MAV: Shrouded Rotor Concept
Because of deteriorating airfoil performance at low Reynolds numbers, MAV-scale rotors suffer from high power consumption and low Figures of Merit. It is well known that enclosing a propeller – or rotor – in a shroud or duct can significantly improve its static (hovering) and forward-flight performance. Also, it is important for the MAV to remain stable in gusting crosswinds, to be able to transition quickly to translational flight, and to have good forward-flight performance characteristics. The asymmetry in the aerodynamic environment between the forward and rear portions of the shroud inlet in forward flight leads to a strong nose-up pitch moment, which not only makes it difficult for the aircraft to attain significant forward flight speeds, but also to maintain a stable hovering position in a gusty environment.
Additionally, the design of a shroud for optimum performance in hover has been found to lead to unsatisfactory performance in forward flight, and vice versa. For example, a sharper inlet lip is advantageous in translational flight because it creates less drag, but can, in hover, cause the flow to separate at the inlet before even reaching the rotor. The shroud additionally would serve as an inherent safety feature, protecting both the rotating blades as well as personnel. We have carried out systematic experimental testing of the effects of the different shroud geometric parameters on the hovering, transition and forward-flight behavior of an MAV-scale shrouded rotor. Parameters investigated include the inlet lip radius, diffuser angle, diffuser length, blade tip clearance, and the use of stator guide vanes in the rotor wake. Work consisted of hover tests of 17 shrouded-rotor models with multiple values of diffuser angle, diffuser length, inlet lip radius and blade tip clearance, and at various rotor collectives.
Measurements were made of thrust, power, variation in pressure along the shroud surface, and axial velocity distributions in the wake of the open and shrouded rotors. The MAV-scale rotor had a diameter of 16.0 cm (6.3 in). Increases in thrust of up to 95% above that of the open rotor were obtained, at the same power consumption, and with corresponding increases in maximum Figure of Merit of up to 75%. The optimal shroud configuration found was one with a lip radius of 13% of the rotor diameter, a diffuser included angle of 10, a diffuser length of between 50% and 72% of the rotor diameter, and the smallest blade tip clearance tested, that of 0.1% of the rotor diameter.
Also, we have carried out forward-flight tests of the open rotor and a single shrouded-rotor model in an open-jet wind tunnel, at fixed collective, at angles of attack from 0 (axial flow) to 90 (edgewise flow), and at flow velocities up to 20 ft/s (14 mph). The shrouded-rotor model tested was the optimum hover configuration, but with a shorter diffuser (31% of the rotor diameter) so as to reduce the model weight. A custom four-component sting balance was designed and constructed to measure the lift, drag, pitching moment and torque (rotor shaft power) of the models. Shroud surface pressure distributions were also measured, on both the windward and leeward sides of the shroud.
At the same collective, the shrouded rotor consumed less power than the open rotor, at all angles of attack, while also producing more thrust at all angles except the axial and near axial flow conditions. The pressure measurements clearly illustrated the asymmetries in the flow-field of the shrouded rotor, which led to higher drag and pitching moments compared to the free rotor, and revealed that a net increase in suction on the inlet was the reason for the increased thrust in edgewise flow.
In the future, the use of active boundary-layer control devices will be investigated. Computational fluid dynamics (CFD) analysis will be refined and correlated with measured data, and then analysis will be used to optimize shroud design.