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Speedometer princible

  1. Oct 3, 2008 #1
    Hi there,

    I ask about the speedometer of a ship or an airplane. The water (or air) flows so how do they know the absolute speed of the ship (plane)?
  2. jcsd
  3. Oct 3, 2008 #2


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    Staff: Mentor

    There is no such thing as absolute speed.

    Ships use turbine flow meters. They are basically a paddle-wheel sticking through the hull. A sensor measures the rate of rotation and converts that to speed in knots. If the navigator wants "true" speed (referenced to the earth instead of the water), s/he uses GPS or adds a correction based on known currents.

    [subsonic] Airplanes use pito-static tubes that measure the velocity pressure of the air and convert to speed via Bernoulli's equation. That gives airspeed. If they want ground speed, they can use GPS or make corrections to airspeed based on altitude and wind (airspeed drops with altitude, according to Bernoulli's equation).
  4. Oct 4, 2008 #3


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    Science Advisor

    that is to say; all speeds must be measured relative to something else. Ships and aircraft measure their speed relative to the fluid through which they travel.

    Please doublecheck my reasoning on this, but Pitot tubes measure both ambient air pressure and velocity air pressure, displaying the difference between the two. I believe this means that no "correction for altitude" is required for giving ground speed; only a correction for wind.
  5. Oct 4, 2008 #4


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    Homework Helper

  6. Oct 4, 2008 #5


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    Staff: Mentor

    Sorry Jeff, I guess I got my aviation terms wrong (or just not specific enough). Affected by altitude is what I meant, so IAS, it is.

    Bernoulli's equation has only three terms here: velocity, pressure, and density. Density drops with altitude, so IAS drops with altitude. What it means for a pilot is that for, say, 100 kts ias at 5,000 ft, the actual speed of the air over the wings is higher than for 100 kts ias at 1,000 ft. But the lift of the wing is the same because, as Jeff said, the mass flow rate of air over the wing is the same.
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