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What causes the vapor cone of an aircraft going M ≤ 1? I know that the vapor is from the air expanding, but why is it in the cone shape? What shock system forms it into that shape?


enter image description here

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    $\begingroup$ There is no vapor cone at Mach > 1. Condensation cones (for airplanes) or condensation collars (for rockets) only happen at transsonic speed, a speed near but still below Mach = 1. $\endgroup$ Mar 17 at 15:24
  • $\begingroup$ Your original version was correct. This is a subsonic aircraft. M<1. $\endgroup$ Mar 17 at 18:28
  • $\begingroup$ @RobMcDonald oh right, fixed. $\endgroup$
    – Wyatt
    Mar 17 at 19:18

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You should check out this reference. It goes into great detail of exactly what is going on in photos like this.

The flow is accelerating as it goes around the aircraft. As it accelerates, the pressure drops and the vapor cloud can form. In this case, the flow accelerates to supersonic speed and then is shocked back down. The shock forms an abrupt pressure rise which causes the termination of the vapor cloud.

The flow does not have to reach supersonic flow or have a local shock for you to see a cloud like this -- but the abrupt termination of this cloud certainly looks like this one is terminated by a shock.

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