Errors in ASI Calibration Article, Kitplanes, Feb. 2009

[Kevin Horton]

The tables of ASI reading vs water manometer height in the "Maintain thy airspeed ..." article in the Feb. 2009 Kitplanes are completely wrong. All the values of water height that I checked are more than 25% in error. Strangely enough, the same author published a similar article in the July 89 Kitplanes, but the earlier article had correct data.

The following formulae give results that are within 0.1% of the "official" specs in AN 05-10-24:

For ASIs in mph, water height (inches) =((1+0.2*(ASI/761.22)^2)^3.5-1)*407.51

For ASIs in kt, water height (inches) =((1+0.2*(ASI/661.48)^2)^3.5-1)*407.51

For ASIs in km/h, water height (cm) =((1+0.2*(ASI/1225.06)^2)^3.5-1)*1035.083

You can download zipped Excel and OpenOffice spreadsheets that convert between ASI reading and water manometer height.

Correct data follows:

Airspeed     Water
Reading     Column
(mph)       (inches)
 40          0.79
 50          1.23
 60          1.77
 70          2.42
 80          3.16
 90          4.00
100          4.94
110          5.99
120          7.13
130          8.38
140          9.73
150         11.18
160         12.74
170         14.41
180         16.17
190         18.05
200         20.03
210         22.13
220         24.33
230         26.64
240         29.07
250         31.61
260         34.26
270         37.03
280         39.92
290         42.93
            
            
Airspeed     Water
Reading      Column
 (kt)       (inches)
 40          1.04
 50          1.63
 60          2.35
 70          3.20
 80          4.19
 90          5.31
100          6.56
110          7.94
120          9.47
130         11.12
140         12.92
150         14.86
160         16.94
170         19.15
180         21.52
190         24.02
200         26.68
210         29.48
220         32.44
230         35.54
240         38.80
250         42.22
            
            
Airspeed      Water
Reading      Column
(km/h)        (cm)
 60           1.74
 70           2.37
 80           3.09
 90           3.92
100           4.84
110           5.85
120           6.97
130           8.18
140           9.49
150          10.90
160          12.41
170          14.02
180          15.73
190          17.53
200          19.44
210          21.45
220          23.56
230          25.77
240          28.08
250          30.49
260          33.01
270          35.62
280          38.35
290          41.17
300          44.11
310          47.14
320          50.29
330          53.54
340          56.89
350          60.36
360          63.93
370          67.62
380          71.41
390          75.31
400          79.33
410          83.46
420          87.70
430          92.05
440          96.52
450         101.11
460         105.81
470         110.63
480         115.57
490         120.63
500         125.81

 

[PCHunt]

I'm assuming you have notified Kitplanes?

 

[Kevin Horton]

I'm assuming you have notified Kitplanes?

Yes

(extra characters to keep some forum software writer happy)

 

[Noah]

Well this is timely. I just inherited a Meriam micromanometer yesterday which indicates .001 inches of water to 20", and I was wondering if it would be useful for calibrating aircraft instruments. Does anybody know if this type instrument is what is used by certificated shops for calibrating these instruments?

http://www.meriam.com/productcart/pc/viewPrd.asp?idcategory=8&idproduct=35

Thanks Kevin!

 

[Bill Dicus]

Readings

Kevin: Thanks so much for giving us the correct info and publishing the links. Bill

 

[DanH]

Kevin, nice spreadsheet! It will make things easier, thanks.

(Ok guys, he's sitting there with a broken leg and an idle mind...somebody suggest a project, quick! <g>)

 

[Kevin Horton]

Kevin, nice spreadsheet! It will make things easier, thanks.

You're welcome. I'm happy to help out where I can.

(Ok guys, he's sitting there with a broken leg and an idle mind...somebody suggest a project, quick! <g>)

I've got lots of projects. I'm working on several articles that will hopefully eventually end up in Kitplanes. I'm experimenting with a more capable data analysis program. I'll be asking for some guys to try out an experimental method to determine engine power using fuel flow, fuel flow at peak EGT, rpm, compression ratio and displacement (more on this in a few days). I'm going to try to puzzle out some niggling minor issues with my data recording scripts. I want to work on making vector graphic files for the paint scheme on my aircraft. And, I may even update the software that runs my web site - I've been putting that off for over two years.

I might also do some more prep work on my own pitch axis autopilot that works via pitch trim, using adaptive gains. I did enough simulation work with X-Plane last year to convince me that the concept had some hope of success. Now I need to buy a Parallax Propeller microcontoller and learn how to program it.

 

[TGRV7]

ASI Cal SS

Thx Kevin for the SS

 

[gasman]

Well this is timely. I just inherited a Meriam micromanometer yesterday which indicates .001 inches of water to 20", and I was wondering if it would be useful for calibrating aircraft instruments. Does anybody know if this type instrument is what is used by certificated shops for calibrating these instruments?

http://www.meriam.com/productcart/pc/viewPrd.asp?idcategory=8&idproduct=35

Thanks Kevin!

This should work just fine if you can read to .00 . You will need some tubing and a T fitting and a bulb from a blood pressure cuff to apply pressure and bleed it off.

 

[DanH]

<<I've got lots of projects.>>

I was kidding. You're the last guy I'd expect to harbor an idle mind. Remember the thread Paul started some time back about "Who do you believe"? You're in my top three. Get well quick.