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Cyl #3 and #4 show hottest CHT, too hot for my taste during climb

Trinidad Drivers:
I am trying to figure out why cylinder #3 and #4 indicate the highest CHT, higher than all other cylinders during climb. While analyzing the data from my engine monitor, I noticed the following:

Number 3 CHT is always highest during climb, with #4 following next. In particular #3 exceeds my programmed redline of 400 C, despite jus standard temperatures.
However, during climb #4 is always hottest and followed by #3. In other words #3 and #4 switch the top position.

What would explain that #3 and #4 switch the hottest CHT position?

The attached profile was at about standard atmosphere. Takeoff was at sea level, and the cruise portion, starting at about 17:40 at 9000 feet. Cruise was at 75%, and leaned right at the point where richest cylinder peaks. In another words “barely” LOP. I have GAMI injectors.

Engine EGT and CHT:

Original URL: http://tinyurl.com/oa5rljz

Flight Summary:

Regards!

P.S. For some reason my inline pictures don’t show. Hence the links.

[Images fixed – they didn’t show because the URL between the exclamation marks needs to be a “pure URL” which just points to the image itself. Yours point to a Onedrive page which then launches some sort of viewer which has a Download option. This system can’t be used to insert images into a webpage (like EuroGA is) because the server here doesn’t know how to get to the images. I just screenshot the images with Hyperdesktop and used imgur.com. See here for more details. There is also another issue with the “original URL” in that it contains some punctuation characters which the text processor here doesn’t like so I used TinyURL to re-pack it]

Last Edited by Peter at 13 Apr 07:21
Last Edited by Lucius at 13 Apr 03:51

Cooling delta BTW FF & Air for climb & cruise?…I see the switch happens during the flight phase…

EBST

The max CHT in cruise seems OK but the spread is quite large. This (together with the OK EGT spread) indicates uneven cooling airflow. I’d have the baffles inspected by someone that actually knows how to do that in the TB (i.e. not by the 95% that would do it). What I found to be very useful is to collect pictures of the baffle setup with all kinds of details from other owners and compare. The upper deck and lower deck should be well isolated with air only passing via the cylinder fins and in order to improve that, you can go further than the manufacturer with additional baffling and high temperature RTV.

As to the large CHT spread during climb, this could probably be improved by optimizing the GAMI spread. Your hottest cylinder is also the one that runs rather lean compared to the others. Try to get the EGT lines closer together. It is quite normal for different cooling characteristics in climb and cruise and thus different cylinders being the hottest.

What was missing in your graph is the fuel flow. Make sure the takeoff/climb FF is at the top of the range given. The TB cooling airflow is rather limited (i.e. optimized for speed), it’s not a plane you can regularly operate at Vx or Vy without frying the engine. That is not a problem though, other airplanes that have a 500°F CHT redline only have that because 30 years ago, manufacturers didn’t understand the harm done by high CHTs. Engine cooling is responsible for about 30% of the total drag which means that all aerodynamically optimized airframes (e.g. the TB) are more problematic in temperature management.

The CHT spread seem fairly normal.

The lowest is 280F and the highest is 365F.

Here is my typical one, showing 290F and 355F respectively

I have re-done my baffles pretty comprehensively – pics here

I have not heard of any suggestion that too-low temps are an issue, though obviously too-high temps would be. But yours are fine. The CHTs also scale directly with the air temperature, obviously.

Here are typical temps

Normally #3 is the hottest. #2 is the coldest. A very small difference in the airflow makes a big difference to the CHT, and the airflow around the two front cylinders will be very sensitive to how well the lower cowling fits in that area.

Last Edited by Peter at 13 Apr 08:51
Administrator
Shoreham EGKA, United Kingdom

@Vref, yes, I meant to say that the switch happens during cruise flight. I mixed up the word “climb” and “cruise”.
@Peter, @achimha thank you for your responses!

Hello,

I have a TB20 GT 2001 aircraft. I have been looking for pictures of the baffles that are located on the front part of the aircraft.

I found some very nice pics of the back, and sides but nothing forward of cylinders 1&2..

My engine works great while cruising but on climb out the cylinders start climbing to the 405-415 range.

If someone could share a picture of what the front baffling looks like, i.e. how does the bottom baffle line up/fit into the front part of the top cowling?

Is there any baffling that is installed around the front part of the engine, pictures please?

thank s in advance.

What airspeed do you use in climb? These effieicient touring aircraft are just not made to climb at near Vy and simultaneously remain will all CHTs below 400 degrees.

Also, you don’t throttle back in climb, do you? Mixture?

Frankfurt (EDFZ, EDFE), Germany

If you do a search for “baffle” you get some hits. The main thread is probably this one

In a TB20 (I have a TB20GT) you can climb at Vx or Vy for a short time, say 1 minute, and then you must transition to 120kt or so, for adequate cooling.

Short climbs, say to 7000ft, are done with all three levers fully forward, all the way up.

Longer climbs are best done with the constant-EGT method, see here

Administrator
Shoreham EGKA, United Kingdom

Why would you not lean the mixture during a climb to 7000 feet?

Frankfurt (EDFZ, EDFE), Germany

Because that

1) Limits your max engine power to 75%
2) Requires a pretty high airspeed (= shallow climb) for cooling

I have to admit that I usually do full rich (almost) full power climbs to the cruise level. The reason is that with the turbo, the climb rate is so much higher that way, that I like to get there fast and then switch to cruise. A cruise climb setting gives a very lengthy and shallow climb.

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