Looking for TAS vs OAT data

[Kevin Horton]

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.

[AlexPeterson]

Kevin, great idea. Another variable is the dewpoint/temperature combo. Hot, humid summertime air contains a lot of water by percentage and reduces power. A dewpoint of 20C means about 2% water in the air by mole.

[Kevin Horton]

Originally Posted by AlexPeterson

Kevin, great idea. Another variable is the dewpoint/temperature combo. Hot, humid summertime air contains a lot of water by percentage and reduces power. A dewpoint of 20C means about 2% water in the air by mole.

Interesting comment Alex. I've always looked at this just from a density point of view, but I need to ponder this some more. Just looking at it from a density point of view, it is quite a small effect. For example, at sea level, standard day (59 deg F), going from completely dry air to very moist air with a dew point of 58 deg F only increases the density altitude to about 210 ft. The reduction in density is only about 0.6%.

But, looking at it from a mole perspective, if there is that much less O2 to burn, that would be more significant. Man, I wish we were having this conversation back when I was in university - I was a lot smarter back then . I've got to dig back into my old text books to help put your comment in perspective.

[Ironflight]

Originally Posted by Kevin Horton

Man, I wish we were having this conversation back when I was in university - I was a lot smarter back then .

You're right about that! I'm lucky to even remember that I have books to go look at !

[keen9a]

Is there a good technical book on reciprocating engine performance available these days? I can tell you quite precisely how to correct for humidity (dew point) for turbine engines, but I'm guessing we don't get the mass flow benefit in our Lycs!

[Norman CYYJ]

If we fly in high dew point wx do we have more horse power or less horse power available. There were once engines that used water injection to increase the power. So if we are filling the cylinders up with water does that cause them to run cooler, does it raise the compression level? Fodder for thought.

[Kevin Horton]

Originally Posted by Norman CYYJ

If we fly in high dew point wx do we have more horse power or less horse power available. There were once engines that used water injection to increase the power. So if we are filling the cylinders up with water does that cause them to run cooler, does it raise the compression level? Fodder for thought.

If the air is humid, it is less dense than dry air, so the engine makes less power. There may also be an additional effect of water effectively displacing O2 for combustion, but I need to ponder this a bit.

If we inject water, this is additional mass, over and above what the engine would suck in all by itself, so the total mass flow is higher than it would be with dry air. A turbine engine can make a bit more power because of the higher mass flow. I'm not sure if this helps a piston engine or not. The water evaporates, and this cools the air temperature, which helps on both piston and turbine engines.

[Kevin Horton]

Originally Posted by keen9a

Is there a good technical book on reciprocating engine performance available these days? I can tell you quite precisely how to correct for humidity (dew point) for turbine engines, but I'm guessing we don't get the mass flow benefit in our Lycs!

Ben - I've got some reference material on piston engine performance at work. I've got a couple of small reference texts from test pilot school, but they are in French. One of our flight test engineers has a reference from Pratt and Whitney (or maybe it was Wright). I'll take a look next time I have some spare time in the office (not until next week, probably).

As far as the effect of humidiy on turbine engine performance, all the info I have shows that dry air is more dense than humid air (assuming the humidity is not due to water being injected into the engine). I would have expected humid air to produce less thrust. You seem to disagree. I'm interested in figuring out the source of the discrepancy and learning from it. But, as turbine engine RVs are quite rare, maybe we should take this discussion off the forum - contact me at kevin01@kilohotel.com - I'd love to learn why higher humidity leads to more thrust in turbine engines.

For piston engines, Richard Shelquist has some info on an "official" SAE way to correct automotive engine dyno power for temperature, pressure and humidity. I'll see if I can track down a copy of SAE reference.SAE J1349 to see if it provides any info on how their corrections were developed. For what its worth, the official SAE correction for automobile dyno data shows that higher humidity results in lower power (in other words, if the air is humid, the SAE correction says you need to multiply the dyno result by a number greater than "1" to correct it to what it would be if the air was less humid). This suggests very strongly that piston engines make less power as the air becomes more humid.

The SAE correction seems to show that the adverse effect of humidity on piston engine power is more than 3 times as great as would be expected just based on the reduction in density. So, there is definitely more to this story than I suspected. Maybe it is Alex's moles at work

[fodrv7]

Kevin,
When we flew out of Hong Kong (Tropics) to Alaska (Arctic) we had no idea about "Cold Temperature Altimetery" until it was bought to our attention by the Canaks.
Subsequently, a section was inserted in the manual for corrections to the Minima under very significant IAS minus temperatures.

Do you want me to dig up the data and send it to you.

Pete.

PS. Carl Morgan flew over from NZ last week and we did some flight testing, engine out, and recorded it to Memeory Stick and then Carl did wizard stuff on my Mac with it and Decoded the data and we plotted it with Excel.
We learnt a lot, mostly on how I need to fly to get more reilable and easily identifieable data.
Carl has gone home and I am going to fly some more engine out descents and we are going to endevour to produce a Polar for MY RV-7. Firstly I shall go through your Pitot Static calibration.
I shall keep you posted.

Pete.

[AlexPeterson]

Kevin, I was only contemplating the displaced O2, not the density change. N2 weighs 28 amu's, whereas H2O weighs 18. The density change for a glob of humid air would really be small, but a 2% reduction in O2 is a 2% reduction in power. This would be noticable at higher speeds.

With the ability to really fine tune fuel flow as a function of egt, we notice lower fuel flows in hot, humid days vs winter by a large amount (at the same MAP/RPM), owing to both density and humidity, density being the larger player. For example, at 80F vs 20F, the ratio of molecules per volume would be (460+20)/(460+80)= 89%

Regarding what we've forgotten from the engineering college days - the only way I keep from crying is to notice that I must have learned something over the years that employers value even more.