In order to study the aerodynamic characteristics of shrouded coaxial rotor propulsion unit hovering near ground

experimental measurements were conducted to investigate the effects of rotational speed

rotor spacing and shroud-diffuser-height from ground. CFD simulations were performed to analyze the velocity field and pressure field distribution around propulsion unit

and the flow-field characteristic of ground effect for shrouded coaxial rotor propulsion unit was studied. Results showed that

when the distance from shroud diffuser to ground was less than two rotor radii

the ground effect began to work; when the distance was less than one rotor radius

the ground effect was very significant

and figure of merit (FM) increased by about 30% at most. As the distance decreased

the shroud thrust decreased due to the declining inflow velocity leading to the decrease of the peak pressure at the shroud lip. The rotor spacing had little effect on ground effect. The thrust and counter-torque of shroud and rotors were approximately proportional to the square of rotor speed.