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Working With Aluminum Skins

These tips for bending and joining the aluminum skins of your kitplane will really pay off in saved time, effort, and frustration.

Bending Trailing Edges

Bending JigI made a bending jig using two 2x4s and some door hinges.  I was worried about bending the trailing edge into too tight radius, so I put a small wood dowel into the bend of the skin while I worked the bend.  But I couldn’t get enough leverage to bend the trailing edges.  So I clamped two bar clamps onto the top 2x4 to give me more leverage.  I also used C-clamps in the middle to get even pressure across the trailing edge.  This worked well.  Bend slowly, and check often until you get a good fit.  The aluminum skins should lay flat and just touch the main spar without you having to push.  Don’t over-bend, or your skin will want to bow in instead of laying flat.

Correct BendingIncorrect bending

Bending Leading Edges

Bending leading edgesThis took a lot of forearm strength.  Just like any other bending you do, do a little at a time, checking the fit often.  I used the ¾” pipe from my pipe clamps to roll the edges.  I don’t think the diameter of the pipe is critical, as long as it isn’t too big.  I duct taped it to a skin, and put that side down on the workbench.  Then I clamped a pair of vicegrips to either end of the pipe to use for leverage.  I started with the skin that did not have the bend.  That way when I rolled the second side, the bent skin didn’t get in the way.  Since the bent skin goes on the inside, I was able to roll it past the first side and let it bounce back to the correct position with the first side out of the way.  

I rolled the skin, making sure to keep down pressure while I rolled.  If you just roll the skin without the down pressure, you will get a bend in the skin where the main spar ends.  I worked the flat side until it was in about the right position, then flipped the entire thing over and did the bent side.  Once they were as close as I could get them with the roller, I hand messaged them until the rivet holes lined up nicely.

Skin Lap Joints

The metal of the aluminum skins is so thin, that when you rivet them together, the edge of the outside skin will curl up.  To prevent this, put a very small bend about ¼” from the edge of the skin.  If you have a break, you can use it for the straight edges, but I found the easiest way was to use the vice grip type edge-rolling tool.  (See Tools)
Make sure you do this step before you dimple.  The dimples will get in the way for bending, but the bend will not interfere with dimpling.

You should do this to every outside edge of every skin.  For example on the F-824 skin, the bottom and aft edges need bending, the front and top will be covered, so do not.   Don’t forget the aft end of the FL-801 flap top skins, or the leading edges of the rolled control surfaces.


I used a scrap piece of thicker aluminum to help set up my microstop.  I put a bit in the microstop, and the microstop into the drill.  I then set the microstop by eye to what looked right, or just a little under the correct depth, and drilled a test countersink.  If it was too shallow when tested with either a rivet, or a dimpled piece of the correct thickness of aluminum, I reset it a little deeper and repeated until it was right.

On subsequent setups, I started setting the depth by pushing the bit into a correctly countersunk hole and setting the microstop to be flush with the surface.  Then tested on a scrap and made adjustments if needed.

You can freehand a countersink without a microstop.  There were a few places I had to so this because my microstop wouldn’t fit.  I mounted the bit in either my 90-degree angle drill or my deburring tool.  Then carefully and slowly drilled or spun the bit until I had the desired depth.  Take this slow, it is easy to remove too much metal.

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