Enlarging the Cowling cut-out to improve CHT cooling.

[bakfly]

Hi All,

My Aerovee Engine has now be running for more than 35hrs (33hrs in the Air)
The engine is running very well during high power and idling. The climb performance is ok but the problem is, I can only climb at maximum climb performance at high power for maybe one or two minutes before especially #3 CHT becoming to hot >216C (420F) outside temp. around 30C (87F) . After climbing at about 80 kn (90 M/hr) and reducing power slightly it stays below 216C. Cylinder #1 is about 10C cooler while the front ones are about 20C cooler. The Baffling and Cowling are made following the Sonex specifications. The EGT stay always in the green.
Has anyone had this problem? I am thinking to enlarge the Cowling cut-out at the bottom to improve cooling. Will that help?
My compression ratio is 1-7 and I am still using 100LL fuel. Cylinder head temps are lower during cooler days.

Cheer,

Peter Bakker,
Sonex #1430, Aerovee 2.1
Australia

[jaywa]

Peter

A standard approach is to try and maintain laminar flow in the inlet section to improve heat exchange - fill and shape the inside of the upper lip entry to prevent turbulence at entry of the cooling air , and try and fill and shape the back fence ( on the underside of the cowling ) where you can try to induce downward smooth laminar flow through the fins.
I have also enlarged the exit only because I have a larger motor ( Revmaster 2300 ) with associated larger heat load

Good luck

[RodgerC]

G'day Peter

I was in the same boat but I seem to have got mine under control. I fly out of Kilcoy QLD. Before you get too distressed though, are you reading from under the plugs or the Sonex recommended position? The first thing I did though was double the area recommended by Sonex. The second thing was experiment with aluminium foil tape to ensure the air MUST pass through the fins on the heads. I converted that now into baffles.

At 40ish hours I'm now seeing 390-400 in extended climbs and 320-350 in cruise. On descent it will fall below 300.

You'll get there.

[bakfly]

Thank you for your reply, I am reading CHT from the Sonex recommended position. Rodger, after doubling the area recommended by Sonex on the bottom of the Cowling how many degrees reduction in CHT did you get.

[MichaelFarley56]

One other point to consider; I had moderately high CHT temps until I finally realized my secondary ignition was slightly advanced. Follow the manual on checking CHT readings in cruise by turning off one system at a time to see if temps match. If your secondary timing is advanced like mine was, your temps will be higher. I ultimately adjusted mine and my CHT in cruise dropped nearly 20 degrees F.

[RodgerC]

Pete

For me things seemed to improve in ~15 to 20F increments. Relocating thermocouplesfrom under plugs =20, Enlarging outlet by 100% =20 and aluminium tape/close-fit baffles =15.

I'm in Vietnam for a couple of weeks but when I get back to Oz I'll take some photos of baffles (including those lower intercylinder) and email them to you.

Regards
Rodger

[radfordc]

I found the same thing as Mike. When I adjusted the secondary timing so that it wasn't quite as far advanced the CHT dropped. Later on the electronic ignition failed on a long cross country trip and I flew on just the magnetrons....and the CHT dropped even further. I couldn't detect any change in the way the engine ran on just the primary ignition.

[gammaxy]

Later on the electronic ignition failed on a long cross country trip and I flew on just the magnetrons....and the CHT dropped even further.

Do you know what caused the electronic ignition to fail? Were you able to start the engine on magnetrons only? I've only made some minor efforts to start on magnetrons only (and not been successful), but it would be good to know that it is an option.

[radfordc]

My electronic ignition system was one of the early ones....Sonex has changed components since then. One of the triggers shorted, which in turn burned up the coil, which in turn blew the fuse. Anyway, it happened in flight and I had no idea anything was amiss. The next flight the engine started "harder" than usual. When I did the mag check I found that the secondary was completely dead. I wasn't able to make the fixes until I returned home so I just continued to fly on the mags. This was when I noticed that the CHTs seemed 5-10 degrees cooler than normal.

After repairing the electronic ignition I tested to see if there really was a CHT change. I took off and flew for 20 min on both ignitions and noted the temps (365 ish). Then I shut off the secondary and flew for another 20 min. Sure enough the CHTs dropped to about 355-360. This was a repeatable observation.

[mike.smith]

When I had high CHTs it ended up being because I was running too lean on the #2 needle, though since your EGTs sound low this may not be the problem. I found out for sure by monitoring my fuel flow, which was 2.8 gph full rich! Changing to the #2.5 needle and explicitly following the AeroCarb instructions fixed all that. 3.8 gph now when leaned back in flight.