The aerodynamic characteristics of a rigid four-bladed rotor in forward flight at advance ratio of 0.1 and 0.6 are simulated numerically with Reynolds-averaged Navier-Stokes (RANS) methods and compared with each other. The computed results indicate that reverse flow occurs near the root area of the retreating side of the rotor, giving rise to the unconventional distributions of pressure coefficient. And the proportion of the reverse flow region, where hardly any lift is contributed, is directly related to the advance ratio and increases with the raise of the advance ratio. Detached eddy simulation (DES) method is used to calculate the unsteady aerodynamic characteristics of a yawed flat blade in reverse flow. It is found that complex attached vortex structure occurs on the surface of the blade and the vortex near the root interacts with that near the tip area under the effect of the radial flow. The lift coefficient is directly proportional to the pitch angle and does not drop after the stall angle of attack.