As the helicopter approaches the ground, the spreading wake will interact with any horizontal wind. This will tend to turn the upwind wake upwards after which it re-enters the rotor. This sets up a type of toroidal flow. The effect is shown schematically in Figure 2.33.
A considerable amount of research has been devoted to this effect, for the reason that when the helicopter is operating where the ground surface is easily raised into the air [13-15]. A cloud of ground debris is entrained by the interacting flows surrounding the aircraft. Visibility is severely restricted making operations very hazardous. Figure 2.34 shows a V22 descending into a cloud of dust caused by the downwash from the rotors.
Figure 2.34 V22 descending into a brownout cloud (Courtesy Eglin Air Force Base)
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