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using induced draft airboat style cowl rather than ram air

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mcurcio1989

Well-Known Member
Joined
Mar 15, 2013
Messages
178
Location
Toledo
The subject here is in regards to using an airboat style cowling for slow pusher amphibian. See pic.

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Updraft cooling on a standard Lyc has been done, but its contrary to the fin configuration of the head. There is a lot of fin area on the bottom near the exhaust port and collecting all that heat and then dumping it on the minimal fin real estate on the top is "backwards". Thats not the main issue though. The main problem is the lack of organized flow around the cylinder. The tighter the baffles and the more organized the airflow, the more likely the head is going to cool evenly. Remember that an air cooled engine is subject to significant stagnation and hot spots that won't show up on a highly localized CHT probe.

If this is a pod mounted engine, I'm wondering why you couldnt do some updated J-3 or Jabaru eyebrow style scoops providing the ram pressure for standard downdraft cooling? I get that you dont "need" a full pressure cowl, but eyebrows would provide much of the cooling benefits and would be pretty easy.

Do you have much or any seaplane experience? I'll proceed as though you don't but you'll know all of this if you do. What concerns me is the downwind taxi scenario where step taxi is not an option. Unlike a land based aircraft where a downwind taxi means you almost no relative wind, do have ram air from the propeller, and can taxi at low power as the wind pushes you along - the configuration of a pusher amphibian mean that when you have a tailwind you have to apply a lot of power to keep the aircraft from weather veining. Your almost at a plow taxi where you have a lot of power but the drag from the water keeps you very slow. You have to do this to keep rudder authority. So you're making a lot of power (heat), have no ram air available, and zero relative wind. The eyebrow was my original thought but this scenario kept me up at night. I also was haunted by the fact that I have never seen a pusher amphibian that didn't have some method of inducing draft pressure. I didn't like the idea of having to make a pressure cowl like the coot and lake have (lake just uses the propeller not a fan). That was when an EAA'r mentioned "how about airboats, what do they do". While I like talking theory as much as you I'm reminded of the words of a senior engineer when I was just getting starter "there is no point standing here and debating theory if we can go out and test it". In this case the scenario has been tested by hundreds of Lycomings in pusher configuration with thousands of hours logged, with positive results. I recognize your concerns and I have my own as well but I think the risk is worth experimenting with - partly because I have confidence from the precedent and partly because I think the alternative options are unfavorable.

I disagree with the idea that this doesn't have organized airflow it is just configured differently. Look at the coot and the lake - both have means of inducing a draft through the engine. Those were more "organized" but this system should create draft pressure that will pull air between the cylinders.
 

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