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Cutting cowels?

Old 09-09-2008, 02:45 PM
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ceecrb1
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Default Cutting cowels?

How much fibreglass of the cowel should I cut for the (glow) motor?

So far I have made the holes for the nosewheel, the exaust outlets, the glow plug and the needle valve.
At the moment It is at the state where I'd like to keep it for scale looks but I dont know if more airflow is needed for cooling?
Motor is an OS MAX 55 ax with a jamara pitts muffler.

In the pics you can see the holes I have made, the original venting and in the rear photo, how the cowling drops below the body of the plane and how I left it open for airflow...


advice?
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Old 09-09-2008, 03:13 PM
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KitBuilder
 
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Default RE: Cutting cowels?

My understanding from what I've read here and mags, etc is to have the exit for the airflow be 2x the intake. Where is the air exiting in this setup? I believe.. but dont quote me it's a matter of measuring the intake hole area and being sure yoiu have at least 2x that area for air exit. It looks like the cylinder will get goof flow around it assuming it has someplace to get out too.

Right now it looks like you have the 3 intakes up front and the exit by the nosewheel cutout and the area between the cowl and plane at the bottom as you said... I think you may be 1 to 1 at best so far.
Old 09-09-2008, 04:20 PM
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Default RE: Cutting cowels?

I agree with Kitbuilder, looks like you need to double the air outlet area. The glow plug acess will allow cooling air to exit before it cools the motor if your outlet air area is not sufficient. With outlet air it is far better to have to much than not enough. If you were to round off your exit area and remove the tab just in front of the nose gear I would say you would be ok. Nice work!
Old 09-09-2008, 04:22 PM
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Default RE: Cutting cowels?

Well in that case.. its back to annoying the neighbours with the dremmel tomorrow!!
Old 09-09-2008, 04:31 PM
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da Rock
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Default RE: Cutting cowels?

The hole directly ahead of the engine is usually about the size of the front of the engine. Your's looks good enough. They're almost always just a bit smaller, and if you don't hover the entire flight will do perfectly.

The exit area should be 1.5 times the intake area or larger. It also helps if the exhaust is into a negative pressure zone. The section of cowl in one of your pictures that has a plugged fuel line hanging about is just that kind of area. The "flap" effect of the lower part of the cowl that moves the air away from the fuselage gives a place for the air to cause some negative pressure. It'll happen right where the cowl stops. The passing air there causes a bit of suction. Kewl setup. What the hole around the nosegear does depend on whether or not it's square to the airflow or faces it or faces aft. Very good chance it'll be a good thing. Closed, it isn't for sure.

What really helps on cowls like that is to either baffle or shut the opening opposite the engine. Cold air moves past hot air like it's standing still. And that's what happens to the hot air pretty much when the cold air finds a way around the hot air and finds the exit. The hot air doesn't move very fast because the cold is in the way. It depends on where you fly and how good the exhaust areas are directly behind the engine whether or not you need to worry about this deal however. Lots of modelers don't break a sweat about it and don't do blocking or baffling. Would their engines run better or last longer? Who knows.

Full scale air cooled engines are often baffled so closely you can't get a finger past the front of the fins. Works for them.

This Cap 232 flew a couple of summers with many days over 100F. Never missed a beat. The engine still is pulling another plane. The baffle went in WHEN the temps got into the 100s. It was acting peckish. The baffle solved that.

The Skybolt got it's baffle during assembly. That engine has run sweet in there since 2006.
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Old 09-10-2008, 07:52 AM
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Default RE: Cutting cowels?

I agree with da rock. I think I would be more likely to close up or redirect some of the intake rather than make a larger exit. Its hard to tell by the pics but I would guess the cutouts will probably be ok.

What I would have done on that cowl is not cutout such a big hole for the glow driver. Cut it just big enough to fit the driver in. Then make a baffle to force air over the cylider head like da rock talked about.

To increase the exit you can get creative. If you trim out the bottom part of the cowl parallel to the fuse you can trim out 1/8th or 1/4 inch along the bottom. Its not very noticeable but bc of the width of the cutout it gives a lot of airflow out. You can also put a flap on the exit and that helps a lot as well.

I really like good looking cowls. I think its a shame to hack them all up.

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