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Why not faster than speed of sound?

  1. Jun 9, 2007 #1
    In my sophomore mechatronics class, the professor mentioned that when we design rotating devices (i.e. a fan), we should make sure the outside edges don't move faster than the speed of sound. I always wanted to ask him why but never got a chance. Since then the question has been bothering me and it comes back to me every now and then, I'm finally posting it! :biggrin:. What's wrong with parts moving faster than the speed of sound in a machine?
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  3. Jun 9, 2007 #2

    D H

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    A shock wave will form if the edges of the fan exceed the speed of sound. This shock will do two things: it will drastically reduce the efficiciency of the fan and it might well tear the machine apart.
  4. Jun 9, 2007 #3


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    Many machines have parts that travel faster than the speed of sound. The tips of the prop on a typical crop duster will break the sound barrier. Also, the tips of the turbine in turbo chargers as well as gas turbines travel much faster than the speed of sound.
  5. Jun 10, 2007 #4
    I think that for helicopters it is one central factor that limits the speed of the helicopter, that is the forward moving tip of the rotor blade (The left blade tip if the helicopter rotor rotates clock wise) that can not move faster than the sound.

    "Things" can move faster than sound, but then they will need to have a supersonic design. Subsonic and supersonic design is quite different. Often flight controls designed for subsonic flights will work in unexpected ways if they reach supersonic speed. I think I read somthing about Speedfire's during the second word that were able to reach the speed of sound when diving down from high altitude.

    When it comes to fighter airplaines and supersonic passenger aircrafts I believe it is certainly not true that the jet engine can have a supersonic speed in the airflow into the engine.

    I think all supersonic fighther airplanes use acombinition of first a supersonic "speed breaker" that will set up a shoch wave in front of the engine in such a way that the airspeed is braked down to approx transsonic speed before entering the air intake.

    Supersonic aircrafts is easy recognized because of the design of the "supersonic speedbreaker" they use to have.

    After the airspeed first is redused trough this "supersonic speedbraker" then the speed will be reduced further down due to the shape and geometry of the airduct/airintake. When the airstream enters the engine it has a speed well below the speed of sound, also when the aircraft is flying supersonic.

    What limits the speed of an ordinary jetliner is also the speed of sound. They can not fly at the speed of sound but slightely under. Something like Mac 0.85-0.90 or arount that. The reason for that is the acceleration of the upper side of the wing to produce lift. Neither this can reach the speed of sound as this will make bad things to happen. (Wibrations, shocwaves, etc.)

    I cant say for sure if the speed inside a jet engine is under the speed of sound, but I will belive so. One should remember that "the speed of sound" is not one exact value, but it is, as far as I remember a function of pressure and temperature. (Or was it only the temperature, I'm not sure.

    The way an afterburner of an figher airplane work (or another supersonic aircraft) is that the temperature in the afterburner increases and the airspeed increases. Because of this the airspeed inside the afterburner will not reach the speed of sound before it reaches the final exaust nozzels.

    Some supersonic aircraft har variable geomertry that forms a supersonic shape at the exaust nozzele while flying supersonic, but I believe some aircraft designed to fly at "slow" supersonic speeds has just a subsonic/transsonic exaust nozzle. (Well most of them, I think has "a touch" of a supersonic disign.)

    When you see the airintake, the ehaust nozzle, and the wings of an aircraft, It is normally rather easy to say approx which speed it is built for.

    All this it because of the strange things that happen with aerodynamics at the speed of sound.

    By the way, I believe most aircraft designers and pilots does not like the transsonic speed area, because everything is rater unstable. It is to fly subsonic to save fuel, and then to cross the limit of speed of sound as fast as possible to enter a more stable condition of supersonic flight.
  6. Jun 10, 2007 #5
    Look at the air intake at the SR71, this is a "classical" supersonic design.
    http://selair.selkirk.bc.ca/aerodynamics1/High-Speed/Page7.html [Broken]
    Last edited by a moderator: May 2, 2017
  7. Jun 11, 2007 #6


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    There's nothing classic about it. I don't know of any other aircraft to date that use the movable spike set up.

    It is a design consideration. You can have components or parts of components going above M1, but you need to design for it. What works for one does not necessarily work for the other and hence you need to make the decision what regime you are going to be ina and accept the pros and cons.
    Last edited: Jun 11, 2007
  8. Jun 11, 2007 #7
    The physical "look" compared with other aircrafts might be different, but the aerodynamical prinsiple that it maintain is the same as many other aircrafts, F15 and F16 as an example.

    (Some people believe that the F-16 does not have it, because it looks like it is not there, but from an aerodynamic point of wiev it is there, allmost invisible, but when it vomes to functionality, it's still there.)

    Supersonic air inlet might be based on fixt or variable geometry, but aerodynamically it works basically the same way when flying supersonic.

    Even though the SR71 and the F-16 air intake looks copletely different, they basically works the same way, when flying supersonic.

    The difference is that the SR71 is an old model that has a physical design where it is qute easy to se how the supersonic design of the air intake is working. That's the reason I call it "classical".

    When it comes to F-16, I think it is not that easy at all to see that this also is an supersonic air ainlet, working according to the same prinsipples.
    Last edited: Jun 11, 2007
  9. Jun 11, 2007 #8


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    You said "design." That is a one of a kind design. It has not been replicated. It's purpose has been used many times, but not it's design. A variable ramp is not the same thing. I am well aware of how the usual intake design is implemented.
  10. Jun 11, 2007 #9
    OK .. then .. I't just a question of the correct use of english words :-)

    Possibly "Tecnical implemantation of a physical prinsiples" would be bether word than design. I guess so. (English is not my language.)

    But the way a F-16 air intake works, it works simularly like the SR71 even though they looks completely different ?!

    I thought that if they are doing the implementation of the identical same aerodynamical prinsiples, even though they look different, the would be "of the same design", like you can say a four wheel car is one "design" and a two wheel motorbike is one other "design".
  11. Jun 15, 2007 #10
    Even in SS aircraft the airflow is brought back to subsonic speed before it reaches the compressor (unless for scramjets but I haven’t seen to many successful designs of those, and they don't have compressors). The inlet is designed to create a series of oblique shocks to reduce the velocity and hence, increase the pressure. This is more efficient than a single normal shock. There are off course several ways to accomplish this design wise but the principle is usually the same.
    Physics Wiz, when airflow goes from subsonic to supersonic the aerodynamic principle is reversed.
  12. Jun 15, 2007 #11


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    From that website - "You can see the convergent-divergent diffuser engine inlet on the F-15 below."

    The SR-71 uses the "Center Body Diffuser" in which the spike moves backward or forward with the speed in order to position the shockwaves appropriate in the diffuser. By the time the air gets to the compressor, it is subsonic.

    The SR-71 was developed as a long-range strategic reconnaissance aircraft capable of flying at speeds over Mach 3.2, while the F15 and F16 have lower top speeds.





    and this on the engine - http://www.powmadeak47.com/planes/psr71inlets.html



    The Convair (now part of Genearl Dynamics) B-58 Hustler used a similar concept. The 4 engine nacelles housed GE's J-79 jet engine and they used a variable spiked in the intake.

    http://www.geae.com/engines/military/j79/index.html [Broken]

    Sorry for getting carried away, but the SR-71 and B-58 are two of my favorite aircraft. :biggrin:
    Last edited by a moderator: May 2, 2017
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