an external force attempts to move it out of it's rotating axis of motion

In this case the external force is that of air friction always present pushing back on the propellor blades against the engine torque before any changes of direction leading to precession come into it

One thing that confused me early on was a lecturer explaining that there is a real force acting in the upward direction when you are standing still, pushing you up... couldn't understand that at all, big mystery. Once you twig that if you are stationary under gravity there MUST be an equal force 'pushing' you upwards against gravity. Or you'd be falling. Forces don't always have to be moving things around. Things can be in a state of balance just lying there but they're being pushed around lots really if you look carefully \:\)

Ming


'You are either a hater or you are not' Roman Halter