Aerodynamic performance and flow interaction of the coaxial rigid rotor in hover

A numerical method based on RANS(Reynolds-averaged Navier-Stokes) equations was developed to predict the aerodynamic performance of coaxial rigid rotor in hover. Validation of the method was carried out on the hovering tests for the subsonic and transonic flows. The simulation of rigid rotor XH-59A in hover showed that CFD prediction of rotor performance agreeed with the flight test data. Compared with single rotor of the same solidity, coaxial rigid rotor had better performance, along with bigger figure of merit. The performance of the upper rotor decreased less than that of the lower rotor. Reasons were related to the suction effect of the lower rotor on the flow and its location in the downwash flow field of the upper rotor, resulting in the decrease of its effect angle attack. Numerical method indicated that the optimum figure of merit for XH-59A was 67%, when the collective pitch angle for both rotors was 14°. Compared with traditional coaxial rotor, the interstage distance of rigid rotor was smaller, leading to a better aerodynamic performance.