Kevin Horton
Well Known Member
Many people do speed testing by adjusting the pressure altitude to achieve a target density altitude. The theory is that this will give the same TAS, no matter whether the OAT is standard or not. This approach would work, if the engine power available was a strict function of density altitude.
However, the typical temperature corrections on engine manufacturer power charts claim that the power varies with the square root of standard temperature over the test day temperature. If this is true, the constant density altitude method will not give a result that is independent of temperature deviation from standard.
If the temperature correction on the manufacturer power charts is accurate, this TAS correction spreadsheet will allow correction of TAS data. The test is to be flown at the target pressure altitude, and the resulting TAS is corrected for temperature. It assumes that the rpm and MP would be the same at the standard day condition as they are at the test day condition. This probably makes it unsuitable for fixed-pitch prop equipped aircraft.
This proposed correction method has not yet been validated by real testing. It may or may not work out in practice. I have solicited data to validate the method, and will add to this thread once I can draw some conclusions.
However, the typical temperature corrections on engine manufacturer power charts claim that the power varies with the square root of standard temperature over the test day temperature. If this is true, the constant density altitude method will not give a result that is independent of temperature deviation from standard.
If the temperature correction on the manufacturer power charts is accurate, this TAS correction spreadsheet will allow correction of TAS data. The test is to be flown at the target pressure altitude, and the resulting TAS is corrected for temperature. It assumes that the rpm and MP would be the same at the standard day condition as they are at the test day condition. This probably makes it unsuitable for fixed-pitch prop equipped aircraft.
This proposed correction method has not yet been validated by real testing. It may or may not work out in practice. I have solicited data to validate the method, and will add to this thread once I can draw some conclusions.