Correcting level flight TAS for non-standard OAT

[Kevin Horton]

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.

[kentb]

Kevin, I got a data error from excel when I tried to load you spreadsheet.

Kent

[Kevin Horton]

Kent - thanks for reporting that. That file had gotten corrupted somehow. I loaded a new version. Let me know how it works please.

[aadamson]

Kevin,

Still get an error on file load when clicking the link. First error is something about potential data loss, then the spreadsheet comes up, but the final calc doesn't seem to work.

I suspect it's related to the labels that are used, but thats just a guess.

[pgroell]

Hello Kevin,

getting the same error here.
Having looked at the file, the B39 formula is missing (calculating K degrees from C or F degrees).
I tried by replacing B39 with the simple formula for ?C to ?K, and it works, giving no error on opening.
I think B39 should read something like :

PHP Code:

=IF(temp_units_in="C";273.15+oat_in;273.15+((oat_in-32)*5/9)) 


Maybe there are some more official formulas for temperature conversions.

[Kevin Horton]

Blasted computers. I created the original spreadsheet in OpenOffice, then saved it in Excel format. It seems that OpenOffice didn't create a valid Excel file. After the error was first reported, I fired up my work Windows laptop, opened the file in Excel and fixed it. I then went through the process to replace the file on my web site with the fixed one. But it looks like I maybe didn't click the Submit button, because now I see that the file name is still the original one.

I uploaded a new file - named TAS_vs_OATv2.xls. If you get that one, it should work.

[kentb]

Now that I have it, is this the way to use it to test my plane?

Fly at a test pressure alt. (lets say 8000ft).
Record the GPS TAS (three runs at least 90 degrees apart).
Record the OAT.

After returning to ground (my PC).

Use GPS TAS spreadsheet to compute GPS-TAS.
Use your spreadsheet to compute OAT-corrected TAS.

This number could be used to test different configuration of the aircraft for performance.

Kent

[Kevin Horton]

Originally Posted by kentb

Now that I have it, is this the way to use it to test my plane?

Fly at a test pressure alt. (lets say 8000ft).
Record the GPS TAS (three runs at least 90 degrees apart).
Record the OAT.

After returning to ground (my PC).

Use GPS TAS spreadsheet to compute GPS-TAS.
Use your spreadsheet to compute OAT-corrected TAS.

This number could be used to test different configuration of the aircraft for performance.

Yes, what you have above is correct. But, be aware that this data analysis technique should be considered experimental, as it has not yet been validated by actual results.

If you are trying to measure the effect of small changes on configuration I would want several flights in each configuration. Don't make any decisions based on only one test point. Wait to see if you have a repeatable result first.

Expect to have some days when you don't get much good data. Some days you are sharper than others. And some days the weather simply isn't good enough. You need smooth air, and it should be neither rising nor descending. The air is descending in high pressure systems, and rising in low pressure systems. So even on days with smooth air you probably aren't quite seeing the true performance of the aircraft. You need to get results from several days, hopefully in different weather systems, and average them.

Also, I much prefer the four leg version of the NTPS spreadsheet, as the extra leg provides redundant data that is used to determine the quality of the test point. The spreadsheet does four calculations using different mixes of three legs. If you have good data, all four calculations yield pretty much the same result. If you have questionable data, the results are all over the place, which is seen as a high standard deviation (the spreadsheet does that calculation for you). You should be looking for standard deviations of 1 kt or less.

If you are trying to measure the effect of small modifications, you need to be very particular about getting accurate results. Below I will repeat the relevant portions of the test protocol that I suggested in the post where I was asking for people to send me data:

1. ... not relevant to this discussion ...

2. Use the same weight and CG for all tests, as close as possible.

3. Set the altimeter to 29.92, so you are reading pressure altitude. Fly all tests at the same pressure altitude.

4. Use the same rpm for all tests, and full throttle.

5. Use the same leaning technique for all tests.

6. Test only in smooth air.

7. Allow the aircraft to accelerate to its max speed. Be patient, as this will take several minutes. Record the IAS, OAT, rpm, MP, pressure altitude and fuel flow (if you have an indicator - this isn't needed for my data analysis, but it provides a quality control check on the leaning technique).

8. Measure the TAS by one of the following methods:

a. Record GPS ground speed and track on four runs in a box pattern. Use the NTPS spreadsheet to calculate the TAS. Report the TAS and the standard deviation from the NTPS spreadsheet.

Or,

b. Do flight testing to establish your airspeed system errors (see my Determing Static System Error page for details on how to do this). Then on each flight test, record the IAS, correct for your airspeed system errors to get CAS, then use OAT and pressure altitude to convert CAS to TAS.