Re: Propellor "reverse thrust"

Date:         29 Aug 97 00:53:41 
From:         Robert Reed <aspect@wcinet.net>
Organization: ASPECT One
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showie@uo.guelph.ca wrote:
> On the subject of reverse thrust, I took a flight on a Manx Airlines
> Shorts 330 a few years back. When the plane landed, a big roar went up
> and the plane slowed dramatically. Do prop planes such as this deploy
> reverse thrust by somehow altering the pitch of the propellor blades to
> the extent they "blow" air forward? Or is something else taking place?

The answer is Yes.

The power levers to the PT6 and most other turboprops have a flight
range where the engines act much like a piston engine using the throttle
levers.

However, on the ground, the levers are lifted and pulled back over a
stop into a mode known as "beta mode".  Movement of the levers in beta
mode give the levers manual command of the propeller pitch while the
engine speed stays more or less at a set speed. The propeller pitch
command in beta ranges from several degrees reverse pitch (I don't
recall exactly how much) through flat pitch to several degrees positive
or forward pitch.  It makes ground handling simple, and the wierd noise
you heard happens near the flat pitch range.  Even near flat pitch the
braking effect is strong.

You can even back up the airplane by moving the levers back into reverse
pitch.  It is not often used for this however, as it can sometimes cause
brake problems if the brakes are applied while going backwards.

Robert Reed
http://www.wcinet.net/~aspect