Throughout my schooling this year, one thing that came up was that materials have a speed of sound. That concept makes sense to me. To my understanding it's the speed at which force can propagate through a material. My question is what happens when you exceed that speed while traveling through air say on a hypersonic plane or similar. Specifically, when the equivalent speed of sound in a material exposed to high airspeeds, is passed by fast airspeeds. I've attached a list of materials here and it can be seen that many directly exposed aircraft materials, would likely not have ever reached the speed in the atmosphere since many are at or above Mach 20. My intuition says that the air would rip apart the material because it would be unable to keep up with the force propagation. However, this seems wrong. What is the actual answer?


1 Answer 1


The speed of sound tells us how fast a changing force will travel through something: materials with high speed of sound will respond faster to external forces than materials with low speed of sound.

In an aircraft body there will be various forces as it encounters turbulence. Since solid objects generally respond faster than air to forces the situation is exactly the opposite from your intuition: they can propagate forces faster.

The body will push back on the air due to its inertia and stiffness, producing a fair bit of force that could lead to damage. The reason it doesn't rip has more to do with the stiffness and other material properties (that indirectly set the speed of sound in the material) than the speed of sound itself. But a solid with a low speed of sound would have a harder time distributing changing forces than a fast body, likely ending up with more focused forces damaging it. And stiff bodies have high speed of sound.


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