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Pusher aircraft Thrust Vector

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B52

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Sep 14, 2015
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Looking at various pusher aircraft I notice different thrust vectors angles. Rutan's pusher aircraft have the engine mounted 2° upwards thrust because the engine axis is slightly above the CG cord an so the upwards tilt is implemented to cancel a downwards pitch when power is applied.

In contrary the DC9 thrust vector which is above the CG is downwards. One would think that it would cause the DC9 to pitch down when power is applied which is not the case.

Explanation appreciated.
Vector.jpg
 

Tiger Tim

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It's possible that the DC-9 engine itself is sitting level but the rest of the nacelle is aligned with the local airflow.
 

Jay Kempf

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If you look close at both pictures of the DC 9 you'll notice that the nozzle is level but the inlet is pointed at the local flow angle above and behind the root rib angle. Smaaaahhhhht people at Douglas.
 

TFF

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All incidences are for balancing their influence on the CG/NP. As for the DC9, as you know it does fly nose high and has to have a wide CG range. I suspect it is for pushing the nose over some for high speed cruise. I also know the plane would Mach tuck and it might be used to push the nose over and keep the flight surfaces positive incidence. I don't remember if they are trying to get some deflection lift off the fuselage angle of attack.
 

Dana

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Push/pull makes no difference, what matters is the vertical distance between cg and the thrust axis.
You beat me to it, as you say push or pull doesn't matter. However, distance from the thrust line to the cg matters for transient effects, but distance from the thrust line to the center of drag is what matters for steady state trim.

The reason people think it matters more for a pusher is that pushers (propeller driven, at least) usually have the engine way up high so the pitch down effect is pronounced, whereas tractor designs usually have the thrust line pretty well in the center of the aircraft.

Dana
 
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