Rotor Wake

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Rotor Wake
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A final element of a helicopter rotor that needs to be examined is the wake it creates as it travels with a forward velocity. In the simplest case, where no vorticity is shed along the blade span, vortices would only be shed at the blade tips and roots. Immediately aft of the helicopter, the tip vortices form helical lines on the surface of the stream tube defined by the rotor. The root vortices will coalesce, forming a single vortex along the axis of the stream tube.

Rotor vortices at low flight speed
Rotor vortices at low flight speed [from Stepniewski, 1979]

Soon after the vortices leave the rotor, they begin to roll up in a two stage process. First, the individual tip vortices combine into concentrated lines as they are shed from the tips. Then, several rotor radii downstream of the rotor, the overlapping spiral vortices combine to form two trailing vortices very similar to those found trailing fixed wing aircraft.

Trailing vortices viewed from downwind of rotor
Trailing vortices viewed from downwind of rotor [from Ghee and Elliot, 1995]

While the trailing vortices depicted above are very similar to those of a fixed wing aircraft, it is important to note the "tighter" vortex on the advancing side of the rotor. These vortices exhibit such a difference because of the variation of the downwash distribution between the advancing and retreating blades. As discussed in the section on flapping hinges, the advancing blades experience a higher velocity and thus a greater downwash at the tip, as demonstrated below.

Downwash distribution on a rotor disk in forward flight
Downwash distribution on a rotor disk in forward flight [from Stepniewski, 1979]





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