Surface Pressure Measurements on an MAV-Scale Shrouded Rotor in Hover

The shrouded-rotor configuration has emerged as a promising choice for rotary-wing VTOL Micro Air Vehicles (MAVs), primarily because of the protection provided by enclosing the rotor in a shroud and also because of the potential for significant performance improvements. Traditional design philosophies based on experimental tests of large-scale ducted propellers may not apply to the low Reynolds number (∼20,000) regime in which MAVs operate. An experimental investigation was conducted to determine the effects of varying the shroud profile shape on the hovering performance of MAV-scale shrouded rotors. Seventeen models were tested, using shrouds with different diffuser angles, diffuser lengths, inlet lip radii and blade tip clearances. All models had a nominal rotor diameter of 16 cm (6.3 in). Our previous paper reported on the overall performance characteristics (thrust, power, figure of merit) of the different models. This paper presents the measurements of the pressure variation along the shroud surface. The measurements showed that about 80% of the thrust from the shroud inlet suction comes from the inner half of the inlet lip. Additionally, strong suction pressures were created by the leakage flow between the blade tips and the shroud wall. Optimizing the inlet shape to take advantage of these factors could enable increases in thrust and reductions in vehicle weight. The inlet pressure profiles were strongly affected by changes in lip radius and tip clearance, and less so by diffuser angle and length. Increasing the diffuser expansion ratio caused the suction pressures in the diffuser to increase, but had little effect on the total thrust produced. The fraction of the total thrust borne by the rotor was found to increase with increasing expansion ratio, contradicting the predictions of the classical momentum theory model of a shrouded rotor.