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  #1  
Old 11-09-2017, 08:37 PM
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sbalmos sbalmos is offline
 
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Location: Maineville, OH
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Default Skybolt and SJ Cowl - Bevel edges?

Hi all,

Possibly a fiberglass newbie question here. I'm using Skybolts and a Sam James cowl. I riveted on the fasteners to the firewall last weekend, and have done a loose fit of my upper and bottom cowl halves. Basically just dropped the top half on top of my baffle rubber. Everything looks good to start. But since the gap thickness between the fastener surface and the top forward fuselage is one skin thickness (0.032"?), clearly the cowl thickness is much more, and I end up with a huge intersection jump.

How much should I bevel down the inner firewall edges of the cowl for proper fit? Or is beveling really more just for visual cleanliness? Some part of me worries about beveling down too far, and not leaving enough along the edge of the cowl for structural holding by the fasteners.
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  #2  
Old 11-09-2017, 09:01 PM
rockwoodrv9 rockwoodrv9 is online now
 
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Default

Scott, I used some aluminum infused epoxy bondo stuff. I think it is Evergreen brand. It forms very hard and I was able to get a good fit. Building up and shimming the foreskin I felt was better than sanding off part of the fiberglass cowl. I did get it pretty close with shims and the Evergreen stuff made it smooth.
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  #3  
Old 11-09-2017, 09:29 PM
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sbalmos sbalmos is offline
 
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Hey Rocky. Yeah I think you alluded to something like that a few months ago to me. So you basically bondo-built up a ramp lip on the skins to match the thickness of the cowl? Any pics of it that you could shoot to me? Thanks!
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Cincinnati, OH
RV-9a slider - Started 1/12/12
N112SB reserved
IO-320 w/EFII and Catto 3-blade
Beringer brakes/wheels/tires
Dual G3X Touch (Stein's bankrupted me!)
Emp - DONE! (90%) (9 mos, 4 days)
Wings - DONE! (90%) (1 yr, 2 mos)
Fuselage - Almost Done
Finish - Engine Prep and misc
AT THE AIRPORT!!! (KHAO)
=VAF= 2016 paid up
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  #4  
Old 11-09-2017, 10:00 PM
rockwoodrv9 rockwoodrv9 is online now
 
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I will get some pics when I get home this weekend.
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  #5  
Old 11-09-2017, 10:18 PM
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Bob Martin Bob Martin is offline
 
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Default Thickness

Seems to me that when I installed the scalloped metal to receive the 1/4 turn fastners, I installed a sheet metal shim strip about 1/2" wide between the skin and the receptical strip so the cowl thickness would be close.

I had removed a standard Vans cowl and piano hinges and the same strip was there for the hinge. I assumed it was standard practice to look ahead to the cowl thinkness and shim accordingly.
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  #6  
Old 11-10-2017, 06:23 AM
BillL BillL is offline
 
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Quote:
Originally Posted by Bob Martin View Post
Seems to me that when I installed the scalloped metal to receive the 1/4 turn fastners, I installed a sheet metal shim strip about 1/2" wide between the skin and the receptical strip so the cowl thickness would be close.

I had removed a standard Vans cowl and piano hinges and the same strip was there for the hinge. I assumed it was standard practice to look ahead to the cowl thinkness and shim accordingly.
Yep, this is what I did and it matches pretty well. A shim under the firewall lip on top of the skylocs.
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  #7  
Old 11-10-2017, 07:02 AM
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DanH DanH is offline
 
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Quote:
Originally Posted by sbalmos View Post
Hey Rocky. Yeah I think you alluded to something like that a few months ago to me. So you basically bondo-built up a ramp lip on the skins to match the thickness of the cowl? Any pics of it that you could shoot to me? Thanks!
No, no, no, no....

Shim the fastener strip inward, as Bob and Bill describe, so the outside surface of the cowl is flush with the forward aluminum skin.

As for beveling the edge of the fiberglass, again, no. If anything, you want to add several plies of glass tape to the inside of the cowl edge to stiffen it all you can. It will scallop outward between the fasteners in flight due to internal cowl pressure. Thinning would make that worse.
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  #8  
Old 11-10-2017, 09:53 AM
rockwoodrv9 rockwoodrv9 is online now
 
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Scott, I may not have been clear. You need to shim to get it as close as you can, then fix the little imperfections with the metal bondo. The shims will get you real close. They are a bit of a pain, but work well. It you ar a purest, the bondo may hide a few rivets causing heads to explode while being judged.


Quote:
Originally Posted by DanH View Post
No, no, no, no....

Shim the fastener strip inward, as Bob and Bill describe, so the outside surface of the cowl is flush with the forward aluminum skin.

As for beveling the edge of the fiberglass, again, no. If anything, you want to add several plies of glass tape to the inside of the cowl edge to stiffen it all you can. It will scallop outward between the fasteners in flight due to internal cowl pressure. Thinning would make that worse.
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  #9  
Old 11-10-2017, 11:29 AM
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maniago maniago is offline
 
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I just finished doing this on the M2 with SJ cowl and Milspecs day before yesterday. Its all about the shims. Bondo? Ah no. I used .020 shim on all of the flanges (w/ .015ss firewall flange and .035 top skin) and in the lower corners had to add a second shim two rivets holes (ie 1.5") in length to make the corner work out properly. For sure dont mess with the thickness of the cowling.

Pretty sure shims are mentioned in the SJ instructions......

Just my .02 however.
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