With reduced power and/or less collective pitch, the helicopter can descend vertically. In doing so, it tends to fly into its own slipstream and if the rate of sinking is rapid, it may come to equal, or nearly equal, the rotor downwash. In this rather dangerous condition not only is the air very turbulent, but a vortex ring may easily develop. The slipstream from the rotor is at high pressure while above the blades the pressure is low. The air flows up round the limits of the rotor disc and re-enters the low pressure zone, to be drawn down


Fig. 15.4 Vertical climb.«creased rotor drag
again through the rotor. The helicopter then is effectively flying in the middle of a strong downdraught of its own making. The entire vortex ring tends to become a self-contained system and sinks through the surrounding air rapidly. (Figure 15.6. The situation is closely analogous to the ascent of a buoyant ring-vortex thermal.) Fortunately, the danger of a crash can be avoided by trimming for a forward (or any other direction) motion so that the rotor does not drop into its own slipstream but constantly enters new, relatively undisturbed air.

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