Why are aircrafts more stable traveling faster than sound than at the speed of sound?
In: Physics
If you go faster than speed of sound, you are cutting through the air like a knife.
If you go slower than the speed of sound, air flows around the plane smoothly.
If you go just at the speed of sound, you can neither cut like a clean knife nor flow around the plane. You are on the edge of chaos. Physics is confused. It’s like trying to slowly cut through the air like a knife, but having the knife get stuck.
Imagine a wave in the ocean. It’s smooth behind the wave and in front of the wave. But it’s not smooth _on_ the wave.
A plane compresses the air in front of it. As it approaches Mach 1 it creates a shockwave right at the nose and it sits there. If you go faster the shockwave moves towards the rear of the plane.
This shockwave is what creates a sonic boom.
Air waves act a lot like water waves.
When on a boat, moving forward constantly creates waves. If the boat moves as fast (or just a little slower) as the waves it is creating, they will build up in front of it. Eventually the built up waves will effectively be an uphill swim for the boat, slowing it down and destabilizing the boat’s speed. It is much easier to just go a little faster, and now the build up of waves happens behind the boat.
Aircraft is almost the same, but in 3 dimensions and higher speed.
If everything is subsonic, designed properly, it’s all stable. If everything is supersonic, and you designed properly (which is different than subsonic), it’s all stable.
While you’re in the middle though, you have portions that are subsonic and portions that are supersonic, which means you have shockwaves all over the place that tend to cause flow to separate and reattach as the shockwaves move around, which causes loads on the vehicle to constantly shift, leading to instability. If the lighting is right you can actually see the shockwave on the upper aft surface of the wing on some commercial jets moving around, but it’s so localized and designed for that it’s a non-issue.
when the plane is moving subsonic, the energy the plane is putting into the air (by slamming into it) is thrown out ahead of the plane. as the plane gets closer and closer to the speed of sound, this is still being thrown forward, but it cant get as far from the plane. so it stacks up. getting to and then beyond the speed of sound means pushing through that wall of energy the airplane itself has created. once it gets past that, the airplane is moving so fast that the energy can’t pile up like that.
this is further complicated by the fact that air doesn’t move over the plane at 1 speed. when you’re near the speed of sound, some of the air behaves like you’re moving supersonic and some of it doesn’t. where these regions are and how they behave is very chaotic.
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