Airspeed Indicator Calibration Check

[John Adcock]

Hello everyone,

I'm hoping someone out there would know what the numbers are to check an
airspeed indicator with a water manometer. Should be something like a table
which provides a correspondence between inches of water and indicated mph.
How to calculate it might also be usefull.

Thanks in advance for any information,

John Adcock
jo...@cyberpass.net

[Randy Van Voorst]

John Adcock <jo...@cyberpass.net> wrote in article
<oZJp2.6348$cR3....@news.rdc1.sdca.home.com>...

John,
Using the Ensewiler formula you can calculate the following:

If you want to calculate from mph to inches of water:
Psf = .002496 X mph squared.
Inches of water = psf/5.208

If you want to calculate from inches of water to mph:
psf = inches of water X 5.208
mph = 20.016 X the square root of the psf.

Randy

[David and Michelle Kitson]

This is great, but at 40Mph, the water column is only 0.7 inches high...
With a meniscus to worry about and other aspects of this method, that would
make for a somewhat innacurate calibration I imagine.

This issue came up in an Australian Ultralights article in September 98. (By
John Gilpin).

That might be worth reading also.
http://www.auf.asn.au/sep98/calasi.htm

Or you can find it through the AUF home page.
http://www.auf.asn.au

It discussed some of the practical aspects of ASI calibration for
ultralights.
(And caused a number of letters to the editor on the subject, both for and
against).

Regards

David Kitson.

Randy Van Voorst <qual...@premier1.net> wrote in message
news:01be456b$c6b1a260$6875...@premier1.premier1.net...

[Randy Van Voorst]

David and Michelle Kitson <dki...@blackice.com.au> wrote in article
<36a7e...@news.highway1.com.au>...

> This is great, but at 40Mph, the water column is only 0.7 inches high...
> With a meniscus to worry about and other aspects of this method, that
would
> make for a somewhat innacurate calibration I imagine.
>

You are absolutely correct about the innacuracy of using a U tube type of
column. If you could get ahold of an inclined monometer, the accuracy
would be increased. I am lucky in that at my business we have digital
Meriam monometers that read out in 0.01 inches of water.

Randy

[John Adcock]

Hello everyone,

And thanks to Randy for his formula. Incidentally, the formula can be
written that inches of water is equal to .002496/5.208 times the indicated
airspeed, in miles per hour squared. The .002496/5.208 is a constant of
proportionality (the Van Voorst Constant) and is equal to approximately
.000479.

Well, despite everyone's skepticism about the accuracy of a water manometer
at these low pressures, I was able to satisfy mystelf that the low budget,
Chinese-made 0 to 80 mph asi that I will soon strap to my butt is, as far as
I can tell, very close.

I built a small plywood test setup, fastened two glass tubes to it, put
yellow food coloring in the water, a piece of white paper behind the tubes,
and used a mm scale (exactly 25.4 mm to the inch) that was attached to the
test setup that I could slide up and down. Of course the two tubes were
parallel and mounted vertically, with a length of plastic tubing connecting
them at the bottom. One glass tube is stationary (and connected to the
asi), the other moves up and down to vary the differential applied to the
asi. You then move the mm scale up to read the difference, meniscus to
meniscus.

Using this arrangement, I feel that I am accurate to within one-half of one
millimeter. The trick for me was to take repeated readings, both increasing
and decreasing in altitude, and lightly tapping on the case of the asi.
Since this asi and the airplane that it is going into have not yet flown,
buying a GPS and flying around in circles for several hours is not
practical. Yet.

Thanks and happy flying,

John Adcock

jo...@cyberpass.net

Randy Van Voorst wrote in message
<01be461d$8a726020$8675...@premier1.premier1.net>...

[kl...@geocities.com]

John Adcock wrote:
>
> I'm hoping someone out there would know what the numbers are to check
> an airspeed indicator with a water manometer.

i realize you're already done, but others may want to peruse jim weir's
article about this process. it is available at,

http://www.rst-engr.com/rst/magazine/

one hint is to use a small bit of soap to reduce the menicus.

fwiw: the formula is g*(rho*h)_water = (1/2*rho*v^2)_air

a sample fortran program follows:

program reduceas

nv = 30 ! number of data points to read

rhoa = 0.002378 ! slugs/ft^3 (@ standard temp & press)
rhow = 1.937 ! slugs/ft^3 (@ standard temp & press)

g = 32.19 ! ft/s^2

do i = 1, nv
read (*,*) as, dh ! indicated a/s (mph) and h2o height (in)
v = sqrt(g*rhow*(dh/12.)/(0.5*rhoa)) ! computed a/s (mi/hr)
& *3600./5280. ! multiply by 1.151 for nm/hr
write (*,*) as, v ! indicated and computed airspeeds
enddo

stop
end

--
bil

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