ORIGINAL: Juice
I originally posted the following question in the "Glow Engines" forum. I got some good suggestions, nobody really answered the question. It's more of a fluid dynamics question than it is an engine question. I don't know where else to ask, so I'll try here...
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Apparently I don't have enough outlet area in my .46-size World Models P-51 with an RCV-60SP. One of the "cons" of the RCV-SP engines is that they are more difficult to keep cool, as I am experiencing.
[link=http://image.rcuniverse.com/forum/upfiles/13495/Bz79072.jpg]Picture #1[/link] is how I cool the engine now. This is the side cross-section of the cowl. Engine is grey. Cowl is black. Cool air is blue. Hot air is red. The outlet:inlet ratio is about 2:1. But apparently the hot air doesn't want to circulate down like I want it to.
[link=http://image.rcuniverse.com/forum/upfiles/13495/Up49024.jpg]Picture #2[/link] is how I want to add additional outlet area at the top of the cowl.
The question is how big should I cut the lip at the top of the cowl? And what dimension should I use to calculate the additional outlet area?
In [link=http://image.rcuniverse.com/forum/upfiles/13495/Pu50610.jpg]Picture #3[/link], should I use dimension A or B to calculate the exit area?
Thanks in advance for your help.
Juice
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You are on the right track with your cooling setup. While it winds around within the airplane it should work. You have the airflow directed over the head to maximize the cooling to the head of the motor, then back down and out of the engine compartment. That area should be large enough to allow for the expansion of the air due to heat.
Now as far as how you size the exit vent to the intake, the exit needs only to be around 1.4 times larger than the intake.
Now if drawing 3 is the vent shape you have in mind, in spite of the restriction of area A, the vent area will be B.
A few observations….
You have the area under the motor just after the intake baffled upward to direct the air upward. In the drawing you have it as a squared angles, that would cause your first restriction to fluid flow, that should be rounded gently. The same would follow at the top of the forward edge of the cowl over the motor, and the same at the top of the cowl next to the firewall, the square corners will cause a high-pressure area to develop.
These tweaks should improve the situation without any external mods. to your bird.
However, the one thing I noted beyond that, is that you show the forward edge of your vent to be higher than the trailing edge that is probably one of your biggest problems. That is going to cause an area of high-pressure right where you need to be developing an area of low pressure. The leading edge of your vent must be lower then the trailing edge, not much but at least 1/8 inch.
Like Dick said, you need to you need to make an air siphon. You exit right now is you major problem you are creating an area of high pressure, the down flow off the nose cannot stop the air from causing a high pressure area at your vent.
I think that the hole on top of the cowl probably won't buy you that much more cooling, and won't look all that great.
Good luck