Re: torque from jet engines?

From: (Keith Barr)
Organization: NETCOM On-line Communication Services (408 261-4700 guest)
Date:         03 Sep 95 23:01:50 
References:   1
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In article <>,
Jay Vassos-Libove <> wrote:
>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?) ...

No.  Counter-rotating propellers are installed on some multi-engine prop
aircraft to eliminate the effects of P-factor and the critical engine.

P-factor is an aerodynaic problem that all propeller driven aircraft
experience when flying in a high angle of attack attitude.  The propeller
is basically just a rotating wing that creates thrust by creating lift in
the forward direction, and like any wing the higher the speed and angle of
attack, the more lift it creates.  When an aircraft is flying at a high
angle of attack, the propeller blade that is moving downward is seeing a
greater angle of attack and a slightly higher airspeed than the propeller
blade that is moving upward.  This moves the center of thrust slightly off
center, in the direction of the downward moving prop blade, which is
normally on the right side of the aircraft when viewed from the pilot's
position in American aircraft (Soviet stuff and some British stuff is the
other way).

When viewed from above:

         Thrust Line
  as created by P-factor
     Down      Up

In a single engine aircraft this tries to turn the aircraft to the left,
so a pilot must hold right rudder to counteract this and 3 other left
turning tendencies seen during takeoff and climbout (the other left turning
tendencies are torque, spiralling slipstream, and gyroscopic precession,
but those are another article, write to me if you want explanations of

Before we can understand the critical engine idea, we need to understand
some multi-engine aerodynamics.  When one engine is dead, the asymetric
thrust created by the good engine creates a moment (force times distance)
that tries to rotate the aircraft away from the good engine.  This rotation
must be counteracted with rudder pressure to keep the aircraft going
in a straight line.  The moment created by the good engine is the thrust
that the engine is creating times the distance between the thrust line and
the centerline (CG) of the aircraft.  The more thrust that is created,
or the greater distance between the thrust line and CG, the greater the moment
that has to be counteracted by the rudder.  As you can see, when P-factor
is brought into play, the distance from the CG to the thurst line is
greater for the engine on the right side of the aircraft than the distance
from the CG to the thrust line on the left side of the aircraft.  What this
means is that the right engine can create a bigger moment than the left
engine.  The bigger the moment, the more rudder that is needed to maintain
control.  So, when both props rotate in the same direction, it is a worse
situation to lose the left engine than the right engine, since we want to
minimize the need for rudder to maintain control.  This fact makes
the left engine the "critical engine."  A critical engine is defined by the
FAA as the engine whose failure would most adversely affect the performance
or handling qualities of an aircraft.

If the right engine is remanufactured to rotate in the opposite direction,
however, the thrust line is then on the left side of the prop hub,
and is then an equivalent distance from the CG as the left engine's thrust
line, and therefore there is no critical engine.

Jet engines do not suffer from P-factor, because they have engine nacelles
that straighten the flow of air hitting the fan disk, thereby guaranteeing
that all blade positions see the same angle of attack.

Keith Barr
Westminster, Colorado, USA