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-   -   Need help with SJ cowl!!! (https://vansairforce.net/community/showthread.php?t=79319)

Gonzo24 11-29-2011 11:54 AM

fiberglass strip and screws
 
Bart... I'm fighting the same fight on a -7 with SJ cowl and IO-375 right now. Was the consensus of your talk with Van's and others that 1/4" is enough clearance at the rockers? I have maybe 3/8" at #2 with hinges at present.

Also - after perusing some Lancair builder's sites... I see many elect to use the fiberglass and screw option for clearance issues on the side parting lines. After looking at their sites I didn't feel so bad about my clearances in the engine room :eek:

i would think there would be advantages to containing air leakage at the sides for cooling air drag reduction also with this method. Ah, the joys of cowling mania!

vmirv8bldr 11-29-2011 06:29 PM

1/4" should be ok.
 
From what I've learned, the engine basically rotates a bit on the mounts so the rocker covers mostly just move up and down under the cowl. One of the sources said that 3/8" side clearance is the target gap, but 1/4" should be just fine. If you have 3/8" right now and nothing's rubbing, I think you're right there with everyone else and should just move on. As for the cooling, if the baffles are sealing correctly, or you're using a plenum, then this area should be low pressure anyway and any opening would have a minimal affect on the cooling air flow.

I removed the horizontal hinges last night and put the cowl back on this afternoon. With no strap, or anything else, the sides sag out a bit. So, I'm going to make a backing strip covered with packing tape and cleco it onto the bottom cowl where the hinge was. Then use tape on the edge and outer surface of the bottom cowl to make an epoxy dam and prevent myself from gluing the halves together. Then I'll put it on the plane with the top cowl on. There's a lens shaped gap between the two cowls due to the sag that about 3/16" at the widest. I'll then fill that void with flox. After it cures, I'll sand a scarf joint and glass it over, then install the Milspecs except at the front and around rocker covers, which will get nutplates. This will get me 1/4" at Cyl #2 which will make me very happy, and I'll move on.

I'll post pics etc along the way. Seems others have had similar problems.

DanH 11-29-2011 06:53 PM

Quote:

Originally Posted by vmirv8bldr (Post 600967)
Excellent idea with the nutplate and fiberglass attach strip......I assume SS screws with tinnermans. Do you have a suggestion for a requisite number of BID layers?

Original thread here, with some good comments:

http://www.vansairforce.com/communit...ad.php?t=36697

Truss heads and nylon washers for me. I'm sure it costs me 0.1 knots ;)

Quote:

if the baffles are sealing correctly, or you're using a plenum, then this area should be low pressure anyway and any opening would have a minimal affect on the cooling air flow.
Not as a low as you think, and getting higher as you pull the exit closed.

vmirv8bldr 12-13-2011 12:06 AM

Progress
 
I have been working on a few things, but have gotten the correct fill and fiberglass strips installed on the bottom cowl. I'm pretty pleased with this really. After taking the halves apart, you can really see how much bend there is in the top cowl, and why the aluminum mounting strips would be less efficient here. Thanks to Dan H for that.

For the fill, I first sanded a scarf joint and about an inch down along the seam. I assembled the cowl in the garage and used tape (LOTS of tape!) as a release agent and mold. I taped over the edge on the top half and allowed for about 2" of overlap. Then I put a piece of tape down the seem and filled it from the back with thick flox. I squeegeed that and let it set. Then I separated the halves and sanded the back of the flox.

I then re-assembled the halves and followed Dan H's method to lay-up the mating strip. 5 plies of 2" 8oz tape. Worked like a charm. I did one on Saturday and the other Sunday. On the right hand side, all five plies run the entire length. On the left side, two plies run the full length of the seam but there is a gap in the other three plies right where the forward edge of the #2 cylinder is (you can see this in the pic.) After measuring the thickness, this was probably unnecessary, but should be ok nevertheless.

Test fit tomorrow!


thiggins 04-12-2020 06:11 AM

Carbon fiber version
 
Here's a Mike Patey video of him doing a similar process in Carbon Fiber. Took me a while to figure out how this worked, but it's very cool.

https://www.youtube.com/watch?v=BuWV5ybVX2Y

( The previous episode on cowl construction: https://www.youtube.com/watch?v=enLrRWV1bIM
)


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