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CHT - where would you set longevity limit ?


My Cessna 182 has a normally aspirated Lycoming IO540.
It is equipped with cowl flaps, and the engine now has near 1200 on it… Without any problem so far.
In the beginning I used to fly 50ROP, and then switched to Peak EGT, while learning from people here.

As this (ending) summer has been pretty hot, I gave special attention to CHT during my different trips.
The climb is never a problem, thanks to cowl flaps, with CHT at 350° or below.
In cruise at higher altitude (FL80+), where power is low and air is fin, and with cowl flaps closed, the hottest cylinder (#3 usually) ran between 370 and 375.
Cylinder #1 has always been the coolest, as it has direct airflow and no propeller governor to hide it.

My general question is: in cruise, how cool rhymes with longevity in your mind ?

Read this article and this article

Last Edited by Steve6443 at 22 Sep 19:39
EDL*, Germany

From above link:

“Historically, cylinders which continuously operate at higher CHTs, say, 400 degrees Fahrenheit and above, for long periods, will tend to need mechanical attention before cylinders that run in the 360s or 380s,” Townsend said.

I agree.

I too have been seeing 370F on the hottest, #6 or slightly less on #3, at high altitudes, despite the low power. No problem during climb, which is curious, but is probably due to thicker air.

Shoreham EGKA, United Kingdom

You are good. I used to see ~ 400F in the cruise on #1 while all the rest were 370-380F. It took a while to reduce the temp of #1.

Another factor to consider in piston aircraft baffling: the air may cool one side of the cylinder head better than the other side. This may result in an ‘out of round’ shape where the piston may begin scuffing on the cylinder wall with very bad effects. The lower the overall temperature, the less this effect is likely to occur. Anyway, the baffling needs to be in good shape; when Gami developed their Liquidair baffling for the IO550 , they placed 6 or more thermocouples on each cylinder, giving a very graphical picture of uneven heating and cooling.


Airflow under a cowling

One would struggle to do anything about that on a certified plane. The only way to install a highly customised airflow control system is to do a lot of testing, followed by a Field Approval, or some complex EASA process via a Part 21 company.

The best one can do is to make sure the baffles are properly sealed.

Shoreham EGKA, United Kingdom
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