Dynamics of a Micro Aerial Vehicle with Shrouded Coaxial Hingeless Rotors

Abstract:
Shrouded rotor configurations provide aerodynamic efficiency benefits that are valuable for aerial vehicles requiring high endurance in hovering flight. The effect of the shroud on the loads produced by a vehicle in response to cyclic pitch are not well known in literature. This paper aims to understand the magnitude and phasing of the steady hub loads produced by shrouded coaxial hingeless rotors in hover due to cyclic pitch. A shrouded coaxial rotor vehicle weighing 0.934 kg and with a rotor radius 0.138 m is used as the test platform. The rotors are operated at a tip Reynolds number of 70,000 and a tip Mach number of 0.18. First, the origins of steady hub loads due to cyclic pitch on a hingeless rotor are described using theory. To quantify the effect of the shroud, different configurations of rotors were tested by giving cyclic pitch to only one rotor. For the open hingeless rotor, cyclic variation in drag force at blade sections contributed to 55% of the steady lateral force. In response to cyclic pitch input, the ducted configurations produced 35% larger lateral forces and 60% (coaxial rotors) to 100% (single rotor) larger lateral hub moments than those produced by the unshrouded configurations. The presence of the shroud also changed the phase angle between the forces and moments. While the lateral force and moment acted in similar directions for the open configurations, the angle between them increased by more than 60° when enclosed in a shroud.

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