I think it has to do with how fast the prop can 'push' air versus how fast
the plane is moving through the air.
Imagine you are in a small boat with an outboard motor. The motor is moving
the boat at 15 mph. You decide you want to go faster so you pull out an oar.
You can only move the oar at a speed of 15 mph so the net force you are
applying to the water is zero. If you were going slower, you could apply
force to the water, thus making the boat inch ahead a little faster.

wrote in message
...

I'm not an engineer, nor do I play one on TV. I do find it interesting
that the speed or velocity of the aircraft is a factor in figuring
thrust in both the propeller and the jet propulsion formulas.

Perhaps someone else can explain in it terms that you and I can
understand.

TC