ORIGINAL: Steve Steinbring
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?
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.