Originally Posted by
H Peacock
PDR. Surely Intake Momentum Drag is the result of the air coming through the intake (not sure it's sucked-in but I know what you mean) and then being 'stopped' at the engine. The engine is producing thrust vertically so the net momentum is rearward, i.e. it would push the Harrier backwards? You therefore need a tad of rearward nozzle to countwract this and remain stationary.
I didn't mention drag, just momentum. Consider an aeroplane sitting stationary in still air with the engine off. Now fire up the engine and some of the air in front of the aeroplane is induced ("sucked" if you will) into the engine. It also creates a big zone of low pressure in front of the aeroplane where all that air used to be (I exaggerate for effect) - lower pressure than the air behind the aeroplane. There are lots of ways you can model or look at this as a static pressure effect, a momentum effect etc, but the net result is a forward force on the airframe which is opposed by a slight net forward moment in the nozzle thrust.
PDR