If the force (pressure) can be transferred/propagated only at the speed of sound, how are supersonic aircraft able to move air out of their way/do work on the air? It seems that they must transfer the force faster than the speed of sound or else they would intersect with the air. Does the speed of sound increase at the shock to allow it?


3 Answers 3


The idea that force can only be transferred/propagated at the speed of sound is a simplification. It's a very reasonable simplification, which works in a very wide array of cases, but it's a simplification.

A fundamental concept in gas mechanics is the "mean free path length." This is how far a molecule can travel on average before it collides with another. This is on the order of hundreds of nanometers for pressures you'd see at reasonable altitudes (68 nm at STP). If you are looking at processes that occur at scales much larger than this, it is very reasonable to model their behaviors as transmitting things at the speed of sound, because these statistical effects lead to a very natural expected speed of propagation. It can be seen as a result of the central limit theorem applied to a very large number of molecular collisions where the drift velocity (average movement of particles) is relatively smooth.

As you push things, this nice model breaks down. As you approach the sound barrier, you get pressure gradients that get shorter and shorter. Near the speed of sound, these gradients start to get into the single-digit-multiples of this mean path length. At this point, we can no longer assume that we can just model everything with the nice clean statistical models we had earlier.

What actually ends up happening in most situations is that you get a "shock." Far from the flying body, the air transmits force at the speed of sound with respect to the unmoving air (or slowly moving, if there's a wind). Right up against the flying body, the air transmits force at the speed of sound with respect to the flying body. At "reasonable" speeds, there's a smooth shift from one to the other, where the pressure gradients aren't so steep that we can't call them pressure gradients.

At supersonic speeds, that smooths shift doesn't happen. Instead, we end up with a thin shock region (on the order of 1 μm or thinner) where it's not reasonable to model the air as having a "pressure." It acts more like a bunch of billiard balls being flung along. This region, a few mean free path lengths wide, is where the usual rules we're used to, break down enough to resolve the physics. On each side of the shock, "normal" speed-of-sound behaviors occur. Inside the shock, it's messy.


It is only the pressure wave that can propagate at the speed of sound. This means that a molecule of "air" that is ahead of a subsonic aircraft can get pushed out of the way without hitting that aircraft. It gets a push from another molecule, which is pushed by chain of molecules until it get to the one which is the one that hit the aircraft. The aircraft only transfers energy to the molecules that it actually hits.

A supersonic aircraft winds up directly knocking all the molecules ahead of it out of its way, in the form of a shock wave. The aircraft knocks them out of the way at whatever speed it is traveling, and the energy transferred to the whole volume of molecules is the source of the drag rise when speeds approach supersonic.

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    $\begingroup$ "these molecules move on their own only at the speed of sound" -- this is not correct. At 25 °C the average speed of a nitrogen molecule is about 515 m/s (calculatiuon here), but the speed of sound is only 346 m/s. $\endgroup$ Commented Sep 24, 2018 at 19:01
  • $\begingroup$ @Henning Makholm When I did 515 * cos(45 degrees) I got the curious number 364. $\endgroup$
    – Joshua
    Commented Sep 24, 2018 at 19:41
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    $\begingroup$ @HenningMakholm There is no such thing as a molecule of air, so this answer is obviously simplified. If Nitrogen could get out of the way of an aircraft at 515m/s then your comment might apply. The mean free path of Nitrogen at sea level is 34nm, so any given molecule does not move at that speed over any significant distance compared to the size of an aircraft. This is like saying that the water molecules in your body are moving at 590m/s as you get hit by a truck going 100km/h (27m/s); it's true but meaningless. In any case I edited the answer. $\endgroup$
    – Pilothead
    Commented Sep 24, 2018 at 20:55

Supersonic aircraft do not push the air out of the way in front of the aircraft. They only push it sideways out of the way after the aircraft has passed the air (for the air beside the aircraft), or at the very moment the air meets the aircraft (for the air directly in front of the very centre of the aircraft, i.e. directly in front of the nose spike). That’s why a shock cone extends from the nose backwards, at an angle. The faster the aircraft, the smaller the angle.

enter image description here

The formula relating mach to the shock cone angle can be found here, at NASA's website.

It's no more complex than that.


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