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. Marking the Skin Joint Line on the Main Spar and the Rib Rivet
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.
Top Skin Application
- 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.
- 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.
- 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.
Wing Walk Area Reinforcing Sheet
- 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.
- Using the template mark and drill #40 the last hole toward the
trailing edge in each rib.
- 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.
- Back drill through the hole at the trailing end of each rib into
- Using the template mark the rest of the rivet locations for the
ribs onto the skin.
- 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.
- 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.
- 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
- 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..
- Drill these holes #41 or 3/32" (#40 if you plan to countersink
- 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).
Aileron Bellcrank Gussets
- 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.
- 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.
- Carefully pull out the reinforcing sheet and replace the clecoes
in the skin.
- 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 Brace Rib and Inboard Aileron Mounting Bracket
- 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.
- 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.
- 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.
- 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
- 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
- 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
- Deburr and prime the gussets and angle. Rivet the angle to the
- 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.)
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.
Aileron and Flap Locator Templates
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.
topskin.doc 9/13/94 Frank Justice
- 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.
- 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.
- 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.
- 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.
- Clamp the bracket in position on the spar and drill #30 through
it into the spar. Remove the bracket.
- 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.
- Remove, deburr, and prime the bracket and short rib.
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