Asymmetric Affects of Single Engine Operation

When an engine on a multi-engine aircraft becomes inoperative, a number of asymmetric affects occur involving the aerodynamics of the aircraft.

Asymmetric Thrust

This behavior is obvious.  When one engine is not operating and the other is in full operation, there is more thrust on the operating side of the aircraft than on the inoperative side.  This can cause other asymmetric symptoms:  Asymmetric Lift, Yaw and P-Factor.

Picture Courtesy the Airplane Flying Handbook - FAA

Asymmetric Lift

When one engine is in operation and another not, the air being forced over the wing of the operating engine causes more lift to be created by that wing than the wing which houses the inoperative engine.  This additional lift causes the aircraft to want to bank in the direction of the inoperative engine causing a turn towards that engine.

Asymmetric Yaw

With a single engine operative, the aircraft will want to yaw towards the inoperative engine for obvious reasons.  The thrust of the operative engine is not being countered by drag or by thrust from the inoperative engine, this causes the aircraft to want to yaw towards the inoperative engine.

One of the first things a pilot will want to do when this occurs is apply rudder pressure on the side of the operative engine to counter this asymmetric yawing affect.  The simple act of doing this will create a forward-slip configuration thus reducing the effectiveness of the rudder.  As shown below, the rudder is not very affective because of the relative wind simply moving over its surface.  The rudder is only affective when it can be placed at an angle to the relative wind - in a fully deflected rudder configuration, the rudder becomes ineffective if its angle of deflection is the same as the relative wind.

The recommended procedure to help reduce this reduction of rudder effectiveness is to bank towards the operative engine 5 degrees or more thus reducing the forward slip and configuring the aircraft in a side-slip; a much more aerodynamically sound configuration.

Asymmetric P-Factor

P-Factor is that aerodynamic feature that causes the downward rotating blade to create more thrust than the upward rotating blade while in a high pitch configuration.

With co-rotating propellers (both blades are spinning in the same direction), an engine-out situation will reduce the affect of P-Factor on the aircraft.  Why?  When two propellers are spinning in the same direction, they are creating 2 times the P-Factor than a single-engine aircraft would who's propellers also spin in that direction.  So, on a multi-engine aircraft with co-rotating propellers, when one engine is disabled, the P-Factor is 1/2 of the total P-Factor that was being produced.

With counter-rotating propellers, P-Factor of one engine is negated by the other aircraft.  So, the total P-Factor is Zero (0).  When one engine is made inoperative, the total P-Factor goes from Zero to the total P-Factor created by the now - single operative engine.

Asymmetric Drag

With only a single engine operating on a multi-engine aircraft, drag becomes one of the worse enemies for the pilot.  Drag is increased on the inoperative side of the aircraft because of the wind-milling propeller.  Additional drag is created if the aircraft is forced to yaw towards the operative engine by use of rudder.  This total drag is called asymmetric drag.

This page was last modified on 12/03/2006