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So I was wondering if speed affects the pressure distribution at the wingtip. The second comment on this answer says (if I interpret it correctly) that speed doesn't change the pressure distribution at the wingtip, but why is that?

My thinking is that if you go faster, the air outside of the tip wouldn't have as much time to react to the low pressure zone at the tip because it's going faster. Kind of like in orbital mechanics, if you go faster by a planet your orbital line doesn't change as much because you're going faster (less time under acceleration). If the air doesn't have as much time to react, then wouldn't less air be bent towards the wingtip, affecting the pressure (and lift) distribution?

Thanks!

Wyatt
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1 Answers1

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Small pressure changes travel at the speed of sound. In sufficiently subsonic flow the pressure distribution at different speeds is self-similar (same shape, different magnitude). Only when tip flows become transsonic will you see a marked change in pressure distribution. This change will become fundamental once the speed component orthogonal to the wing's leading edge becomes supersonic.

See it this way: The faster-moving wingtip will produce pressure variations of greater magnitude, so the air will experience more force over a shorter time. Both effects cancel each other and the flow pattern stays the same. Your gravity analogy would need increasing gravity with higher orbital speed to be suitable here.

Peter Kämpf
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  • ah that makes sense, thanks a lot. One small question : when the wing reaches transsonic speeds, what would make the pressure distribution change? (I'm assuming the transsonic speeds you're talking about is around mach 0.6, because of other answer I read) – Wyatt Dec 12 '23 at 00:09
  • @Wyatt Yes, the transsonic regime starts strictly at Mach 0.3, but the effects only become noticeable at around Mach 0.5 to 0.7, depending on local pressure. Please read this answer to get an idea what changes. Essentially, disturbances will reach further sideways and once local speed exceeds the speed of sound, it will accelerate further where in subsonic flow it would decelerate. Local shocks will do that deceleration eventually. – Peter Kämpf Dec 12 '23 at 12:23
  • ah, okay thanks. I read the answer you linked and it clarified some things. One last question though : What makes disturbances reach further sideways with higher speeds? My thinking was that lower density air (from higher speed) would prevent disturbances from reaching as far. (I did read the tube analogy in the linked answer) – Wyatt Dec 13 '23 at 00:47
  • @Wyatt Since density drops at higher subsonic speeds, more tubes need to be involved at making way for the object. Each tube contracts less, so more of them need to accelerate their flow for the needed contraction. – Peter Kämpf Dec 13 '23 at 07:20
  • ah okay that clarifies things, thanks. I have another question posted here (related to this question) and would greatly appreciate your insights on that as well, if you don't mind. Thanks in advance! – Wyatt Dec 13 '23 at 20:29