verifying airspeed indicator

[Heino & Deanne Weisberg]

I do not believe my airspeed indicator is accurate. Is there a way to
verify/calibrate an airspeed indicator on the ground?
Heino

[[email protected]]

Heino & Deanne Weisberg wrote:
> I do not believe my airspeed indicator is accurate. Is there a way to
> verify/calibrate an airspeed indicator on the ground?
> Heino

What sort of ASI do you have?

Have you looked to see if this is addressed in the manual?

For what range of speeds do you want to check/calibrate it?

In the air you can check it vs GPS.

--

FF

[Nathan Young]

On Mon, 17 Oct 2005 07:07:56 -0500, "Heino & Deanne Weisberg"
> wrote:

>I do not believe my airspeed indicator is accurate. Is there a way to
>verify/calibrate an airspeed indicator on the ground?

It might be easier to calibrate in the air (that is unless your
homebuilt is not flying!).

You can determine true airspeed empirically through the use of a GPS
while flying. Specifically, go to the following website, follow the
instructions to capture the data and then use their 'true airspeed
applet' to calculate true airspeed.

http://www.reacomp.com/true_airspeed/

If you note temperature, pressure, you can work backwards to figure
what the indicated airspeed should have been.

-Nathan

[Kublai]

I don't think testing against GPS makes sense at all, as the ASI will
surely work properly if indicating wrong ground speed at headwind - so
what information do you gain ?

Kublai

[[email protected]]

T o d d P a t t i s t wrote:
> "Kublai" > wrote:
>
> >I don't think testing against GPS makes sense at all, as the ASI will
> >surely work properly if indicating wrong ground speed at headwind - so
> >what information do you gain ?
>
> This question is hard to understand, but GPS testing does
> make sense. If the aircraft is flown at a constant ASI
> indicated airspeed, and the GPS groundspeeds are recorded,
> the calculator will give true airspeed. From that you can
> determine if the ASI is off (with appropriate
> altitude/installation error corrections).
>

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.

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.

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

--

FF

[[email protected]]

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.

--

FF

[Scott]

GPS will show groundspeed only, so it would only be accurate in ZERO
wind conditions. However, I've read that you can compensate for wind by
flying N, S, E and W by using a compass for pointing your plane. You
point the nose in each direction and do NOT correct for wind drift (and
do not use the track readout on your GPS). In other words, you point
the nose for 90 degrees, even if your GROUND TRACK may be 98 degrees
because of wind. Take the 4 readings you get and average them. This
should get you in the ballpark.

Scott


wrote:
> Heino & Deanne Weisberg wrote:
>
>>I do not believe my airspeed indicator is accurate. Is there a way to
>>verify/calibrate an airspeed indicator on the ground?
>>Heino
>

>
> In the air you can check it vs GPS.
>

[Morgans]

"T o d d P a t t i s t" > wrote

> Another method that I used in my
> experimental glider was to switch the ASI to a trailing
> probe.

I don't have a clue what you mean by that. Clues accepted, gladly! <g>
--
Jim in NC

[[email protected]]

I do not know but if you found the answer please tell me.
regards

[[email protected]]

I am ehsan ashrafi,student of aerospace eng.I would like to share my
experiences with you
where do you live and what is your course
regards

[GTH]

Morgans a écrit :
> "T o d d P a t t i s t" > wrote
>
>
>>Another method that I used in my
>>experimental glider was to switch the ASI to a trailing
>>probe.
>
>
> I don't have a clue what you mean by that. Clues accepted, gladly! <g>

Jim and all,

I believe the trailing probe Todd is talking about is a specially
designed pitot/static assembly, trailed some distance behind the
aircraft in order to operate clear of the wake and disturbance.
It is typical flight-test hardware.

Hope this helps,

Regards,
Gilles Thesee
Grenoble, France
http://contrails.free.fr

[[email protected]]

GTH wrote:
> Morgans a écrit :
> > "T o d d P a t t i s t" > wrote
> >
> >
> >>Another method that I used in my
> >>experimental glider was to switch the ASI to a trailing
> >>probe.
> >
> >
> > I don't have a clue what you mean by that. Clues accepted, gladly! <g>
>
> Jim and all,
>
> I believe the trailing probe Todd is talking about is a specially
> designed pitot/static assembly, trailed some distance behind the
> aircraft in order to operate clear of the wake and disturbance.
> It is typical flight-test hardware.
>

A little googling turns up some information on "drag probes".

--

FF