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So why can a thrust vector control system increase my maximum g-load capacity? Because still n=L/G is relevant and if I can not provide enough lift (pointing up) to counter balance the thrust (pointing down to spin a missile around c.g.), why should I use it?

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    $\begingroup$ Because sometimes you just desperately want to turn, e.g to get out of a spin or stall, regardless of how much lift is available. $\endgroup$ – user3528438 Nov 23 '20 at 14:07
  • $\begingroup$ " . . . my maximum g-load capacity?' refers to you as an individual, or the airplane? $\endgroup$ – skipper44 Nov 23 '20 at 15:06
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    $\begingroup$ Your question isn't very clear. i.e. "why should I use it?" Are you designing a missile for a class project, or flying combat simulator games and wondering how to be more effective in defending against them? $\endgroup$ – Michael Hall Nov 23 '20 at 16:33
  • $\begingroup$ It looks like you have created multiple accounts. Be aware that you won't be able to fully participate to the site if you don't gather enough reputation. It is easier to do so by having one single registered account: aviation.stackexchange.com/help/merging-accounts $\endgroup$ – Federico Nov 25 '20 at 7:44
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The point of thrust vector control is post-stall maneuvering.

Control surfaces lose much of their effectiveness when the flow over them is separated. Pointing the jet exhaust in different directions works almost regardless of angle of attack. All that is required is good intake flow, and by placing the intake below the forebody (as done on the X-31, the F-16 or the EF-2000), this is still possible even at high angles of attack.

Post-stall maneuvering enables the airplane to point its sensors and weapons in the direction of an opponent within seconds of detection. Compare that to flying a turn at Mach 0.8 over maybe 150°. At 6g (= 80° bank angle), this takes 11.5 seconds, assuming a speed of 256 m/s (which is Mach 0.8 at medium altitude) and an instantly achieved maximum turn rate.

By attaining a very high angle of attack in order to slow down the airplane and thrust vector controlled rotation of the fuselage into the direction of the opponent (Herbst maneuver), an airplane can turn much quicker and, consequently, shoot earlier. This is critical for winning in air-to-air combat.

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A max G limit is generally imposed due to the load the wing is capable of. Specifically, the bending moment it can sustain.

Think about it, in a hard turn the wing and fuselage experience forces in opposite directions: The wing is creating lift in the direction of the turn, while "centrifugal force" wants to sling the fuselage to the outside of the turn. This makes the wing bend.

However, in the same turn if you had a thrust nozzle that could generate thrust acting on the fuselage towards the inside of the turn you would reduce that bending stress on the wing, increasing the margin for G available.

ADDENDUM: While I think my explanation is useful, (based on my interpretation of the question) it is highly unlikely that a VTOL type jet would use this technique at high speed to counter the risk of structural overstress. Rather, aircraft like the Harrier can use this to its advantage when maneuvering at lower speeds to reduce the risk of an accelerated stall.

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  • $\begingroup$ Most aircraft with TVC actually deflect thrust upwards to increase turn rate, as they are mainly trying to do nose-up maneuvers to bring their weapons and sensors to bear. Aircraft that can use their thrust to aid the wing in creating lift are just VTOLs. Harriers did VIFFing (vectoring in forward flight) ostensibly to push the opposing Mirages and Etendards to fly slower, where they are not optimized to maneuver, and beat them in dogfights. Even for the Harrier, VIFFing does not necessarily increase turn rate, but it helps push the enemy aircraft to fly way slower than their optimum. $\endgroup$ – Efe Ballı Nov 23 '20 at 17:35
  • $\begingroup$ @Efe Balli, I will admit that I am not well versed in combat tactics of thrust vectoring aircraft, but your first sentence about deflecting thrust upwards to increase turn rate would only be applicable in a negative G turn. Most fighters have much more positive G available, and it is far more comfortable on the pilot to stay positive. Unless I am missing something... $\endgroup$ – Michael Hall Nov 23 '20 at 18:21
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    $\begingroup$ They deflect thrust upwards to create more nose-up pitching moment, it is not directly related to creating more lift. It reduces net lift as the thrust is negating some lift, but the point is just to keep the aircraft in control post-stall (as in the first answer) or to turn the aircraft further, to bring the guns to bear. In other words, TVC for frontline fighters does not serve to augment max-G turning, it is mainly for slow speed, post-stall control. $\endgroup$ – Efe Ballı Nov 23 '20 at 18:26
  • $\begingroup$ Ok, I get it. You meant tailpipe thrust deflected upwards to pivot the nose. I had in my mind something forward of the CG, like a Harrier... This highlights that the question isn't really clear, and my answer might not be appropriate to what they are asking. $\endgroup$ – Michael Hall Nov 23 '20 at 18:30
  • $\begingroup$ TVC of frontline fighters (which the Harrier is not) works this way, they use actuators on the nozzle for that purpose. The F-35B, the only other VTOL fighter, to my knowledge cannot use the rear nozzle or the LiftFan in maneuvering flight. $\endgroup$ – Efe Ballı Nov 23 '20 at 18:34

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