Science Term: airspeed

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airspeed

Definition/Summary

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]

Equations

Scientists

Recent forum threads on airspeed

Breakdown

Physics
> Classical Mechanics
>> Newtonian Dynamics

See Also

vector triangle

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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 V_GA + V_AP = V_GP.

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])

ie:

[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?

Commentary

Filip Larsen @ 08:15 AM Mar13-10

In aviation the speed of a vehicle relative to the airmass it passes through is usually denoted True Airspeed (TAS) and the direction of the air speed vector is called the heading of the vehicle. Its speed relative to the ground is denoted Ground Speed (GS) and the direction of the ground speed vector is called the track.

The direction of the wind is, in aviation, specified as the direction the wind comes from and not the direction it travels toward, which is opposite. When constructing wind vectors based on aviation wind directions it is therefore important to either reverse the vector before adding it to the airspeed vector when calculating ground speed vectors, or alternatively, subtract it instead.

Also note, that true airspeed should not to be confused with indicated airspeed (IAS) or calibrated airspeed (CAS) which are related to the air speed indicated on a gauge that measures air speed based on dynamic air pressure which besides the true airspeed also depends on air density (varies with vehicle altitude, air pressure and outside air temperature) and compressibility (significant at speeds comparable to the speed of sound in the given air).