Characteristics of the Rotor Wake in Forward Flight

As in hover, a helicopter rotor wake in forward flight is found to be dominated by the blade tip vortices. However, because of the free-stream (edgewise) component of velocity at the rotor plane in forward flight, the wake is now convected behind as well as below the rotor and it takes on a more complicated (nonaxisymmetric) form. The rotor wake geometry in forward flight is found to be sensitive to the rotor thrust, advance ratio, tip path plane (TPP) AoA, the presence of other rotors (such as a tail rotor, or another main rotor as in tandem or coaxial configurations), and rotor-airframe interference effects. While much is now known about the general features of rotor wakes in forward flight, much more is still to be learned about the details of the flow, especially before mathematical models of the rotor wake can be adequately validated.

There are a variety of techniques that can be used to visualize rotor wakes in forward flight, and some of these have been discussed previously in reference to hovering wake studies. Other techniques have included cavitation from the blade tips in a water tunnel, such as used by Larin (1973, 1974). Lehman (1968) and Landgrebe & Bellinger (1971) have used bubbles to trace out the tip vortices trailed from a rotor in a towing tank. Jenks et al. (1987) have used stratified layers of dye in a towing tank to observe some aspects of the wake roll-up. In a wind tunnel, smoke injection from the blade tips can provide good evidence of the epicycloidal tip vortex trajectories – see Muller (1990a, b). Laser sheet smoke flow visualization, such as used by Ghee & Elliott (1995), is generally con­sidered one of the more accurate ways of documenting the spatial locations of the tip vortices.

An example documenting the general features of a helicopter rotor wake in forward flight is shown in Fig. 10.8, where smoke trailed from the blade tips was used to mark the vortex locations. It will be apparent from the top view (x-y plane) that the tip vortices are initially laid down as a series of interlocking epicycloids. Mutual interactions between the individual filaments results in some distortion of the vortex positions, but mostly in the z direction normal to the plane of the rotor (see side view). This distortion is particularly strong at low advance ratios, where the tip vortices are closest together. Notice from Fig. 10.8(a) that along the lateral edges of the wake the individual tip vortices begin to roll up into vortex bundles or “super vortices,” as mentioned previously. Another example of this phenomenon is shown in Fig. 10.9. Notice also in Fig. 10.8 that the effects of the tail rotor can be seen as an expanding turbulent wake embedded inside the main rotor wake.