[mort]

Originally Posted by niky

The prop thrust decreases as speed increases because the propeller is pushing against the air, and can't generate as much thrust at higher air speeds because it has to push harder to generate more thrust.

Hi niky,
Hardly. For some propeller shapes it is almost the opposite: at some high propeller speed the air over the blade starts to stall. Lift, in this case in the forward direction, falls suddenly, but l/d doesn't disintegrate. The sudden drop in lift is accompanied by a drop in drag, so the engine revs higher (if not well governed) moving the prop further into stall. Above the critical tip speed the propeller fails like it was slipping on the drive shaft. - like the engine was suddenly unloaded. The prop is going faster, but loading the engine less.

A simple explanation of propeller theory.
Or you could have checked out the original source - from www.epi-eng.com

To find the speed which can be reached with a known engine HP, prop efficiency and airframe drag (thrust = drag in steady state level flight):

KTAS = ( HP * eff * 326 ) / Drag

It is clear from the relationship between power, thrust and speed, that if power and propeller efficiency are held constant, then propeller thrust decreases as true airspeed increases.

In case you were wondering, the hump in those curves is due to the fact that at low airspeeds, prop efficiency is very low. As airspeed increases, so does efficiency, quickly at first, then more slowly, up to it's max (about 85-87%).

And:

...the prop efficiency begins to decrease dramatically when the prop is operated at a helical tip velocity in excess of 0.85 Mach. That occurs because the local air velocity over the surface of the prop (near the point of maximum airfoil thickness) will reach Mach 1, and create a shock wave, separating the flow and dissipating prop energy.

At constant power thrust falls with increasing speed is basic physics. You won't be taken seriously about anything if you continue to deny the definition of power = speed times force.

from wiki.answers.com

Power is the time rate of change of energy, dE/dt= power= Force x velocity=Fv.

from wikidot.com

Mathematically we define work as:

W = F · s (1)

from physicsclassroom.com


from wikipedia

P(t) = F · v

For constant thrust power increases with speed.
It is plain that F = P/s: For constant power, as speed increases thrust will fall.
-mort