Making a Wing Leading Edge Skin
by Brian Meuser, February 2012
Here's a photo step-by-step on fabricating and installing a leading edge aluminum skin for a wing, in this case for a Curtiss-Wright Model 12W biplane. This is my first time making a skin, so suggestions and criticism is welcome - Brian.
Layout and Prep Prior to Bending
First I measure for the size of the skin, the placement of the ribs and any other things at need to be transferred to the skin (in my case I have inspection openings in my leading edge. The skin is cut to length and width. Mine was 16.5 inches by 107.5 inches.
I create a layout for the ribs as seen below. Each rib is measured in three locations (in case there is variation). Top edge of the rib, the nose of the rib and the bottom of the rib. I measure from the last full rib (before the wing tip bow) out to the three locations of each rib.
Using tape the same size as the width of the rib and my layout chart, I place the tape on the skin where each rib should be, then layout and pre-drill the nail holes.
Using a router I freehand cut relief semi circles slightly deeper than the bend on the edges (represented by the 1/4 inch tape). I like using a tape line as a measuring tool.
Utilizing a metal straight edge on the top and bottom, I clamp the skin with a 1/4 reveal. Then using hard wood and a little wax, I bend over the edge about 45 degrees. I work the area between the semicircle cuts one at a time.
The semi circle relief cuts eliminate the need to bend this area back flat where it crosses and ribs. I think it improves the look of the fabric in these areas.
Skin has been cut to size, edges bent and holes drilled for ribs. Make sure you have orientation marks to you keep track of which end is the tip and which way the sheet will be bent (easy for me to get confused).
Bending the Skin
Before going to the bending jig, I clamp wood stringers to the edges. This keeps the sheet flat, makes it easier to move, and protects the edges from the straps during initial bending. (Unfortunately, shop neatness isn't one of my qualities!)
You should have transferred the center line of the nose ribs to the skin. This line is used for orientation in both the jig and the wing. Place the line on the center of the pipe and clamp with a wood stringer. Note: I ran one side of my stringer through my planer to take out any imperfections like knots. It is amazing what will transfer to the aluminum. Any deformity spells trouble and will result in a ripple or oil can. Cut this stringer a little short so you can see the lines on each end.
The bending jig is made from 2x4's cut to exactly the same length with cradle cuts for a 2 inch pipe (pipe size will vary depending on the size of the nose of the rib. Do a test before committing a 9 foot sheet). The 2x4's were placed 24 inches apart. The leading edge is clamped in position with a top stringer and additional stringers are place on either side (up tight against the center stringer, The edge strings also remain in place at this time. Straps are located at each 2X4 upright. Straps between the uprights should be avoided as they could bow the pipe and create deformity in the leading edge.
Straps are uniformly and slowly tightened. The skin will try to rotate (the longer edge will try to bend first) and will need to be adjusted as the straps are tightened. This is easily done by grabbing the edges and shifting the skin. Keep the line on the top of the pipe! Note: The three stringers at the top are still together.
As you tighten the straps and the edges move inward, you can remove the edge stringers. Once the straps are tight, I use a rubber mallet and slowly, carefully, and evenly hit the stringers to the right and left of the center string downward moving them to the edges of the pipe. The idea is to pull the aluminum down around the pipe. I keep tightening the straps at every opportunity. Remember the straps need to be tightened evenly without creating deformation.
Because I am using .020 2024 T3 I will get significant spring back. If you were doing this with 3003 or another non-tempered aluminum, you would not need to pull down nearly as far. Always do a test first. Also, make sure your pipe is smooth and straight or the deformities will be transferred to the skin.
I let the skin sit in the bending jig overnight. I call "Letting it Soak."
In the same manner the skin was placed in the jig, it is transferred to the wing. Straps are placed at compression members. Once again, alignment is important. You should have reference marks to make certain the skin stays in the correct location as it will try to move in the same direction the straps move when being tightened. I also put my tip skins in place (without nailing them) prior to installing the leading edge skin. This allows for the extra space the skin will need at the rib where the two skins intersect.
If you need more straps than you have members put some temporary blocks between the spars. I should also mention that care should be taken to make certain the wing is straight and level before applying the skin. Remember, don't create deformities when tightening the straps. Watch your ribs. They should not bow.
Just like in the jig, the top and bottom wood strips are moved away from the center wood strip. I do the top first and move it until the first nail hole is exposed. I use a rubber mallet the move the strip uniformly a little at a time. There should be enough tension on the straps to keep the skin next to the rib but not so much to create deformities. Hopefully you marked and drilled your nail holes in the correct locations.
As with the jig, there will be a point when you need to get rid of the edge boards. You will probably need to loosen the straps to get them out. Continue to move the top strip past each row of nail holes and nail off. The idea is to keep pulling the skin away from the nose towards the trailing edge. Once the top is done, I do the same thing on the bottom.
You won't know for certain if you have any deformity or oil cans until you remove the straps and wood strips. I lucked out. No problems!
Skin has now been Scotch-Brite'd and coated with epoxy varnish.
As this is my first experience making and installing leading edge skins, any comments or suggestions are always welcome!