On Wed, 07 Jan 2004 10:13:19 -0800, Jay wrote:

Curious why you would do that in the first place? Couldn't you get away
with GPS speed (average of 2 runs in opposite direction to pull our
windage) to check your differential air pressure based airspeed
instrument.


Averaging the ground speed from two runs in opposite directions only
provides the right answer if the runs are aligned exactly with the wind.
But you have no way of being sure exactly what the wind direction is, so
your average speed will be off by some amount.

Example 1, if the TAS is 100 kt, you have a wind of 270 deg at 20 kt,
and the runs are done on headings of 360 and 180. The ground speed on each
run is 102 kt, so have an error of 2 kt in the test technique.

Example 2, use the same TAS and wind, but do the runs on headings of 300
and 120. The ground speeds are 83.3 and 117.7 for an average of 100.5.

But, small errors in the heading on one run can make a big difference -
how accurate is your compass swing, and how closely can you fly a heading
while doing this test? E.g. same TAS and wind, but the actual headings
flown are 305 and 120 (5 deg error on one run). The ground speeds are now
84.4 and 177.7 for an average of 101.

Yes, the error can be small if the wind speed is low, and you are somewhat
aligned with the wind, but why not use a technique that gives the exact
answer, rather than an approximation?

For an accurate method, see:
http://members.rogers.com/khorton/rv...y/TAS_FNL4.pdf

Spreadsheet to do the calculations for the above method, and lots more
info on calculating TAS using GPS data at:

http://go.phpwebhosting.com/~khorton...ex.php?&PID=49

--
Kevin Horton RV-8 (finishing kit)
Ottawa, Canada
http://go.phpwebhosting.com/~khorton/rv8/
e-mail: khorton02(_at_)rogers(_dot_)com