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Naf, Mike, et al,
Each of the other methods utilize some interesting assumptions and complex math. I am using the equipment I have and simple arithmetic.
Here are my assumptions:
TAS - True speed through undisturbed air. This is calculated by adjusting indicated air speed (IAS) for temperature (related to speed), pressure altitude and compressibility (related to speed). This would be ground speed (GS) if there were no wind or movement of the airmass within which the airplane is flying. Rocky Mountain Instrument's (RMI) TAS is calculated using all three components noted above and its accuracy is dependent on, among other things, its OAT probe. My OAT probe is located just ahead of the left wheel well door and its support and wiring is in the wheel well. This area is unaffected by sun or heat generated by the airplane or cockpit.
IAS - Measured by the difference between the ram air collected by a pitot tube and static (ambient) air pressure. I utilize two static ports located as recommended by the factory. The static lines were checked and exhibit no leakage. The pitot is a standard heated "L" shaped tube located on the left aileron bellcrank door sufficiently far from the wing and propeller blast to eliminate any peculiar disturbance. The pitot line was also pressure checked. IAS is cross checked with RMI and standard airspeed indicator and are within 1 Kt.
GS - Readout from Garmin 430, 12 satellites, Selective Availability turned off meaning 49 feet RMS position error and .1 Kt RMS speed error in a steady state.
Stabilized flight - Otto will do the flying in altitude hold, heading control (autopilot DG bug) after stabilized hand flight is trimmed. This sometimes takes ten minutes to set up just right. Otto is so much better than me. This is done in smooth air. There is no precession of the DG and no altitude hunting.
The power is set and leaned, heading selected, flight stabilized, auto-pilot engaged and, when observation indicates stabilization, readings of IAS, TAS and GS are taken. The heading bug is turned 120 degrees and, when stabilized on the heading, readings are taken. This is repeated for the third leg. Note that this technique gets a reliable three legged data set and keeps you over the same terrain to boot (unless the winds are hellacious).
CAS or calibrated airspeed can be reasonably determined by averaging the data and applying the difference of GS and TAS to IAS.
Here is my recent test:
Overcast, wind at the ground was 180@11 (Mag Var = 0), 2500 ft, 7 deg C, 30.05 hg.
Test 1 - 2500 rpm, 25"
DEG IAS TAS GS
270 176 180 168
150 176 180 183
030 176 180 194
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538 540 545
Avg 176 180 181 CAS=177
Test 2 - 2290 rpm, 18.1", 10 degrees of flap
DEG IAS TAS GS
270 114 116 102
150 115 117 115
030 113 115 125
------------------------------
342 348 342
Avg 114 116 114 CAS=112
All of this is close enough. Even the other techniques are off by a Kt or two when crosschecked.
Scott Krueger
N92EX
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