Just curious 🙂
In: Physics
Sound moves at a fixed speed, much much slower than light. That’s why you hear the crack of thunder after you see the flash of lightning.
When you have a plane trying to cross from flying slower than sound to flying faster than sound, it experiences stress from the air “piling up” just in front of it, as the sound of an airplane is a compression wave in the air. Many planes were destroyed figuring out how to do this, thus the term “sound barrier”. It turns out that only some shapes of planes can go through this transition without being damaged.
It isnt really a “Barrier.” Its simply a way to refer to the actual speed that sound travels at. By saying you have “Broken the sound barrier” you are really just saying that you are now moving faster than sound travels
Shock waves coming off the leading edges of control surfaces and wings start to make the way they work completely different as an airplane gets close to the speed of sound. WWII fighter jets had a problems with flight controls as they neared the speed of sound in a dive, and many crashed due to it. Suddenly, the ailerons wouldn’t work because pressures against the surfaces were stronger than the pilot, or the wing would flex opposite them, rendering them ineffective. This was a “barrier” that conventional aircraft design couldn’t pass. A complete redesign of aircraft finally allowed them to push past this barrier. Pointy noses, thin wings with sharp edges, and rear control surfaces that moved as a whole were among the necessary changes.
Sounds travels as a vibration through the air. It is basically each molecules bumping into the next molecule, which then bumps the next one, etc etc. Because molecules are a certain distance apart from each other, this takes time to move along. The speed of sound is the speed at which this wave moves.
Going faster than this speed (e.g. in a fighter jet) is commonly known as breaking the sound barrier. It’s kind of known as a barrier because as you approach that speed, the air bunches up and causes more drag so the plane needs a lot of thrust to get through that (this is often what afterburners do – they provide that extra push to ‘get through’ the barrier). Think like pushing your finger against some saran wrap – while it’s slack you get no resistance but as it pulls taut you get some resistance until you push hard enough to break through. (this is a gross simplification but enough for ELI5)
You may notice that the speed of sound changes at different temperatures or altitudes. That is because under different conditions, molecules are closer or further apart, meaning the wave moves faster or slower.
Imagine a pond, if you drop a rock into it, a wave spreads out at a certain speed. Sound moves through the air in a similar way.
Say instead of dropping a rock, you pull a stick over the surface, you can move the stick as fast, or faster than the speed of the waves. You’ll notice that the waves bunch up and form a wake behind the stick. The same thing happens with a supersonic plane.
As a plane goes faster than the speed of sound, it begins to move through the air faster than the pressure waves it produces, this actually changes the way the air behaves around the wings and the engine of the plane, this means to make a supersonic plane, you can’t just strap a more powerful engine to a plane and make it fly supersonic, you have to design the plane in a special way. This is why figher jets have that triangle shape, instead of the straight wings on slower aircraft. This is what was referred to as the “sound barrier”.
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