As the CHT Turns

[VictorBravo]

After spending 6 months building an "improved" set of cooling baffles, which did not work, I refurbished and reinstalled the original baffles. This I have already mentioned.

I test flew the airplane using the refurb'ed old cylinder baffles and my new lower crankcase (oil) baffle the other day. I expected the CHT gauge to read something like 300F or 325 F on a cool day.

It read 450 F on climbout instead.

Everyone under the sun has told me that the "spark plug washer" type CHT probes read very high compared to the other type.

So, to settle the dispute, I went and bought a couple of packets of those little stick-on labels that change color at a certain temperature.

I stuck 7 of these stickers on the hottest cylinder (#1) at various locations. I had to stick them to the cylinder itself instead of the fins as much as possible, soi I didn't get an artificially cold reading. I used a combination of 340-379 F range dots, and 390-435 F range dots.

Attached is a sketch of the results of this flight. The A-65 manual says that the max CHT is 550 F which is very very high in my opinion. They don't give a recommended CHT for climb and cruise, but most of the airport bums here say 325-350 on a cool day is good. But nobody has any info as to what the temps should be at various places on the cylinder. So, if one place on the cylinder is 400 degrees and another place is 340, what is the CHT??? When is it too hot and when is it fine? Any EXPERT knowledge and opinions are very sincerely requested.

By the way, the "oil baffle" works great. The oil temp stabilized at 168 F with 2/3 of the oil opening covered by an aluminum plate!

[Pearson]

Re: As the CHT Turns

Bill,

Those temps don't seem too high to me, especially if that was the hottest one. Continentals are built with a lot of tolerance, maybe that is why. Since the oil temp is good, especially with 2/3 of the opening closed off, I would think everything is good. Is there someone you know that rebuilds these engines professionally that might have a more detailed opinion?

[fearofpavement]

Re: As the CHT Turns

They aren't new cylinders or rings are they? If so, expect very high initial temps until they wear in some.

Are these temps different than what you had experienced before or haven't you previously flown this aircraft?

I've been following this project somewhat but don't remember the history of it.

[VictorBravo]

Re: As the CHT Turns

They aren't new cylinders or rings are they? If so, expect very high initial temps until they wear in some.

Are these temps different than what you had experienced before or haven't you previously flown this aircraft?

I've been following this project somewhat but don't remember the history of it.

They're not new, they're at least 300 hours old. The problem is that I did not have a CHT installed before. So I have no idea what the temps were other than oil temp. The big question I have is where on the cylinder should the "real" CHT be measured? I know that the temp is highest at the bottom spark plug, where the CHT probe is.

My question is if it is A degrees at the bottom plug, B degrees at the top plug, C degrees on the surface of the cylinder near the intake port, D degrees near the exhaust port, E degrees near the intake elbow flange, and F degrees on the cylinder near the exhaust flange, which one should I consider as the "real" CHT?

I made a decision to make an end run around the problem once and for all. I don't want to mention the actual solution here for a certain reason, but I believe that I will no longer EVER have a shortage of cooling air flow again. I'm tired of this particular problem, it cost me too much already, so I am going to shoot a rabbit sized problem with a deer sized rifle and be done with it. Please do not count the number of knobs in my cockpit next time you see this airplane!. No, I will not be planning to STC it until the FAA changes how it looks at little old airplanes

[Ragwing nut]

Re: As the CHT Turns

Naturally the temp will be higher around the exhaust port. Temps don't look that out of line. Since the combustion chamber is not a perfect hemisphere and the plugs are not centered, you won't have even temperature distribution. Then add cold air coming in from the top and hitting the front cylinders, and heated air being passed to the rear cylinders before going through the fins at an elevated temperature. taylorcraft cowling is not very efficient at all and it definitely becomes a problem when you put an O-200 under the stock cowl. You could have tried increasing the jet size in the carburetor, or even possibly add an oil cooler. I plan to try using the Ercoupe Alon oil tank baffles on the next one we build to force cold air around the oil tank.

Mike

[flyguy]

Re: As the CHT Turns

Bill,
Where are your cold junctions (where it changes from thermocouple wire to copper.) Mine are in the cockpit and as the gauge is calibrated for a cold junction of 75F then if the cockpit temp is 50F the reading will be 25 high. You probably know that, but I thought I would mention it.
Did I say that before I made some changes that my two bottom rear plugs were straddling 400F while climbing. One a little above and one below. I have yet to check it on a really warm day.
Darryl

[Len Petterson]

Re: As the CHT Turns

Just a opservation I did yeaterday. When I was cleaning pistions I found what all 4 pistons had hard a carbon deposits at the same points under the rings. Centerd at about 10 deg of the vristpin.
Is what a high heat point?
My CHT always read 350 to 375. Sensor is under the plug.
I have flown with the lower air intake covered and it has not affeted the CHT but the oil temp is up from 130 - 160 to 180 - 190.Good for the winter and meduim temp in the summer.
I consider the Oil temp accurat and CHT a guidline for operation.(;f
If anyone of the 2 goes out of normal range it is truble.
Len

[crispy critter]

Re: As the CHT Turns

Bill,
The CHT red line is around 550F,you are showing about 450F(on the gauge) during climb on the hottest cylinder and at or near it's hottest point....that's really well within reason. What does it read when you level off? You stated you had the probe hooked to the #1 cylinder at the lower spark plug....this is the hottest cylinder on the airplane with the least amount of airflow,the bottom plug recieves the least amout of cooling and it is right next to the exhaust port which is normally running about 1200-1400F.......your #1 cylinder is always going to be the hottest and it seems to me that yours is well within reason. I can't see where you have a problem at all....I also can't believe you've got your oil temps to run that low with partial coverage unless the outside air temps are 40F or below. One other thing.....your oil temps on continetal engine has nothing to do with the cylinder head temps....one does not affect the other.

[Jim Herpst]

Re: As the CHT Turns

To solve this "problem" I have a good solution. Remove the CHT gauge. lol from a flatlander that does not have use for a mixture control on an A-65 which would be my only reason to have a CHT (Sorry Bill I could not resist....) Have some fun being scientific and checking every thing out.....

[VictorBravo]

Re: As the CHT Turns

Thanks for the opinions guys...

Darryl, I never heard of cold junctions, thank you for the education!! Mine are inside the cockpit. I was flying on a 70 degree day.

Mike, I know that the T-craft cowl and baffle system is not great, and that's why I went on a Jihad for 6 months to try and improve it. But the way I went was the wrong way somehow.

Len, I am trying to get the results you got... a CHT reading of 350-375. Mine reads 450F when I am climbing with full power and it might even go higher if I let it.

Crispy, I understand that 450 is well within the published limit, but I do feel the published limit is very very high. When I level off, on a 70 degree F day at about 3000 feet MSL (which means about 65 F at that altitude) at 2100 RPM or maybe 2050, it reads 400 F. I got my oil temp that low (168 F in cruise, 175 on climb) because I made a MUCH better lower crankcase baffle than what it had. Mine is sealed to the case, even in between the pushrod housings, and it is sealed to the cowl with weatherstrip, and at the back end it deflects the oil straight down the front of the oil tank. I believe it is working about as well as that type of baffle can work. I disagree about no connection between oil and CHT. Especially because Continental and Taylorcraft built an airplane with only oil temp as the sole indication of engine operating temperature.

Jim, I'm almost sorry I put the CHT in now!