source of cooling air?
01-11-2007, 07:11 PM
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source of cooling air?
Here is the question:
When an uncowled engine is on a plane moving forward at 50 mph, what is the source of the cooling air?
Is it prop wash off of the propeller or from the forward speed of the airplane?
I am trying to design an engine cooling shroud for an exposed 4 cylinder and depending on the source of the cooling air, the shroud needs to be designed differently.
Elson
When an uncowled engine is on a plane moving forward at 50 mph, what is the source of the cooling air?
Is it prop wash off of the propeller or from the forward speed of the airplane?
I am trying to design an engine cooling shroud for an exposed 4 cylinder and depending on the source of the cooling air, the shroud needs to be designed differently.
Elson
01-11-2007, 09:07 PM
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RE: source of cooling air?
Elson--from some basic experiments I have done with an on board temp recorder I have found that the difference between the front of the cylinder and the back of the same cylinder is about 30-40 degrees.
I am currently doing a baffling job similar to Chip Hydes on his DA 200. Basically it is a flat plate going from the firewall to the cowl with a tightly cut out for each cylinder. (place lower mid cylinder) The air is forced/pulled from the top through/out the bottom. I will be creating a low pressure zone in the bottom of the cowl with a lip on the exit area and hopefully have a pretty good air seal all the way around so the air has to pass through all the fins from top to bottom.
I don't know if this will work for you. If your cylinders will always be exposed and not cowled, I would use some sort of basic scoops to force air over and down the backs of the cylinders.
I am currently doing a baffling job similar to Chip Hydes on his DA 200. Basically it is a flat plate going from the firewall to the cowl with a tightly cut out for each cylinder. (place lower mid cylinder) The air is forced/pulled from the top through/out the bottom. I will be creating a low pressure zone in the bottom of the cowl with a lip on the exit area and hopefully have a pretty good air seal all the way around so the air has to pass through all the fins from top to bottom.
I don't know if this will work for you. If your cylinders will always be exposed and not cowled, I would use some sort of basic scoops to force air over and down the backs of the cylinders.
01-11-2007, 09:28 PM
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RE: source of cooling air?
RTK,
If the majority of the cooling air is from forward motion, then the majority of the air has no twist and a simple scoop on the top of both cylinder pairs which curves around the back of the final cylinder will force air down between the cylinders and around the back of the hind cylinder.
However, if the majority of the cooling air is due to prop wash, then the air has a sever twist to it and the scoops need to be alternated to catch the twisting air off the prop. The scoop on the left cylinder pair needs to be on top and the right cylinder pair needs the scoop on the bottom of the cylinders.
The engine will have no cowl and will be working hard pulling nets with a lot of prop slipping. Radar speed is 56 mph with nets deployed. Engine rpm is in the high 6Ks with a 10 pitch prop.
This past year, we used the alternating scoop idea on 150 and 200 twins with no heating problems. We have a 212 - 4 cylinder on the shelf which will also get used this summer and the cooling is more critical on a 4 cylinder than a twin. That is the reason for the posting of the question.
Elson
If the majority of the cooling air is from forward motion, then the majority of the air has no twist and a simple scoop on the top of both cylinder pairs which curves around the back of the final cylinder will force air down between the cylinders and around the back of the hind cylinder.
However, if the majority of the cooling air is due to prop wash, then the air has a sever twist to it and the scoops need to be alternated to catch the twisting air off the prop. The scoop on the left cylinder pair needs to be on top and the right cylinder pair needs the scoop on the bottom of the cylinders.
The engine will have no cowl and will be working hard pulling nets with a lot of prop slipping. Radar speed is 56 mph with nets deployed. Engine rpm is in the high 6Ks with a 10 pitch prop.
This past year, we used the alternating scoop idea on 150 and 200 twins with no heating problems. We have a 212 - 4 cylinder on the shelf which will also get used this summer and the cooling is more critical on a 4 cylinder than a twin. That is the reason for the posting of the question.
Elson
01-11-2007, 09:35 PM
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RE: source of cooling air?
Yes and yes for me too. If the engine is mostly uncowled, how about using the scoop method used on full scale J-3 Cubs. It forces all the cooling air through the cooling fins of the cylinders and has been proven over time. For the most part, a J-3 cowl just covers the crankcase and the base of the cylinders.
Roger S.
Roger S.
01-12-2007, 10:08 AM
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RE: source of cooling air?
Elson,
The prop mostly churns up the air in front of the engine. This massive turbulence provides excellent air "scrubbing" of the cooling fins. It does however nothing to get the air moving through the fins.
The morsels of wisdom are documented in early NACA files. When radial engines were introduced and the power shot up, cooling became a problem and limited the usable plane power. In the years between 1918 and 1940, many tests were carried out comprising a wealth of knowledge. The final naca cowl with flaps that resulted, uses the airblast from the outer prop half to create a vacuum behind the engine baffles, and thus pull the air trough at low plane speeds like taxi and ground manoevring. At speed, these flaps were all but closed, because ram air pressure took over.
With the low speeds we employ, suction is more important than ram air pressure, and a well planned flow of cooling air through the jugs finning is a must in your applications.
see http://www.prme.nl/forum/viewforum.p...7bf59910085ac5
The prop mostly churns up the air in front of the engine. This massive turbulence provides excellent air "scrubbing" of the cooling fins. It does however nothing to get the air moving through the fins.
The morsels of wisdom are documented in early NACA files. When radial engines were introduced and the power shot up, cooling became a problem and limited the usable plane power. In the years between 1918 and 1940, many tests were carried out comprising a wealth of knowledge. The final naca cowl with flaps that resulted, uses the airblast from the outer prop half to create a vacuum behind the engine baffles, and thus pull the air trough at low plane speeds like taxi and ground manoevring. At speed, these flaps were all but closed, because ram air pressure took over.
With the low speeds we employ, suction is more important than ram air pressure, and a well planned flow of cooling air through the jugs finning is a must in your applications.
see http://www.prme.nl/forum/viewforum.p...7bf59910085ac5
01-12-2007, 11:51 AM
#8
RE: source of cooling air?
I think Roger may be on to something as that system worked on the older Cubs.
I cannot comment with any practical experience on model aircraft, but will offer you some information about how they cool the full scale stuff.
Newer piston powered aircraft with multiple cylinders cannot be flown without the cowling and associated baffling installed, they'll overheat severely and are consequently not type certificated for flight without the cowl and baffling installed. In fact ground running time for such things as setting up fuel controllers (which requires full power fuel flow checks) is limited by engine operating temperatures, take too long and you have to shut it down and wait for it to cool off.
Those aircraft use a system of baffling which is designed to both route and slow down the airflow going through the engine compartment for cooling purposes. Those high performance aircraft have adjustable cowl flaps to control the amount of outflow air coming from the cowling. Those cowl flaps must be open for high power settings or the engine will overheat, and closed for cruise settings or the engine will get too cold.
You may be stuck with using an onboard monitoring system and playing with various shapes and configurations to get something that will work for you, or at least that's what it looks like to me...
I cannot comment with any practical experience on model aircraft, but will offer you some information about how they cool the full scale stuff.
Newer piston powered aircraft with multiple cylinders cannot be flown without the cowling and associated baffling installed, they'll overheat severely and are consequently not type certificated for flight without the cowl and baffling installed. In fact ground running time for such things as setting up fuel controllers (which requires full power fuel flow checks) is limited by engine operating temperatures, take too long and you have to shut it down and wait for it to cool off.
Those aircraft use a system of baffling which is designed to both route and slow down the airflow going through the engine compartment for cooling purposes. Those high performance aircraft have adjustable cowl flaps to control the amount of outflow air coming from the cowling. Those cowl flaps must be open for high power settings or the engine will overheat, and closed for cruise settings or the engine will get too cold.
You may be stuck with using an onboard monitoring system and playing with various shapes and configurations to get something that will work for you, or at least that's what it looks like to me...
