Plywood Skin Repairs

hole and place it so that the mark is concentric with the hole. Nail it in place with nailing strips, while holding a bucking bar or similar object under the doubler for backup. Place waxed paper between the nailing strips and the skin. Cloth webbing under the nailing strips facilitates removal of the strips and nails after the glue dries. 7. After the glue has set for the installed doubler, and you have removed the nail strips, apply glue to the inner half of the doubler and to the patch plug. Drill holes around the plug’s circumference to accept No. 4 round head wood screws. Insert the plug with the grain aligned to the surface wood.

8. Apply the pressure to the patch by means of the wood screws. No other pressure is necessary. 9. After the glue has set, remove the screws and fill the nail and screw holes. Sand and finish to match the original surface. The steps for making an oval plug patch are identical to those for making the round patch. The maximum dimensions for large oval patches are 7-inches long and 5-inches wide. Oval patches must be cut, so when installed, the face grain matches the direction of the original surface. [Figure 6-32] Scarf Patch A properly prepared and installed scarf patch is the best repair for damaged plywood and is preferred for most skin repairs.

The scarf patch has edges beveled at a 12:1 slope; the splayed patch is beveled at a 5:1 slope. The scarf patch also uses reinforcements under the patch at the glue joints. Much of the outside surface of a plywood aircraft is curved. If the damaged plywood skin has a radius of curvature not greater than 100 times the skin thickness, you can install a scarf patch. However, it may be necessary to soak or steam the patch, to preform it prior to gluing it in place. Shape backing blocks or other reinforcements to fit the skin curvature. You can make scarf cuts in plywood with various tools, such as a hand plane, spoke shave, a sharp scraper, or sanding block. Sawn or roughly filed surfaces are not recommended because they are normally inaccurate and do not form the best glue joint.

The Back of the Skin is Accessible for Repair When the back of a damaged plywood skin is accessible, such as a fuselage skin, repair it with scarf patches cut and Outer edge of doubler Butt joint of patch to skin Plywood skin (Two rows of screws and nails required for large patch.) Inner edge of doubler Nail holes Screw holes—to be filled before finishing A C B DE F Plywood doubler (grain parallel to skin) Butt joint of patch to skin Plug patch (grain parallel to skin) 1" 1½" 2 ³∕8" 1½" 7∕8" 3" 4½" 2" 3 ³∕8" 2 ½" 1³∕8" 5" 7" Small Large D E FA B C PATCH DIMENSIONS 6-23 installed with the grain parallel to the surface skin. Details for this type of repair are shown in Figure 6-33.

scarf between frame members using permanent backing and gussets. When the damage follows or extends to a framing member, support the scarf as shown in section B-B. When the scarf does not quite extend to a frame member, support the patch as shown in section C-C. Damage that does not exceed 25 times the skin thickness (31⁄8-inches for 1⁄8-inch thick skin) after being trimmed to a circular shape can be repaired as shown in section D-D, provided the trimmed opening is not nearer than 15 times the skin thickness to a frame member (17⁄8-inches for 1⁄8-inch thick skin). A temporary backing block is carefully shaped from solid wood and fitted to the inside surface of the skin. A piece of waxed paper or plastic wrap is placed between the block and the underside of the skin. The scarf patch is installed and temporarily attached to the backing block, being held together in place with nailing strips. When the glue sets, remove the nails and block, leaving a flush surface on both sides of the repaired skin.

The Back of the Skin Is Not Accessible for Repair To repair a section of the skin with a scarf patch when access to the back side is not possible, use the following steps to facilitate a repair, as shown in Figure 6-34. Cut out and remove the damaged section. Carefully mark and cut the scarf around the perimeter of the hole. Working through the cutout, install backing strips along all edges that are not fully backed by a rib or spar. To prevent warping of the skin, fabricate backing strips from soft-textured plywood, such as yellow poplar or spruce, rather than a piece of solid wood. Use nailing strips to hold backing strips in place while the glue sets. Use a bucking bar, where necessary, to provide support for nailing. A saddle gusset of plywood should support the end of the backing strip at all junctions between the backing strips and ribs or spars. If needed, nail and bond the new gusset plate to the rib or spar. It may be necessary to remove and replace an old gusset plate with a new saddle gusset, or nail a new gusset over the original.

Unlike some of the other type patches that are glued and installed as one process, this repair must wait for the glue to set on the backing strips and gussets. At that point, the scarf patch can be cut and fit to match the grain, and glued, using weight for pressure on the patch as appropriate. When dry, fill and finish the repair to match the original surface. 6-24 Backing T 12T C C-Section Patch 3T Backing Framing member Clamp and bond backing to frame and skin B B-Section Patch Backing Clamp and bond backing to frame and skin 12T 3T T 30T 12T T Plywood saddle gusset minimum thickness “T” bonded in placeA A-Section Patch 3T (¼" minimum)8T T T 12T D D-Section 3T Waxed paper or plastic wrap Nailing strips C C B A B B D D C Nailing strips Maximum diameter 25T Temporary backing Minimum thickness bonded in placeT Saddle Gusset Temporary backing block-shape to fit skin 6-25 PatchT Patch 30T 12T T Plywood saddle gusset nail and glue in place (minimum thickness ) Plywood skin A A-Section Rib cap8T (1" minimum) B B C A B B A B B A C Rear spar Front spar Ribs Saddle gusset T 3T (¼" minimum) Plywood skin Section Section T 12T Plywood skin 3T (¼" minimum) 12T Rib cap Plywood or spruce Spar B - B C - C 6-26