Has anyone made baffles in a cowl to direct the air over a gas engine. I have a MXS-R and hait to cut a large exit hole in the cowl to cool the engine. Any suggestion and pictures would be helpful. Thanks

I don't think you can avoid having a large exit hole. I mean, even if you have baffles, the air needs somewhere to go!

There is a large exit area in the cowl.. the problem is where the air enters.... there relitivly small (see pic)

The exit area needs to be twice the intake area. There is no way around it. Without that, air won't flow into the 3 holes in the volume that it should.

Please post a pic of the exit. What needs to happen is the air should be drawn through the cowl and out. If you don't have enough exit area there will not be a partal vacuum created at the exit and no flow past the engine. As previously stated, 2 to 1 exit to intake is needed. The size of your inlets should be fine as long as you get airflow. I once made some baffles for an SBACH by carving some ovals out of foam and then covering with packing tape. I waxed the packing tape and laid some glass cloth and epoxy resin over that. Once cured the part can be split and peeled off the foam. I even tinted the resin red to match. In the end it only dropped the temp 10 degrees. This was on a twin BTW.

I am not sure about gas engines, but the golden rule for nitro engines is 1 in and 3 out. You can install baffles that will direct the intake air over the engine head which helps in cooling, but you do need a place for the heated air to exit. Good Luck, Dave

Even thro there are 3 inlet holes... only the bottom one is directly in front of the engine fins. It's 1" dia. The exit area is 5"x1" . See photos

I have made baffles in a number of planes to direct the air flow, usually in planes like the Sukhoi though. An engine doesn't require a lot of air flow but everyone is correct, you need about 2 to 1, sometimes more. From your photos I would try flying the plane before I worried too much about it. Looks like good flow in and out in that cowl. Give it a try beforwe you start doing anything.

That inlet hole is pretty small. Is the engine overheating? From what I can see you really can't open up the inlet so some baffling would be a good idea. It can be as easy as a box that is glued to the cowl that comes back from the inlet and encloses the cylinder

Unless you are totally dead set against working some more material out of the way, I, personally, would open up the intake in front of the engine cylinder some more.. Maybe with a sanding drum on a Dremel to open that hole to the point where the cowling starts to roll back toward the tail.. maybe take the sides out a little more just to be sure the whole jug is getting fresh air. That is where I would start, it would take the least amount of time. Unless, like I said, you are dead set against opening any more holes on it..

You can also baffle the upper cooling air inlets to help. If you want to get VERY efficient with baffling, split the cowl as would be on a full scale. Some work adding attachments, but you can build some first class, effective baffles.

Bedford

Hot air is hard to move. Cold air is easy to move and will route itself around hot air, trapping the hot air where it is.

Cowlings like yours present a problem because of the above. You've got 3 intakes, but two of them are not in front of anything that needs to be cooled. They are going to do no good and some bad. The air they let in is going to go straight to the exhaust opening. Hot air doesn't move cool air very well at all, so if it gets to the exhaust opening, it'll find it blocked by the cool that got there with momentum.

If you leave any intake open that doesn't point directly at something that needs to be cooled, you're actually hurting the intakes that do.

I've used screen in excess openings. When the openings in front of the engine are marginal, the other openings get baffles or plates behind them to direct their air at the engine.

The reason the 3-1 ratio is so popular is because of the above. Nobody wants to spend the time baffling the inside of the cowl, so they provide excess exhaust opening and cross their fingers. Very often that large exhaust hole is so large that it doesn't work well as an exhaust. Quite often they're cut out on surfaces that experience positive pressure. They're actually less effective when that happens. Sometimes putting a baffle back at the exhaust that shuts off one side of the inside of the cowl works good. Those work well if they block the useless intakes from getting a straight run at the exhaust hole.

Unfortunately, living in Florida can have some downsides. If you lived in Minnesnowta, your summer temps wouldn't usually cause any problems with what you've got there. I'd try angled baffles behind the two clear openings first. If you can see the baffle from the front and see all baffle, you'd be getting the most good out of them for your effort. Have then angled so they 'bounce' the air toward the engine.

BTW, did you know using lower nitro fuel actually can increase your engine temps? Nitro burns cooler than alcohol. Of course, running higher rpm counteracts that. Nothing is dead simple, is it.

There are two issues at play... cooling and keeping the pressure in the cowl from going positive and affecting the carb tuning. In order to avoid pressure, the 2:1 ratio must be adhered or a nipple added to the carb vent port and a line run to neutral pressure inside the fuselage.

As to the cooling, others have laid that issue out well... the intake has to be directed at the cylinder and head. I'd close (damper) the two upper ports about 80% and open the bottom one as much as esthetically possible. If that isn't to your liking, then it is a matter of baffling (redirecting the air flow) to redirect the intake of the upper two holes so that the air coming in them is forced over the cylinder and head.

The upper holes don't do much and may actually hinder the cooling. Temporarily plug the upper (inlet) holes and follow previous advice for exit (outlet) hole. Run engine and check temps with and without cowl in place. Open lower hole and corresponding exit hole as needed. Once satisfied remove one of the temporary plugs and check temps. If no change do the same with other. I have a 'gut' feeling the upper holes will need to be permanently plugged unless you build a plenum around head. Then you can duct air from upper inlets to plenum if you want. I know some of the CL speed guys have used pressure cowls but I have no experience with that, personally. The idea is to 'pressurize' air inside cowl around head so that more heat is absorbed (?) before it exits smaller outlet. I could be wrong about this. You pays yer money so you takes yer chances- right?

here what i did to my cowl and then i expoys some screening in

Baffles definately work.  Full scale break the rules of thumb many have stated here because of the baffling.  If you keep the air close to the engine you do not need as much air.  That is because with no baffling, much of the air going past has little to no cooling effect.  I would think 1" would be marginally enough with good baffling.  Might want to baffler air from the other two holes toward the engine for good measure.

close of the two non working holes with a balsa sheet circle. paint it black then install with some CA. all good and only took 5-10 mins of work.

This is true even at higher rpm....High nitro requires a richer needle setting. Richer also means richer in oil which carries heat away more efficiently.

The OPs engine is a gasser. A lot of intake air isn't needed to cool an engine, gas or glow. How the air is directed and exits is the key factor. With that cowl the main intake air is about perfect and the two top holes should help it exit and bring in extra air to help with the cooling of the inside of the cowl. Why chop up a nice cowl when one flight of the plane will tell the OP if the engine requires any more cooling. Just a thought.

Gray, I would agree that he will most likely get enough air through the botom inlet. The problems I see are that the upper inlets are doing nothing and may be hurting. Imagine filling a funnel too fast. At some point the liquid fills the funnel and drains slowly. Think of the cowl in the same fashion. If it gets flooded with air then the air will pass through the cowl slowly and not transfer much heat out. So the first thig would be to block off the upper inlets. Next once the air gets into the cowl from the bottom inlet it will go the path of least resistance which is around the cyilinder. There needs to be 3 sided baffle built into the cowl that directs the airflow through the fins. Of course this all depends on if the engine overheats at all. I think the OP should mix up some 32 to 1 gas, fire her up and get the needles set and then go do a 5 min flight, land and check the temps. Worry about it if he see's greater then 200 degrees after landing.

I think that is what I have been saying. You are correct but I think the outlet is big enough to allow all the air flow from all three holes to be pushed out? I may be wrong but there is only one way to know for sure and you just hit that nail on the head. you would know after the first flight if anything at all is required. If the engine boggs down at all in flight I would just shut it down to idle and land, let it cool off then get out that old silver tape and see what happens if the top two holes are plugged? I see no reason for baffles but a flight test would tell if they were needed too. I'm sure the planes designer has tested out the cowl during the design and flight testing. I wouldn't cut or do any mods to the cowl until after that first flight test.

Plug the top holes. The do nothing for you except add air you don't need. If the exit area is at least three times the area of the inlet you should be fine. The cylinder is directly in front of the bottom inlet so airflow should not be a problem. Fly the plane with and without the cowl to compare the running of the engine. If it "leans" out with the cowl on you have a cooling issue. If you need to close the top holes, just cut some round wood, paint it black and insert from the inside of the cowl and tack glue in place. No chopping required!

Good luck on your project!

Ken