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Old 04-20-2010, 11:48 AM
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wjcalhoun
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Default Engine cooling with cowl aircraft

Hi guys
I have a Wild Hare baby extra; OS AX 1.20 up front with 16x8 APC. The plane flies great, but I have a problem with cooling, and don't know if I need more inlet air, or more outlet area, or outlets in a different place. After I land, the engine smokes very lightly for about 15 seconds. It is not the cowl, not tubing, and not debris that is smoking. Obviously something is too hot. Never had this problem with my other cowled aircraft.

The mixtures seem about right - I am 3-4 clicks rich of peak (150-200 rpm down from peak), and the pinch test still gives me a burst of speed, so I think the hi speed needle is ok. It idles well, and takes throttle without stumbling or cut out, so I think the lo speed needle is about right also.

I know the rule-of-thumb ratio of 3:1 exit area to inlet area. The intakes are about 3 square inches surrounding the spinner, but part of that area is blocked by the spinner. I have about 17 square inches of outlet area, along the bottom of the cowl, so I am better than 5:1.

The OS AX 1.20 muffler can be mounted transversely, so it would almost fit into the cowl, but I opened the cowl forward of the muffler, to about 1 inch forward of the cowl ring. It is open on the bottom about 4.5 x 4", so there is a lot of exit area.

So... the question is, should I cut a few slots in the top and front of the cowl for more cooling air inlet over the head, or do I somehow need more outlet area. Is there any reliable method to tell which is the problem?

Thanks in advance




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