It's due to the fact that the forces of Lift, weight thrust and drag are not acting through exactly the same axis, mainly because the centre of pressure, and centre of gravity rarely coincide. Weight acts through the centre of gravity, where as lift acts through the centre of pressure.
In most aircraft the CofG is slightly forward of the CofP, and therefore the opposing forces of lift and weight create a nose-down pitching moment. Normally this force is balanced by the fact that the thrust line is slightly lower than the drag line, and so when the a/c is straight and level, with thrust=to drag and lift=weight, there is no pitching moment.
If you cut the power, you reduce the nose-up pitching effect of the thrust-drag couple, and so the lift-weight couple causes the nose to pitch down.
Most light aircraft are designed this way so that if the engine fails, the aircraft will begin to adopt a glide attitude without the tendency to loose speed.
I hope this makes sense, it's really hard to explain without a diagram. Can anybody help me out here?
Last edited by VNAVSPD; 8th May 2006 at 21:05.