Kevin Horton
Well Known Member
Some people are interested in keeping a close track of their aircraft's performance in level flight, so they can evaluate the effect of small modifications. This requires some way of dealing with the fact that performance varies with temperature, and the ambient temperature varies quite a bit over the course of a year.
Conventional wisdom suggests that on days with non-standard temperature, the test altitude should be adjusted to achieve a target density altitude. This approach would work very well if the engine power available was a strict function of density altitude. If this were true, we should get the same TAS, no matter how the temperature varied, as long as we kept the density altitude the same.
However, the temperature correction that Lycoming publishes on their power charts suggests that the engine power is not a strict function of density altitude. If we have a warmer than standard day, and we lower the test pressure altitude to achieve our target density altitude, the power available will be higher than if we tested at that density altitude on a standard temperature day. Thus the TAS will be higher than it would be on a standard temperature day. All the above assumes that Lycoming knows what they are talking about with their temperature correction.
I would like to gather some test data to determine if Lycoming's temperature correction fits real world data. I need several people with aircraft with constant speed props who are prepared to do TAS tests on several different flights on days of different temperatures at altitude. No changes that could affect drag or power should be made between flights. Once I have enough data, I will analyze it and publish my conclusions here. I have an idea for a better way to correct TAS for non-standard temperatures, but I need to get some data from several aircraft to see if my concept will work.
Interested parties should adopt the following test protocol:
1. Choose a pressure altitude that you will use for all these tests. It should be high enough to offer smooth air, but low enough that we get as wide a variation as possible in air temperature. Use your local knowledge to pick the pressure altitude - it doesn't matter to me what altitude you use, as long as you tell me what it is, and use the same one for every test. The test series may go on for several months, to help get a range of temperatures.
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.
Send me the data from all your flights where the data is of good quality. I want to get IAS, CAS (if you used method b above), TAS, standard deviation (if you used the NTPS method), OAT, pressure altitude, rpm, MP and fuel flow (if your aircraft has a fuel flow indicator). Also tell me what model engine and propeller you have. My e-mail address is khorton01 AT rogers DOT com.
It may also be interesting on the same flights to do another TAS test at the density altitude that is equal to the target pressure altitude. This would allow the consistency of results from the conventional constant density altitude method to be compared to my proposed constant pressure altitude + analytical correction method. If anyone is interested in doing this, contact me and I'll provide a table of OAT vs pressure altitude to give your target density altitude.
Conventional wisdom suggests that on days with non-standard temperature, the test altitude should be adjusted to achieve a target density altitude. This approach would work very well if the engine power available was a strict function of density altitude. If this were true, we should get the same TAS, no matter how the temperature varied, as long as we kept the density altitude the same.
However, the temperature correction that Lycoming publishes on their power charts suggests that the engine power is not a strict function of density altitude. If we have a warmer than standard day, and we lower the test pressure altitude to achieve our target density altitude, the power available will be higher than if we tested at that density altitude on a standard temperature day. Thus the TAS will be higher than it would be on a standard temperature day. All the above assumes that Lycoming knows what they are talking about with their temperature correction.
I would like to gather some test data to determine if Lycoming's temperature correction fits real world data. I need several people with aircraft with constant speed props who are prepared to do TAS tests on several different flights on days of different temperatures at altitude. No changes that could affect drag or power should be made between flights. Once I have enough data, I will analyze it and publish my conclusions here. I have an idea for a better way to correct TAS for non-standard temperatures, but I need to get some data from several aircraft to see if my concept will work.
Interested parties should adopt the following test protocol:
1. Choose a pressure altitude that you will use for all these tests. It should be high enough to offer smooth air, but low enough that we get as wide a variation as possible in air temperature. Use your local knowledge to pick the pressure altitude - it doesn't matter to me what altitude you use, as long as you tell me what it is, and use the same one for every test. The test series may go on for several months, to help get a range of temperatures.
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.
Send me the data from all your flights where the data is of good quality. I want to get IAS, CAS (if you used method b above), TAS, standard deviation (if you used the NTPS method), OAT, pressure altitude, rpm, MP and fuel flow (if your aircraft has a fuel flow indicator). Also tell me what model engine and propeller you have. My e-mail address is khorton01 AT rogers DOT com.
It may also be interesting on the same flights to do another TAS test at the density altitude that is equal to the target pressure altitude. This would allow the consistency of results from the conventional constant density altitude method to be compared to my proposed constant pressure altitude + analytical correction method. If anyone is interested in doing this, contact me and I'll provide a table of OAT vs pressure altitude to give your target density altitude.
Last edited: