It is recommended that no more than 8 hrs. be permitted to elapse between final surfacing and gluing. The gluing surfaces should be machined smooth and true with planers, jointers, or special miter saws. Planer marks, chipped or loosened grain, and other surface irregularities are not permitted. Sandpaper must never be used to smooth softwood surfaces that are to be glued. Sawed surfaces must be similar to well-planed surfaces in uniformity, smoothness, and freedom from crushed fibers.
Tooth-planing, or other means of roughening smooth, well-planed surfaces of normal wood before gluing are not recommended. Such treatment of well-planed wood surfaces may result in local irregularities and objectionable rounding of edges. Although sanding of planed surfaces is not recommended for softwoods, sanding is a valuable aid in improving the gluing characteristics of some hard plywood surfaces, wood that has been compressed through exposure to high pressure and temperatures, resin-impregnated wood (impreg and compreg), or laminated paper plastic (papreg).
PLYWOOD SKIN REPAIRS Most skin repairs can be made using either the surface or overlay patch, the splayed patch, the plug patch, or the scarf patch. Probably the easiest to make is the surface patch. Surface patches should not be used on skins over ½-in. thick.
To repair a hole by this method, trim the damaged skin to a rectangular or triangular shape depending upon the exact location of the damage relative to the framing members (figure 5-113). Where Front spar Minimum radius = Trimmed - 5 times openings skin thickness Rear spar FIGURE 5-113. Typical shapes for damage removal.
the framing members form a square corner and the damage does not extend to the next parallel member, a triangular opening should be made. The corners of the cutouts should be rounded with a radius of at least five times the thickness of the skin.
Doublers, made of plywood at least as thick as the skin, are reinforcements placed under the edge of the hole inside the skin. The doublers are nailed and gluded in place. The doublers are extended from one framing member to another and are strengthened at the ends by saddle gussets attached to the framing members.
A patch large enough to extend at least 12 times the skin thickness beyond all edges of the opening is cut from material of the same kind and thickness as the original panel. The edges of the patch are then beveled (scarfed), as shown in figure 5-114.
It is usually impossible to use clamps when gluing an external patch; therefore, the pressure is applied by some other means. It is usually done by placing heavy weights on the patch until it is dry.
Two or three small nails driven through the patch will prevent slipping during the drying.
After a surface patch has dryed, it should be covered with fabric. The fabric should overlap the original plywood skin by at least 2 in.
Surface patches located entirely aft of the 10% chord line, or which wrap around the leading edge and terminate aft of the 10% chord line are permissible. The leading edge of a surface patch should be beleved with an angle of at least four times the skin thickness. Surface patches can have as much as a 50-in. perimeter and can extend from one rib to the next. The face-grain dircction of the patch must be in the same direction as the original skin. Surface patches should not be used on skins over ⅛-in.
thick.
Flush Patches In places where an external patch would be objectionable, such as on wing coverings or fuselage skin, etc., a flush patch can be used.
Plug Patches Two types of plug patches, oval and round, can be used on plywood skins. Since the plug patch is strictly a skin repair, it should be used only for damage that does not involve the supporting structure under the skin.
A plug patch has edges cut at right angles to the surface of the skin. The skin is cut out to a clean round cr oval hole with square edges. The patch is cut to the exact size of the hole, and when installed, 235
Front spar Trimmed opening іґ B Saddle gusset Rear spar -30 T -Plywood skin Patch 12 T (1" minimum) ЗТ (¼" minimum) -Rib cap
- Plywood saddle gusset Section A-A Minimum thickness = T Nailed-glued in place I21 • I platch - 12T- - Patch -Rib cap Spartas Spar Unsupported Section B-B Section C-C FICURE 5-114. Surface patches.
- Grain direction of skin, patch, and doubler
•Saw cut in doubler
- Butt joint of patch to skin
- Inner edge of doubler Outer edge of doubler
- Nail holes Screw holes - to be filled before finishing
- Butt joint of patch to skin I Plug patch
- Plywood skin - Saw cut in doubler
• Plywood doubler Dimensions A BC Small circular plug patch 2· , 2 1% Large circular plug patch 3% 3 2% (Two rows of screws and nails required for large patch) FIGURE 5-115. A round plug patch.
the edge of the patch forms a butt joint with the edge of the hole.
A round plug patch, shown in figure 5-115, can be used where the cutout hole is no larger than 6 in, in diameter. Large and small circular patches have been designed for holes of 6 and 4 in. in diameter.
The steps for making a circular or round plug patch are:
(1) Cut a patch of the correct dimension for the repair. The patch must be of the same material and thickness as the origina.
skin. Orientation of the face-grain direction of the round plug patch to that of the skin surface is no problem, since the round patch may be rotated until grain directions match.
(2) Lay the patch over the damaged spot and mark a circle of the same size as the patch.
(3) Cut the skin out so that the patch fits snugly into the hole around the entire perimeter.
(4) Cut a doubler of ¼-in. plywood so that its outside radius is g in. greater than the hole to be patched and the inside radius is ⅜ in. less. For a large patch, these dimensions would be ⅛ in, each. The doubler should be of a soft plywood such as poplar.
(5) Cut the doubler through one side so that it can be inserted through to the back of the skin. Apply a coat of glue to the outer half of the doubler surface where it will bear against the inner surface of the skin.
(6) Install the doubler by slipping it through the cutout hole and centering it so that it is concentric with the hole. Nail it in place with nailing strips, using a bucking bar or similar object for backing. Waxed paper must be placed between the nailing strips and the skin.
(7) After the glue has set in the installation of the doubler, apply glue to the surface of the doubler where the patch is to join and to the same area on the patch. Insert the patch in the hole.
(8) Apply pressure to the patch by means of a pressure plate and No. 4 wood screws placed at approximately 1-in. spacing.
Waxed paper or cellophane placed be- Outer edge of doubler Butt joint of patch to skin
- Inner edge of doubler
- Nail holes Screw holes - to be filled before finishing Patch dimensions
ABCDE F Small 2% 1¾ 3 4% Large 2 3% 1% 5 7 Two rows of screws and nails required on large patch Butt joint of patch to skin
- Plug patch (grain parallel to skin) ....
- Plywood doubler (grain parallel to skin)
- Plywood skin FIGURE 5-116. An oval patch.
tween the plate and patch will prevent glue from sealing the plate to the patch.
(9) After the glue has set, remove the nails and screws. Fill the nail and screw holes, sand, and finish to match the original surface.
The steps for making an oval plug patch, figure 5-116, are identical with those for making the round patch. The maximum dimensions for large oval patches are 7-in. long and 5-in. wide. Oval patches must be prepared with the face grain carefully oriented to the same direction as the original skin.
Splayed Patch A splayed patch is a patch fitted into the plywood to provide a flush surface. The term "splayed" denotes that the edges of the patch are tapered, but the slope is steeper than is allowed in scarfing operations. The slope of the edges is cut at an angle of five times the thickness of the skin.
Splayed patches may be used where the largest dimension of the hole to be repaired is not more than 15 times the skin thickness and the skin thickness is not more than 1/10 in.
Lay out the patch as shown in figure 5-117. Tack a small piece of plywood over the hole for a center point. Draw two concentric circles around the damaged area. The difference between the radii of the circles is five times the skin thickness. The inner circle marks the limit of the hole and the outer one marks the limit of the taper.
Cut out the inner circle and taper the hole evenly to the outer mark with a chisel, knife, or rasp.
Weights or clamp Pressure plate %" T = 1/10" or i" plywood or less - Waxpaper or cellophane ST • 5T L plywood skin FICURE 5-117. A splayed patch.
Prepare a circular patch, cut and tapered to match the hole. The patch is of the same type and thick ness as the plywood being repaired.
Apply glue to the beveled surfaces and place the patch into place with the face-grain direction matching that of the original surtace.
After the patch is in place, a pressure plate cut to the exact size of the patch is centered over the patch, with waxed paper between the two, and pressed firmly against the patch with a weight (sometimes a sandbag) or clamp. Since there is no reinforcement behind the splayed patch, use care to prevent excess pressure. After the glue has set, fill, sand, and finish the patch to match the original surface.
Scarf Patch A properly prepared and inserted scarf patch is the best repair for damaged plywood and is preferred for most skin repairs. The scarf patch differs from the splayed patch in that the edges are scarfed to a 12 to 1 slope instead of the 5 to 1 used with the splayed patch. The scarf patch also uses reinforcements under the patch where the glue joints occur.
Much of the outside surface of plywood aircraft is curved. If the damaged plywood skin has a radius of curvature greater than 100 times the skin thickness, a scarf patch can be installed. Backing blocks or other reinforcements must be shaped to fit the skin curvature.
Figures 5-118 and 5-119 illustrate methods for making scarfed flush patches. Scarf cuts in plywood are made by hand plane, spoke shave, scraper, or accurate sandpaper block. Rasped surfaces, except at the corners of scarf patches and sawed surfaces, should be avoided since they are likely to be rough or inaccurate.
When the back of a damaged plywood skin is accessible (such as a fuselage skin), it should be repaired with scarf patches following the details shown in figure 5-118. Whenever possible, the edge of the patch should be supported as shown in section C-C. When the damage follows or extends to a framing member, the scarf should be supported as shown in section B-B.
Damages that do not exceed 25 times the skin thickness in diameter after being trimmed to a circular shape can be repaired as shown in figure 5-118, section D-D, provided the trimmed opening is not nearer than 15 times the skin thickness to a framing member. The backing block is carefully 239
• Saddle gusset Nailing strips
- Maximum diameter 25T Minimum . distance.
~ 15T Temporary backing 30T Patch -12T Backing Plywood saddle gusset (·" minimum ) Minimum thickness = T Nail-glue in place Section A-A 121 Patch • Patch Backing Framing member - Backing Clamp and glue backing to frame and skin Section B-B Section C-C Nailing strips
- 127.
Temporary backing block, shape to fit skin Section D-D Cellophane or waxpaper.
FIGURE 5-118. Scarf patches, back of skin accessible.
Front spar Ribs Saddle gusset Rear spar
- 30T -12T- f Plywood skin 81 3T (Ke minimum) - L Rib cap Plywood saddle gusset (1" minimum ) Section A-A Nail and glue in place (minimum thickness = T) Plywood skin 121- Patch -12T - Patch T -T Rib cap ЗТ (¼ minimum ) Plywood skin Plywood or spruce Section B-B Section C-C FICURE 5-119. Scarf patches, back of skin not accessible.
shaped from solid wood and fitted to the inside surface of the skin, and it is temporarily held in place with nails. A hole, the exact size of the inside circle of the scarf patch, is made in the block and is centered over the trimmed area of damage. The block is removed after the glue on the patch has set, leaving a flush surface to the repaired skin.
When the back of a damaged plywood skin is not accessible, it should be repaired as outlined in figure 5-119. After removing damaged sections, install backing strips along all edges that are not fully backed by a rib or a spar. To prevent warping of the skin, backing strips should be made of a softtextured plywood, such as yellow poplar or spruce rather than solid wood. All junctions between backing strips and ribs or spars should have the end of the backing strip supported by a saddle gusset of plywood.
If needed, nail and glue the new gusset plate to the rib. It may be necessary to remove and replace the old gusset plate with a new saddle gusset, or it may be necessary to nail a saddle gusset over the original.
Attach nailing strips to hold backing strips in place while the glue sets. Use a bucking bar where necessary to provide support for nailing. Unlike the smaller patches made in a continuous process, work on the aircraft must wait while the glue, holding the backing strips, sets. After the glue sets, fill and finish to match the original skin.
Fabric Patch Small holes that do not exceed 1 in. in diameter, after being trimmed to a smooth outline, can be repaired by doping a fabric patch on the outside of the plywood skin. The edges of the trimmed hole should first be sealed, and the fabric patch should overlap the plywood skin by at least 1 in. Holes nearer than 1 in. to any frame member, or in the wing leading edge or frontal area of the fuselage, should not be repaired with fabric patches.
SPAR AND RIB REPAIR The web members of a spar or rib can be repaired by applying an external or flush patch, provided the damaged area is small. Plates of spruce or plywood of sufficient thickness to develop the longitudinal shear strength can be glued to both sides of the spar. Extend the plates well beyond the termination of the crack as shown in figure 5-120.
If more extensive damage has occurred, the web should be cut back to structural members and repaired with a scarf patch. Not more than two splices should be made in any one spar.
A spar may be spliced at any point except under wing attachment fittings, landing-gear fittings, engine-mount fittings, or lift-and-interplane strut fittings. Do not permit these fittings to overlap any part of the splice. Splicing under minor fittings such as drag wire, antidrag wire, or compression No fittings within these limits 5 to 1 slope SA Longitudinal crack Direction of grain of spruce or face grain of plywood FIGURE 5-120. Reinforcing a longitudinal crack in a solid or internally routed spar.
strut fittings is acceptable under the following conditions:
(1) The reinforcement plates of the splice should not interfere with the proper attachment or alignment of the fittings. The locations of pulley support brackets, bellcrank support brackets, or control surface support brackets should not be altered.
(2) The reinforcement plate may overlap drag or antidrag wire or compression strut fittings if the reinforcement plates are on the front-face of the front spar or on the 10A
• No fitting within these limits
SPLICING OF SOLID RECTA GULAR SPAR BA-
- 10A No fitting within these
SPLICING OF SOLID ROUTED SPAR 6B 10B
- 10A
SPLICING OF BOX SPAR FLANGES FICURE 5-121. Spar splicing.
rear-face of the rear spar. In such cases it will be necessary to install slightly longer bolts. The inside reinforcement plate should not overlap drag strut fittings, un- • 10A Inserted web section less such overlapping does not require suf- 10A ficient shortening of compression struts or + 1" (1/6 flange adjustment for proper rigging. Even changes in drag-truss geometry to prevent - ЗВ Plywood cover strip same thickness as A 4B depth) B~ though takeup is sufficient, it may be necessary to change the angles on the fittings.
Space the splices so that they do not overlap. Reinforcement plates must be used as indicated on all scarf repairs to spars. The 4B desired slope for the scarf is 12 to 1, but a slope of not less than 10 to 1 is accepta- Filler ble. The plates are held in place by glue blocks Spar and must not be nailed. web Figure 5-121 illustrates the general method for splicing common types of wooden spars.
Always splice and reinforce plywood webs with the same type of plywood as the original. Do not use solid wood to replace plywood webs. Plywood is stronger in shear than solid wood of the same thickness because of the variation in grain direction of the individual plies. The face-grain of plywood replacement webs and reinforcement plates must be in the same direction as that of the original member to ensure that the new web will have the required strength. One method of splicing plywood webs is shown in figure 5-122.
BOLT AND BUSHING HOLES All bolts and bushings used in aircraft structures must fit snugly into the holes. Looseness allows the bolt or fitting to work back and forth, enlarging the hole. In cases of elongated bolt holes in a spar or cracks in the vicinity of bolt holes, splice in a new section of spar, or replace the spar entirely.
Holes drilled to receive bolts should be of such size that the bolt can be inserted by light tapping with a wood or rawhide mallet. If the hole is so tight that heavy blows are necessary to insert the bolt, deformation of the wood may cause splitting or unequal load distribution.
Rough holes are often caused by using dull bits or trying to bore too rapidly. Well-sharpened twist drills produce smooth holes in both solid wood and plywood. The twist drill should be sharpened to approximately a 60° angle. All holes bored for bolts which are to hold fittings in place should match exactly the hole in the fitting.
FIGURE 5-122. Method of splicing a box-spar web.
Bushings made of plastic or light metal provide additional bearing surface without any great increase in weight. Sometimes light steel bushings are used to prevent crushing the wood when bolts are tightened. The holes for bushings should be of such size that the bushing can be inserted by tapping lightly with a wood or rawhide mallet.
A J 1-31-0te - 10A • Face grain of plywood side plates
- Spruce block FIcURE 5-123. A rib cap-strip repair.
RIB REPAIRS A cap strip of a wood rib can be repaired using a scarf splice. The repair is reinforced on the side 10A opposite the wing covering by a spruce block which extends beyond the scarf joint not less than three times the thickness of the strips being repaired. The entire splice, including the reinforcing block, is reinforced on each side by a plywood side plate as shown in figure 5-123.
When the cap strip is to be repaired at a point where there is a joint between it and cross members Face grair of plywood of the rib, the repair is made by reinforcing the scarf joint with plywood gussets, as shown in figure
5-124.
A, B, C, D, and E are
•ЗАТ - 5A + ЗА- original dimensions
- E Direction of face grain of plywood A, B, C, D and E are original dimensions. FIcURE 5-125. A rib repair at a spar.
Reinforcement plates shall be plywood glued and nailed.
- No fittings within these limits Splice plate Edge damage repaired Spruce block Bevel 5 to 1 . ЗА - FIcURE 5-124. A rib repair at a joint.
When it is necessary to repair a cap strip at a spar, the joint should be reinforced by a continuous gusset entending over the spar as shown in figure
5-125.
Edge damage, cracks, or other local damage to a spar can be repaired by removing the damaged portion and gluing in a properly fitted block, as shown in figure 5-126, reinforcing the joint by means of plywood or spruce blocks glued into place.
The trailing edge of a rib can be replaced and repaired by removing the damaged portion of the cap strip and inserting a softwood block of white pine, spruce, or basswood. The entire repair is then reinforced with plywood gussets and nailed and glued as shown in figure 5-127.
- Longitudinal crack FicURE 5-126. The repair of cracks and edge damage on & solid spar.
Compression ribs are of many different types, and the proper method of repairing any part of this type of rib is specified by the manufacturer. Figure 5-128 shows a typical repair made to a compression rib built up of a plywood web and three longitudinal members, the center one of which has been 245
it is possible to replace a damaged member, it is always better to do so than to attempt a repair.
FICURE 5-127. A rib trailing-edge repair.
repaired by a properly reinforced scarf joint and an outside layer of plywood. pression members, braces, and rib diagonals should Such members as glue blocka, filler blocks, cora- FIGURE 5-128. A compression-rib repair.
not be repaired, but should be replaced. Wherever 246