Both the advancing rotor blade and the retreating blade on a helicopter
cause problems limiting maximum airspeed.

In many case, it is the retreating blade that limits first. Retreating
blade stall is announced by rotor roughness and vibration. In fully
articulated rotor blades, the retreating blade flaps down to the limit.
Then, because of precession, the nose will pitch up and the helicopter will
roll over.

At about the same time, the advancing blade will be moving at close to the
speed of sound. A sharp rise in drag produces shock waves which can cause
structural damage.

A number of years ago, in England, a pilot and crew were testing a
helicopter. At 11,000 feet and an IAS of 46 knots, the helicopter was
deeply in the retreating blade stall. It did two barrel rolls so large that
at the end of the second roll the aircraft collided with the ground.
Miraculously, the pilot and crew member survived. (This information comes
from "Principles of Helicopter Flight" in the Retreating Blade Stall
section.)

I have heard there is a jet powered gyrocopter that can fly at 350 knots or
so. Gyrocopters eliminate a lot of problems facing helicopters, but they
are still not very good at soaring.

Colin N12HS


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