Any rule of thumb to use in the amount of cooling ventilation to be cut in a cowling?

I am getting ready to cut a new airplane's cowl. Currently the only cowl opening is for the prop shaft. There are a couple of flat areas to either side of prop shaft opening I am planning on cutting open for air flow. I am installing an OS 120 AX and will have to make a cylinder head side opening cut as well.

You'll need more exhaust area (hot air out) than air intake area by about 3:1. Air expands as it's heated, this accomodates that. That said, glow engines get only a small part of their cooling from the cylinder fins, most is taken away by the fuel and oil, so they're not terribly easy to overheat. The exposed head will take care of alot of things. I'd suggest an opening in the bottom rear of the cowl to help draw air through the cowl.

If this were a gasser, it'd be much more critical, and you'd need to learn about baffling. Not the case here.

J

Steve Steinbring,

Here are a couple pics of my Aeroworks YAK 54 .90-1.20 running the OS 120AX. The head of the engine is not exposed but there is more exhaust air going out than there is coming in.

I do 2:1...twice the hole out as in..never had a heat issue

Zippi,

Thanks, may I ask what prop you are running with that airplane setup?

I'm building up a GP Venus II for a trainer pattern ship. After attending a pattern meet some months ago it is capable of doing more than I can right now.[] I want to slow the aircraft down and still have good vertical, so the proper prop is an issue.

Many years ago I flew some ballistic pattern, but my eyes and reactions are not at that level anymore[]

Thanks again for the help!

Steve

I'm in the same boat as you. My eyes are not as good as they once were. I am running an APC 17X6 prop with a Bisson pitts style muffler. It runs somewhere in the neiborhood of 9300 rpm. This give the plane unlimited vertical and the speed is not to bad.

Cooling isn't a simple deal for most folks.

You really need to have an intake area the size of the front of the engine, and you need to place that area in front of the engine as well. Then you need to have the exhaust opening behind that engine. If not directly behind, then as close as possible. You want the air coming in to be directed and routed close to the engine when possible.

Control line proved years ago that the heat exhaust opening can be as small as 1.4X the intake opening and work great. But that's with the "pressure cowl" rules just listed.

It also helps to have the heat exhaust opening in an area of negative pressure. Lot's of times, if the exhaust area sees enough negative pressure, the intake really works great.

Also, what you don't do is as important as what you do. Don't have openings that let cool air in where there is nothing to cool. If you do that, the cool air will basically trap the hot air around the engine and the hot air stagnates, doing more damage than it would otherwise. Openings that aren't in front of what needs to be cooled can screw up the picture.

Cool air is easier to move than hot air. So cool air coming into the cowl will move around the hot air that needs to be pushed out, and winds up pushing out whatever cool air it finds. And that flow of cool air basically can take over the exhaust opening.

What da Rock said is good advise and the use of baffles is a good idea and probably more useful on gassers in my experience. With out going into a lot of cutting and baffleing during the build it's best to set the plane up the way you think it will be getting enough air flow and fly it. If it developes a heating problem then it needs to be addressed but most of the time they'll probably be fine. I've been flying since the early 80's and have flown planes from Cox 049 engine to Zenoah G62 gassers and I've never used a baffle. I'm not saying that they are not needed but just use a little common since during your build. I have seen planes at the field shut off in the air from the engine getting to hot and after looking at the plane there was plenty of air coming in but the only opening on the bottom of the plane was were the exhaust tubes came out. Like I said, you have to use a little common since and let the air coming in have room to get out.

Zippi,
Experience is an excellent teacher, thanks for sharing.
I mentioned the baffles because often there is only one heat exit and it isn't behind the engine. When there are two intakes and one exit, or exit area is restricted, the solution is often to either block the off-side intake or to baffle it. Baffles are really easy and quick to make. And forcing the air over to the engine is usually better than simply blocking it.

You know, the best plan for a good cooling setup starts with the exhaust and moves forward. My two pictured models could easily be setup with 1.5x exhaust area considering the opening in front of the engines, but that was about it unless some massive hacking was done to get more exhaust area. So both models needed less intake to have a good intake/exhaust balance. With the Decathlon, the intake area in front of the muffler actually had better flow to the exhaust than the engine intake. So that one really didn't want or need that offside opening that wasn't in front of the engine or the muffler. That opening was definitely not wanted if any hope of having at least 1:1 was the goal, and 1:1.4 is the minimum goal.

I'm flying today and the prediction is for temperatures in the 100s. It's funny, but the guys talk about my models having "closed openings" and they always wonder how my engines run cooler than theirs. A couple of them have cooked engines. They usually blame the last needle setting.

It's actually a simple design layout. At least 1.4X size exhaust to intake. That means making a decent exhaust AND closing off or baffling intakes that are "surplus" and in the wrong place.

But Zippi's idea is good too. Try what you think should work and find out.

Thanks gents great imput! On my cowled airplanes I have always provided a larger exit than inlet causing a venturi effect (or at least I hope so). Never had any concrete numbers I'd heard 3 to 1 used around the field.

da Rock,

If I were to cut the offside of the cowl on three sides bending in the fourth side to form a duct adding some light plywood to deflect the air over the cylinder head. Would that essentially be what your calling making a baffle?

Steve Steinbring,

If your changing the air flow from what it was originally, that is baffling. I kinda look at it like this. If you made a change and it works or maybe you think it works and your satisfied with what you did and the engine isn't over heating then what you did was the correct thing to do no matter what anyone else says. That's the fun of this hobby, "experimenting".

da Rock sounds like a man with experience as well and has some very good comments. Like a fellow once told me, "You are your own boss" Take some advise from us, do a little experimenting on your own and you can't go wrong.

Zippi,

Thanks! I was just making sure I was on the same page. I've never used the term baffling, just ducting and venting. A baffle to me would be a deflector of some sort in a vent pipe or duct.

Been around awhile myself, but hate to reinvent the wheel if I don't have to so I ask questions and go from there as you suggested!

Good flying and thanks again gents.

If you look at the 3 pictures I attached to post #7, the 1st and 3rd pictures show baffles in place. Those two cowls have openings both sides of the spinner. The engines happen to be behind one of the two holes. The openings are more than adequate by more than twice. That is to say, the opening in front of the engine is absolutely adequate, more than adequate to allow enough air in to cool the engine. And darned if there isn't another hole equally as large on the other side doing nothing except allowing a huge amount of cool air into that cowl.

Now think about the suggested sizes for exhaust openings. If you go with 3:1 as many people suggest, you'll wind up having no cowling left behind the engine. If you look at the middle picture, you'll see a opening that is about 1.5 times as large as ONE intake opening on that airplane. And it's pretty big isn't it.

The baffles in pictures 1 and 3 could have been made of scrap wood, or cardboard, or balsa or anything. I used scrap expanded bead insulation sheet. I use it for lots of things because it's almost zero weight and glues nicely with epoxy or canopy glue. And it sands great. I fit it to things by sticking a scrap of sandpaper into where I want it to go and rub the foam around like I was snugging it into place. The sandpaper fits it perfectly and it takes about a minute to do. You can buy the stuff at Lowes or Home Depot or any home improvement. I also make building stands out of it. It's fuel proof.

In both the Cap 232 and the Decathlon, those baffles basically are about as big as what you see through the opening in the cowl. They angle back slightly to divert all the air that can make it through the opening on that side of the cowl directly at the bottom of the engine. They'd work just fine if they actually plugged that hole tight.

When you leave those offside holes open with nothing behind them, the air that comes in there blows right to wherever the exhaust opening is. The warm air from the engine really gets blocked in the cowling. Cool air is easier than warm to move. And warm air really has trouble moving cool air. Cool air has no trouble moving cool air. So the cool coming in has no trouble filling whatever vent area you made available because cool air is going out that hole.

Baffle or block the offside holes. Your engine will be happier for it.

I used the fake engine of the F4U Corsair to block about 3/4 the opening in that cowl. There is a large hole in the fake engine right in front of the OS91FX, and two small holes at the very bottom of the fake. They're right in front of the pitts muffler. It's good to have air hit it too.

da Rock,

That is similar to the way I set up make QQ YAK 54 for cooling. Works great.