ORIGINAL: iron eagel
This is the actual side area the duct is enclosed all the way back to just about the stock COG.
The actual side area of the airplane without the fairing and duct would be what's in the included picture. When I first looked I thought you had a pipe in there so I included one. The more I look, the less I think you have a pipe on the original. No big deal, what is important is that lots of those airplanes are setup with the engines sticking up and some with them down. Either way, the side area of the engine is side area and does what side area does. And in that area, you're really not adding a heck of a lot more side area with the cowling around the engine. Yes, the duct will be adding side area, but it's in an area behind the engine that would see a heck of a lot of disrupted air without the duct there.
BTW, lots of speed models of all types ran streamlined cowls up to engine's head. They went straight back about about twice as far as they went forward. They disrupted the air appreciably less than the dirty engine shape and were very sufficient at what they did. Nobody chose to extend them with a duct. I'll see if I can find a picture or two of my old models that used those cowls.
I would suggest that the duct isn't needed unless you plan to use a pipe or muffler. And suggest the duct isn't needed even with a pipe. And mention that enhancing the cooling of an engine can be a bag of worms. If you have too effective a negative pressure at the exhaust, it is entirely possible that the engine will run too cool. And will give you fits because of that. Make sure to keep the ratio of intake to exhaust around 1.5:1 no matter what the rear of the cowl/duct looks like.
It is a tapered wing that is why I am concerned. Stock it has a real fast roll, although I have cut down the ailerons so it may not be as responsive in roll. Although I am setting up the ailerons on pull/pull so I can mount the servos 8 inches further inboard than stock to make the wing tips lighter, and keeping the most mass close in to the roll axis of the plane.
perhaps this view will help
OK, tapered matters. It probably will snap nicely. And it's stall characteristics would play against a loss of lateral stability. But my experience is that it won't be much. And my feeling is that your cowling isn't going to be anything like a yaw stability problem.
I'm really looking forward to hearing how this works out. These discussions offer almost as much learning and fun as doing maidens on other guys airplanes.
