"Bravo Delta" wrote in message
...
Can somebody please explain to me the technique to measure your
plane's true
airspeed by flying in 3 or 4 different directions for a while and
recording
GPS ground speed ? Do you just fly the same amount of time each leg
and
average the groundspeed readings ?


Assume you want to know TAS for a given IAS. You'll have to repeat the
following recipe for each IAS you're interested in. ALL speeds must be
in the same units (knots, mph, ft/sec, or whatever).

You will fly N and get GPS ground speed g1, then E to get g2, then S to
get g3.

Now it's time to calculate. There are three preliminary calculations
needed, for P, A, and Q, as follows:

P = (g1^2 + g3^2)/2

A = arctan(g3^2 - g1^2, 2*g2^2 - g1^2 - g3^2) [you have to use the arc
tangent function which gives the angle counterclockwise from the
positive x axis, usually called something like ATAN2(x,y) in
spreadsheets]

Q = (g3^2 - g1^2)/(4*cosA)

Then, at last,

TAS = sqrt((P + sqrt(P^2 - 4*Q^2))/2)

Also, wind speed (assumed constant!!) = absolute value of Q/TAS and wind
angle = 360 deg - A (mod 360).

Hope this helps. If still uncertain, see Performance of Light Aircraft
p. 31.

John T. Lowry, PhD
Flight Physics