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Is this enough engine cooling?

Old 08-15-2006, 09:16 PM
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Default Is this enough engine cooling?

Putting together my aeroworks Edge and was getting to the cowl. With a Tower 75, the engine fits very nicely inside the cowl, such that I can either use a remote glow or cut a small hole for the glowdriver. My question is whether there is enough cooling over the head to avoid cutting a hole in the side of the cowl for the head? In the 1st pic, using the clear cowling you can see the position of the head, while the 2nd pic shows the two inlet holes in the front of the cowl. Wondering if those holes alone will provide enough engine cooling.
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Old 08-15-2006, 09:32 PM
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Default RE: Is this enough engine cooling?

Should be as long as you have a large enouf air outlet
Old 08-15-2006, 09:49 PM
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Default RE: Is this enough engine cooling?

For PROPER cooling you should use a 3:1 ratio. For every square inch of air inlet, you should have 3 square inches of air outlet. Depending on how cold it is outside when you fly, you MAY be able to get away with a 2:1 ratio. I wouldn't use this ratio for summer flying since the chance of overheating is greatly increased.
Old 08-15-2006, 10:08 PM
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Default RE: Is this enough engine cooling?

jzinckgra,

As the others have said, the outlet area is what is important. I'll just add that you may want to block off the hole in the cheek cowl opposite where motor cylinder is. That way your exit hole can be smaller.
Old 08-15-2006, 10:15 PM
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Default RE: Is this enough engine cooling?

Thanks for the responses. I am not sure on the exit hole size, but it should be pretty big, as that's where the muffler will exit the cowl. I can always cut away a little more than I need to clear the muffler. Just seems like a waste to cut a big hole in the side if I don't need to.
Old 08-16-2006, 01:41 AM
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Default RE: Is this enough engine cooling?

The side hole for the head is not needed and you will get no ram air effect at that location. In other words no cooling help. I would however use a head hole for plug serviceing. With a 135 degree mounting it is important with the added possibility of hydro lock with careless handling.

This is a full pressure cowl and putting all the exit area around the muffler header area like you described will be a major heating liability. The air must be allowed to flow past the engine then out. Doing as you described will dam up considerable air behind the engine and become an oven.

Here is a picture of the outlet on an airplane that I use up to 40% nitro in and notice Its also a 135 degree mount with the outlet aft and favoring the side behind the cyclinder and a slanted baffle to smooth the outlet flow. No air can dam up in this way. This works very well and on this one the outlet area is around 2.5 times the inlet if you discount the cylinder hole, through which very little air goes in or out at that point.

John
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Old 08-16-2006, 08:01 AM
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Default RE: Is this enough engine cooling?

john,
thanks for the response. maybe the pic is deceiving, but my muffler will exit directly in the middle, underside of the cowl, not off to the side like yours. if the exit hole has to be directly behind the head of the engine, that wouldn't leave much choice, as the firewall is in the way (see 1st pic). other than the muffler exit hole, I can't see how else to devise the best cooling setup. Like you said, cutting a hole for the head will not provide any benefit.
Old 08-16-2006, 09:54 AM
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Default RE: Is this enough engine cooling?

A couple of suggestions.

Either block off or place a deflector in the cowl opening that's not in front of the engine. It's going to allow air into the cowl, but the air will flow right through and have no reason to go anywhere near the engine.

"Pressure cowls" are cowls that have all their interior blocked except for very close around the engine. They block completely except immediately next to the engines' cooling fins. They usually only use sheet metal because all they're doing is blocking airflow that would bypass the engine if allowed to.

Cool air is easier to move than hot air. Cool air is coming in the front, and if it runs into hot air, it simply goes around the hot air if it has a way to do it. You need to block those easy paths.

I'm including a couple of pictures of my Sukhoi. It's got a HUGE open cowl. I used 1" thick pink insulating foam board I picked up at Lowe's Home Improvement. It was very easy to work with. I filled all of the cowl that wasn't taken up by the engine and muffler. The foam also made the lousy-made, warped fiberglass cowl round again and gave it tremendously more strength.

You'll note that the opening in front of the engine is still huge. Sorry I don't have a picture from angles that'd show the exit holes, but trust me, they're not greatly larger than the "intake" area. The ratio of intake to exhaust only takes into account the area available to the air to pass at it's most restricted area. You can easily get by with a ratio of 1 to 1.5 if you block off the cool air bypasses. A "pressure cowl" design does that.
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Old 08-16-2006, 10:06 AM
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Default RE: Is this enough engine cooling?

The exit hole can be just about anywhere if the airflow is forced to pass near the engine. It greatly helps to have it directly behind the engine, but if it's at least near that side of the fuselage, it's good.

You also want to have some negative pressure around the exit hole. Most CAPs have an oddly shaped bulge on the bottom rear of the cowling. If you leave the leading edge of that bulge as a "lip" and remove the flat portion of the bulge, the lip will help turn that area into a negative pressure area. Exhaust openings on airplanes very often have deflectors immediately ahead of them to do just that, cause negative pressure downstream.
Old 08-16-2006, 10:14 AM
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Default RE: Is this enough engine cooling?

Looking back at the two pictures you included................

There is a somewhat square bulge in your lower cowling. I assume that's where the muffler will be. The front of that bulge will work as a deflector. If you remove cowling behind that, the resulting opening should be in negative pressure.

Don't go crazy making the opening look huge from the bottom view. The area that matters is basically "frontal area" since the intake openings are "frontal". To speak "geometrically" (like we had to in Geometry class), the plane of the intake opening is the plane on which you'd measure the intake area. The exhaust area has to be figured on that same plane, only further back where that opening is.
Old 08-16-2006, 10:19 AM
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Default RE: Is this enough engine cooling?

Just block of the left hand intack and make the outlet area 3x intake. Some say 2 times, some say 4 times.
Don't worry about the hole on the bottom, nobody will see it... unlike your dead stick landings if you haven't got it big enough.
Old 08-16-2006, 10:40 AM
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Default RE: Is this enough engine cooling?

Here are some pictures to illustrate.

The picture from the front partly shows the baffle I placed behind the "worthless" opening. The baffle is about an inch back from the opening and is at an angle. If you could see it from the inside (sorry, but I didn't feel like pulling the cowl for pictures inside) it is glued to the top of the cowl and down the side.

There are two pictures of the bottom. One looks straight down on the bottom. The area of the exhaust opening, if measured at that angle, would be worthless. That opening could be hacked all the way to the tail of the airplane and it wouldn't do one bit better as an exhaust. The picture that looks up inside comes closer to showing the actual size of the true exhaust area.

I've guessed at the true area of the exhaust and it's actually maybe not even equal to the two intake openings. If it is larger, good deal. As it happens, I've been flying in the hottest days we've had this summer and that engine acts like it did in the spring.
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Old 08-16-2006, 10:49 AM
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Default RE: Is this enough engine cooling?

BTW, one very easy way to block the offside opening would be to use screen. It's easy to fit, and easy to epoxy to the back of an opening like that.

The screen will allow air to enter, but once the other, unrestricted opening starts to flow, the screen-blocked side will be virtually closed.

However, whatever you do, if you do not block the areas inside a cowling that's got a lot of open area inside, the engine is not going to be cooled by the airflow through that cowl. Since hot air is harder to move, the entering air simply follows the other cool air that's nowhere near the engine, the cool air that's passing out the holes at the rear and that also never came near the engine.
Old 08-16-2006, 12:42 PM
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Default RE: Is this enough engine cooling?

Jeez, thanks guys for the help. I never thought there was so much to consider to keeping a cowled engine cool. And I have been flying for quite some time It probably explains why a couple of my other cowled planes run like crap on hot days. Somtimes I go from 1/2 to full throttle and there just isn't any more umphh in the engine. This may explain a lot.

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