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Thrust vectoring changes the direction of exhaust gasses to provide force in the direction the nozzle is pointing. Going back to the ancient Greek steam jet, the aeolipile, building pressure inside a chamber and releasing it through a nozzle will create a force (action/reaction) in the opposite direction, based on the principles of momentum.

But, the mass flow changes direction in the curved nozzle. Will this not produce an additional pitching moment in the opposite direction of the curve?

Would this not have to be applied to over-all pitching moment calculations for an engine mount?

The curiosity stems from this recent question about an aircraft with a dorsal single jet engine mount.

Robert DiGiovanni
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  • Isn't it the point of thrust vectoring? To add pitch (and sometimes yaw and row as well) authority when the elevator isn't very efficient (or nonexistent, like a rocket)? – user3528438 Apr 28 '22 at 14:21
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    To answer your question directly, no it will not product a pitch moment (torque) in the opposite direction of the curve, but a force towards the inside the curve. That force can produce a torque on any direction depending on where your center of gravity is. – user3528438 Apr 28 '22 at 14:33
  • Are they better off doing all that flow bending on the air intakes? Why not 1 on top and 2 on the sides (where form drag mitigation is most effective), keeping the (further aft and faster flow rate) exhaust tubing straight? – Robert DiGiovanni Apr 28 '22 at 18:04
  • Engineering is all about compromises. The size of this plane doesn't allow for a L-1011 style S-shape intake and would make the plane too tail-heavy. While the DC-10 style mounts the nozzle too high. Twin engine or even triple engine would need two or three smaller engines which may not be available at this thrust class and would naturally be more complex and expensive. And to be honest, more engines around the tail means higher chance of damaging the tail, should one of the engines blows up. – user3528438 Apr 28 '22 at 18:36
  • @user3528438. Not multiple engines, just multiple intakes. Straight pipes seem to help motorcycles. Something about less back pressure. Intakes could be designed in XS so the (curving) airflow is nice and slow. – Robert DiGiovanni Apr 28 '22 at 19:35
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    Seems to me that force (and torque) produced by bend in duct is in same direction as force and torque from directed exhaust nozzle, not opposite. – quiet flyer Apr 28 '22 at 20:52

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No, the change in mass flow direction does not cause an additional pithing moment. The mass flow changes direction, and creates forces that will have to be absorbed by the internal support structure of the exhaust nozzle, like the aileron hinges must be supported by an appropriate construction. The mass flow change force is an internal force.

So the dynamics of the mass flow turning a corner are not of consequence to the aeroplane flight path, kind of like a pilot pressing on the side of the cockpit also does not alter the course of the plane.

Koyovis
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