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CHTs vs OAT

jjconstant

jjconstant

Well Known Member
When testing the effects of cooling modifications on CHTs, is there a way to "normalise" OAT? In my experience the same climb at the same speed and weight on a 60 degree day and on a 100 degree day will not yield exactly 40 degrees increase in CHT at the same final altitude. This could simply be flawed observation on my part...

Do humidity and pressure come into play or are the differences minor (less than 5 degrees to my way of thinking when looking at temps between 350 and 430 degrees).
 
I got the the answer from Kevin Horton in another thread...this is cross posted in the flight testing section for the archives.

Kevin Horton said:
The standard correction for CHT testing is found in FAA Advisory Circular 23-8C, section 23.1047 Cooling Test Procedures for Reciprocating Engine-Powered Airplanes.

The test data is corrected to determine predicted CHTs on a day with 100deg F at sea level, and normal lapse rate from sea level upwards, as follows:

Predicted CHT = Test CHT + (100 - 0.0036 * test pressure altitude - test OAT).

Example:
Test CHT = 400 deg F
Test pressure altitude = 4000 ft
Test OAT = 75 deg F

Predicted CHT on a day where the temperature is 100 deg F at sea level is 400 + (100 - 0.0036 * 4000 - 75) = 411 deg F.

In other words, the standard correction assumes that if the OAT increases 10 deg, the CHT would also increase 10 deg. This AC suggests the same correction would apply to oil temperature. For cylinder barrels, they apply a 0.7 factor to the correction - i.e. the cylinder barrel temperature is predicted to increase 7 deg F for a 10 deg F increase in OAT.
Click to expand...
 
Kevin's is (not surprisingly) the same as the Lycoming chart. I just did the back calculations a few days ago to see how they plotted the cooling curves.

Good luck.
 
Air density is an important factor. Some certified turbocharged aircraft have their service ceilings limited by cooling difficulty at high altitude, despite the fact that it is cold at those altitudes.
 
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