When an aircraft approaches the speed of sound, this happens:
- Locally, the flow becomes faster than the sound and causes shocks.
The position of these shocks can oscillate, shake the aircraft and even overload the structure locally.
If the high-altitude fin is not movable, it can happen that an interception becomes impossible. From the end of the 1930s, many fighter jets were lost in falls from high altitude.
And most importantly, resistance will increase in such a way that more power will do little to help fly faster.Even the slender Concorde needed afterburners to break through Mach 1.
Why is that?
Imagine that the air flowing around a body flows through a stack of flexible hoses.The walls of the hoses are impenetrable, infinitely thin and follow the local streamlines. When the body approaches at subsonic speed, the air in the pipes near this body makes room for it by acceleration: this reduces the required cross-section and lowers the static pressure, so that the overall pressure remains constant. On the back of the body, the air slows down again and the hoses regain their old cross-section and static pressure, as is the case after the Bernoulli effect.
However, when the speed approaches the speed of sound, the acceleration is additionally increased in density.Nevertheless, the air near the body accelerates, but this does not change the cross-section as much as before, because now this speed increase is associated with a loss of density. The cross-section is still decreasing, but not as strong as before. More hoses need to bend away from the body and the air in them needs to get faster so that the body can squeeze through. More generally, a change in body thickness (more precisely: the second derivation of its cross-section) will affect more hoses, so that their effect does not decrease as quickly as at subsonic speed with increasing distance. The aircraft needs to push more air to the side, which leads to more resistance.
At the speed of sound, the cross-sectional reduction is precisely compensated by the loss of density due to the speed change, so that the same air mass requires more volume and eats up the entire gain from the speed increase.Now there is an air wall that cannot give way as the body approaches. That is the sound barrier. In reality, the speed around this body does not reach the speed of sound at the same station in all tubes, so there are slightly under- and supersonic sections that allow it to push through. Nevertheless, the air resistance is greatly increased and strongly depends on details in the body contour.
At supersonic speed, the density changes more than the speed, so that in order to reduce the cross-section, the air in the pipes becomes slower when it makes room for the body.Since she does not receive any warning for the approaching body, she does so suddenly, in a push. As a result, the cross section of our pipes can now decrease as the density in this slower air increases after the impact. The static pressure also increases, so that the overall pressure can remain constant again.
This thought experiment was explained to the researchers of NACA Langley by Adolf Busemann in 1951.One person in the audience, a young man named Richard Whitcomb, used the insights to formulate the rule a few weeks later.
In supersonic flight everything becomes normal again, but the aircraft must now be retrimmed for the rear shifted buoyancy center.Pilotswho broke through the sound barrier in the constructions of the forties and early fifties all reported that all of a sudden all the shaking stopped and the flying was gentle and calm again as soon as they were in supersonic flight.
What is the “sound wall” and why was it such a challenge?”
Actually, the speed of sound is only the maximum propagation speed of a weak disturbance in a medium and a sound wave is just one example of this.The propagation of a wave is hindered because the “speed of sound” is simply the transition point in a particular medium, where the effect of the density and speed changes cancel out each other.
The whole trick of supersonic flight is that different parts of the aircraft reach the local speed of sound at different flight speeds.The smoothing of the cross-sectional distribution above the flow direction and the use of flat contour gradients, wherever possible, also reduce the maximum resistance at the speed of sound. But you had to learn this slowly and painfully.