Bottom cowl skin aluminum

[Gary Wallace]

I screwed up my bottom cowl skin trying to shrink the bottom corners to follow contour of boot cowl . Cant quite get the right shrinkage without distortion. I am thinking of using .032 3003 h14 for bottom skin for better shrinking results in those corners instead of 2024t3 .025. I believe this will be sufficient. Thoughts? Thanks.

[whee]

Personally I try to avoid shrinkage.

Is it possible you are over doing it by thinking you need to shrink the corners? Mine is just pulled around the corner and held in place with the cowl screws. Once trimmed to the proper overlap I think it looks good.

[Gary Wallace]

Thanks whee. Long story but i need shrinkage...if i dont ask my wife. The skin is out roughly 3/8 inch off boot cowl skin in bottom corners with the 1 inch overlap. In my case i need to shrink it to make me happy and need to know what metal i should use. Hoping for a quick reply or going with 3003 h14 as im frustrated and need to move on.

[Bdflies]

I used .032” 2024. After producing lots of scrap aluminum, I came to realize good shrinking meant very tiny nibbles. Lots of very tiny nibbles, not big bites.

Bill

[tailwind]

You may not like this thread. If the cowl stands proud by 3/8 in. Over a 1 in overlap perhaps the problem is not with the cowl but in the boot cowl?
I do not like to shrink 2024 with a hand shrinking tool. The face of the shrinking dies have a straight knurl and cause stress cracking at the scratch marks. When I do I have used 80 grit emery cloth sandwiched between the sheet metal and the dies. Sounds like a lot of metal to move in a small area in that alloy without heat treating.
I guess 3003 is an option. Much more plastic range and you can heat it and get some plastic range back as work hardening occurs. Little nibbles as stated above.

[Bdflies]

I agree about the straight knurl affect creating 'issues'. The smaller bites minimize the depth of the affected area and the .032” is thick enough to sand off all remnants of the shrinking marks. 3003 is pretty soft. How about 6061?

Bill

[Gary Wallace]

Thanks. Will give her another shot early next week. With. 032 2024. Not looking forward to it

[Jim Douglass]

Something you may wish to try--I am assuming you have the front of the bottom cowl attached to the nose bowl--if so unattach it from the nose bowl. Put one attach screw or cleco at the back of the bottom skin and see if the skin can be persuaded to lay flat against the boot cowl. It may be that the bottom cowl skin is not happy where it is being forced to lay because of the location of the nose bowl or boot cowl attach screws. I use a hole finder if the nutplates are installed in the boot cowl and the nose bowl to find where the new holes are to be located--again assuming the skin will lay flat and it is happy there in a new location. If this works, start with a slightly over-sized new skin and start with a new attach point at the point where you are having trouble and work outward both way ways, Then attach the skin to the nose bowl. This probably sounds easier than it really is.
I don't think 3003 alum would be a good choice for an engine cowl. Consider 5052 before 6061 or 3003.

[Jim Douglass]

I should have mentioned this in my earlier post. There are advantages in building the bottom engine cowl with 2 halves instead of one piece. If you are building by yourself you can easily handle a half side. Fitting the skin to the boot cowl is much easier because you are working only one side at a time. When both sides are installed to your satisfaction, you can run a strap around the bottom skins and pull the 2 sides together at the centerline, mark the overlap, cut off the excess and rivet the 2 sides together with a butt patch inside with flush rivets. The strap allows adjustments for the the air box and carb cutouts in the bottom cowl.

[Gary Wallace]

Thanks for the input Jim. Im working away here trying to get things ironed out. Very slowly but hopefully surely.