Great Planes Piper J-3 Cub 20 Kit - GPMA0158 - Manual (Page 14)

D D 1. Use a T-bar or flat sanding block with 150-grit
sandpaper to lightly sand the top edges of the ribs to
smoothly blend the ribs to the main spar and remove
any bumps.
D D 6. To provide a gluing surface for the sheeting, carve
a slight bevel on the wing tip where the sheet will meet
the tip.
D D 2. Butt one of the 1/16" x 1-1/2" x 30" balsa wing LE
sheets against the leading edge. For the strongest glue
joint, sand or cut a bevel at the front edge of the sheet to
match the angle between the leading edge and the ribs.
D D 3. Mark the sheet so that its aft edge is 1/16" ahead of
the aft edge of the top forward balsa spar. The exposed
spar will provide a "step" for the cap strips to glue to later.
D D 4. Before attempting to bend the sheet so it may
reach the outer wing tip, the sheet must be thoroughly
wetted from R5 outward. Use a spray bottle or a sponge
to liberally apply water to the tip area of the sheet outboard
of rib R5. Carefully "work" the sheet with your fingers by
bending and twisting it in the direction required to meet the
wing tip. Replace the sheet on the wing and test bending
the sheet into position. Don't force the sheet - add more
water if necessary.
D D 7. Position the leading edge sheeting against the rear
edge of the LE and covering rib R2. Using thin CA, glue the
front edge of the sheet to the LE. Slightly wet the entire
sheet to bend it to the spar. Apply a bead of thick CA to the
forward spar. Working quickly, bend the sheeting to the
spar holding it down with something flat like a T-bar sander
or flat block of wood until the CA cures. Do not glue the
sheet to the LE past rib R5.
D D 5. Starting at rib R5 the aft edge of the sheeting will
have to be trimmed to keep it parallel with the front spar
(notice that the sheeting bends backward as it is curved to
meet the tip). Trim and test fit the sheet until it conforms to
the desired straight line.
D D 8. Lift the wing off the building board. Wick thin CA
between the ribs and the sheet from the inside of the wing.
Add a fillet of medium or thick CA along the inside of
the LE.
D D 9. Wet the sheet one more time in the tip area - the
water may have evaporated from the balsa. Test bend the
sheet to the tip. Apply thick CA where the sheet will contact
the outer tip. Add thin CA to the joint between the sheet
and rib R6. Fill the small seam between the LE and the
front of the sheet between ribs R5 and R6 with a scrap
piece of balsa.
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