The table of ASI reading vs water manometer height in the "How Fast is Fast" article in the April 2007 issue has errors. The errors are small at low speed, but there is about a 4 kt error at the highest speeds in the table. I'm not sure where those numbers came from.
The CAS for a given pressure is equal to asl*SQRT(5*((P/Psl+1)^(2/7)-1)) [reference test pilot school notes].
where:
asl = speed of sound at sea level (661.48 kt or 761.22 mph)
Psl = atmospheric pressure at sea level (407.513" H2O at 20 deg C, or 10350.8 mm H2O)
The above values are from NASA Reference Paper 1046, Measurement of Aircraft Speed and Altitude, Tables A19 and A27.
Corrected ASI vs water manometer height:
Speed | in. | mm | | | Speed | in. | mm |
(mph) | H2O | H2O | | | (kt) | H2O | H2O |
15 | 0.11 | 2.8 | | | 15 | 0.15 | 3.7 |
20 | 0.20 | 5.0 | | | 20 | 0.26 | 6.6 |
25 | 0.31 | 7.8 | | | 25 | 0.41 | 10.4 |
30 | 0.44 | 11.3 | | | 30 | 0.59 | 14.9 |
35 | 0.60 | 15.3 | | | 35 | 0.80 | 20.3 |
40 | 0.79 | 20.0 | | | 40 | 1.04 | 26.5 |
45 | 1.00 | 25.3 | | | 45 | 1.32 | 33.6 |
50 | 1.23 | 31.3 | | | 50 | 1.63 | 41.5 |
55 | 1.49 | 37.9 | | | 55 | 1.98 | 50.2 |
60 | 1.77 | 45.1 | | | 60 | 2.35 | 59.7 |
65 | 2.08 | 52.9 | | | 65 | 2.76 | 70.1 |
70 | 2.42 | 61.4 | | | 70 | 3.20 | 81.4 |
75 | 2.78 | 70.5 | | | 75 | 3.68 | 93.4 |
80 | 3.16 | 80.2 | | | 80 | 4.19 | 106.4 |
85 | 3.57 | 90.6 | | | 85 | 4.73 | 120.1 |
90 | 4.00 | 101.6 | | | 90 | 5.31 | 134.8 |
95 | 4.46 | 113.3 | | | 95 | 5.91 | 150.2 |
100 | 4.94 | 125.6 | | | 100 | 6.56 | 166.5 |
105 | 5.45 | 138.5 | | | 105 | 7.23 | 183.7 |
110 | 5.99 | 152.1 | | | 110 | 7.94 | 201.8 |
115 | 6.55 | 166.3 | | | 115 | 8.69 | 220.7 |
120 | 7.13 | 181.2 | | | 120 | 9.47 | 240.4 |
125 | 7.74 | 196.7 | | | 125 | 10.28 | 261.1 |
130 | 8.38 | 212.9 | | | 130 | 11.12 | 282.6 |
135 | 9.04 | 229.7 | | | 135 | 12.01 | 304.9 |
140 | 9.73 | 247.2 | | | 140 | 12.92 | 328.2 |
145 | 10.44 | 265.3 | | | 145 | 13.87 | 352.4 |
150 | 11.18 | 284.1 | | | 150 | 14.86 | 377.4 |
155 | 11.95 | 303.5 | | | 155 | 15.88 | 403.3 |
160 | 12.74 | 323.7 | | | 160 | 16.94 | 430.2 |
170 | 14.41 | 365.9 | | | 170 | 19.15 | 486.5 |
180 | 16.17 | 410.8 | | | 180 | 21.52 | 546.5 |
190 | 18.05 | 458.5 | | | 190 | 24.02 | 610.2 |
200 | 20.03 | 508.9 | | | 200 | 26.68 | 677.6 |
210 | 22.13 | 562.0 | | | 210 | 29.48 | 748.8 |
220 | 24.33 | 617.9 | | | 220 | 32.44 | 823.9 |
230 | 26.64 | 676.7 | | | 230 | 35.54 | 902.8 |
240 | 29.07 | 738.3 | | | 240 | 38.80 | 985.6 |
250 | 31.61 | 802.8 | | | 250 | 42.22 | 1072.4 |
As a sanity check on the above calculated values, NASA Reference Paper 1046 has tables of pressure vs ASI readings, with the pressure in units of mm Hg. At 250 kt, table A23 has a pressure of 78.744 in Hg. At 20 deg C, 1 mm Hg is equal to 0.53577 " H2O. So, 250 kt would be 42.19" of H2O. This is within 0.1 kt of the values I have provided above. NASA Reference Paper 1046 is available for free download from NASA.
http://hdl.handle.net/2060/19800015804 (14 MB PDF file)
I have created Excel and OpenOffice spreadsheets that will convert back and forth between ASI reading and water manometer height. They are available at: