RCU Forums - View Single Post - Thrust line, Datum line, Center line?

05-18-2009 | 08:36 PM

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From: Ithaca, NY

RE: Thrust line, Datum line, Center line?

I’m sorry to cause anyone to read my writing four times. I wish I could draw diagrams but I can’t, so I’ll try a different approach. Let’s put aside, for the moment, the question of whether downthrust works because of the vertical vector component of the thrust acting on the nose of the plane, or because of an interaction between the prop wash and the stab.It won’t matter for this discussion.I like the suggestion above of looking at the wing as the “player”. The AOA of the wing in flight depends on the speed and weight of the plane, the wing characteristics, air density, etc. So changing the “angle of incidence” by shimming the wing has no effect on the AOA in flight. All it changes is the angles of the other components going along for the ride.Suppose you decrease the angle of incidence of the wing.Now the whole plane has to fly at a more positive angle to keep the wing at the same AOA. That includes the engine. So now, by decreasing the angle of incidence, you have raised the angle of the thrust line. If you started with 2 degrees of downthrust and you decreased the angle of incidence of the wing by 2 degrees, you’ve just wiped out your downthrust. I’ll stop for now. Does that make sense?

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