0
$\begingroup$

As a thought experiment to try to illustrate my question below:

If a fan was moving forward at the speed of sound but was rotating such that the low pressure created by the fan's rotation was equal to the high pressure from the impact of the fan moving through the air, would they in any way cancel out? I have been learning about sonic booms lately and am wondering. Why forward airplane air intake (ie compressors that pull in a large volume of air such as a jet engine does) are not used to create a low pressure zone in front of the aircraft?

I understand that it is the pressure change (N wave) that is the problem with the boom, a sudden up/down of pressure too close together. And it seems the boom itself is a result of a high pressure zone off the front of the craft and the main way scientist are trying to mitigate this effect is to "spread out the pressure change" across a longer smoother bottom of the craft.

Perhaps the amount or velocity of air one would have to intake, or energy required make this an impossibility, but I would love to know which part of physics is responsible for this not working. Or perhaps the shapes edges will still create enough boom that you cannot remove. Perhaps the turbulence or pressure now created behind the plane will make a sonic boom. Or many other things.

Any thoughts are appreciated!

$\endgroup$

1 Answer 1

0
$\begingroup$

Fans do not create a low pressure area ahead of them in flight. Only in static conditions is such a low pressure created and needed, so air will start to move towards the fan.

Once air is rushing into the direction of the fan, it will not suck air in but reject some. The drag created by this even has its own name: Spillage drag. All the fan does is to add some more pressure to air which already has been compressed by slowing down ahead of the fan. This slowing down is the task of the intake: The Mach number at the fan of the Concorde engines was only Mach 0.38 in supersonic flight. At cruise, the intake would lift the pressure of the air at the compressor face by a factor of more than six over ambient by efficiently decelerating the flow. The compressor added a compression ratio of 12, so the pressure in the combustion chamber was 80 times higher than ambient.

Shocks are the result of air being pushed away by the approaching aircraft. To mitigate shocks, it is best to make this pushing as gentle as possible. Sucking away the air would need to compress it at the same time (so the volume can be reduced), and the energy needed for this would make such a concept extremely inefficient.

$\endgroup$
1
  • $\begingroup$ Thanks for the explainer Peter. That is really fascinating that the air intake of the Concord in supersonic flight was just Mach 0.38, a far cry from Mach 1. $\endgroup$
    – RC Stuff
    Aug 20, 2022 at 11:26

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.