Propeller thrust question

In article ,
"xerj" wrote:

Could you re-phrase your questions?

Ahhh yeah. Just realised you'd never hit the zero thrust speed. Woops!

The thing that STILL puzzles me though is that the thrust created *by the
propeller* goes down (as measured in Newtons for instance) as the velocity
increases.

As detailed here http://www.mh-aerotools.de/airfoils/propuls4.htm , the
thrust at 252 knots for a theoretical plane is 17.2N, whereas at 108 knots
it's 31N.

That's what's got me confused. Higher speed = more drag, yet the measured
thrust from *just the prop* is less. Is the extra thrust to match drag
coming from the momentum of the already moving airframe?

Not quite.

The plane accelerates (and the drag increases and the thrust decreases
just as you observe) until the thrust and drag are exactly the same. At
that point the airplane stops accelerating, that is, its velocity
becomes constant. The momentum of the plane doesn't provide any extra
*thrust*, but the momentum is (more or less) what keeps the plane moving
at that point. (The reason I say "more or less" is that what really
keeps the plane moving is the need to maintain a particular speed in
order to balance thrust and drag. If the plane slows down then thrust
exceeds drag and the plane speeds up again, and vice versa. This would
happen even if the plane's momentum (i.e. its mass) were zero. In this
case, the plane would reach its final velocity instantly once the prop
was turning, and would stop instantly as soon as the prop stopped
turning.)

(Note that it doesn't actually matter that the thrust decreases with
velocity, only that the drag increases faster than the thrust as
velocity increases so that at some speed they become the same.)

rg