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Old 01-16-2012, 09:33 PM   #11
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Re: Round Cowl Engine Cooling

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The problem with cowled motors, round included, is that the air will always find the easiest path, and with an unbaffled installation this is usually around the bottom of the motor. This is the case with pic #1. After 35 flights without the cowl, (learning the habits on my first 4-stroke) the Saito powered Skybolt seized the first run with it installed. Overheating was definitely the cause and why I finally put the effort into building the air system. In that pic you can obviously see the obstruction created by the motor and the free path it had below the crankcase.

On my round cowled motors I usually opt for the fake radial kits and cut out one cylinder, positioning that opening in line with the engine cylinder. That alone will not ensure good cooling as you need to keep the air tight till AFT of the cylinder. I do this by backing the cylinder kit with a wood frame and then adding air dams to the cylinder. The key is to seal the entire front end so the air can ONLY go past the cylinder. Maintaining airflow where you need it is exactly as it is done on real ones.

What this gives you is a high pressure area ahead of the engine and low pressure aft where that heated air has a chance to expand and escape. To do that the rule is exit area in the cowl has to be 3X that of the inlet.

The second pic is again the Saito powered Skybolt, showing how tight the cowl follows the motor contour. It will still leak air a bit, but most is still forced around the cylinder by the system in pic 3. The lip you see in the second and last pic was added to augment the flow at low speeds, (hovering, etc.) by creating low pressure just at the outlet. In that last Skybolt pic you can just make out the lower air dam tight to the jug. Sorry for the mess. That plane now has close to 300 trouble free flights with NO overheating.

The Harvard and Rare Bear are true examples of what I was describing. Unfortunately I never took pics of the back of the baffle designs, something I hope to correct the next time the cowls are removed.
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Old 01-17-2012, 07:43 AM   #12
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Re: Round Cowl Engine Cooling

most areas have been covered but I would like to add . If you don't baffle, it is possible to keep most engines cool by making them run excessively rich however proper baffling allows the engine to run at it's correct temp with the proper fuel mixture, this helps the engine develop it's greatest power.
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Old 01-19-2012, 11:27 AM   #13
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