Some years ago, I found an 1944 "Education Manual EM910 - Elements of Aeronautics" for the US Armed forces in a used book store (with the original paper compass in tact!). While it covers detailed math on topics of airpseed and such, I did find an interesting estimation tool within. On the topic of how airspeed is measured
From Part I, Chapter 4 "Basic instruments used in flying", Pages 35-36
"Air-speed indicators are graduated (scaled) to indicate the speed of the airplane at sea level at normal temperature (16deg C). But at higher altitudes, where the air is less dense, the difference in pressure between the two tubes* is less for a given air speed than it is at sea level. Hense the air speed indicated at altitudes above sea level is less than the true air speed. (A means of determining altitude is described below.)
The reduction in indicated air speed is about 1.5 per cent for each 1000 feet. Hense to find the true air speed an approximate correction can be made to the indicated air speed by adding 1.5 per cent to the indicated air speed for each 1000 feet of latitude. Thus, if at 6000 feet the air speed indicated in 100 miles an hour, the correction is 6 X 1.5 per cent, or 9 per cent. Nine per cent of 100 miles is 9 miles. Hense the true air speed is about 109 miles an hour.
Roughly, the rule works the other way also. That is, if the true air speed at 6000 feet is 100 miles an hour, the indicated air speed is about 9 per cent less, or about 91 miles an hour."
While this is an estimation, I find it fairly accurate for the GA planes at lower air speeds. You would want to cross test it against the GPS to see what it's practical ranges are, and then could use it to provide an corrected IAS at altitude to use with ded reconing. The chart in the book shows a graph from - to 20,000 feet, with speeds from 40 to 160 mph.
Sea Level -- 99.8
1,000 ft -- 101.3
2,000 ft -- 102.8
3,000 ft -- 104.3
4,000 ft -- 105.8
5,000 ft -- 107.4
10,000 ft -- 115.9
15,000 ft -- 125.3
20,000 ft -- 135.9
30,000 ft -- 161.1
40,000 ft -- 193.3
, 05-14-2008 at 02:30 PM|
Last edited by , 05-13-2011 at 11:47 AM
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