# What is airspeed

1. Jul 23, 2014

### Greg Bernhardt

Definition/Summary

Airspeed is the speed (or velocity, $V_{AP}$) 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: $V_{GA}$.

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): $V_{GP}\ =\ V_{GA}\ +\ V_{AP}$

Equations

Extended explanation

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, $V_{GP}$)
= the velocity of the air relative to the ground (windspeed, $V_{GA}$)
+ the velocity of the plane relative to the air (airspeed, $V_{AP}$)​
ie:
$V_{GP} = V_{GA} + V_{AP}$​

"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?

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