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Is there a particular reason for which thrust vectoring is not used in airliners as with military aircraft besides weight and complexity factors?

I understand that on military aircraft maneuverability is a core component but, by implementing this technique also on commercial airliners, wouldn't there be any benefit? For example one thing that comes into my mind would be reduction of stress on the tail of an aircraft at high speed/high altitude turns, thus reducing maybe maintenance.

Another option would be increased safety, whereas if maneuverability would be compromised by loss of the tail of the plane (for example American Airlines 587), you would still have some sort of control, even though reduced, on the plane with directional thrust.

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weight and complexity is a very big factor for deciding to include something or not, also what would happen when (not if) it fails –  ratchet freak Apr 20 at 12:14
    
True. But if it fails there would be redundancy. At least that's my point of view. –  Fabrizio Mazzoni Apr 20 at 12:17
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@FabrizioMazzoni; "fail" doesn't necessarily mean "stop"; if everything would simply stop when they failed, our lives would be a lot simpler. One of the horror failures in flying is runaway trim, which is where the trim motor doesn't stop, and moves the trim surfaces to their maximum. Not fun. –  falstro Apr 20 at 13:15
    
Agreed with falstro. Putting vectored thrust on airliners would probably cause more accidents than it prevented. Also, the backup yaw authority can already be obtained with the engines by using differential thrust. –  reirab Apr 21 at 6:00
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Asking if there's any particular reason besides weight and complexity is a bit like asking if there's any particular reason not to eat cyanide, besides it being poisonous. Once there's a significant reason not to do something, there's no need to look for others. –  David Richerby Apr 21 at 8:45
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2 Answers 2

up vote 15 down vote accepted

Thrust vectoring is for flying outside the "normal" envelope (which is characterized by attached flow). Airliners should really never leave this envelope, so they are perfectly fine with regular control surfaces.

If you want to add thrust vectoring, it would make most sense if the engines are at the back of the airplane. On most airliners, it is really better to put them on and ahead of the wings, because in this location they help both with flutter damping (mass ahead of the elastic line helps) and bending relief. Putting the engine mass right where lift is created is better than carrying stresses all around the airframe, which would be the case with rear-mounted engines.

The redundancy point is valid, but it would be more helpful to have redundant control surfaces, and this is exactly what airliners have. At some point, every airplane has to come down for a landing, which requires to throttle the engines. No thrust, no control!

Most extreme case: If one tail surface breaks off, I wonder if thrust vectoring would be up to the job of trimming the aircraft even with cruise thrust. Not only have the raw forces to be sufficient, the reaction time to control setting changes has to be quick enough to suppress oscillations. I am sure, however, that thrust vectoring will certainly not suffice in all flight phases.

Thrust vectoring is perfect for situations at high angle of attack where you quickly want to point the nose of your aircraft at your adversary, so you get lock-on first. This is quite different from what an airliner has to do.

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Do commercial airplanes actually have redundant rudders? I don't think I've ever seen a jet airliner with redundant rudders (or at least haven't noticed it if I have seen it.) I'm assuming that the rudder was the primary control surface that the OP was referring to, since that's the one that separated in the crash he referenced (actually, the entire vertical stabilizer separated because of excessive rudder input.) –  reirab Apr 21 at 5:39
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@reirab: Yes, indeed. If you look closely, the rudder is split in a lower and upper part, and the same goes for the elevator on each side (inner and outer part, of course). See training.deicinginnovations.com/wp-content/uploads/2012/04/… for an example. –  Peter Kämpf Apr 21 at 9:23
    
Larger planes like the 747 and A380 (in your linked picture) are unique in having separate rudder surfaces. This actually caused a problem on this flight. Smaller planes have only one rudder section. –  fooot Apr 21 at 16:52
    
@fooot: The smaller ones compensate this with redundant linkages and actuators. This is a requirement for certification. –  Peter Kämpf Apr 21 at 18:09
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Thrust vectoring enables extreme maneuverability, airliners don't need that. Personally I don't want to be subjected to 9 Gs on the redeye from JFK to LHR! It's also very expensive technology to build and maintain, so your airfare would go up substantially. You also need a huge amount of power to make it work, so you'd have to fit afterburning engines to your commercial aircraft, which isn't workable.

Anyway, differential thrust is already an option on airliners and other aircraft which have engines outboard on the wings. A B-52H lost its tail in the 1960s and made it back, the pilots made good use of differential thrust. UA232 used differential thrust after they lost hydraulics.

So it's a technology that is expensive to implement, and isn't going to help prevent an accident.

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