Baffle Mod

[Toobuilder]

Great! It will be nice to see the results.

One thing I've never understood is the lengths the automotive, fan cooled engines (VW, Porsche, Corvair) go to to push air all the way around the cylinders with very elaborate baffling while the aircraft engines simply go only half way. With the much more demanding duty cycle of the aircraft engine, it would seem they would benefit the most. And for all practical purposes, the cooling scheme is the same - i.e. high pressure on one side, lower pressure on the other. The car drives a fan to deliver this pressure differential, the airplane uses forward motion. Why are the baffles different?

[dealfair]

Am going to duplicate your bypass duct Dan. Thank you for the good photos.
The washer has given me much improvement. I'm fascinated with how much cooling improvement is gained by just paying attention to the baffling details. Thanks to all for the info.

[BillL]

Here are some picts of my parallel valve. It appears to have some (small) depth for airflow. Do you think the either bump, or alternate method, or nothing needs to be applied to avoid temp imbalance?

Great thread BTW, very timely bump for me as the baffles are coming out of the box now.

[DanH]

Originally Posted by Toobuilder

One thing I've never understood is the lengths the automotive, fan cooled engines (VW, Porsche, Corvair) go to to push air all the way around the cylinders with very elaborate baffling while the aircraft engines simply go only half way.

The best (i.e. most recent) data for air cooled engine design is probably found with the SAE. I'm not a member and not curious enough to spend non-member money for SAE books.

Heat transfer from metal to air is governed by temperature delta. The air entering a full wrap baffle is at ambient temperature, so heat transfer from the hot metal is high. Further around the cylinder the air is now much warmer, and heat transfer is lower. The result is less and less cooling as the air proceeds further around the cylinder.

There's an early NACA work illustrating the issue. See Table 1 and Fig 2:

http://ntrs.nasa.gov/archive/nasa/ca...2009013404.pdf

It is interesting to note that our stock baffle scheme is something like Fig 2a; small exit radius, close entrance width, no exit duct...the worst configuration tested in terms of uneven cooling and energy loss.

To be fair the deformation of the cylinder from perfectly round to some unknown shape may be minimal. I don't know. It should also be noted that there is a difference between cooling cylinders and cooling cylinder heads. Obviously the fin area is shifted toward the exhaust side, and aluminum has a far higher ability to transfer heat by conduction as compared the steel barrels.

Like you I'm tempted to try full wraps. I have hesitated because of economics, not lack of curiosity; IO390 cylinders are very expensive. I've mostly concentrated on extreme sealing and a bit of airflow control to the regions left lacking. I do have partial wraps on the barrels between cylinders, thus each cylinder has symmetrical airflow.

Some of the EZ bunch have done radical cooling experiments. You might look there.

[DanH]

Originally Posted by BillL

Here are some picts of my parallel valve. It appears to have some (small) depth for airflow. Do you think the either bump, or alternate method, or nothing needs to be applied to avoid temp imbalance?

Again, nobody really knows exactly how much clearance is optimum. Field experience suggests that it's more than Lycoming has provided. Would I build in some additional room for airflow to the neglected bottom fins? Yes indeed.

Keep in mind that you want to keep the air between the fins all the way to the exit on the bottom of the cylinder. It's common to see sloppy baffle wrap fit.

Here the air just leaks out the side (small arrow) rather than proceeding all the way to the base of the head. The factory's positioning of the rubber anti-vibration comb didn't help at all:



Sometimes your engine shop contributes to the stew. Here they helpfully added a blob of orange silicone so the center baffle plate would not wear against the fin tips. As a result air flow to the preferred exit is compromised (big arrow), encouraging it to instead flow out the open side (circled arrows):

[dpansier]

Photos of a Formula One Continentals using both Silicon and Aluminum cylinder wraps.

[az_gila]

Originally Posted by DanH

Again, nobody really knows exactly how much clearance is optimum. Field experience suggests that it's more than Lycoming has provided. Would I build in some additional room for airflow to the neglected bottom fins? Yes indeed.

Keep in mind that you want to keep the air between the fins all the way to the exit on the bottom of the cylinder. It's common to see sloppy baffle wrap fit.


Sometimes your engine shop contributes to the stew. Here they helpfully added a blob of orange silicone so the center baffle plate would not wear against the fin tips. As a result air flow to the preferred exit is compromised (big arrow), encouraging it to instead flow out the open side (circled arrows).

I've seen an engine shop simply use a piece of this stuff -

http://www.aircraftspruce.com/catalo...baffleseal.php

bonded to the baffle to both prevent leaks at the edges and prevent wear. Spruce says it's good to 550F so contact with the cylinder should be OK.

[DanH]

Originally Posted by az_gila

I've seen an engine shop simply use a piece of (silicone baffle seal) bonded to the baffle to both prevent leaks at the edges and prevent wear. Spruce says it's good to 550F so contact with the cylinder should be OK.

Yep. The lower sections of my aluminum baffles are fully lined, so I don't depend on the aluminum to keep air between fins. The aluminum wraps are not much more than a backup to ensure the fiberglass/silicone wraps stay in place. Best I can tell none has shown any sign of debonding.

This is Loctite 598 (aka Permatex Ultra Black) rolled into ordinary 9 oz plain weave fiberglass between plastic sheets, then cut to size with scissors and stuck to the fins while wet.

[scard]

Originally Posted by DanH

Yep. The lower sections of my aluminum baffles are fully lined, so I don't depend on the aluminum to keep air between fins. The aluminum wraps are not much more than a backup to ensure the fiberglass/silicone wraps stay in place. Best I can tell none has shown any sign of debonding.

image clip

Wow, great minds... I used baffle material similarly between the baffle and fins. It is effectively self "bonded" in there to the fins from the heat. Of course mine aren't nearly as pretty as this powder puff .

[Toobuilder]

Originally Posted by DanH

...There's an early NACA work illustrating the issue. See Table 1 and Fig 2:...

Good stuff Dan. Fig 2(i) appears to have the best design from a TLAR perspective, and the the numbers seem to agree. This would be fairly easy to accomplish around the cylinder barrels, but the heads are going to require some thought. In any case, I think that there is a LOT to be gained without much additional work.

The "problem" I'm running into however, is that every improvement in cooling "power" also starts begging for a variable geometry cowl outlet. As I stated before, my most recent flight started out at 120 degrees on taxi, and was a still very warm 71 degrees at 8500 - yet I had two cylinders under 295 in cruise. Thats a bit too cool for best efficiency.