Numerical study of aerodynamic performance and flow interaction of coaxial rigid rotor in hover and forward flight

A numerical method based on Reynolds-averaged Navier-Stokes(RANS) equations is employed to predict the aerodynamic performance of rigid coaxial rotor in hover and forward flight. Validation is carried out separately, on the subsonic and transonic flow of hovering test, and subsonic forward flight wind tunnel test data with acceptable accuracy. The simulation of XH-59A rotor in hover shows that CFD prediction of rotor performance agrees with flight test data. It indicates that the optimum figure of merit for XH-59A is 69%, when the collective pitch for both rotors is 14°. The flow interaction of the coaxial rotor is investigated via the comparison of single rotors at the same working conditions. In hover, coaxial rotor has a better performance, bigger figure of merit and axial induced velocity than single rotor having same solidity. Compared to single rotor having half solidity, the performance of the upper rotor decreases less than that of the lower rotor in hover and low advance ratio flight. When advance ratio increases, the interaction decreases, the performance of both rotors is nearly identical (μ>0.6). The interaction is composed of two parts: the strong downwash flow from the upper rotor to the lower one and the weak suction effect of the lower rotor to the upper one. When advance ratio increases, both parts deviate from the location of the other rotor, but oscillation of the coaxial rotor thrust and torque increases significantly.