"Capt. Geoffrey Thorpe" The Sea Hawk @See My Sig.com wrote:
"John" wrote in message
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It's supposed to be fairly stable because the thrust reaction point
is well above the CG, so there is a strong pendulum effect. They
claim it's better than a helicopter.

An often made, completely wrong assumption - "pendulm effect" - ain't
no such thing for an object in free flight.

Technically speaking, I don't know under what assumptions one could ever
claim an object moving in a fluid is ever moving "freely". More below on
the importance of this point....

Early rocket experimenters often attempted use "tractor" engines
assuming that it would provide stability - Dr. Robert Goddard's first
liquid rocket is an example. It didn't take them long to figure out
that they were wrong.

But a rocket and an rotorcraft aren't equivalent under all cases of
interest. For example, if your rotorcraft's engine fails, then because it
is traveling through a fluid the craft will rotate so the center of
aerodynamic pressure is above the center of gravity.

So if your craft normally flies with the c.g. below the center of pressure
(e.g. rotors above fuselage) then on engine out I would expect you'd
probably count on little change in attitude while autorotation would
ideally slow your descent somewhat.

But if your craft normally flies with the c.g. above the center of pressure
(e.g. rotors below the fuselage) then on engine out I would expect some
nasty rotations that are unlikely to dampen out before you strike the
ground. Even if they did dampen out, you're upside down and the rotors
would actually have to reverse direction to provide autorotation. drag.