Leading Edge Skin Assembly

by Frank Justice

Read the construction manual section on building the leading edge skin for additional information.

If you intend to equip your RV for instrument flight, you will need to install a heated pitot tube. You may find it easier to mount somewhere in the leading edge rather than on the main spar, depending on what kind of mounting you can find/make. The best place in the leading edge is between the two innermost ribs and just forward of the main spar where space is tight but support is good. It is suggested that you buy your pitot tube and prepare a place for it before attaching the leading edge assembly to the spar, although it is possible to do it later by removing the tank. Several mail order places sell new pitot tubes but you will either have to make a mounting bracket or get one from an aircraft salvage place.
Rib Assembly and Alignment
  1. Lining up the leading edge ribs properly requires some attention to detail. Misalignment does not affect the strength of the wing, but it makes it very difficult to get the skin-to-rib rivets close enough to the middle of the rib flanges for adequate strength and easy bucking. There are several causes for problems; twist remaining in the ribs after fluting, rib webs not exactly on a right angle to the spar long axis, shifting of the ribs as you put the skin in place, and varying width of the rib flanges. Also it is difficult to make a straight rivet line along the top side of the wing where the skin curves more. The following procedure will minimize the effects of these problems.
  2. Attach all the leading edge ribs to the spar. Put clecoes in from the bottom side. Drop bolts in all the bolt holes.
  3. Mark centerlines on the rib flanges and bring the lines down onto the top skin and spar flange.
  4. Make a rib spacing template as follows: On the top side of the wing place one end of a strip of wood or metal against the leading edge of the top main skin with the end aligned with the tip edge of that skin. Mark the position of the centerlines of all the ribs on it. Place the template on the bottom side of the wing and do the same; use the tip rib centerline as the reference point here.
  5. Secure the ribs vertical to the spar and properly spaced at the noses. You can cut blocks of wood or plasic pipe to just the right length and secure them with a piece of wire through the tooling holes as suggested in the manual. Make sure the spacers do not hit the raised area of the ribs near the tooling holes or a twist will be introduced. An easier method is to run 3/16" threaded rod with large washers on it through the top tooling holes in the ribs so there is a washer against both sides of every rib. Use several short rods connected with threaded couplers to save time in getting the nuts down to the right places. Use washers no larger than 3/4" outside diameter on the side of the ribs with the raised area.
  6. Working on the bottom side of the wing secure the nose end centerlines of the ribs to exactly the same spacing as the rear edge centerlines using the bottom side template. Then verify that the rib flange centerlines on the top side are properly spaced using the top side template; twist the ribs by hand or re-flute if necessary.
  7. Make a rivet spacing template for the top of the leading edge skin by transferring the marks made on the rib layout for fluting to a strip of aluminum at least 1" wide. At the spar end mark it "SKIN EDGE" and use the skin joint as the reference point. Be sure to mark the different position for the first rivet on the innermost three ribs. If necessary mark alternate positions for the rivts at the tip in the innermost rib. Hold the template up to the ribs to insure that the rivets all fall in a flat spot and not too close to a flange gap. Mark this template "TOP". Make a similar template for the bottom of the leading edge using the lower edge of the main spar as the reference point and mark it "BOTTOM".
  8. Make five spacer blocks out of 2x2 or 2x4 wood, 7 5/8" long. Tape these blocks in place under the spar between the large flanges, about in the center of the gap between the ribs.
  9. Cut five pieces of the strap material long enough to go completely around the leading edge skin with about one foot extra. Place of these straps in the middle of each of the gaps between the ribs; pull out several of the clecoes holding the main skin on so the strap can go underneath it. See SK-36. The straps must go across the wooden blocks previously put in the spar. Later after the skin is put in place tie the ends of the straps together with the buckles as shown in SK-36. The buckles should be about 10 inches above the spar on the top side.
Positioning the Leading Edge Skin
  1. Mark "top" and "bottom" sides of the leading edge skin for easy reference. The longer side is the top.
  2. Put some kind of tape that is thin, transparent, and slippery on the tips of the ribs to allow the skin to slide freely over them without being scratched. Make sure you can see the centerline marks through the tape. Place the leading edge skin on top of the ribs with the top side trailing edge against the main topskin edge (the straps will hold it there); pull it down loosely with the strap clamps. Align the outboard edge with the outboard edge of the main skin.
  3. Secure the leading edge skin onto the skeleton with the straps. Mark the positions of the centerlines of the rib flanges onto the leading edge skin at the trailing edges using the marks previously made on the spar and topskin. Using the rib spacing template mark the rib centerline positions near the nose on both the top and bottom. Mark the positions of the rivets on the bottom side of the skin using the rivet spacing template.
  4. Check for any gaps between the leading edge and top skins due to uneven shearing of the metal. Remove and trim the leading edge skin edge if necessary.
  5. Remove the leading edge skin. Draw a line 1/4" up from the trailing edge on each side of the skin for the skin-to-spar holes.
  6. Hold the skin up to the wing as a sanity check.
  7. Using the rivet spacing template mark the positions of the skin-to-rib rivets on the top side of the skin. Drill the rib rivet holes #41 or 3/32" at the following locations on both top and bottom: the ones closest to the spar, the leading edge two, and two more equally spaced between the leading and trailing edge. Deburr the holes.
  8. Note: The construction manual suggests drilling all of the holes before placing the skin, but it is quicker and safer to drill only a few reference holes at this point. The skin alone is hard to hold for drilling, and since the ribs must be drilled also you would have twice as much work.
  9. Align the rib centerlines with the holes in the skin, Tighten down on the straps a little at a time while watching to make sure the centerlines stay visible. Insert the W-623 Joint Plate strip between the leading edge skin and the innermost rib, with 3/4" extending past the skin edge. See drawing 19. Line up the strip so that there will not be a skin rivet through or past its end in the gap between it and the spar flange. This gap will not cause a problem.
  10. Tighten the straps until the trailing edge of the skin meets the main skin on the top side and the skin is fairly tight against the ribs on the top side.
  11. Mark the positions of the top side skin-to-spar rivets 1/4" in from the skin edge and lined up with the holes in the main skin Reposition if necessary those holes where the rivets would be difficult to buck because of the tiedown bar or stiffener angles in the spar.
Drilling the Leading Edge Skin
  1. Drill the top side skin-to-spar rivet holes #41, starting from the tip end and working inboard. Pull down on the leading edge of the skin by hand if necessary to keep the trailing edge tight against the main topskin.
  2. Drill the first row of skin-to-rib holes working center to ends. Then drill by rows in the same way up to the leading edge. Put pressure on the skin next to the drill bit to keep the drill chips from forcing the skin away and upsetting the alignment.
  3. If the rib centerlines on the back side are not well aligned with the pre-drilled holes or the marks on the main skin you can shift the rivet line and/or reach in and realign the rib centerlines with the holes.
  4. Tighten the straps more to make the bottom side trailing edge meet the line previously marked on the spar or get as close to it as possible.
  5. Drill the skin to rib holes starting at the leading edge and working down.
  6. Mark and drill the skin-to-spar holes on the bottom side starting with the ones that line up with the rib lines. Make sure you do not put holes where you can't buck the rivets. These holes are 1/4" from the edge and 7/8" or less apart.
  7. Locate the center of the hole for the tiedown ring (presently hidden by the skin) and drill a small hole. Enlarge the hole a little at a time and grind one side if it looks like the hole is not centered. Make sure the final hole is slightly larger than a 5/16" bolt, then finish the edge with a countersink bit.
  1. Remove all of the clecoes from the leading edge skin. Deburr the outside holes. Remove the straps. Remove the skin and deburr the inside. Dimple all holes. Clean and prime, at least the inside. Clean, dimple, and prime the Tank Attach Flange piece on both sides.
  2. Deburr and dimple the ribs, then remove them from the spar (or some other order if it suits your tools). Deburr and dimple the skin attach holes on the bottom side of the spar; countersink those you can't dimple.
  3. A cradle of some sort that holds the leading edge securely, leading edge down, is highly recommended for riveting unless you have a helper to hold it. This assembly and the tanks are very difficult to rivet neatly without a holder of some sort.
  4. Put the ribs in the leading edge skin with the tank attach flange strip and rivet together. Leave out the second and third ribs from the root to start with, then put them in after the others are riveted; this makes it easier to buck the rivets on the first rib which is very close to the second.
Attaching to the Spar
  1. If you are using a heated pitot tube install its support between the innermost two ribs now. (left wing only)
  2. If you are going to put landing lights in your leading edge it is easier to do it now.
  3. Put the leading edge assembly back on the spar held with clecos. Don't worry about the fact that on the top side the spar holes are not dimpled yet but the leading edge skin is.
leadedge.doc 9/13/93 (8/17/94 ok) Frank Justice

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