Sonex aerovee cowling

[Brett]

Hi all,

The age ol post coming up again. Going through the first few flight process and seeing high CHT,s.

I had really hoped this wouldn't happen. Initial flight indicated lean but not excessively. I never had EGT,s over 1400. I now have the mixtures slobbering rich and EGT's are around 1100. I managed about 1 hour total flight now by taking off without too much ground running getting to about 1000ft then backing off to 2500rpm @ 24-26 " mp. This works out to be about 95kts. If I try to climb at any sort of real power setting it will skyrocket the CHT's straight past 450f . I have also tried the secondary ign off with no real change.

Question is , what cowling outlet size do people with cool engines have?

I have very strictly followed the Sonex plans down the exact shape of the inlets as described. I can't see any leaks possible in the seals and have plugged up any slight gap. Top mount oil cooler as well.
The lower outlet seems awfully small.

Suggestions for exact outlet sizes would be much appreciated, or a photo or two would be fantastic.

Thanks


-Brett

Btw

Typical day temp around here lately has been about 28-30c. Now we are coming into winter perhaps I can get the engine broken in to aid the problem

[wlarson861]

The recommended outlet size is probably correct except they don't figure in the space taken in by the engine mount cross member at the bottom of the firewall. I added an inch forward on my opening and lowered my temps considerably. Also remember that a new engine will be very hot until about 40 or 50 hours(my experience). If you can keep the CHT's under 450 for 50 hours you will be surprised how different the engine behaves after that.
The big thing is the advance on the ignition. retard it and see what change it makes.

[Brett]

Thanks Bill, that does make sense about the engine mount right in the outlet area. I hadn't given that much thought before.

Seeing as it's now a weekday I have sent tech support an email so will see what is suggested and possible go to work on the cowling with a dremel on the weekend.

[jjbardell]

I am also struggling with CHT temps. I have talked to Kerry and just sent a follow-up. To recap my issues:

Mine were running 420-450 with RPMs of 2,600 at 31"MAP. Speeds of 110mph. I couldn't climb past 300ft/min. I tried the tuning per the directions several times. So, we tried a different approach. Yes I am aware I am open to ridicule from everyone (so please keep the negative comments out...It's experimental and I am experimenting). The directions just were not working. I set the mixture rich on the ground (just to the point of black smokey exhaust), then leaned it to eliminate most of smokiness, but not all. I performed a run-up and readjusted my idle speed (as I would expect, the idle dropped down to 600...so I tuned it back to 800). I have to lean the mixture control to keep the engine alive on the ground. It runs rough on the ground and I have to keep the RPMs up (1100+). I can climb at 700+ft min.

But, when I took off, the temps dropped from 415 to 355-365. My RPMs jumped from 2600 to 3000 and speeds went from 110 to 135-140.

I thought that was my solution. Landed and observed slightly higher idle RPMs. I figured this was the warm oil and lower oil pressure making the engine spin easier. This was Saturday.

Sunday I went flying again. Changed nothing on the plane. Idle speed was still slightly higher. I went for a 3 hour flight. First half the temps were slightly warmer (380s) but everything else was great. I stopped for lunch and flew home after. Idle speed was higher yet (back to 1100) and temps jumped right after takeoff. Mixture was full rich but it clearly felt lean again. Temps were 400-410 for the short flight home with 31" map at 2600rpm. Same issue as before.

It seems as though the mixture is leaning out as I fly. I confirmed the set screw was tight against the needle. I am at a loss. I can richen it back up and then go fly and all is good. I am on the 2.5 needle and contemplating the 3.0 needle. I don't have the cross bar (tri-gear) but am also considering going from 50sq in to 60 sq in on the exit.

Let me know if you find the magic solution.
Josh

[Waiex 49]

For what it is worth:

Back in the day when I was still flying with an AeroVee, I was having high CHT's.

I looked at the cylinder head fins, and found a great number of the cooling air passages were blocked by "flash" material left over from the manufacturing (casting?) process.

I carefully removed this excess material (I actually removed the heads so I could do this on a work bench) and was able to open up the cooling air passages to their designed specifications. I believe there is a brief mention of this in the AeroVee manual, but (as best I can recall) there are no specific instruction steps.

With this step, and with eliminating air leaks in the fence baffles, I was able to bring down the CHT's.

Best of luck,

Don Bowen
Waiex 49

[Rynoth]

I looked at the cylinder head fins, and found a great number of the cooling air passages were blocked by "flash" material left over from the manufacturing (casting?) process.

I carefully removed this excess material (I actually removed the heads so I could do this on a work bench) and was able to open up the cooling air passages to their designed specifications. I believe there is a brief mention of this in the AeroVee manual, but (as best I can recall) there are no specific instruction steps.

Now this is interesting... I have the same casting material on my heads (it basically divides the top and bottom sides of the fins) and it seems pretty thick. I always thought airflow would be better without that material, but it's so uniform and thick that I never once considered actually removing it, thinking it was an integral part of the construction of the heads.

[John Usher]

Steve Bennett from Great Plains A.S. discusses this in his EAA video. The video is a must watch for anyone building a VW aircraft engine regardless of the "brand".
http://www.eaavideo.org/video.aspx?v=2117083972001

[edit: new link https://www.eaa.org/videos/2117083972001]

[radfordc]

Yes, you do need to open up the passages in the head and also sometimes between the molded fins. I used small drills to drill through the flashing and then needle files to open them up. You can use an angle grinder with a thin cutoff disk to clean out between the fins. Be very careful not to cut into the exhaust ports.

[kevinh]

Yes, you do need to open up the passages in the head and also sometimes between the molded fins. I used small drills to drill through the flashing and then needle files to open them up. You can use an angle grinder with a thin cutoff disk to clean out between the fins. Be very careful not to cut into the exhaust ports.

In fact, I had to do this with my brand new lycoming clone engine on my RV7. It really helped with high CHT temps there as well, so even PMAed castings have this issue. ;-)

[Waiex 49]

Here is a before/after photo of rework done on a VW cylinder head.
Not sure if this is the exact same head as used on AeroVee engines, but it should serve to illustrate the issue with casting flash.