Cowl Fit

[skelrad]

I'm still slogging through the cowl fit on my 9A. I fit the bottom cowl sides and curve at the bottom of the fuselage as best as I could, but the shape of the cowl just doesn't fit the fuselage all that well. The bottom portion of the cowl has a bit of a wave in it and takes pretty significant force to attach it to the fuselage (and then it looks horrible because it doesn't lay flat between the attachment points). What's the best way to remedy this? I don't know if this is something heat can help with, or if the best approach is to cut a few slits in the bottom and corner so I can reglass it into a better shape. I originally tried to push the whole cowl up when I drilled the sides, but that then made the curve at the fuselage corner even worse.

My top cowl fit is...tolerable - it fits the curve of the right side of the fuselage, but the left side wings out about a half an inch. That makes taking it on and off a real pain, so I also need to figure out the best way to reshape that portion as well.

What's the best approach to fixing issues like this?

(before anyone jumps on it - the bottom of the fuselage IS flat and the rivets are nice and flush as well. The photo makes it look much worse than it is.)

 

[DanH]

I've never heard of a cowl which really truly didn't match the fuselage profile. It would require an enlargement of the cowl mold, or a shrunken fuselage. Why would this one be different?

Generally, cowl rework is limited to the flat behind the spinner, and adjustment of mating flanges. Perhaps it would be best to call in some skilled help for a good look at fitting before cutting into the aft edge.

 

[wirejock]

Dumb question, but is the cowl bowl/ spinner position set?

 

[jerry98b]

Mine didn't match up prefect either. I couldn't do much on the bottom where is bulges up between the screws but I did put a little filler in the corners.
Did you put the thin strips of aluminum between the hinges/lower mounts and the fuselage?

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[rvbuilder2002]

(before anyone jumps on it - the bottom of the fuselage IS flat and the rivets are nice and flush as well. The photo makes it look much worse than it is.)

Even though most of your photo is not in focus, it is fairly easy to see the skin is not totally flat.
It probably is, relative to the overall width of the bottom of the firewall because of the 1/8” thick aluminum angle it is rivited too, but it can be seen that the dimples are not fully formed (this is fairly common on this .040” thick skin which is the thickest on the entire airplane that gets dimple countersunk, making it the most difficult to dimple properly).
This may be causing a distortion in the skin shape near the outboard most rivet on each side, leading to the molded cowl shape not matching up.

 

[skelrad]

Even though most of your photo is not in focus, it is fairly easy to see the skin is not totally flat.
It probably is, relative to the overall width of the bottom of the firewall because of the 1/8” thick aluminum angle it is rivited too, but it can be seen that the dimples are not fully formed (this is fairly common on this .040” thick skin which is the thickest on the entire airplane that gets dimple countersunk, making it the most difficult to dimple properly).
This may be causing a distortion in the skin shape near the outboard most rivet on each side, leading to the molded cowl shape not matching up.

My come and go building skills make anything possible! I did take a contour gauge and compared the right and left fuselage corners. They are at least close enough in profile that I couldn't really tell a difference. When I put the gauge on the cowl though, the left side corner is kind of opened up - meaning the angle from the side to bottom is not as acute as the right side (just to illustrate, if it's supposed to be 90 degrees, it's instead 100 degrees relative to the other side).

I've never heard of a cowl which really truly didn't match the fuselage profile. It would require an enlargement of the cowl mold, or a shrunken fuselage. Why would this one be different?

Generally, cowl rework is limited to the flat behind the spinner, and adjustment of mating flanges. Perhaps it would be best to call in some skilled help for a good look at fitting before cutting into the aft edge.

I had a repeat builder who's built both RVs and a number of composite planes come look at it. After we tried and tried to manipulate the cowl into the best possible position without requiring excessive force to hold the shape in place, his take was that that corner/angle had somehow been flattened out. His only guess was maybe it was how it was stored over the years. He said if that's the case, I could try heat, but he wasn't holding his breath that it would really work well. His suggestion was to just build up the thickness of the corner and bottom from the inside, then sand to shape and blend from the outside. That sounds reasonable to me.

 

[wirejock]

If you're sure the cowl is in exactly the position you want... Sand a scarf. Cut some fingers. Layup the patch. I think folks here are just trying to make sure before it is chopped up.

 

[skelrad]

If you're sure the cowl is in exactly the position you want... Sand a scarf. Cut some fingers. Layup the patch. I think folks here are just trying to make sure before it is chopped up.

Yeah, the rest of the cowl is about as good as it can possibly get. Even so, it's going to need a lot of lipstick in the end. As is always the case, looking back I can see how I could have saved a little bit of fitment work here or there by doing things in a slightly different order, but that's the nature of an education. I was just thrown by this last portion of the fit since nothing I did seemed to make a difference. Onward and upward.

 

[wirejock]

Yeah, the rest of the cowl is about as good as it can possibly get. Even so, it's going to need a lot of lipstick in the end. As is always the case, looking back I can see how I could have saved a little bit of fitment work here or there by doing things in a slightly different order, but that's the nature of an education. I was just thrown by this last portion of the fit since nothing I did seemed to make a difference. Onward and upward.

Don't feel left out. I followed the manual and cut the horizontal edges. The bottom cowl ended up at least an inch shy. I had to add a scarffed section. The bowl didn't fit right either. Don't get me started on the empennage fairing. It sounds like Vans bashing, but there is no way they can make parts fit every airplane. That's the beauty of fiberglass. It can always be fixed.