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From: paj@gec-mrc.co.uk (paj)
Newsgroups: sci.military
Subject: Re: Autorotation of Helicopters (was ... AV-8B Harrier)
Message-ID: <1990Dec13.034019.20161@cbnews.att.com>
Date: 13 Dec 90 03:40:19 GMT
Sender: military@cbnews.att.com (William B. Thacker)
Organization: AT&T Bell Laboratories
Lines: 41
Approved: military@att.att.com



From: paj <paj@gec-mrc.co.uk>

I have been following this discussion and am somewhat puzzled.

First, a note on my expertise: I fly a hang-glider.  I have never even sat
in a helicopter.

Gliders fly by descending such that (for constant speed) their drag:weight
ratio equals the glide slope.  I was under the impression that helicopter
blades are aerofoils and hence capable of gliding (rather than being simple
flats which would be far less efficient).  Hence the principle of
autorotation is to lower the collective such that the "glide angle" of the
blades is equal to the ratio of drag:weight where drag includes engine and
pitch mechanism and weight includes the aircraft itself.  Hence the
helicopter will descend slowly.

In a conventional aircraft (glider or powered), one lands by flying close to
the ground and then leveling out (with power right down) to allow the aircraft
to slow down until it stalled onto the ground.  This ensures that the landing
is at the absolute minimum speed (amongst other things).  I expect that
in a helicopter stalling the blades is not necessary but the inertia
would be useful for decreasing vertical speed in the last seconds.

I would have thought that the big thing to avoid when autorotating a
helicopter is stalling the blades.  This would cause the aircraft to loose
lift until the rotor can be unstalled.  This would have to be done
by lowering the collective as far as possible and praying.

There is a kind of aircraft known as an "autogyro".  It consists of a
helicopter rotor with no drive and a conventional aircraft propellor for
thrust.  The rotor is driven entirely by autorotation.  A gentleman named
Frank Wallis (no relation to Barnes) has been pushing these for military
use with little success for some years, and was also the builder and
stunt pilot of "Little Nellie" on one of the Bond films (Thunderball I
think).

Have I completely misunderstood how helicopters fly or am I about right?

Paul.