Why is CHT so dependent on the RPM?
Reading this again after a long time reminded me of several flights this summer, when I was having trouble managing the CHTs during high altitude (FL180-FL200) flights, working hard to stay above nasty convective tops but suffering high CHTs (approaching 400F) due to the very low IAS (around 90kt; ~130kt TAS).
Despite the crappy weather all over Europe, summer 2012 was particularly notable for high temperatures at high altitudes. I recall a flight over the Alps at FL160 at 0C, which is ISA+17.
At such altitudes, one has to run at max RPM just to get the engine to suck enough air to produce enough power...
One thing I noticed was that the last 100RPM, say 2475-2575, really affected the CHT. Going back to 2400 which is where I normally fly, would drop the CHTs by some 30F.
Can anyone think of why?
Well, as you are not turbocharged, you can't easily compare the same power output with high MP/low RPM on the one hand and low MP/high RPM on the other hand at such altitudes. This would be an interesting comparison. I have not done it at FL200 but I generally see a bit lower CHTs when using higher MP/lower RPM to produce a certain power output. Higher friction and more ignitions per second (i.e. less time to dissipate heat) could be reasons.
I would guess that the cooling airflow is just too weak at your density altitude. The TB20 does not have cowl flaps, does it?
I generally fly with the lowest RPM possible to achieve a certain power setting. My economy cruise is 2100RPM at 23" and increasing to 25" when going a bit faster.
did you fly LOP ?
Peak EGT, as always.
Hi CHT has also to do with fuel. At high alt you will be at a lean mixture, this won't help keeping your CHT low. The more fuel is going through your engine the lower your CHT will get. The fuel will help with cooling your engine. At such altitudes the energy generated by high RPM's will add significantly to CHT, so dropping to 2400 rpm will help in reducing the energy generated as Achimha already explained. This is only part of a contributing factor to high CHT.
Running LOP does not lead to higher CHTs. Running lean while ROP does. Peter, why do you run at peak?
At high altitude (FL170+ for me) one has to go ROP for best power - 125F ROP or so, because the operating ceiling is achieved at the highest engine power setting (obviously) which means max revs (2575) and 125F ROP.
Below that altitude I fly at peak EGT or a little LOP. I don't fly "deep LOP" because
I also fly at lower revs if max revs are not needed, and 2400 is nice and smooth. 2200 gives the best economy, for very long flights with no headwind.
if you fly about 70 °F LOP at FL80 and above, you don´t harm your engine. Running rough indicates an ignition problem or an uneven fuel distribution.
What Spark Plugs are u running? Finewire or massive from Champion or different? Had that rough running problem as well and changed to Tempest Finewire. A huge Improvement! What is the measured difference ing gal/h between 1st and last cylinder peak? Should be less than 0.5 gal/h. If it is more, uneven fueldistribution may be a contributing factor.
Try to read Mike Busch or John Deakin at EAA or AVWEB. In the last 10 years, there have be mode a lot investigations about piston engines. Running engines at Peak EGT or even 50F rich of Peak is the biggest stress for the engines (red box).
Hard way to leave the old teached way, but look at it, might change fuel consumption and engine life (to the better end) .
p.s. the high CHT at FL170 may be caused by high AoA as long as you are non turbo. With higher AoA engine cowling does not fill quite good with even thinner air. Check baffling as well.
Sorry! Should be: if you fly about 70 °F LOP at FL80 and below .....
I've been reading Deakin for 10 years and know of the various issues and methods. Have GAMIs too before after. (the comments in the graphs re unstable fuel flow were to do with a defective fuel flow turbine location).
Plugs are iridium Tempests; after the defective resistor debacle with Champions I won't touch them again with a bargepole. But I put them in only recently and don't recall testing "deep LOP" recently.
However, my original question was why does CHT go up such a lot at the top end of the RPM range. I guess it is a combination of the extra power produced and the reduced airflow at ~FL200.
Obviously I have to go to 100-125F ROP (best power) to be at FL200 anyway, so the whole LOP discussion is immaterial in that case.
Why should FL080 above/below apply? Do you mean this being the Lyco-specified 75% power level? I don't think this has any significance, beyond preventing pilots who do not know anything about engine management from leaning the engine when above 75%.