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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
         Indicator      Column
          Reading       Height
           (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
         Indicator      Column
          Reading       Height
            (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
         Indicator       Column
          Reading        Height
           (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

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