Explanations of shockwave for the common folks (youtube videos, googling) all tend to focus on successive sound waves generated by the air craft traveling outward in circles (sphere).

That to me, only explains why the boundary of a sockwave is a straight line (cone). It doesn't really explain:
[1] why the sudden pressure or velocity change of the air after the boundary.
[2] I can't use that to explain the shockwave on wing surface.

I have a feeling that shockwave on wing is not so much related to noises made by the plane, but it is related to solid cutting into liquid forcing the liquid into some behaviors, that in turn acts back onto the surface.

What I think I understand: sound wave being a planar wave of pressure, bernoulli effect and lift.

so on those foundations, could some one explain how is sockwave formed, and why is it parpendicular to the wing surface.

if possible:
I prefer microscopic understandings, as in layers of particles, starting from the layer touching the surface of the wing and explain our way layer by layer away from the surface.


1 Answer 1


Here is a slightly different way to think about this, which I hope will help. First imagine a wing traveling through air at subsonic speed. Our point of view is attached to the wingtip and looking straight down the leading edge, so what we see is parcels of air flowing towards the leading edge and splitting into two streams- one goes up and over the top surface and the other goes down and under the bottom surface.

those parcels of fluid air must pick up a velocity perpendicular to their original direction of travel so they can get out of the way of the leading edge of the wing. Now the fastest speed any parcel of fluid can assume here by itself is the speed of sound, but since the wing is subsonic the parcels of incoming air can easily flow up or down as required to get out of the way of the advancing wing. the velocity vector of any parcel then is the vector sum of its incoming velocity relative to the wing and this sideways component. New parcels of incoming air can feel the sideways velocity component of the parcels already in contact with the leading edge and so the splitting process actually begins slightly ahead of the leading edge.

All this changes when the wing approaches supersonic. There's less time available for setting the incoming air into sideways motion ahead of the leading edge and so the incoming air begins getting squeezed between the wing's leading edge and the parcels of incoming air behind it. The splitting point moves closer to the leading edge as this compression effect takes hold.

When the leading edge goes supersonic, the presence of the advancing wing can no longer be communicated in advance of its arrival and the air just ahead of the leading edge has no way to get started moving sideways before it smacks into it, and all the while, new parcels of incoming air are similarly piling up just ahead of the leading edge and getting compressed too.

A thin wall of compressed air then builds up sideways at the tip of the leading edge and gets forced to move forward by the leading edge into the incoming air, which cannot get out of its way.

That thin wall is the shock wave.

  • $\begingroup$ Thank you, I think this is a very reassuring microscopic explanation of the shockwave on the tip of a super sonic object. However, what about the shockwave on the surface of the wing during subsnoic flight? $\endgroup$
    – eliu
    Sep 9, 2018 at 15:12
  • $\begingroup$ your explanation caused me to realize my lack in other area of knowledge: When the surface of a plane squeeze/impact an air percel, how is sound wave generated? When we speak, violin strings vib, cords are pushing the air molecules back and forth, every cycle of push and retract cause the planar wave to form and propagate. How does a moving object cause sound wave? Imagine a perfect infinite flat object travel in constant speed (flat face facing the fluid) into perfect fluid in subsonic speed, will there be wave propagating out? but, what will be the frequency? $\endgroup$
    – eliu
    Sep 9, 2018 at 16:01
  • $\begingroup$ I recommend an introductory physics text on sound. a very good one is by Rossing, Moore and Wheeler, "The Science Of Sound" 3rd edition. it contains answers to all your questions. $\endgroup$ Sep 9, 2018 at 18:17
  • $\begingroup$ a shock wave occurs in air anytime the local flow velocity exceeds the speed of sound. since parcels of air must accelerate as they flow over the top of a wing, there will come a point where the flow over the top of the wing goes supersonic even though the flow over the rest of the wing is not. $\endgroup$ Sep 9, 2018 at 18:19
  • $\begingroup$ Thank you, I will check out the book. However, during subsonic flight, the wing upper surface's shockwave boundary does not form AT the point of supersonic flow, but AFTER the point of supersonic flow. There is always a supersonic flow region before the shockwave boundary. $\endgroup$
    – eliu
    Sep 9, 2018 at 20:06

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