Insane CHT's during Lancair Test

[E1453]

I've just read an article in CAFE FOUNDATION's Aircraft Performance Report about the Lancair IV-P homebuilt.

Well, the Lancair seems to be an incredible single, now the first in my list of future aquisitions, just above the SR22.

But an important detail shocked me: the peak CHT during climb to FL250: 466F ! Just insane!

The engine was a Lycoming TIO-540:

360BHP, 8,5:1 compression ratio, double supercharged, intercooled, 35in/2700rpm at full power.

I found this compression ratio strange, but, the manifold pressure is not so high anyway and the injection system is designed for a super-rich mixture (the TO fuel flow is 40 gal per hour!). It looks like a superenrichment is taking place, for cooling, maybe because of the high compression ratio, wich I think is a necessary to keep power at high altitudes (is it?).

Well, how can anybody see 466F CHT and think it's normal? Oh, maybe because the red line and the LIMITATIONS section in the POH (wich is just what many pilots read, anyway), says that anything below 475 F is within the "green arc".

[Genghis the Engineer]

I'm not sure that I get your point.

The aircraft is in a max power / max rate climb and is showing a CHT just below the red line. It's quite common on aircraft with less than perfect cooling (which is often the case with something like this primarily designed for performance) for the best climb speed to be set as slow as they can commensurate with not overheating the engine.

Now if it was showing that in the cruise, I'd be a bit worried, but not at a best rate climb.

G

[411A]

All TCM turbosupercharged fuel injected engines use fuel enrichment at full throttle/max power, and I would expect the Lycoming engines do as well.
No separate ADI for cooling, so fuel is used, and is a well known procedure...for a very long time.

The upper CHT limit on my TCM turbo motors is 460degrees F, and at a full power climb (no 5 minute limit with these engines) the CHT approach 440.

Having said this, these engines have aluminum cylinder heads, and the aluminum alloy used will exhibit stress cracking if the temp exceeds 410 F(approximately) for prolonged periods.

This leads to compression leaks/cylinder failure down the line...and big bucks to fix.

Better to keep the CHT 380F or below, for optimum results.

[E1453]

I would never let my engine's CHT's reach 466F, for any period of time, even during takeoff. Althought the manual says that below 475 (Lycoming) or 460 (TCM) is within limits, Lycoming says, at its website, that, to achieve the recomended TBO, the CHTs should not exceed 400F.

From Lycoming website:
"Minimum in-flight CHT should be 150ø F (65ø C), and maximum in most direct drive normally aspirated Lycoming engines is 500ø F (260ø C). Some of our higher powered more complex engines have a maximum limit of 475ø F (245ø C). Although these are minimum and maximum limits, the pilot should operate his or her engine at more reasonable temperatures in order to achieve the expected overhaul life of the powerplant. In our many years of building engines, the engines have benefited during continuous operation by keeping CHT below 400ø F in order to achieve best life and wear of the powerplant. In general, it would be normal during all year operations, in climb and cruise to see head temperatures in the range of 350ø F to 435ø F."

Well, 466F during climb is not normal to me, and not for Lycoming, too. Also, a less perfect cooling must be fixed, or the climb procedure revised, in order to achieve a temperature that don't compromise the cilinder's life.

A maintenance/safe/cost-wise pilot would not climb with such high CHT.

[dirkdj]

I fully agree that 466°F CHT is criminally high, even if Lyc 'approves' it. Engine longevity is not one of their priorities apparently.

On a TCM IO550 engine, somewhere in the 420-430°F range a thermal runaway will occur if the CHT's are allowed to go that high. What happens is that the cylinder gets heated unevenly and becomes oval while the aluminium piston inside stays round. This will cause scuffing and because of higher friction, more heating will occur, with more scuffing until after a short time the cylinder and piston will be damaged.

This phenomenon was observed on an instrumented engine with eight (8) CHT sensors around the circumference of every cylinder.

Even with a JPI you have only one CHT probe per cylinder. So, when the runaway heating occurs, it is too late already and only a massive power reduction can save the cylinder.

My personal CHT limits are 380°F for operation with 400°F as a never exceed limit (do something right now).

This is an area where the manufacturers redline doesn't mean a thing, it was determined when the manufacturer could prove to the FAA that his engine would not go into detonation at this CHT level. This is on an engine test stand with each cylinder individually cooled by shuttered air hoses, an optimal condition never achieved in a real cowling.

It looks like the Lancair builder will have to spend some more time on baffling, cowl flaps, etc if he wants a long life out of his engine.