1

Hypothetically, if aerodynamicists had the opportunity to somehow raise the speed of sound in air to an infinite level so that air was no longer compressed, how would this affect aircraft related aerodynamics? Are there cases a higher speed of sound would be helpful rather than a hindrance?

Would subsonic aircraft designs be able to fly much faster without design changes?

Would we be able to design axial compressors with much higher pressure ratios per stage?

Perhaps much higher thrust could be generated from the same size propeller?

Are there cases where this would not be desirable? Such as spacecraft re-entry for instance?

usernamechecksout
  • 651
  • 4
  • 13
  • 1
    I don't think they reference a Mach speed in outer space. – wbeard52 Jan 27 '24 at 19:40
  • 3
    To "somehow raise the speed of sound in air to an infinite level so that air was no longer compressible" sounds more like WorldBuilding than Aviation to me. – Ralph J Jan 27 '24 at 21:19
  • @RalphJ You’re right, however I feel asking what if questions can increase our understanding. It is taken for granted that air compresses, what if it wasn’t compressible? Would this be a much better world for the aerodynamicist? – usernamechecksout Jan 28 '24 at 15:46
  • @usernamechecksout wouldn't that be water? – Aadiraj Anil Jan 28 '24 at 16:14
  • @AadirajAnil Water is compressible, just less so. Generally the less compressible the medium the less time it takes for “information” to pass through it. – usernamechecksout Jan 28 '24 at 17:24
  • If air was not compressible, nothing could move at all. Solids are not compressible, and we can't move through them. – wbeard52 Jan 28 '24 at 17:37
  • @wbeard52 Air does not need to compress to allow objects to move through it. Air allows objects to pass since it is a gas with no inherent molecular structure and therefore no "resistance". – usernamechecksout Jan 28 '24 at 20:12
  • @usernamechecksout I think you missed the point. If air cannot be compressed any further, it becomes a solid and nothing can get through it. If there was a path for any object to go through the super compressed air, by definition, the air can be compacted / compressed more. – wbeard52 Jan 28 '24 at 20:51
  • @wbeard52 I changed the sentence from "so that air was no longer compressible" to "so that air was no longer compressed". Does this change your argument? In other words, if the speed of sound was raised high enough (through hypothetical means of course), air would no longer be compressed when being decelerated. – usernamechecksout Jan 28 '24 at 21:05

2 Answers2

4

I think there's some misconception, and I hope you correct me if I'm wrong, the title is "infinite Mach", but you're saying you want to raise speed of sound to infinity, this means mach no. is zero, because $M = \frac{v}{a}$.

But why do you think decreasing compressibility is useful in aviation?

However, if I understood your question then the answer is: no, subsonic airplanes won't fly much faster if speed of sound $a=\infty$, actually they won't be able to fly at all. As people reffered out in the comments, a matter with infinite speed of sound is more likely to be a solid rather than a fluid. Of course, solids don't have infinite speeds of sound, but they're considered to be so in the context of studying fluid mechanics, because comparatively, speeds of sound in solids is much much higher than in any normal fluid.

Another way to think about it is: assuming $a=\infty$ might be somehow similar to assuming the velocity of the airplane is zero $v=0$, because both assumptions lead to zero mach no. $M = 0$ which means there's no motion of the body inside the domain that was once considered as a fluid.

أحمد صلاح
  • 51
  • 3
  • 1
    Thank you for your answer. I realize that my title was asking the wrong question. I do mean an infinite speed of sound, not Mach number. I disagree that Mach=0 means no motion of the body. A Mach of 0 would mean there are no compressibility effects, not 0 motion. Mach isn't a measure of velocity. – usernamechecksout Jan 28 '24 at 20:09
  • Welcome to Aviation SE! This answer's good enough to be your hundredth, not your first :-) – Camille Goudeseune Jan 28 '24 at 20:13
  • 1
    @usernamechecksout I agree that mach number is by definition: a measure of compressibility, but may be there's something I don't understand, why does a higher speed of sound mean higher velocity?

    What I know is: I can only play with two things (independent variables in our context): velocity and speed of sound, so if I change the speed of sound of a fluid, this means that mach no. (dependent variable) changed, but it doesn't mean that velocity of airplane changed at all, because it's still independent of speed of sound.

    Again, correct me if I'm wrong

     – أحمد صلاح Jan 29 '24 at 23:26
  • @CamilleGoudeseune you're too generous sir, thank you a lot – أحمد صلاح Jan 29 '24 at 23:30
  • @أحمدصلاح A higher speed of sound wouldn't change velocity. It only changes the ratio of velocity to the speed of sound. The lower the ratio the lower the Mach number, or in other words the compressibility. V could be any value you choose. If you're solving for V of course it wouldn't make sense for Mach to be 0 otherwise infinity*0=undefined velocity. Suggesting the use of infinity may have been a mistake on my part. I really meant "infinite" in more of the common english definition not the mathematical one. I am learning all of this as we go so correct me if I'm wrong. – usernamechecksout Jan 31 '24 at 02:13
  • @CamilleGoudeseune Ok so I agree that increasing speed of sound means decreasing mach no. this is totally right, despite of the velocity still what I don't understand is: why is decreasing mach no. useful anyway, why is a less compressible air better for the airplane? – أحمد صلاح Feb 05 '24 at 21:14
  • Less compressibility improves flight for the reasons listed in sophit's answer's last paragraph. I couldn't say it better than that. – Camille Goudeseune Feb 05 '24 at 22:28
3

Hypothetically, if aerodynamicists had the opportunity to somehow raise the speed of sound in air to an infinite level so that air was no longer compressed, how would this affect aircraft related aerodynamics?

The aerodynamics would just look like incompressible aerodynamics i.e. just like any aerodynamics happening at Mach numbers smaller than 0.3. There wouldn't be any difference in respect to what it is already known about that particular branch of aerodynamics.

If you want to keep the fluid incompressible (Mach number <0.3) but travel anyway faster, you just have to increase the speed of sound either by playing around with the fluid and/or its temperature: this is something normally achieved in "cryogenic wind tunnels".

Obviously getting rid of all the side effects of compressibility (wave drag, shift of the aerodynamic centre, shockwaves,... ) would have, in general, a net positive effect, mainly less drag.

sophit
  • 11,796
  • 1
  • 28
  • 61