Super Sonic Flow and the Modern Airliner
F/A-18C Super Sonic at 100 feet, a great video. It happens fast, you can see the shock wave as a ball of vapor as the Hornet flashes by. Let’s take a closer look.
In this picture you can see the classic shock wave caused by compression of the air. Due to humidity and pressure change the vapor cloud presents a fantastic visual. The first super sonic fighters had a hard time punching through the number due to drag from the wing in the tran-sonic region, .92-.96 Mach for the F-4 Phantom II. It is called Critical Mach and varies aircraft to aircraft. Basically, due to flow dynamics, parts of the aircraft go super-sonic first. It creates a lot of added drag which prevents the entire aircraft from punching through the number into the super-sonic region. Because the wing is producing lift, it creates the biggest wave drag. So do things like the canopy, notice the separate shock wave behind it in the picture above.
Just a bit of techie stuff, the Area Rule in a nut shell:
The area rule says that two airplanes with the same longitudinal cross-sectional area distribution have the same wave drag, independent of how the area is distributed laterally (i.e. in the fuselage or in the wing). Furthermore, to avoid the formation of strong shock waves, this total area distribution must be smooth. As a result, aircraft have to be carefully arranged so that at the location of the wing, the fuselage is narrowed or “waisted”, so that the total area doesn’t change much. Similar but less pronounced fuselage waisting is used at the location of a bubble canopy and perhaps the tail surfaces.
So what does that mean? It means you can arrange the fuselage and wing to minimize the area disruption, drag, and punch the aircraft through the number with less power. Or delay the Critical Mach number, to cruise faster with less power. Look at the fuselage where the wing is joined. By indenting the fuselage, giving it a coke bottle shape, the area is greatly reduced.
Delaying critical Mach also delays creation of a sonic boom. Sonic Boom? That is the thunder clap you hear when the shock wave reaches the ground. Look closely at the picture below.
Note the ocean surface, the disturbance on the water is caused by the shock wave. The closer you are the louder it is. Again, also aircraft dependent. The loudest Sonic Boom ever recorded I can proudly say was an F-4 at 100 feet. Why? The older aircraft bludgeoned the air into submission. Modern fighters with computer designed fuselages don’t have to. Note the smaller wing and no visible “coke-bottling”.
So what does all this have to do with modern airliners? Simple, the airlines want the fastest Mach Cruise for the least amount of fuel burn. They want a high Critical Mach number to get to their destinations quicker. It is not just fuel that is saved; every minute flown is a cost in maintenance, crew costs, etc. Going from a cruise Mach of .78 to .84, for the same fuel burn is a huge savings.
Recognize the shape of the fuselage where the wing joins on this Airbus 380?
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