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I'm told that planes can actually stall when the airflow over the wing goes past Mach 1? Why does this happen and how do you design an aircraft to avoid it?

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  • $\begingroup$ stall happens on high angle of attack at any speed, you avoid it by not pitching up or down in supersonic flight $\endgroup$ – ratchet freak Aug 4 '14 at 23:00
  • $\begingroup$ I believe what you are asking about is the coffin corner. Is this what you are interested in? en.wikipedia.org/wiki/Coffin_corner_%28aviation%29 $\endgroup$ – JerryKur Aug 4 '14 at 23:49
  • $\begingroup$ @ratchetfreak: The supersonic flow separation occurs at any angle of attack. $\endgroup$ – Jan Hudec Aug 5 '14 at 4:59
  • $\begingroup$ @JerryKur: Existence of coffin corner is a consequence of supersonic flow separation, but supersonic flow separation is not limited to it. It occurs on supersonic planes as well. $\endgroup$ – Jan Hudec Aug 5 '14 at 6:16
  • $\begingroup$ A high speed stall is something that happens close to, but still below, Mach 1. It involves supersonic flow over the wing, but the whole airplane still travels at subsonic speed. $\endgroup$ – Peter Kämpf Aug 5 '14 at 9:50
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When the velocity of the airflow locally exceeds speed of sound above the wing a shock wave forms and the flow detaches beyond this shock wave.

supersonic flow separation

Similarly to stall, the supersonic separation of flow removes the component of lift produced by decrease of pressure on the upper surface of the wing and so the effects are similar.

I causes reduction of lift and because centre of pressure is about quarter chord on the upper surface, but midchord on the lower, it causes a significant pitch-down moment, which might be impossible to recover even if the post-stall lift is otherwise sufficient to keep the aircraft flying straight. This effect is often called Mach tuck. Supersonic planes often have all-moving elevators to have sufficient control authority to compensate for it.

A difference from normal stall is that after supersonic flow separation the lift remains proportional to angle of attack and so the aircraft continues to behave more or less normally except for the change in trim.

Mach tuck may occur as low as Mach 0.7 depending on aircraft design, because the air moves faster over the wing. It can be delayed by using swept wings, because the shock waves only form when the air velocity component perpendicular to the wing exceeds speed of sound.

Supersonic aircraft eventually encounter supersonic flow separation, but the lower surface lift is sufficient to balance the aircraft weight at that speed and altitude and the aircraft can continue flying with the upper surface flow separated.

That's not the case with most subsonic aircraft, which for efficiency reasons tend to cruise at altitudes where they have very small margin to stall. As the stall speed increases with altitude while speed of sound slightly decreases with the lower temperature there, the stall speed will eventually equal critical mach number, which creates the coffin corner and absolute ceiling for the aircraft. For most civilian transport aircraft the range between stall speed and critical mach number (where drag increases and supersonic flow separation would begin) reduces considerably at cruise altitude, but they usually don't have enough engine power to reach the actual coffin corner.

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    $\begingroup$ Mach tuck is the phenomenon of trim change due to the rearward march of the center of lift. It can happen in a vertical dive, even when the wing does not need to produce any lift. A stall is always when the desired lift cannot be produced and the aircraft leaves the linear range of the lift curve slope. $\endgroup$ – Peter Kämpf Aug 5 '14 at 8:05
  • $\begingroup$ @PeterKämpf: I reformulated the answer to avoid using the term "stall" for anything related to supersonic flow separation. $\endgroup$ – Jan Hudec Aug 5 '14 at 12:07
  • $\begingroup$ @PeterKämpf A minor point, but mach tuck has nothing to do with trim. It refers to the nose-down pitching moment caused by the aft shift of the center of lift, which gets progressively worse because as the nose drops the speed increases even more, which leads to the C/L shifting further aft, etc, etc. Trim isn't involved (other than for aircraft with Mach Trim systems which are designed to prevent mach tuck from happening). $\endgroup$ – Lnafziger Aug 7 '14 at 4:48
  • $\begingroup$ @Lnafziger: I should have selected a different word. For me, trim is the balance of forces around an axis. For you, trim is the action of changing them, or maybe even the control for doing this. My understanding of the word comes from Naval English, and it seems it does not apply to aviation. $\endgroup$ – Peter Kämpf Aug 7 '14 at 5:31
  • $\begingroup$ @PeterKämpf: Ahh, I understand. To a pilot, trim has a very specific meaning, which is to use the control which moves the trim tab/movable control surface other than with the yoke, which changes the trim of the aircraft. Since that is an action that we perform (we trim the aircraft) it is How most likes will understand that comment. $\endgroup$ – Lnafziger Aug 7 '14 at 10:45
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Aircraft stall when the wing cannot produce enough lift to sustain flight. This can happen for two reasons:

  1. Flow separation due to high angle of attack. The lift curve slope, which is positive and linear at low angles of attack, becomes negative, such that an increase of the angle of attack results in lower lift. This is caused by viscosity effects.
  2. Flow separation due to strong shocks on the wing close to Mach 1. This is caused by Mach effects.

The first is a low speed stall, but it can happen at any speed. High speed stalls are the second variety. They can be provoked by

  1. flying faster at the same attitude, normally in very thin air and at speeds around Mach 0.8.
  2. Or they can be provoked by demanding more lift at the same speed, e.g. by initiating a turn.
  3. A third way is by climbing, such that the air gets thinner and colder, and the wing needs a higher angle of attack to produce the same lift as before.

In all cases, the initial flow over the wing was locally mildly supersonic and produced a weak shock. Either by increasing speed (more precisely: The flight Mach number) or angle of attack, the shock becomes stronger and can cause flow separation, such that the wing produces less lift than before. The aircraft stalls. In case of the U-2, the tail would still work, only the wing would produce less lift, so the plane pitches down and accelerates. Acceleration makes things worse, because now the shocks grow even stronger. Now the pilot is locked into a dive which he cannot end. A lot of fun when a few kilometers below the pilot sees MiG-17s, performing pull-ups from high speed to get close enough to open fire.

Thankfully, the drag increase due to the stronger shocks limits the speed increase, and after dropping for maybe 2km, the air becomes dense enough for the U-2 pilot to successfully stop the dive and start the climb back to the safer, higher altitude.

How to avoid this? Generally, this is impossible to avoid altogether, it can only be mitigated. Wing sweep is the most effective way to limit lift loss due to local shocks, but even a highly swept delta wing will have a reduction of its maximum lift coefficient around Mach 1. An optimized airfoil shape helps to push the limit up, but when this new limit is exceeded, the lift loss will be heavier. In the end, most modern aircraft resort to limiting the flight envelope and using an electronic FCS to ensure that the envelope is not exceeded.

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  • $\begingroup$ Good answer (+1!) but is this actually called a "high speed stall"? $\endgroup$ – Lnafziger Aug 5 '14 at 11:47
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    $\begingroup$ @Lnafziger: If the dynamic pressure is not sufficient, it is a low speed stall. No matter if you pull 8g, the best way out is a speed increase. A high speed stall is when increasing Mach number effects reduce lift below what is required, either by accelerating or by pulling gs with sufficient dynamic pressure. $\endgroup$ – Peter Kämpf Aug 5 '14 at 17:02
  • $\begingroup$ That doesn't actually answer my question.... $\endgroup$ – Lnafziger Aug 5 '14 at 17:06
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    $\begingroup$ @Lnafziger: Yes, this is called a high speed stall. At least to my understanding. Does this answer your question? $\endgroup$ – Peter Kämpf Aug 5 '14 at 17:52
  • $\begingroup$ According to "how", stall does not imply the lift not sufficient to sustain flight, only that the wing is in the flying under conditions where the lift no longer increases with angle of attack. Not sure whether there is any "official" definition though. $\endgroup$ – Jan Hudec Aug 5 '14 at 19:06

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