Originally Posted by
jcomm
Or... am I completely missing something here ???
Yeah, Newton's third law of motion. For every action there's an equal and opposite reaction.
So, if 15,000 inch pounds are being applied to the prop, than the prop has to be in turn applying 15,000 inch pounds to *something*. About the only thing that "something" could be is the engine. I suppose that you could make a case that in an straight thru axial flow turbine like the Allison 501, torque is applied to the engine and to the exhaust gasses flowing out the tailpipe which would result in rotational motion of the exhaust gas. I don't have any figures, but I suspect that the rotational momentum of exhaust gas exhausting from a turbine is pretty small, in no small part because the stators between the turbine stages are designed to reduce rotational flow. On second thought, the 15,000 inch pounds isn't being applied to the prop, that would be the torque the engine is applying to the reduction gearbox (RGB) as that is where tormenter measurements are taken on the 501. The engine-gearbox collectively would be applying about 17000 *foot* pounds (12 times larger than an inch-pound of course) of torque to the prop. (sorry, I'm still working on my first cup of coffee this morning) So if the rotational momentum of the exhaust gas stream form a small portion of the 15,000 inch pounds of turbine to RGB, it is a *very* small amount of the 17,000
foot-pounds of torque applied to the prop.
Oh, and re-reading you post, don't mix up the various torque action-reactions. You have to view them as individual systems: 1) Engine applies 17,000 ft-lb of torque to the prop, prop applies 17,000 ft-lb of torque to the engine. Now the prop applies some forces to the air which can't be as neatly summed up as torque on a shaft, and those forces are equal and opposite the 17,000 ft-lb of torque at 1021 rpm, but that's a separate interaction than the engine-prop interaction as far as Newton is concerned.