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Old Dec 24, 2010, 03:51 PM
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Exit air holes in cowling

Hi

It has been my understanding that a cowl should have about 3 times the exit air area more than the intake area. Is that true for both glow and gas engines?

In my particular case I am about to build (assemble) a 30cc Yak, gas powered. It has a round fiberglass cowl with a 5 inch diameter hole. If the 3 to 1 area is required I would have to remove a massive amount from the bottom of the cowl which would look unsightly and substantially weaken the cowl (i think).

The only thing I can think of is to glue in some balsa baffling around the intake hole to reduce that area. Is there any other way to accomplish proper cooling without having to butcher a huge amount of the cowl to allow exit air to escape.
I am trying to keep cutting holes in the cowl to a minimum.

Thank you
GMB48
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Old Dec 24, 2010, 04:55 PM
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I've seen estimates as high as 3-4 times bigger for the exhaust holes of electrics.

My Aeronautical Engineer friend says that's not needed, has never been proven to be the case, and recommends the same size as the intake.

Oops, I just realized you said glow..... sorry for the inane post....
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Old Dec 24, 2010, 10:24 PM
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Originally Posted by GMB48 View Post
Hi

It has been my understanding that a cowl should have about 3 times the exit air area more than the intake area. Is that true for both glow and gas engines?

In my particular case I am about to build (assemble) a 30cc Yak, gas powered. It has a round fiberglass cowl with a 5 inch diameter hole. If the 3 to 1 area is required I would have to remove a massive amount from the bottom of the cowl which would look unsightly and substantially weaken the cowl (i think).

The only thing I can think of is to glue in some balsa baffling around the intake hole to reduce that area. Is there any other way to accomplish proper cooling without having to butcher a huge amount of the cowl to allow exit air to escape.
I am trying to keep cutting holes in the cowl to a minimum.

Thank you
GMB48
The spinner takes up some of that area doesn't it?
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Old Dec 24, 2010, 11:06 PM
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Yes, it certainly does. I had not given that any thought. Thanks for that input. I guess I will open up the bottom "X" amount and see if I have engine overheat problems. I can always enlarge the opening a little at a time if needed. I plan on a DLE30 engine turning a 19X8 prop, 32/1 gas to oil mixture, and I have a 3 1/2 inch aluminum spinner for it. I was hoping someone who has already done a Goldwing Yak 55m would chime in with what they did.

GMB48
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Old Dec 25, 2010, 03:12 AM
hul
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It has been my understanding that a cowl should have about 3 times the exit air area more than the intake area.
this is a myth. See what Mark Drela says about this: http://www.rcgroups.com/forums/showp...02&postcount=8

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Old Dec 25, 2010, 08:05 AM
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Yes, there is a lot of valid data showing that the outlet needs to be about twice in inlet area. Even more important is the ducting to route the airflow around the engine; just having it come in and out the cowl area is not sufficient. You must duct it so that you get proper air flow over the cooling fins, if the fins sit in stagnant air, little cooling is accomplished no matter how much air flows through the cowl.
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Old Dec 25, 2010, 10:54 PM
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another thing to consider to make your intake smaller and look good is a dummy radial and only cut out the space between the fake cylinders that is in front of the engine so that it directs the air at the engine
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Old Dec 26, 2010, 07:33 AM
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this is a myth. See what Mark Drela says about this: http://www.rcgroups.com/forums/showp...02&postcount=8

Hans
Mr. Drela stating the tried and true to be a myth, makes me wonder why this idea/formula has worked so well since the beginning of time?

GMB48, If this were my project, I'd mock up a radial engine for the cowling and direct air past the cylinder(s) with baffles and have an exit that is at least twice the area. I should think this to be a good place to start.
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Old Dec 26, 2010, 10:19 AM
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I don't remember where I read it, but I remember reading that the P-51 oil coolers had a problem with too small an outlet for the size of the inlet. The air would "pack" into the cooler, and not flow; therefore, no cooling.

Les
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Old Dec 26, 2010, 06:27 PM
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There is a big difference in full size and speed practice and what is needed for model size a d speed.
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Old Dec 26, 2010, 07:19 PM
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Mr. Drela stating the tried and true to be a myth, makes me wonder why this idea/formula has worked so well since the beginning of time?
The "tried and true" method of selecting motors for our planes was always based on the watts per pound formula. This "tried and true" method has been blasted out of the water so many times as to make it almost a joke today. Times change, and what may have barely worked at one time gets replaced with much more accurate methods.
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Old Dec 27, 2010, 03:53 AM
hul
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you need to keep two things separate:
1. necessary airflow for good cooling
2. getting this airflow in and out of the plane with minimum losses

A given engine needs a certain airflow past the cooling fins. Whether this air goes through small in- and big outlets or vice versa doesn't matter for cooling.

Big inlets and small outlets are more efficient, cause less parasitic drag. See the professor's post.

This is perhaps less important on a 3D plane, but doing this right makes the plane faster and more fuel efficient. Most full size warbirds adjusted cooling by throttling the cooling air outlet.

There are of course limitations. On a scale plane that has a small inlet, an even smaller outlet is not an option because that would reduce cooling air flow. The only way to get enough air to the engine would be a large outlet.
But if the inlet is huge, why have an even bigger outlet?

I'd say in the case of GMB48's Yak, the 3:1 rule doesn't make sense. Internal ducting to get the air to the cylinder is important though.

Hans
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Old Dec 27, 2010, 08:28 AM
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With a huge inlet, and small outlet, the air can form a dam, keeping in the heated air.

When the air outlet is greater than the inlet, a vacuum is formed, sucking the heated air out.
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Old Dec 27, 2010, 03:01 PM
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Keep in mind too that much of that front opening will not be needed for your single cylinder compared to the full sized plane's radial engine. Putting a dummy engine into about 2/3 of the opening with just an air inlet in front of the actual engine's cylinder means that you can use about half of the outlet area and do just fine. And a hearty "DITTO!" for the idea of using sheet metal ducting to direct the air over the cylinder and head fins. In particular the head since that is where most of the heat is concentrated on any engine.
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Old Dec 29, 2010, 05:44 PM
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NFZ, I don't see where the original poster of this thread was asking about electric power! In fact isn't he specifically asking about "both glow and gas engines"?
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Last edited by Mode One; Dec 29, 2010 at 05:58 PM.
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