verifying airspeed indicator

T o d d P a t t i s t wrote:
wrote:

I think his point is that flying into a headwind or with a
tailwind will give you the same true and indicated airspeed
but different groundspeeds as determined by GPS.

Your post isn't a model of clarity either, but I think the
answer is "so what." I'll try to be clear. I think you're
saying that wind does not affect true or indicated airspeed.
That's true. (You might be saying that IAS=TAS in head and
tailwinds, which is not true.) It's also true that the GPS
groundspeed will be different from IAS and TAS in the
presence of wind. (In specific situations, it could be the
same)

You are correct that is what I was thinking. I expect that
OP was thinking the same thing too.


So I come back to: "So what?" I mean isn't that the point?
You use GPS to find out what the wind is, then get rid of
the effect of wind to get the real TAS which equals what the
GPS groundspeed would have been if there was no wind, then
compare the real GPS derived, wind eliminated, TAS to what
your suspect airspeed and suspect ASI was telling you. If
the GPS derived TAS is 90 knots and your ASI said 100 knots,
you've got a problem.

Yes, you understand. However it would appear that OP did
not understand that, so I suggested the Gedenkin below to
help him understand.


You could fly at constant indicated airspeed and
altitiude in a wide circle and note the headings at which
the groundspeed (GPS) is maximum and minimum. For a steady
wind, the true airspeed corresponding to that indicated
airspeed should be the average of the two.

Here you are trying to solve for the wind to get TAS.

Actually I was eliminating it from the result, without
actually solving for it but you could also solve for it
easily enough, it would be half the difference between
the max and min ground speed.

That's what the cited web page did - it solved the equations
for you. Your circle method would work, but it requires you
to spot the min/max, and the GPS has to respond quickly
enough, and you have to do this in a turn. The better
method is to fly straight lines and solve the equations.
The web site just makes the "solve the equations" step easy.

Agreed, the described method is more practical but less
intuitive than the wide circle.


The algorithim at the aformentioned webpage appears to do
something a bitmore sophisticated than that by numerically
solving for the wind.

Exactly!


Plainly we agree. Hopefully OP gets it too.

A water manometer is still dead simple to make and use, but it
won't help if you have a problem with the orientation of the
pitot as for instance, at high AOA. Calibration with a manometer
will not help you calibrate the low speed flight regime of a
STOL aircraft, the GPS method could.

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FF