Effects of the rotor tip gap on the aerodynamic and aeroacoustic performance of a ducted rotor in hover

Ducted rotors are configurations known to outperform their unducted reference baselines when aerodynamic performance is concerned. Aside from aerodynamic benefits in hover, a duct also affects acoustic emissions. One of the most contended design parameters of a duct-rotor assembly is the radial distance between the blade tip and the duct wall, referred to as the “tip gap”. The present study explains how the aerodynamic performance of a ducted-rotor system is affected by the tip-gap distance, taking into account the performance of the rotor and those of the duct's inlet lip and diffuser sections. Separate thrust measurements of the rotor and duct establish that the latter can generate up to half of the total thrust of the assembly. Static wall-pressure measurements along the inner wall of the duct reveal a low pressure suction zone over the duct's inlet lip area. This allows the assembly to generate more thrust than the rotor alone, even though the duct's diffuser section generates a drag component (negative thrust). From the velocity fields it is further shown that the performance-deterioration with an increasing tip gap distance is associated with a contraction of the rotor slipstream in the duct diffuser. • A duct generates a large proportion of the total thrust of a ducted-rotor system. • Aerodynamic and aeroacoustic performance of a ducted-rotor system is strongly affected by the tip-gap distance. • A smaller tip gap distance increases the aerodynamic (thrust) performance. • A smaller tip gap distance benefits the pressure recovery in the diffuser. • An enhanced pressure recovery in the diffuser yields a higher flow velocity at the duct inlet, and leads to more duct-thrust.