I am putting together an ARF that has a cowl and I am hanging a .91FS on it. This is the first model I have where I have to deal with a cowl. While I have learned a little about 4-strokes, I know even less about 4-strokes inside cowls. So I have come to the experts here on RCU for some instructions.

The engine is mounted 90* from vertical and I have a pretty decent sized opening for the heads and access to the glow plug. With this configuration, the muffler is to exit the cowl thru the bottom. I guess the first/main thing I need to know is how big the opening for the muffler should be and why? Do I cut it just big enough to fit the muffler or do I need it bigger to allow for plenty of air movement to help keep the engine cool. Do I just maybe want to cut a long oval type opening so that the cowl will slide on past the muffler and give it even more "air"?

I appreciate any and all help with this as I really don't want to run the risk of burning up a new 4-stroke engine.

Thanks,

HI Crash,
My Great Planes Cap with an OS .70 cowl-enclosed doens't have any cooling problems. I have opened up both sides of the cowl nose area as in the full scale, made a hole for the cylinder head that is big enough for the glow driver and a hole for the high speed adjustment needle. The exhaust area is just a moderately sized semi-circle that allows the exhaust pipe/muffler to just barely exit. I had a rubber exhaust extension attached to the 4 stroke but that only lasted for about 7 flights before burning off. I haven't bothered to replace it. Many recommend that the exhaust holes for cooling are 2times the area of the intake holes. I can't really say that they do in this case. Here is a photo, but unforunately it doesn't show the exhaust area. Hope this helps.

Crash,

The hole in the bottom of your cowl (the one that lets the air out) needs to have 3 times the area of the front two holes combined. So, if the holes in the front of the cowl are 1 square inch each, you will need 6 sq in of exhaust area.

Hope this helps,

Bryan

Garyss that Cap looks good. I'm about to start covering mine. How about a few more pics? I presume you used monokote on the fus and wings and lutrekote on the cowl?

I don't want to hijack Crash Master's question, but here are a couple. Yes, I used Monokote Dark Red, black and white. The Lustercote paint doesn't match extremely well, but I might have used gray primer rather than white. I shamelessly copied the colors from the last photo's CAP. If you need more e-mail me. Thanks