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What is airspeed

  1. Jul 23, 2014 #1

    Airspeed is the speed (or velocity, [itex]V_{AP}[/itex]) of a plane relative to the air, or (same thing) relative to the wind.

    Moving air is like moving water: an object will tend to be carried along at the same velocity as the moving air or water. In air and water (and any other fluid), that can be called a current. In air, it is more usually called wind.

    Confusingly, windspeed is the speed of the air (or wind) relative to the ground: [itex]V_{GA}[/itex].

    The true speed of the plane (relative to the ground) can be found from a vector triangle, or by adding the windspeed and airspeed (as vectors): [itex]V_{GP}\ =\ V_{GA}\ +\ V_{AP}[/itex]


    Extended explanation

    Adding velocities as vectors:

    All vectors are relative.

    For a position vector, that's obvious … the vector is from one position to another.

    But it's also true for a velocity vector … it's from one velocity to another!

    So just as you can write a position vector as AP, and get equations like GA + AP = GP,

    you can write a velocity vector as AP, and get equations like GA + AP = GP,

    except perhaps it's clearer if you emphasise that they're velocities by writing VGA + VAP = VGP.

    For example, if A P and G represent (the velocities of) the air a plane and the ground, then:
    the velocity of the plane relative to the ground (true speed, [itex]V_{GP}[/itex])
    = the velocity of the air relative to the ground (windspeed, [itex]V_{GA}[/itex])
    + the velocity of the plane relative to the air (airspeed, [itex]V_{AP}[/itex])​
    [itex]V_{GP} = V_{GA} + V_{AP}[/itex]​

    "airspeed" and "windspeed":

    "windspeed" is the correct term for the speed of the wind, but "airspeed" isn't the correct term for the speed of the air! silly name isn't it? :wink:

    * This entry is from our old Library feature. If you know who wrote it, please let us know so we can attribute a writer. Thanks!
  2. jcsd
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