My understanding is that if you apply power (increase the thrust) for a jet in level flight, it will momentarily pitch up, and then gain some speed.

So, if a fighter pilot lights their afterburner at a certain time when in a max-G level turn, does this further increase the achievable angle of attack and Gs, due to the increased thrust from afterburners helping to pitch the aircraft up, or does it just allow the max G-force to be sustained for longer?

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    $\begingroup$ I'm having a really hard time understanding this question. Can you edit it so that it is more comprehensible? $\endgroup$ – Jpe61 Sep 21 '19 at 7:45
  • $\begingroup$ My understanding is that if you apply power (increase the thrust) for jet engine aircraft ,(in level flight, and some AOA ~11, it will momentarily pitch up to the ~14AOA, due to the additional power , so if afterburner is open then it will be possible to get the more AOA /G's or just sustain the G's already have it before doing this power increase? $\endgroup$ – George Geo Sep 21 '19 at 10:13
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    $\begingroup$ this did not help at all. is your question if the aircraft exceeds it max G if the afterburners are switched on while turning? $\endgroup$ – Christian Sep 21 '19 at 12:47
  • $\begingroup$ The way I see it it work is that, the increase in AOA with afterburner on , will add drag so will not be "significant"G 's just an number of @0.5G. and keeping it longer than without afterburner.My question, is this is true (possible) ? $\endgroup$ – George Geo Sep 21 '19 at 17:35

Lighting the afterburner will only cause a pitch change and AOA change if the thrust line of the engines does not pass though the Center of Mass of the aircraft. If the thrust line pases above the center mass, increasing thrust will cause a nose-down pitch change, if the thrust line is below the center mass it will cause a nose up pitch change.

But, more significant, is the angle of the aircraft thrust vector with respect to the aircraft's velocity vector. Since you posit that the aircraft is in a high G turn, that means that the thrust line is definitely tilted with respect to the aircraft's velocity vector, so increasing the thrust will increase the force turning the aircraft, by an amount = to the delta in thrust times the sin of the angle between the thrust vector and the flight path (velocity) vector. This might momentarily increase the g-force, but since you posit that the aircraft si already at Maximum G, the pilot would ease off on AOA to let the G-force settle back to the aircraft's maximum G. What the increased thrust would allow, is performing the turn at the aircraft' placard maximum load factor (G) at a lower airspeed than would be possible without the increased thrust...

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    $\begingroup$ That is the way I believed it will the jets what have afterburner benefit from used in the mix of turning fight Although it is possible for the pilot to not unload at all the G's he /she pulled because of the P/S at the time. Thanksfor your time. $\endgroup$ – George Geo Sep 25 '19 at 21:10

TL;DR: Maybe. Maybe not. It depends on the particular fighter jet, and how it's loaded, and the effect will, in any case, be very small.

An increase in thrust causes an airplane to pitch up via two mechanisms (more details in this PDF):1

  • An increase in thrust causes an increase in speed; for airplanes with positive speed stability (which is true for all civil aircraft, but is often not true for newer fighter jets), this causes the airplane to pitch up and slow down.2 This changes the slope of the airplane's flightpath (making it point more up), but (at least at first approximation3) does not change its angle of attack.
  • If the thrust from the airplane's engines is not exactly aligned vertically with the airplane's center of mass, an increase in thrust will create a change in the airplane's pitching moment, which does change the airplane's attack angle; if the engines apply their thrust below the airplane's CoM (examples: 737, B-58), increasing thrust will pitch the airplane up to a higher AoA, whereas, if the engine thrust is applied above the CoM (examples: DC-9, Learjet 23) increasing thrust will pitch the airplane down, decreasing the AoA. Most fighter jets have engines that are very close to the vertical location of the CoM, so the change in pitching moment from a thrust increase is very small, and its exact direction and magnitude will depend strongly on the specific aircraft and loading used (for instance, burning off fuel could change the vertical location of the airplane's center of mass - and, thus, the magnitude, and, possibly, even the direction, of the change in pitching moment caused by a thrust increase).

Thus, going from maximum dry thrust (no afterburners) to maximum thrust (with afterburners) could, depending on the aircraft, produce a slight increase in angle of attack and vertical4 load factor, but:

  • the effect would, at best, be quite small, and would depend strongly on the specifics of the aircraft and its loading;
  • with certain aircraft and/or under certain loading conditions, this effect could well be negative (in which case engaging afterburner would decrease the maximum achievable AoA and vertical load factor); and
  • the main effect would be (as you, in the latter part of your question, surmised could possibly be the case) to allow a high-G manoeuvre to be sustained for longer (due to the increased thrust helping to counter the high drag of a high-attack-angle manoeuvre).

1: For the purposes of this discussion, I am ignoring aircraft with thrust-vectoring capability.

2: Most airplanes will initially enter cyclic phugoid oscillations with anything other than a very slow, gradual thrust change, but the phugoid eventually damps out, and, even during the phugoid cycles, the "constant-attack-angle" part of the situation remains approximately true.

3: In practice, an airplane undergoing phugoid oscillations with a non-infinite period will undergo continual small changes in its attack angle, due to various damping effects, but the AoA does remain approximately constant.

4: Vertical from the aircraft's perspective, that is.

  • $\begingroup$ Increasing thrust increases specific power, but it does not guarantee that Excess Specific Power is positive. Since op posits that the aircraft is at Maximum G, it is quite likely it is operating at or near corner velocity, where excess specific power may be very negative, and the aircraft may (dynamically) be rapidly decelerating. Lighting the afterburner in this case will not cause airspeed to increase, it will only slow the rate of decrease. So the effect on pitch due to airspeed may be completely the opposite. $\endgroup$ – Charles Bretana Sep 25 '19 at 18:48
  • $\begingroup$ Also, by definition, if the aircraft is at maximum G, it is at or above corner velocity, where increases in speed (if maintaining Maximum G), will cause a decrease in AOA and pitch attitude. $\endgroup$ – Charles Bretana Sep 25 '19 at 18:48

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