I think I have my answer and I really appreciate this discussion. What prompted it was that little hole that you found on a lot of the 70's planes (before mufflers) on the top of the nose just behind the engine compartment. I used them and many of the folks I flew with used them to help cool the engine case and to provide a drain for all the crap that built up there from the exhaust. I don't see them on any of the current ships probably because engines run cooler and cowlings stay dry inside. The one I used was patterned after the one Bob Gieseke used which was a version of drawing "A". I have been convinced here that two of these will probably vent enough air to keep the ESC from overheating and I don't need to add the blisters.
Ken
I think picture "A" is all you need. I have learned through the years from observation and reading, that you need more exhaust opening than you need intake. You have air flowing down the sides of the fuselage past that opening creating low pressure at the opening. By not having any restriction through the area where your components are, any air that comes in would pack a positive pressure build up in that area. Unless you allow that pressure build up to exhaust, the air stays in the compartment. If you have an intake opening of say, 1 square inch, and you have exhaust opening or multiple openings of 2 square inches or more total, should should be "cool." Add that to the already low pressure at the opening, and you have flow. If you have a similar set up for an IC power plant through the engine compartment, you should be "cool" also. If the head and cylinder fins are doing their job along with the oil in your fuel, cooling for the bottom end of the crank case is not needed. One way to check that is, if you have a metal spinner on your model, as soon as your airplane rolls to a stop, grab the spinner to feel if it's hot, cold or what ever. You have to do it immediately. If you do it quick enough, and it feels cold, in a matter of seconds it will get warmer, almost hot, as the heat from the top end creeps down the crank case, to the crank shaft, forward to the drive plate and then the spinner ( as long as the spinner back plate isn't rubbing the nose ring.). That is the second main reason engines are made from aluminum, because it conducts heat so well and rapidly. When the engine is running, the venturi effect of the air and fuel being drawn down the intake and through the bottom end of the engine keeps things pretty cool as long as the engine keeps running. When the engine quits, the physics of convection takes over and it heats up quickly. Want a demonstration of this effect? Take a bowl of warm or even hot water. Soak a rag in it and get it thoroughly soaked. Wring it out, and the rag will be pretty warm. Grab a corner and swing it around like you are rooting on your favorite sports team for about 5 or 10 seconds, then feel the rag. it will be very cool to almost cold. I figured this out, oddly enough, by racing Fox .35's on Sky Ray .35's at the SIG contest. Fox .35's can be pretty hard to start hot. That is, I believe, because if you come in after running at a leaned out condition with a 9-7 prop loading things down, the top end gets really hot. When landing for a pit stop, it takes just seconds for the bottom end to get hot enough to boil fuel. Have you even noticed that castor never get baked on to the bottom end of a Fox .35? I noticed other guys were squirting syringes of fuel on their engine to cool it. I remembered from science class that alcohol has a much lower boiling point that water and does a lousy job of that, so i started to take a couple of syringes full if ice water to use for that purpose. Plane lands, crew recovers the model, hits the engine with the full load of water as quickly as possible, fuel, flip and fly. On a profile model, there is nothing getting in the way of the engine radiating heat out to ambient air, unlike a cowled engine. I know why the think they put that hole their in the old days, and maybe it did help, but if everything else about the engine installation was correct, there was no problem in the first place. Read some of Al Rabe's old articles on how he dealt with cooling issues on his early scale stunters. Pretty interesting reading.
Type at you later,
Dan McEntee
Topic: Exhaust Suction (Read 2696 times)