Re: torque from jet engines?

From: (Gregory R. TRAVIS)
Organization: Indiana University, Bloomington
Date:         17 Aug 95 04:58:45 
References:   1
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In <airliners.1995.1242@ohare.Chicago.COM> (Jay Vassos-Libove) writes:

>In a multi-engine propellor driven aircraft, the props on
>opposite sides of the fuselage might counter-rotate so as
>to cancel out the torque that would otherwise be imparted
>to the aircraft when the engines are running (right?) ...

>So, do jet engines develop torque on the airframe like this?

Counter-rotating props are not used to cancel torque.

Consider the rotating prop blades as tracing a disk that is perpendicular
to the aircraft's fuselage.  Normally the "relative wind" (the airflow
over the aircraft in flight) strikes this disk head-on.

However when the aircraft is at a high angle of attack (such as when
taking off), the relative wind strikes the disk at a slight angle.

This angle effectively INCREASES the angle of attack of the downward
roating prop blades (causing them to produce more thrust) while it
DECREASES the attack on the upward rotating blade.

		Looking from above at the prop disk where the
		angle of attack is high and the prop is rotating
		counter-clockwise (when viewed from the front)

			|  <--  More thrust being produced here
			|  <--  Than here

This causes the prop to want to twist counter-clockwise (when viewed from the

On a multi-engined aircraft this twisting moment is very undesirable because
it causes the fuselage to want to rotate.  Under certain conditions
(such as engine failure) this twisting can make the aircraft uncontrollable.

Therefore designers try to minimize the LEVER (not the torque) of the
twisting moment by ensuring that the "more thrust side" is as close to the
fuselage as possible.  This is done by having engines and propellors that
rotate opposite of each other on opposite sides of the fuselage.  Note that
counter-rotating props completely eliminate the twisting force if both
engines are operating and minimize it if one engine fails.