Many of you would remember the Iconic scene from the 2013 movie Man of Steel, where Henry Cavill’s Superman Character is learning to Fly and and is shown to eventually break the sound barrier.
It should be noted that flight characteristics change dramatically as an object goes from subsonic speed i.e. slower than sound speeds to supersonic speed. So let’s explore that.
You throw a stone in a pond and a ripple is created that propagates in all directions. Imagine now that a stone or any object is moving in a particular direction in water. As it moves and speed increases, the object will get closer to the ripple in front of it and further away from the ripples behind it.
The ripple has its own speed. This is equivalent to the speed of sound in that medium. Now think about this. Is it possible that the object moves fast enough to outspeed the ripple it created? Well In water that is harder to do so, because the speed of sound in water is very high (about 1500 m/s). Now let’s consider air.
The air around us also acts like water in that any disturbance in it also creates invisible ripples.
It is easier to outspeed the ripples in air because the speed of sound in air is relatively slower that is 343 m/s. When an object outspeeds the ripple it created, this results in a bang like sound. And is because of creation of shock waves. This sound is called the sonic boom. Interestingly the crack of the whip is also a sonic boom at a small scale. When the whip is given a jolt, the ripple created increases in speed as it reaches the end. The end of the whip can move 30 times faster that the initial movement. When the end of the whip reaches a speed faster than sound, the cracking sound is produced. Likewise, when a bullet is fired it also creates a mini sonic boom.
Let’s learn how flight dynamics change with speed. We all know that when you pass a air through a nozzle it tends to speed up. If we pass it through a diffuser it tends to slow down. If however the flow entering the nozzle is supersonic, instead of further speeding up the flow, the nozzle actually slows it down. On the other hand, let’s consider a diffuser. If the flow at the inlet is supersonic, its speed would further increase as it comes out of the diffuser. That is the reason diffusers not nozzles are seen at the back of a space shuttle.
This is just one aspect of the change in flight dynamics. If you look at the design of aircraft that can go at supersonic speeds, you will note that they are shaped differently to aircraft that fly relatively slower. Supersonic aircraft are sleeker. Even the aerofoils, i.e the shape that make the cross section of the wings, are flatter. When you compare this to the cross section of a subsonic aircraft and you will notice that their wings have a more thicker profile.
An ideal aircraft should be therefore able to change its shape as it speeds up from subsonic to supersonic. This has been achieved to some extent through variable sweep wings and adjustable nozzle shape in fighter jets like F 14 Tomcat , Panavia Tornado and the Mig 27.
These aircraft can sweep their wings back after reaching supersonic speeds. Some variable sweep wing aircraft were also produced by Russia such as Sukhoi Su 17 and Su 24. , but they also adopted another approach to improve flight dynamics at supersonic speeds. They brought out the forward swept aircraft. Forward Swept wing gives excellent agility and manoeuvrability to the aircraft even at high speeds. The Sukhoi Su-47 is an example of that.
This brings us to a fun question for you guys. Do you think Superman’s posture should be arms close to the body (or in the after swept position ) as he is flying through the air or should they be extended forward? i.e. is the forward swept position. Research about this and let us know. Best answer would be pinned at the top in the comment section.
On a side note, the tip speed on the wind turbines can also reach close to speed of sound. It should be remembered the the linear velocity of blades may not be very much but at the tip of they blade linear velocity is very high. The tip speed therefore is a limiting factor for the rotational speed of the turbine and Large wind turbines are deliberately made to rotate slowly.