPM. While this type of configuration has potential performance benefits, it also has potential airframe vibration challenges. A flight test vehicle was designed and constructed. During flight testing, the vehicle encountered severe vibrations that caused multiple instances of airframe damage. Structural modifications of the frame helped to mitigate vibration issues with the fuselage but came at the cost of weight and reduced aircraft performance. A mathematical reduced-order model that couples the structural response of the airframe with the inherent aerodynamic damping of the rotors is developed. The results show that out-of-plane vibration modes more heavily damped than in-plane modes, making in-plane modes more susceptible to damage. This study also presents an active method of vibration attenuation, using excess control power due to the over-actuated nature of the configuration. This is not in lieu of, but in conjunction with passive methods of vibration attenuation, such as damping elements incorporated in the frame. Stability analysis of the vehicle shows that the stability characteristics are similar to convention rotorcraft. This over-actuated vehicle enjoys multiple control allocation strategies. The control allocation problem is analyzed through the volume of the reachable set from each control allocation scheme. The control allocation analysis shows that the addition of blade pitch control significantly increases the size of the reachable sets of the vehicle, when compared to RPM control as the only method of control. While cyclic pitch only had a modest impact on the size of the reachable sets, it can be argued the cyclic-pitch control would be integral to expanded flight envelope operations, such as autorotation flight. Regardless of the commercialization of this exact configuration, this study builds intuition that can be extended to various multirotor aircraft designs.

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