Hover and Wind-Tunnel Testing of Shrouded Rotors for Improved Micro Air Vehicle Design

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2008-08-29

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This dissertation describes an experimental investigation of the effects of varying the shroud profile shape on the performance of MAV (Micro Air Vehicle) -scale shrouded rotors. Hover tests were performed on seventeen models with a rotor diameter of 16 cm (6.3 in) and various values of diffuser expansion angle, diffuser length, inlet lip radius and blade tip clearance, at various rotor collective angles. Compared to the baseline open rotor, the shrouded rotors showed increases in thrust by up to 94%, at the same power consumption, or reductions in power by up to 62% at the same thrust. These improvements surpass those predicted by momentum theory, due to the additional effect of the shrouds in reducing the non-ideal power losses of the rotor. The uniformity of the rotor wake was improved by the presence of the shrouds and by decreasing the blade tip clearance, resulting in lower induced power losses. Strong suction pressures were observed on the shroud inlet surface, at the blade passage region; taking advantage of this phenomenon could enable further increases in thrust. However, trade studies showed that, for a given overall aircraft size limitation, and ignoring considerations of the safety benefits of a shroud, a larger-diameter open rotor is more likely to give better performance than a smaller-diameter shrouded rotor.

A single shrouded-rotor model was subsequently tested in translational flight at various angles of attack. In axial flow, at the same collective, the net thrust and the power consumption of the shrouded rotor were lower than those of the open rotor; in edgewise flow, the shrouded rotor produced greater thrust than the open rotor, while consuming less power. Measurements of the shroud surface pressure distributions illustrated the extreme asymmetry of the flow around the shroud, with consequent pitch moments much greater than those experienced by the open rotor. Except at low airspeeds and high angles of attack, the static pressure in the wake did not reach ambient atmospheric values at the diffuser exit plane; this challenges the validity of the fundamental assumption of the simple-momentum-theory flow model for short-chord shrouds in translational flight.

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