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Suppose I have an engine with propeller. I know there is aerodynamic force pushing the propeller opposite direction the engine is turning it. One component of this force is creating lift, but second is braking the prop. Now these forces are applied at some distance from engine shaft, thus they're creating torque, which is turning the whole engine if it wasn't fit somewhere.

And now my problem: how to describe what happens, after fitting this engine out of mass center. For example take a ruler, that has pivot somewhere and at the end of it, there is this engine with propeller. How do I describe momentum at the center of mass - the pivot, instead at the engine (engine has no weight in this example)?

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I don't understand your use of the word "fit", as in "after fitting this engine out of mass center". Any such question can be answered by considering a propeller blade to be a rotary wing. – Mike Dunlavey Jun 11 '13 at 12:10
Fit = mount. I tried to pinpoint and generalize my problem, but in fact we're discussing tricopter here (probably will be easier to imagine, I don't have mouse right now and drawing on touchpad is pain). Engines there are offset from center (which is in the middle of tricopter) on arms. So how do I get from torque at engine to torque at center. I'd still like the answer to be more general then for this special case, so I can use it as concept in future.. But rotary wing seems like step in good direction. – Raven Jun 11 '13 at 12:41

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