User Guide

GROUND SCHOOL
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Diagram 6.14: Vortex rings
wing at any angle of attack there will be a critical speed below which lift suddenly collapses
– the wing stalls. For any reasonably powerful or streamlined helicopter, the maximum safe
airspeed is determined by the speed at which the retreating rotor blade starts to stall.
In this situation, you suddenly start to lose lift on the retreating blade side, and the
helicopter rolls towards it. Provided that you are not diving too steeply, you may be able to
reduce speed by lowering the collective and pulling back on the cyclic before you lose
control, but beware of violent panicky maneuvers, which may stall more blade area or set
the blades flapping violently.
Like all aerodynamic effects, retreating blade stall is affected by air density and by
temperature. It will happen at lower speeds at higher altitudes and/or higher temperatures.
Vortex Ring Effect
The commonest visible example of a vortex ring (or ring vortex) is a smoke-ring. In fact they
are quite a widespread phenomenon, but like all flow patterns they are usually impossible
to see directly. All you need to start one is a stream with a more or less circular cross-
section (like your rotor downwash) which satisfies certain other conditions.
In a helicopter, you can unintentionally create a vortex ring around your main rotor if you
make a sustained high-speed descent vertically or at a steep angle. Once the ring has
formed, it is surprisingly stable, and moves with you as you descend. The extra downwash