The growing demand for efficient propulsion systems that operate primarily in hover, has led the ducted rotor configuration to emerge as an increasingly popular design solution. One of the major design parameters for the ducted-rotor assembly is the radial distance between the blade tip and the circumferential wall of the duct, referred to as the tip gap. How a variation in the tip gap affects the performance of the ducted-rotor assembly is addressed in this current research work.

The aerodynamic characteristics and performance of the ducted-rotor assembly are experimentally investigated for a parametric variation of the tip gap. The aerodynamic performance of both the rotor and the duct are characterized through force measurements with load cells. These types of measurements reveal that, for typical operating conditions, the duct generates more than half of the total thrust of the assembly. The thrust performance of the duct was also characterized with a detailed mapping of the static pressure distribution over the duct inner-wall, confirming that the thrust-generating mechanism of the duct is a direct consequence of the relatively low pressure over the inlet area of the duct. Even though the diffuser of the duct generates a small amount of pressure drag, its presence is critical for the below-the-rotor pressure recovery and for establishing a high axial throughflow. From detailed flow measurements using the particle image velocimetry method it was furthermore found that the performance deterioration—with an increase of the tip gap—is associated with a contraction of the rotor slipstream in the duct-diffuser, since the less rapid breakdown of the tip vortices reduce the axial momentum near the diffuser wall. A set of preliminary acoustic experiments were also conducted and show that the noise radiated by the system is also strongly dependent on the size of the tip gap.