An F22 has a higher thrust to weight ratio. The F22 has around ~70,000lbf thrust from its F119 engines, where as the F15 has around ~48,000lbf thrust from its F100 engines.

But every metric I can find indicates that the top speed of the F15 is higher than the top speed of the F22 (though I have not found a concrete reliable source on either).

How is this the case?

  • 2
    $\begingroup$ With stealth, the goal is to shoot before the other plane realizes you are there. If you actually get into a dog fight, you've already made several mistakes. $\endgroup$ – zeta-band Sep 19 '19 at 20:22
  • 3
    $\begingroup$ I am aware of this fact and not sure how it at all relates to the question of speed. $\endgroup$ – ScottF Sep 19 '19 at 20:23
  • 1
    $\begingroup$ Higher thrust to weight ratio mainly mean better acceleration and climbing rate, not higher top speed. $\endgroup$ – Manu H May 13 '20 at 8:05
  • $\begingroup$ @ScottF Before stealth, the enemy could see you coming, so you had to get there, strike and get away before they could react. If they can’t see you, though, you can conserve fuel (and extend range or increase payload) by flying slower. $\endgroup$ – StephenS Aug 11 '20 at 2:41

Aerodynamic heating will damage or destroy the composite wing of the F-22, especially around the leading edge where compression heating is highest. See here for the temperatures which supersonic flight causes.

Therefore, the F-22 has been restricted to Mach 1.8 for short duration and Mach 1.6 for prolonged flight. More speed will not be needed, anyway.

The internal weapons storage of the F-22 makes it bulkier than the F-15 which causes more wave drag. Also, the engine intakes were not designed to operate at speeds above Mach 2, so pressure recovery in them drops at higher speed. These two factors will affect the top speed of the F-22 negatively, so I would not be surprised if the F-15 is faster even without restrictions on the F-22 speed.

  • $\begingroup$ If I understand correctly, wave drag is the direct answer to the question. The first part (no need to go faster) explains why 70.000 lbf is sufficient $\endgroup$ – MSalters Sep 20 '19 at 11:56
  • $\begingroup$ @MSalters: I have no drag polars of both, but argue from first principles. I think that the intakes have even more to do with the lower speed. And temperature is still an issue. $\endgroup$ – Peter Kämpf Sep 20 '19 at 12:39
  • $\begingroup$ Is the plane hard limited to mach 1.8, or could a pilot override that in certain scenarios? $\endgroup$ – ScottF Sep 20 '19 at 15:11
  • 1
    $\begingroup$ @ScottF: Hard limiting the top speed of an aircraft is impossible. Even if the engines are automatically cut to idle, a simple dive lets it accelerate again. Therefore, an override is possible. $\endgroup$ – Peter Kämpf Sep 21 '19 at 22:51
  • $\begingroup$ @Peter Kämpf: But if the hard-limited top speed is already faster than the terminal velocity of the aircraft, then cutting to idle and diving would not cause acceleration, right? From Mach 1.8, wouldn't cutting to idle cause a deceleration of the aircraft, regardless of pitch or orientation? $\endgroup$ – Aaron Holmes Aug 11 '20 at 1:21

The F-15 was specified to have a maximum speed of Mach 2.5. However, I believe this was reached only once during Developmental Test and Evaluation and they had to change the engines afterwards. On the production models, there was an overheat switch in order to get to this speed, but the A model only had 11,300 lbs of gas so having this capability was a joke. And the switch had a wire the pilot had to break to throw it. All supersonic aircraft with the exception of the SR-71, the F-111 and Soviet MiG-25 did not have enough gas to make top speed usable. People forget the three main parts of fighter aircraft performance, thrust to weight ratio, wing loading, but equally important is its fuel fraction. Most jet fighters have fuel fractions between 25% to 30%, that is, at takeoff gross weight you divide that into how much internal fuel the fighter can carry and you get the fuel fraction. Why is this important? Well the fuel fraction tells you how far you can fly or stay in air combat and return. In the case of the F-111 it had a fuel fraction of 38% so it could stay supersonic for awhile. The surprising thing about the MiG-25 is that its fuel fraction is only 26% while the SR-71 fuel fraction is 56%. The MiG-31 has a fuel fraction of 47%. The other part of this is how fast the engine burns fuel measured in Thrust specific fuel consumption, that is,how much fuel does the engine need to produce the required thrust for a given altitude and speed condtion.


Not the answer you're looking for? Browse other questions tagged or ask your own question.