Comparison of the Rotor Wake Modeling in Hovering Motion

The time marching free wake model of the helicopter rotor in hovering motion is compared with the prescribed wake model. The flows are assummed to three-dimensional unsteady potential flow. In prescribed wake method, an empirical formula from experimental results is used to calculate the wake geometry until the computed thrust coefficient is well agreed with the given thrust value. However, in free wake method the wake shed from the trailing edge of the rotor blade makes the global wake geometry without any particular restriction. For the case of the single blade rotor in hover, the thrust and lift distribution along the span are obtained and the radial and axial wake geometries that is, the inner vortex sheet and the tip vortex line are calculated. The numerical results are in good agreement with the results obtained from the prescribed wake geometry. The computed free wake geometry is also at least qualitatively in good agreement with the experimental wake geometry which is represented by three wake region; revolutions of well defined tip vortex lines, recirculated and expanded zones of far wakes.