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Another Cowl Inlet Duct Post

snoopyflys

Well Known Member
Hi all,

In my ongoing quest to become friends with my pink cowl and less intimidated dealing with fiberglass, I am turning my attention to fitting the Cowl Inlet Ducts. As others have mentioned in posts going back in time, the builders guide and drawings seem to basically leave it to builders desecration. I saw a few posts here in the forum where some trimmed others did not. Having a Titan ECI XIO-360 horizontal induction and the snorkel, could someone offer a non-engineer type their experiences with figuring out the best position of the inlet ducts to optimize the cooling or refer me to some other posts that I may help me here? I have my oil cooler mounted on the baffle as per plans. Here are a few pics for reference, both inlet ducts are not trimmed and guessing on the positioning. Do they get centered on the inlets and trimmed to match the inlet opening width or it does not matter? Not sure how it will/will not affect the baffling if at all. Any insights/experiences shared will be greatly appreciated.
Thanks,
 

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Dan, lots of those ramps have been glued in without much regard for detail. They work, but like most things, average effort nets average results. Or you can optimize....that "experimenting" stuff so fiercely defended.

The third photo illustrates something I think needs attention. See below. There is no virtue to a reduction in flow area above the leading edge of #1. Move the ramp forward and reshape the upper part of the inlet. While there, think about how you'll optimize sealing at the sides of the inlet.
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Learning continues…

Thanks Dan.

I was heading in that direction on the #1. I’ve trimmed and moved it forward to where I now have about 3 fingers width between the #1 cylinder and the duct. The #2 duct has right at 4 fingers untrimmed. About a 1/2” difference. Crude way of measuring depth but is relatively consistent for me. Would it be best to further optimize where both ducts offered the same amount of clearance to each cylinder?
 
Once you get them trimmed the way you want, and bonded in (and they don't really fit perfectly anywhere, you just have to try to find a good place), an inportant detail is to close off at least one side (end). What I did was just extend the contour of the diffuser piece outboard to intersect the side of the cowl.
You can make a surface to lay some fiberglass plies on by taping a piece of thin cardboard to the "inside" of the diffuser that extends outward, and trim it to fit next to the side of the cowl, or carve a piece of foam, or anything to get a surface to put fiberglass on. Then, 3 or 4 plies of 6 oz plain weave glass cloth with epoxy will do nicely.

This does two things. Most important, by closing off one end of the diffuser, you block off the tunnel that can flow a lot of air. The details of what happens depends a lot on the particular arrangement of the baffles and what they seal against, but it is common to have the baffles seal against the diffuser surface on the outboard end, and seal against the upper cowl around the front of the engine and up to the side of the inlet duct. If you don't close off the end(s) of the diffuser, there is now a big leak path from upper cowl to lower cowl area by flowing thru that tunnel.

The other thing that the extension of the diffuser does is provide more smooth diffuser area for the cooling flow to come in through the inlet and slow down and spread out in the upper cowl area, with out encountering any abrupt area changes with corners or edges.

Make the outboard side baffles seal against this extended diffuser surface all the way to the inlet.
 
Thanks Dan.

I was heading in that direction on the #1. I’ve trimmed and moved it forward to where I now have about 3 fingers width between the #1 cylinder and the duct. The #2 duct has right at 4 fingers untrimmed. About a 1/2” difference. Crude way of measuring depth but is relatively consistent for me. Would it be best to further optimize where both ducts offered the same amount of clearance to each cylinder?

Probably, but no one has data to quantify the answer.

Here's a twist. As you optimize the rest of the system, notably (a) reduce leakage, and (b) increase heat transfer (i.e. really good baffle wraps), mass flow will be reduced. That pushes diffusion more external, which lowers inlet velocity, which minimizes any sort of "imperfection" in the ramp arrangement.
 
Final draft…

I sealed up the inboard sides for the cowl ramps and applied some additional micro to transition the inlets to the ramps and the ramps to the cowling. Hoping this will help maximize the cooling. Time will tell.

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Cowl ramps first

Cowl ramps first. #1 ramp needs to be a s far forward as possible. Then cut baffles to fit.

My ramps are too far aft, but baffles are cut. So I am going to modify the center section of ramp and not disturb where baffles seals. JMHO
 
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