# Am I correct with my thoughts of supersonic vs. subsonic flow?

So I'll just list a few things that I thought were correct. If there's anything I'm wrong about, I'd appreciate if someone could explain where I went wrong.

In supersonic (vs. subsonic) flow, the lift distribution changes because shocks form on the wing.

In places where supersonic flow would accelerate, subsonic flow will decelerate. This is due to the supersonic flow going faster over the wing. Probably sounds like a bad explanation, but I was going off of this answer.

Those probably seem very vague, but I just wanted to see if the basic principles I thought of were correct.

• "This is due to the supersonic flow going faster over the wing" faster than what? Compression shocks slow down the flow, so the reverse sounds more correct. Dec 23, 2023 at 20:18

If you’re solely concerned about wing and lift at SS, here’s a past stack answer:

In pure subsonic flow the pressure distribution is the combination of the flow around a body, where air has to speed up locally to make way for that body, and the flow around an inclined flat plate. Things will become more complicated if the rear end of the body is blunt and causes the flow to separate locally.

If you raise the Mach number, density changes will become greater and will require larger speed changes to let air flow around the body. Please remember the tube explanation from this answer to explain what happens in detail.

Things become really different once the local flow speed becomes supersonic. When before a contraction in the body would cause the flow to slow down, it will now accelerate further. This will lead to a local drop in pressure when before, at subsonic speed, pressure would increase. Eventually, this supersonic pocket will collapse in a shock, so speed drops to subsonic while pressure rises immediately. Below I copied an example of typical speed distributions from US patent 3,952,971 by Richard Whitcomb.

Speed and pressure distributions are very similar, only that pressure changes with the square of speed.

This extension of the low pressure area on the wing will shift the center of pressure backwards. If you increase flight speed further, the supersonic area will increase until at Mach 1 it will extend over the whole length of the airfoil.

On a straight wing this will happen the same way over wingspan. Only on swept wings will sweep effects make root and tip pressure distribution look different. But what happens can all be explained by the same basic principles:

-Low pressure areas suck in more air, so they attract the flow.

-Conversely, high pressure areas will divert the flow.

-At subsonic speed air needs to speed up to make way for the airplane.

-At supersonic speed air needs to slow down to make way for the airplane.

• Welcome Nick. If you are quoting someone else's writing, please add complete attribution. Jan 7 at 12:40

Lift doesn’t change significantly. Drag does though. The complicated part of supersonic flight is not wing design, but rather controlling the speed at which the air enters the engine/intakes, which is why most SS aircraft feature variable intake angles.

• Welcome Nick. Although "complicated" is subjective, optimal wing design for supersonic vs. subsonic flight is quite different. Jan 7 at 12:39