Quote:
Originally Posted by SHIPCHIEF
The side of the cowling is probably one of the lower pressure areas on the airframe. ......
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I agree.
The attached image depicts the side Bonanza vents I copied and built and they are a reason the IO360 cht's and oil run cool, too cool in fact. Beech invented them about 50 years ago and they work.
The vents were installed to facilitate cooling with the Subby H6 and were almost removed when the Lycoming was installed. But one test flight changed my mind. I taped them shut with duct tape wanting to observe temperatures just using the stock cowl exit. There was no change and I thought coming in to land, they will be removed.
But after shut down I saw why there was no change. The internal lower cowl pressure had blown the duct tape off the cowl and they were taped tight. With that I decided to leave well enough alone. I don't think there is any drag with them as the airplane goes as fast as Vans numbers and somedays a bit faster. The max CHT observed so far is 405 and that was a WOT climb to 10,000' last summer to see how long it would take to get up there. (8 minutes, 26 seconds

) Oil temp with that climb was 185 or 190.
I believe you get good cooling by running air across the engine, lots of it, and one way to do that is having a copious amount of exit area. The standard Vans inlet is about 45.5 inches square and the exit 49 which equals 107%. That works for many guys.
My cowl was modified at least 4 times trying to cool the H6. I undid some of it going to the IO360 but I still have the Bonanza vents and a 5X16 bottom exit. Total area is 108 inches which computes to an exit ratio of about 237%.
Now I know that much exit area is not suppose to work in terms of drag and there is no science to qualify why the engine runs cool and airplane flies like it does, but it does. I suppose I could make some changes to perhaps see an improvement in performance. But I spent about 4 years trying to cool that H6 (never did conquer the beast) and flying around with a Lycoming that never over heats is such a pleasure, I think I will leave it all alone.
At present the side vents are taped shut on the inside to facilitate getting the oil temp up a bit. Even with the air shut off, oil temp still runs around 160. This is with the SW8432R mounted like in the RV-10 with a 4" scat tube from aft of #4 baffle.
I also believe another reason this thing is running like it is, is because it is not a 200+HP engine. (it turned up 187 on the Barrett dyno) There is a lot more heat generated with engines like what Alan Judy and others are running. And of course the faster you go, the more drag becomes an issue.
