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Aircraft: velocity + altitude measurement; simply or not?

  1. Mar 6, 2014 #1
    Hi!

    I'm breaking my head about the following problem:

    Consider an A/C at a certain velocity flying at a certain altitude. The A/C measures it's speed via the difference in total- and static-pressure and applying bernoulli.

    Now the aircraft accelerates. What happens?

    At this point I get uncertain and look forward to hear your opinions:

    - I assume that the measured static pressure drops
    --> measured velocity increases (no problem)
    --> But: static pressure drops: altitude decreases (big problem!)

    The other opportunity would be:

    - The total pressure increases
    --> The static pressure remains constant & only dynamic pressure increases


    Edit: Additional fundamental question:
    Does a static pressure sensor see reduced static pressure if a flow is doubled in speed? If yes, this would again mean that the static pressure sensor indicates lower altitude due to higher velocity...

    Thanks in advance!
     
    Last edited: Mar 6, 2014
  2. jcsd
  3. Mar 6, 2014 #2

    rcgldr

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    Not if it's designed properly. The static pressure port is normally a flush mounted hole perpendicular to the flow of air. It "hides" under a boundary layer and the pressure at the port is the same as the static pressure of the ambient air, regardless of relative speed.
     
  4. Mar 6, 2014 #3

    cjl

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    As rcgldr said, the static pressure stays approximately constant (assuming no altitude change), and the total pressure increases. This can get somewhat more complicated if you're looking at a supersonic aircraft, but for subsonic flow, you can pretty much just assume that the measured static pressure is independent of airspeed.
     
  5. Mar 7, 2014 #4
    Okay I understand. And if I had no A/C to "hide" my static pressure port behind in a moving fluid, is there a chance to measure the pressure altitude?

    Somehow this always brings me to the total pressure because if I bring the fluid to rest, I know that static pressure recovers to total pressure. Why isn't this true for the total pressure / static pressure we measure?
     
  6. Mar 7, 2014 #5

    rcgldr

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    Which is why forward facing pitot / static tubes are used. The static port is an opening on the side of the tube, and this eliminates trying to find a spot on the fuselage that isn't changing the speed or direction of the relative air flow.

    The fluid has momentum and a stagnant zone forms at the front of a pitot tube, bringing a portion of the flow to rest. This is called impact or stagnation pressure. At speeds below 1/3rd mach, the simpler formula based dynamic pressure can be used. Wiki article:

    http://en.wikipedia.org/wiki/Impact_pressure
     
  7. Mar 9, 2014 #6
    The pitot-static probe has to be properly aligned with the flow so if you're flying at angle of attack or at sideslip angle you're note measuring something meaningful. And that's there reason why aeroplanes have more than 2 pitot tubes at different positions and angles.

    Also, sqrt_9, if your aircraft accelerates it is quite probable that you'll start gaining altitude unless you decrease the angle of attack.
     
  8. Mar 10, 2014 #7
    Okay, I get that.
    So, bernoulli isn't applicable because the static pressure port is out of the streamline. Is that right?

    Anyway, thanks for the answers so far!
     
  9. Mar 10, 2014 #8
    Well, it relates back to the proper alignment of the equipment with the flow direction as well as to which static port is used because you have to use the right one... I have to revise, I don't remember all the details.
     
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