[quote=Harri Pihl;70657]At the beginning speed is zero (V=0) so the formula:
T = (n*W)/V
can't be used but as example the one for V=0 in the Hamilton Standard book (above formula for V>0).
So, at 1kt/hr Thrust is near maximum?, at 2Kts the thrust is coming down and at 300kts it is at minimum?
This is mostly true for the jets but not for the propeller planes. We can assume that power remains fairly constant (save the RAM and altitude effects), but thrust will vary according to formula for V>0.
Power is basically constant, Thrust (Force along positive horizontal axis exerted on the stsyem by the engine/propeller combination) is basically constant for this system from V=0 kts and V= 300kts and every velocity in between.
It does not start at 1kt (or 10 or 100kt) at maximum and decrease with Velocity. If you think so, tell me at what velocity you want to start using the THp formula?
This part is only partially true, the plane will reach equilibrium but while the velocity increase, the thrust at constant power will decrease.
This part is also only partially true because thrust increase when the speed decrease.
The power is constant, however, the thrust depends on speed.
IMHO the main problem here is that you have apparently played with the jets.
Actually true, and I mostly played with engineering and physics.
Force = Thrust = d/dt(MV) ..
in this system (through a propeller plane) the thrust is the net mass flow rate throught the prop plane. Unlike a jet engine it has no additional combustion energy converted to the system as thrust all the engine torque must be converted through the propeller system to impart energy onto the stream tube entering the prop plane where it dramatically accelerates the mass flow rate of the stream tuve from Vfreestream to Vpropstream.
I am discounting exhaust thrust from this discussion as it is negligible to the thrust of the engine prop system until you get to the critical altitudes of the airframe/engine system.
It's not just my use of this very basic formula but also Hamilton Standard, Hoerner, NACA, NASA, RAE... you name it. Below is quote from Hoerner.
Harri - I know the equations. Look at the polar diagrams of the same ship (c-54, B-17) etc that shows the the effect of added weight to the same system and engine capabilities on cruise speed and range.
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Thp is different from Thrust. Thrust of a system is required to do a free body diagram on the system to understand the result of variations in drag due to Weight.
The equations are about equating 'push' of a jet to transmitted Hp from an engine generating torque on a propeller. At 375 mph a system that generates one hpxprop efficiency= equates to 1 pound of thrust = 1 Thp.
Forget about Thp, Hp, and Thrust of a jet.
Focus on Force(s) retarding acceleration versus Force causing acceleration until equilibrium is attained. This is what I say doesn't decrease as a function of Velocity.
It is the same at zero mph, 20mph, 200mph and 340mph for that P-51B-15 with a 1650-7 at 67"/3000 rpm at sea level. The difference is that the Force > Drag until equilibrium is achieved.
If you wish we can agree to disagree - I can live with that.