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Design => Engineering board => Topic started by: Jim Thomerson on June 17, 2011, 03:54:37 PM
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Maybe this belongs in engines, but is an engineering question, I suppose. Engines almost universally have the head fins oriented parallel to the crankshaft. Is this the direction of airflow across an exposed engine head? Or would rotating the head X degrees make the fins parallel to the airflow off the propeller and thus provide better cooling? :!
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Speaking as an engineer*, thinking of the airflow as being anything other than directly along the line of flight makes my head hurt.
Clearly, what's there is mostly good enough or we'd be burning up engines. Clearly, once you cowl an engine it's the ducting, not the prop, that's going to have the major impact on airflow. Beyond that -- I dunno. Try attaching a few bits of yarn to the head of an engine, fire her up, and see which direction they point -- keeping in mind that the airflow direction will change with relative wind velocity and prop details. If you've got a Schnuerle ported engine with a round head, then it should be easy to cant the fins 60 degrees off of "straight ahead".
* Well, electronics engineer.
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I don't know, and I'm too lazy to look it up, but you got me thinking. Start with the Adkins and Liebeck paper: Design of Optimum Propellers; Adkins, Charles N. and Liebeck, Robert H.; American Institute of Aeronautics and Astronautics, 1983. An easier way would be to mount the fuselage on an automobile and put a wee streamer ahead of the head.
What I was thinking was that wind superimposed on the velocity relative to the ground has a bigger angular effect on the flow than does propwash, but still too puny to affect cooling. However, you could use a weathervane atop the plane to operate a valve to open and close cooling to the head, or-- better yet-- to operate a valve in the exhaust, to vary the engine run as a function of where the airplane is relative to the wind.
I once wrote a short monograph on calibration of such a weathervane (known in our jargon as a beta vane, beta being sideslip). I was known as the Master Beta.
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Orientation is not as big a factor as is surface area. It was once believed that color played a significant part. Black was said to dissipate more heat. So a bunch of people painted engines black. Well Black absorbs IR nad UV better than light or reflective colors,and that leads to an object heating up more and quicker but that hardly applies to the heat generated by the engine. What they did was actually make the engine run hotter because the paint added a insulating factor. Utlimately it is the number and size of the cooling fins. Expose as much material to the cooler surrounding air as possible. A rough sand blasted bare metal finish is better than a smooth highly polished one (The tiny pits in the surface adds surface area) Small holes drilled into the fins (so long as you do not remove more surface area than you add) also helps, noted by the drilled and slotted disk brake rotors on racing cars, which is partly done to aid shedding heat which leads to brake fade. Air flow is secondary, you just need enough to move the heated air from around the engine, after a certain point blowing more air past the motor does not gain anything as the metal can only conduct and shed heat just so fast.
Orienting the head fins in line with the anticipated airflow, just helps move the heated air surrounding the head more efficiently. It prevents low airflow areas where on fin sheilds the others from direct air movement. But as I mentioned a head fin design that maximizes surface area would be the best solution.
Of course weight needs to be realistic as well. One way to cool a engine is to cool the fuel, liquid carries away heat much more efficiently than air. I have no Idea how this will effect the engine run, perhaps an experiment with some fuel on ICE can be tried. Just remember that when you cool somethng condensation forms, so your fule container has to be capped tight (alcohol loves to absorb moisture) Perhaps a quart at a time chilled down in a cooler with ice. Fill the tank immediately before the flight and try it. It would be interesting to discover the results, one things for sure the engine will run cooler.
Note: It needs to be remembered that in our application retained engine heat is what aids in combustion, cool it too much and engine no go so good.
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The pesky 5 litre flat 12 Porsches just watercooled the heads when turbocharged .
Valve temperature and drivle.
Wonder how a Termosyphon would go for circulation.
Got any thoughts on Evaporative plus condensor . Light & closed circuit .
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On thing to keep in mind if you were to pursue the "cool" fuel Idea, cold fuel will not atomize as nicely as warm fuel,, in my experience,, In stock cars, cooling the fuel allowed us to get MORE fuel through the carb, cooling the airflow allowed you to get more fuel into the engine with the more dense charge. but you had to balance the atomization aspect.
In my Humblest opinion, cooling on our motors is best accomplished by reasonable airflow, and the use of castor,,
or on my new plane, just keeping the batteries in the airflow works S?P
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Read an article on the 'Supercool Prop' site once that did a test on radial fins running vertically along the length of the barrel.
The engine has purpose built metal ducting that diverted the airflow 90º so that it ran from the crankshaft centre line and exited at the head.
It is theoretically superior to make the air flow along the straight walls of the cylinder rather than around its curved sides but in all cases it seems to be the ducting that makes the difference, even with heads.
Anyway, I seem to remember motorcycle head fins that were canted off from the direction of travel and it was said to improve cooling, but to me it was just a marketing exercise.
Cheers.
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Do some research on "Rotors & Stators" in a gas turbine compressor section. The prop being the rotor and the cyl head fins being the stator.
The fins really need to be slightly curved to match the slipstream be correct. Maybe a niche model engine builder will see the light.
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If you are really interested in cooling the engine look into how full scale pressure cowlings work. The Reno record setter, Rare Bare, uses a huge spinner to direct the into a pretty narrow gap in the cowling. Inside the cowl the air makes a 90 deg. turn towards the heads, slows down, and is ducted into the proper places on the engine for maximum cooling. Behind the engine the air is collected and ducted in tapered ducts(to allow for the expansion of the heated air) for additional thrust. All the cowled light plane engines do something similar. The air comes in the intake ducts, makes a turn around a baffle to slow it down, and then is forced by other baffles to flow through the cooling fins.
On most model engines the fins are too closely spaced to do much good. The air that does flow between them goes into laminar flow and acts like a plug, forcing almost all the air to simply go around the OD of the cylinder. I read somewhere that the fins need to be at least 1/8 in. apart and no more than 1/4 in. or so deep to get airflow through them without ducting. The only engines that come close are some of the big 4 strokes and Henry Nelson's designs.
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Consider too that a lot of testing was done during WWII on air cooled engines and the British found that tapered fins (thinner at their extremity) were far more efficient at shedding heat.
But with a total loss oil system on our model engines most of the heat is shot out of the exhaust and I seem to remember Team Race engine builders concurring that only about 30% of the engines heat is actually radiated.
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There are enough engines around that do not have head fins at all, suggesting to me that fins are better places around the cylinder. Especially around the area of combustions and slightly below.
One of the hardest things to do effectively is to cool the rear of the cylinder, in both cowled and uncowled applications.
Much has been written about how to provide the rear of the cylinder with a shot of cooling air.
As a "By the Way" a speed flier mate of mine made a new head for his 21 model, and ran the mill across at a 45* angle by mistake. End result, was no difference in heat, but too much embarrasment to keep it on . He made a new one with the traditional North South fins.