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First gasser. Have cooling questions. - 2/1/2013 5:46 PM   
Kingrich09


 

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Status: offline 		 I am finishing up my first gasser build which is a 30cc eg raven with an os 33gt. I bought the plane used with the landing gear ripped out of it and it is completely rebuilt now but I am unsure of what to cut out for cooling air to exit the cowl. The previous owner had cut out some of the covering in the belly section so the air would go through the area where the canister would be. Is that the best solution or should I just cut some exit vents in the cowl. I hate to do either where this is such a beautiful plane but I know the air has to exit somewhere. 

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RE: First gasser. Have cooling questions. - 2/1/2013 6:18 PM   
ggraham500



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Status: offline 		As a basic, a gas engine (and I suppose glow engine, too) the exit area around the cowl from the exhaust should be 3x or 4x the air intake area for the engine.

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RE: First gasser. Have cooling questions. - 2/1/2013 7:01 PM   
Kingrich09


 

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Status: offline 		It's not a question of how much but where to put the cutouts.

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RE: First gasser. Have cooling questions. - 2/1/2013 8:31 PM   
airraptor


 

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quote:

ORIGINAL: ggraham500

As a basic, a gas engine (and I suppose glow engine, too) the exit area around the cowl from the exhaust should be 3x or 4x the air intake area for the engine.



Wrong Wrong Wrong Wrong Wrong Wrong......

Kingrich post some pictures of the air inlet of the cowl installed on the plane and then one or two pictures with the cowl off.

You have to direct the air around and thru the fins of the engine. a hole hole in front of the engine and a 3x-4x hole i nthe bottom can reduce the cooling of the engine. the placement of the cooling exit, size and direction of airflow all need to be considered. Like I said send some pics ok

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RE: First gasser. Have cooling questions. - 2/1/2013 8:51 PM   
Kingrich09


 

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Status: offline 		I will take some pics when I get home tonight and hopefully I can figure how to upload them from my iphone right into my posts

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RE: First gasser. Have cooling questions. - 2/1/2013 9:29 PM   
3136


 

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Status: offline 		Have a look at this http://www.rcuniverse.com/forum/m_11071695/mpage_1/key_/tm.htm

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RE: First gasser. Have cooling questions. - 2/1/2013 9:57 PM   
Kingrich09


 

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RE: First gasser. Have cooling questions. - 2/1/2013 10:02 PM   
MTK



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quote:

ORIGINAL: airraptor

You have to direct the air around and thru the fins of the engine. a hole hole in front of the engine and a 3x-4x hole i nthe bottom can reduce the cooling of the engine. the placement of the cooling exit, size and direction of airflow all need to be considered. Like I said send some pics ok


Airraptor is on the money.... Directing airflow through the fins is one of the best things you can do. What you don't want up front is those additional holes in the cheeks which are there specifically to cool twin opposed cylinders. On a single cylinder set-up they are there only for show or scale effect. If these are also opened, they could easily reduce cooling air to the engine

On large cowlings, some form of ducting should be fashioned to direct the airflow. The ducting may actually touch the fins' edges since fin depth is 1/2" or so for most of the 30cc class engines. I often use 1/32" balsa, but depron works fine too. Probably not a good idea to use metal. 




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RE: First gasser. Have cooling questions. - 2/1/2013 10:04 PM   
Kingrich09


 

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Status: offline 		So I should try to block off them "cheek"  holes and cut a bigger hole in front of the cyl then maybe block off the tunnel where the canister would go



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RE: First gasser. Have cooling questions. - 2/2/2013 12:41 AM   
speedracerntrixie


 

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Status: offline 		If it were mine I would run it as is. The entire head is outside the cowl and the canister tunnel will provide plenty of exit area provided some of it is open. You didn't post a pic of the bottom so we have no idea if it is open or not. If not I would open a section around 4"X3".  My Laser appears to not have alot of exit area but airflow goes past the engine and out the canister opening. The setup runs quite cool.




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RE: First gasser. Have cooling questions. - 2/2/2013 1:39 AM   
Kingrich09


 

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Status: offline 		That's how the previous owner had it. It had a big opening in the rear if the canister hole. It is all covered up now after the repairs I have done. I was just trying to find the best and most effective solution writer that be a big hole in the canister opening or block the canister section off and vent the cowl.

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RE: First gasser. Have cooling questions. - 2/2/2013 6:20 AM   
airraptor


 

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Status: offline 		If it was my plane I would block the scale holes. Have the one open just in front of the cylinder. I would build up a some balsa ducting and glue it to the cowl on the inside coming from that small hole. These will angle out some and just touch the cylinder fins. You can do the angle two ways. one is to where it touches the front of the cylinder with them around 1/4" less on each side than the width of the cylinder. This is the easiest method that provides great cooling. The other way is to have the balsa ducting go around the cylinder walls and extend back past the cylinder. Then another piece of wood to direct the flow down toward your exhaust exit hole.

Just remember air will take the path of least resistance... you want to produce a slight negative pressure in the cowl. Forcing the air through the engine just makes it run very hot.

Now some more food for thought:

If you want the max power of your engine you need to make sure the cylinder has equal temperature al the way around. Some will think that it’s solid and will have the same temperature. It doesn’t. Just run an engine with the cowl of and measure the temperature with an IR temp gun. You will find that the highest temperature is just right above the exhaust port or a rear exhaust engine. On a side exhaust engine with normal rotation will have the highest temp just above the exhaust port and the rear of the engine around 4-5 o’clock position. Now that you know this you will want to try to cool the engine evenly. Oh I forgot the coolest spot on the cylinder fins will be at the 11 o’clock position about half way down the cylinder.

Now why does this matter? Well most engines have a round bore and a round piston. As we all know metal expands with heat and different metal expand faster than others. So if the front of the engine is cooler than the rear the cylinder will not be round and take forever to break in and losing power with the combustion pressure is down ever so slightly and the extra drag near the front of the cylinder. I know this very small scale to just running the darn thing lol but I think you will gain life on the ring and gain some power by cooling the cylinder evenly.

Now to cool it evenly you can direct more air to the rear of the cylinder and limit the air to the front. Do this by using aluminum tape and block some of the fins on the front or just grind them down some. Take them down to half the original or even less. Leave the fins stock length in the rear in the areas' above. Now for some you say that crap well look at the OS AX line of engines. They are progressed from the square or round cylinder and heads to one that is angled and more fin area behind the engine. Also look and many of the nova Rossi engines where that have extended the length of the cooling fins behind the engine and around the exhaust port.


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RE: First gasser. Have cooling questions. - 2/4/2013 12:33 PM   
Kingrich09


 

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Status: offline 		  This is what I have come up with soo far. Still need to block off the cheek holes though that's tonight's project along with re routing the choke lever. 

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RE: First gasser. Have cooling questions. - 2/4/2013 3:25 PM   
MTK



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quote:

ORIGINAL: Kingrich09

  This is what I have come up with soo far. Still need to block off the cheek holes though that's tonight's project along with re routing the choke lever. 

Yes that will work well on the hottest of days. It's important to understand the discharge hole cut in the existing raised bump in the cowl. Having a small dam in front of the hole creates a slight negative in the cowl when the plane is flying. It actually will draw the hot air  out. It's similar to hooded or louvered exits we used to see in vintage car hoods.

BTW- when the engine runs hot, it will sag. The engine is doing most work and is hottest in verticals. Once happy with the ducting, that's the test to run...the extended vertical. You don't want any sagging. If you get the combination of a hot day, a good loading (large prop) and extended vertical, say, twice as long as you would normally fly, then it would be a good test of whether there's adequate cooling. One more thing, the OS has more than enough cooling fin area to cool down enough.

Some other engines may not have enough, which means that you can take all the precautions and still the engine won't run happy in hot days

Another BTW- The balsa sheet used to cover around the negine box is fine. What I tend to do is to dope in medium silkspan or ply span esaki tissue to cover holes. Works great, is super light weight, and gives the front end a finished look, especially if you paint the box and firewall with epoxy paint like Klass Kote


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