Wing Top Skin Fitting

by Frank Justice

There are as many different opinions as to how and in what order to rivet on the wing skins as there are builders. These instructions are optimized for riveting the bottom skin last; using any other order will cause many of the steps in here to be difficult to do, and a new order for almost everything must be found. Both top and bottom skins as well as the flap and aileron attachment are completely prepared before skin riveting starts. One piece top skins are used by some builders because the finished appearance is better and they are easier to install. Van's at present will not ship them because of their size. The top skin(s) is drilled as the first step and left attached for much of the rest of the procedure.

As soon as the top skin(s) is clecoed on the whole wing assembly becomes very rigid. The leading edge and tank skins can then be pulled down to meet the main skin(s) with no misalignment gaps or trimming. While it would be a little easier to put on the leading edge skin and fit the tank without the main skins in the way, having the skins present is helpful in getting rid of any gap between the main and leading edge skins. Just be sure that the whole wing structure is very well supported until one set of main skins is riveted in place.
Marking the Skin Joint Line on the Main Spar and the Rib Rivet Lines
  1. Mark about ten points along both sides of the main spar flange, 1" down from the top of the main web of the spar. This is where the leading edge of the main skins will be.
  2. Verify that the ribs are still straight; modify the fluting if necessary. Put marks in the center of each rib flange at both ends. Lay a straightedge along these points and draw a line all the way from the top of the main spar to around the bottom of the rear spar. Insure that the line is at least 3/16' from the edge of the flange and not too close to the web for proper bucking of rivets later.
  3. Mark rivet stay-out areas along the main spar (where the spreader angles and tiedown bar will make it difficult to buck the leading edge skin rivets) where they will be visible after the top skin is put in place.
Top Skin Application
  1. Make a rib rivet location template out of a strip of aluminum 29" long (or some stiff material with a straight edge). Make sure the marks correspond to the flat spots on the ribs. Spacing is approximately 1 1/4". Make the leading edge of the top skin correspond to the end of the strip. Include a mark 1/4" from that end (for the rivet line along the main spar) and another one in the middle of the rear spar flange.
  2. Using the template mark and drill #40 the last hole toward the trailing edge in each rib.
  3. Hang the top skin (or the outboard top skin) on the wing skeleton with the tip edge extending 5/8" beyond the tip rib web and the leading edge aligned with the marks previously made on the main spar. Secure with tape and clamps as necessary to make the skin conform very closely to the skeleton. The one-piece top skin makes a very accurate straight-edge. If the leading edge of the skin does not line up all the way across with the marks previously made on the main spar, adjust the supports under the rear spar until it does.
  4. Back drill through the hole at the trailing end of each rib into the skin.
  5. Using the template mark the rest of the rivet locations for the ribs onto the skin.
  6. Make a line 1/4" down from the leading edge of the skin and mark that line of rivets 7/8" or less apart. Put a mark on every rib centerline and evenly space the other rivets between them; this keeps them away from the rib reinforcing angles. Arrange the spacing so none of the marks are in the stayout zones.
  7. Drill #41 or 3/32" at all the marked locations (#40 if you plan to countersink instead of dimple the skin). Start in the center of the wing at the main spar and work outward toward the ends. Then work downward along the ribs, again starting in the center. To insure accurate placement in the rib flanges first drill only through the skin at the rib center and look for the line marked on the center of the flange. If necessary push or tape the rib so that the line is in the center of the hole while drilling the rest of the holes. Check occasionally from the back side to insure that the holes are coming out in the center of the flanges. If you have the two-piece top skins do not drill the holes at the edgfe of the skin for the innermost rib.
  8. Make a line on the skin for the rivets that will go into the rear spar flange. Make sure it is placed so as to give adequate room to buck rivets at the root end where the spar flange strips and doubler could get in the way, yet far enough in that the edge distance in the spar reinforcing plates at the middle and tip is adequate.
  9. Mark on the skin where the edges of the spar reinforcing plates lie. Also mark a point 46 7/8" in from the web of the tip rib. Mark the rivet locations 1" apart or less, making sure they do not lie too close to the edges of the reinforcing plates and not within 3/8" of the 46 7/8" mark..
  10. Drill these holes #41 or 3/32" (#40 if you plan to countersink the skin).
  11. If you have the two-piece top skins put the inboard skin in place. It should overlap the outboard skin by 1 3/16" Place a long straightedge along the edges of the skins to insure that they are lined up with each other. Drill this skin, including the extra row of rivets at its outboard end (see SK-48).
Wing Walk Area Reinforcing Sheet
  1. Remove the clecoes holding the skin to the five closely-spaced ribs at the root and to the spars in the same area. Clean out the chips out from between the skin and the ribs. Insert the wing walk area reinforcing sheet (13" x 27.5" x.025") under the skin, making sure its edges are not too close to the rivet holes.
  2. Drill through the skin holes into the reinforcing sheet. Try to make the drill stop as soon as it breaks through and use only moderate pressure; this will keep the bit from enlarging the holes in ribs which might now be slightly misaligned with the skin.
  3. Carefully pull out the reinforcing sheet and replace the clecoes in the skin.
  4. Deburr the reinforcing sheet holes. If you plan to dimple the top skin, dimple the reinforcing sheet. Prime this skin on your next painting day; you won't need it for a while.
Aileron Bellcrank Gussets
  1. The two gussets for the aileron bellcrank support can be made and installed anytime after the top skin is in place and must be done before the bottom skin is riveted on. If you do not have the right tools you may not be able to rivet the braces to the ribs, so doing the following procedure next is advisable.
  2. If your kit did not come with them, obtain the new W-425 gussets from Van's; they are easier to install, stronger, and less likely to deform the skin. There is however a slight problem with them in that the lower flange of the lower gusset is right where the platenuts for the bellcrank access plate are on the lower skin reinforcing ring. Drawings 19 and 21 depict these items separately but not the fact that they interfere. This requires that two large chunks be cut out of the lower gusset flange. Van's instructions call for using a 0.040 spacer on the flange of the lower gusset to cause it to be placed in far enough that it will not interfere with the reinforcing ring. A 0.040 spacer is then used over the part of the gusset flange where the ring isn't to allow the bottom skin to be riveted to it. An alternative method is described below; it consists of not using a spacer, cutting away the interfering part of the flange completely, and replacing it with a reinforcing angle where it will not be in the way.
  3. Carefully measure the distance between the ribs where the gussets will go and then bend the outboard end flange down so the gusset will fit without forcing the ribs in either direction.
  4. On the top-side gusset drill the rivet holes #41 or 3/32" (or #40) for the skin and the gusset-to-rib rivet holes #30. Put the top side gusset in place and drill through it into the top skin. Then clamp the gusset securely to the ribs and drill the gusset-to-rib holes.
  5. On the bottom-side gusset drill the gusset-to-rib holes #30. Clamp it to the ribs in a position such the surface of the flange will lay on the bottom skin when it is put on. Drill the gusset-to-rib mounting holes.
  6. Remove the gussets. On the bottom-side gusset cut off most of the skin-side flange, going about 3/16" into the main part of the gusset, leaving about 4' of flange at the small end, Drill #41 a piece of 3/4" x 3/4" x 0.025 angle about 8" long to the gusset on the opposite side from the existing flange and about 1/4" in from where the skin will be. This keeps it out of the way of the rivets that hold the reinforcing ring to the bottom skin.
  7. Deburr and prime the gussets and angle. Rivet the angle to the bottom-side gusset.
  8. Remove the two ribs that attach to the gussets from the wing assembly. Rivet the gussets to the ribs. Put this assembly back into the wing. (You may have to remove the wing supports, pull most of the rear spar to rib clecos, and bow the rear spar down to do this; it is possible to rivet the gussets to the ribs with them still in the wing but only if you have the right offset rivet set and two people.)
Aileron Brace Rib and Inboard Aileron Mounting Bracket
The Inboard Aileron Mounting Bracket shown in drawing 16 is supplied already assembled. The outboard aileron bracket cannot be done at this time because the rear spar jigging tab is in the way.

Fit the bracket to the rear spar using the distance from the center of its bearing to the bottom skin line extended (5/16" as shown in drawing 16). Use the template described below.
Aileron and Flap Locator Templates
  1. The above step cannot be done accurately without a template. Later on you will need two airfoil templates to help mount the ailerons and flaps, so make them now and and fix one to be used for a hinge bracket locator. The templates are V-shaped, four feet long, made out of 1/4' or 3/8" plywood. Use the airfoil template supplied with the wing kit as a pattern. One one of them draw a line 5/16" below the lower edge of the opening in the area where the hinge bracket bearing will be.
  2. Place the template under the wing and position it where the bracket will go. Then trim the bracket as necessary until you can see the line on the template through the bearing when it is in place on the spar.
  3. Clamp the bracket in place with the centerline of the bearing 46 7/8" from the web of the tip rib (the centerlinbe is simply the inboard edge of the bracket where it is not doubled up). This should result in the opening for the aileron control rod being about 1/8" from the bracket. This dimension was derived from dimensions on drawing 16. Place the short rib W-625 oriented as shown in drawing 10 with its rear flange centered on the long angle of the mounting bracket.
  4. Mark the eight mounting holes in the bracket 5/16" from the edge of the angles as shown in drawing 21, except use smaller spacing so that the rivets will not be too close to the ends of the flange of the short rib that will be held by the five rivets. Remove the bracket and drill the holes #30.
  5. Clamp the bracket in position on the spar and drill #30 through it into the spar. Remove the bracket.
  6. Clamp the short rib in place, and drill through the spar into the rib. Drill #41 (or #40) the top skin-to-rib rivet holes 1 1/4" inches or less apart.
  7. Remove, deburr, and prime the bracket and short rib.
topskin.doc 9/13/94 Frank Justice

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