Okay... I have a G-90 that I'm putting in a CMP Hellcat. I've put it on it's side with a bisson pitts style muffler.. turns out that the entire thing will fit inside the cowl even the exhaust so I"ll have to get some of those rubber exhaust extensions I guess... now I know I need some holes for the exhaust and atleast for the glow ignitor.. the front of the Hellcat is very big and open, being modeled after a radial... I'm not using a dummy radial so it's all open. There are smallish holes on the side/rear of the cowl where the scale exhaust would be. So my question is.. do I still have to cut a big hole in my cowl? it's so pretty... I will if I need to ofcourse.. but if I don't have to.. well.. you know the story..
Thanks.
Casey

Maybe you could post some pictures? As long as air can flow through the cowl (from front to rear) during flight, you will be OK.

Hi casey,

Do you realise the war bird police will come and take you away if you put a two stroke in a war bird? Your family will never hear from you again.

In this case the ideal would be a dummy radial with one cylinder removed in line with the barrel of your motor and exit holes in line behind with a ratio as close to 1/3 front opening and 2/3 rear vents. I know it sounds counter intuitive but if you leave the whole front open the air doesn't flow past the barrrel and the motor will overheat. You may need to experiment with ducting to get sufficient cooling air flow. Don't forget to run the motor a bit rich to aid cooling and this is one of the few instances I would recommend some castor in the mix as lean run insurance.

Cheers,

Colin

yeah.. that's why I posted it here.. so the warbird police wouldn't come get me LOL... I just can't afford a big 4 stroke and the g-90 was 120 bucks... I may do a dummy radial later.. but it looks fine like it is... the front opening is about 4 or 5 inches round with another intake just underneath.. I was hoping the holes I'd be cutting for the exhaust would be enough.. I'll make the exhaust holes one big hole.. would that do the trick? Oh.. and I don't want to post pics yet because it's turning out sooooo good... my best work yet I think.. and I wanted to wait until it's done before I post pics...

In order to provide PROPER cooling for a cowled engine you will need an area for the heated air to exit that is 2 - 3 times the area of the inlet air. If the muffler is also fully enclosed, as in your case, I suggest using the 1:3 ratio for cooling.

Usually, the exit hole will be in the bottom of the cowl where the exhaust extensions come out. Since you already have holes in the sides of the cowl, the hole underneath may not need to be that large.

You may also want to consider mounting the cowl so it is slightly forward of the fuselage. This way there will be exit holes all the way around the cowl.