4.1 Plywood covering4.10 GENERAL. Nearly all wood aircraft structures are covered with stressed plywood skin. The notable exceptions are control surfaces and the rear portion of lightly loaded wings. Shear stresses are almost always resisted by plywood skin, and in many cases, a portion of the bending and normal loads is also resisted by the plywood.
4.11. JOINTS IN THE COVERING. Lap, butt, and scarf joints are used for plywood skin.
When plywood joints are made over relatively large wood members, such as beam flanges, it is desirable to use splice plates, often called aprons or apron strips, regardless of the type of joint. It is desirable to extend the splice plates beyond the edges of the flange so that the stress in the skin will be lowered gradually, thus reducing the effect of the stress concentration at this point.
Splice plates (fig. 4-1 above) can be made to do double duty if they are scalloped corresponding to rib locations so that they may act as gussets for the attachment of the ribs.
Scarf joints are the most satisfactory type and should be used whenever possible. Scarf splices in plywood sheets should be made with a scarf slope not steeper than 1 in 12 (fig. 4-2 below). Some manufacturers prefer to make scarf joints in such a way that the external feather edge of the scarf faces aft in order to avoid any possibility of the airflow opening the joint.
If butt joints (fig. 4-3 below) are made directly over solid or laminated wood members, as over a spar or spar flange, experience has indicated that there is a tendency to cause splitting of the spar or spar flange at the butt joint under relatively low stresses. A similar tendency toward cleavage exists where a plywood skin terminates over the middle of a wood member instead of at its far edge.
Lap splices (fig. 4-4 below) are not recommended because of the eccentric load placed upon the glue line. lf this type is used it should be made parallel to the direction of airflow, only, for obvious aerodynamic reasons.
4.12. TAPER IN THICKNESS OF THE COVERING. Loads in the plywood covering usually vary from section to section. When this is so, structural efficiency may be increased by tapering the plywood skin in thickness so that the strength varies with the load as closely as possible (fig. 4.5 below). To taper plies in thickness plies should be added as dictated by increasing loads. In doing so, the plywood should always remain symmetrical. For example, plywood constructed of an odd number of plies of equal thickness can be tapered, and at the same time maintain its symmetry by adding two plies at a time. This method is suitable for bag molding construction. Stress concentrations should be avoided by making the change in thickness gradual, either by feathering or scalloping. In bag molding construction, the additional plies are often added internally so that the face and back are continuous. (Bag molding refers to the molding of shaped laminates using a vacuum or pressure bag to hold the material in a form whilst curing.)
When flat plywood is used, the usual method of tapering skin thickness is to splice two standard plywood sheets of different thicknesses at an appropriate rib station with a slope of scarf not steeper than 1 in 12 as shown in figure 4-6.
4.13. BEHAVIOUR UNDER TENSION LOADS. Because the proportional limit in tension and the ultimate tensile strength of wood are reached at approximately the same time, plywood skin loaded in tension must be designed very carefully. Observation of various static test articles has indicated that square-laid plywood (plywood laid so that face grain is parallel or perpendicular to the direction of the principal bending stresses) has a tendency to rupture in tension before the ultimate strength of the structure has been reached. Diagonal plywood, however, seldom ruptures before some other structural member fails.The reason for this behaviour is probably due partly to the fact that none of the fibers of the plywood are in pure tension. The failure under tension load at 45° to the grain is almost entirely a shear failure, and the fibers, which have a definite yield beyond the proportional limit in shear, may undergo enough internal adjustment to permit the plywood to deflect with the structure until some other member becomes critically loaded. Square-laid plywood does not yield because some of its plies will fail in tension almost immediately after the proportional limit has been reached. This drawback of square-laid plywood becomes less important when the skin is designed to carry a greater proportion of the bending loads. For the limiting case of a shell structure without flanges, square-laid plywood is preferable.
Rupture of the skin is also influenced by its relative distance from the neutral axis. If the beam or beams are located so that part of the skin is appreciably farther from the neutral axis than the beam flanges, the skin is more likely to have a premature failure than if the flanges are located at the greatest outer fiber distance. Such a condition is illustrated by wing spars placed at the 15 and 65 percent chordwise stations of a normal airfoil.
Where the spanwise plies of plywood covering are of a wood species different from the beam flanges, it is, of course, desirable that such plies have a ratio of ultimate tensile stress to modulus of elasticity equal to or greater than that of the beam flanges.
4.14. BEHAVIOUR UNDER SHEAR LOADS. Diagonal plywood (face grain at 45° angle to the edge of the panel) is approximately five times stiffer in shear than square-laid plywood and somewhat stronger. When shear strength or stiffness is the controlling design consideration, diagonal plywood should be used (see 4.22).
4.15. PLYWOOD PANEL SIZE. In certain cases the size of plywood panels is dictated by the magnitude of directly computable stresses. These occur, for example, in spar webs, D-tube nose skin, and fuselage side panels subjected to high shear. In many other cases, however, the design loads are insignificant. It then becomes necessary to choose combinations of skin thickness and panel size which will stand up under expected handling loads, have acceptable appearance, and aerodynamic smoothness. The typical values given in table 4-1 have been employed by experienced manufacturers.
4.16 CUT-OUTS. When cut-outs are made in plywood skin for windows, inspection holes, doors, or other purposes, sharp corners should be avoided, and for all but small holes in low-stressed skin, a doubler should be glued to the skin around the cut-out. For some types of cut-outs a framework can be installed to carry the shear load and doublers need not be used (figures 4-8, 4-9, and 4-10).
4.2 Beams4.20. TYPES OF BEAMS. The types of beams shown [below] have been used frequently as wing spars, control surface spars, floor beams and wing ribs. The terms "beam" and "spar" are often used interchangeably and both are used in this chapter.
The wood-plywood beams (box-, I-, double I-, and C-) are generally more efficient load-carrying members than the plain wood types (plain rectangular and routed). A discussion of the relative merits of these various beam types is presented in succeeding paragraphs.
The box beam is often preferred because of its flush faces which allow easy attachment of ribs (see section 4.32). The interior of box beams must be finished, drained, and ventilated. Inspection of interiors, is usually difficult. The shear load in a box beam is carried by two plywood webs. By checking shear web allowables by the method given in section 2.73, it will be seen that for the same panel size a plywood shear panel half the thickness of another will carry less than half the shear load which can be carried by the thicker panel.
The preceding statement points to an outstanding advantage of the I-beam since its shear strength is furnished by a single shear web rather than the two webs required of a box or double I-beams. Also, the I-beam produces a more efficient connection between the web and flange material than the box beam in cases where the web becomes buckled before the ultimate load is reached. This is because the clamping action on the webs tends to reduce the possibility of the tension component of the buckled web cleaving it away from the flange.
The double I-beam is usually a box beam with external flanges added along that portion where the bending moments are greatest. This type allows a given flange area to be concentrated farther from the neutral axis than other types.
The C-beam affords one flush face for the flush type of rib attachment but it is unstable under shear loading. C-beams are generally used only as auxiliary wing spars or control surface spars.
Plain rectangular beams are generally used where the saving in weight of the wood-plywood types is not great enough to justify the accornpanying increase in manufacturing trouble and cost. This is usually the case for small wing beams, control-surface beams, and beams that would require a great deal of blocking.
Routed beams are somewhat lighter than the plain rectangular type for the same strength but not so light as wood-plywood types. Usually this small weight saving does not justify the increase in fabrication effort and cost.
In determining the relative efficiency of any beam type, reduction in allowable modulus of rupture due to form factors must be considered.
4.21. LAMINATING OF BEAMS AND BEAM FLANGES. Beam flanges and plain rectangular and routed beams can be either solid or laminated. A detailed discussion of methods of laminating beams and beam flanges is in section 2.4 of ANC Bulletin 19, Wood Aircraft Inspection and Fabrication (ref. 2 - 24).
Since the tension strength of a wood member is more adversely affected by any type of defect than is any other strength property, it is recommended that all tension flanges be laminated in order to minimize the effect of small defects and to avoid the possibility of objectionable defects remaining hidden within a solid member of large cross section.
4.22. SHEAR WEBS. Although square-laid plywood has been used extensively as shear webs in the past, the present trend is to use diagonal plywood (fig. 4-12) because it is the more efficient shear carrying material (see section 4.14).
It is desirable to lay all diagonal plywood of an odd number of plies so that the face grain is at right angles to the direction of possible shear buckles. In this way the shear web will carry appreciably higher buckling and ultimate loads because plywood is much stiffer in bending in the direction of the face grain and offers greater resistance to buckling if laid with the face grain across the buckles (fig. 4-13). This effect is greatest for 3-ply material
Figure 4-14 illustrates various methods of splicing shear webs. If the splices are not made prior to the assembly of the web to the beam ' blocking must be inserted at the splice locations to, provide adequate backing for the pressure required to obtain a satisfactory glue joint.
4.23. BEAM STIFFENERS. Shear webs should be reinforced by stiffeners at frequent intervals as the shear strength of the web depends partly upon stiffener spacing (fig. 4 -15). In addition to their function of stiffening the shear webs, the ability of beam stiffeners to act as flange spreaders is very important, and care must be exercised to provide a snug fit between the ends of the stiffeners and the beam flanges. External stiffeners for box beams are inefficient because of their inability to act as flange spreaders.
Stiffeners are usually placed at every rib location so that the web will be stiffened sufficiently to resist rib-assembly pressures.
4.24. BLOCKING. Any blocking, introduced for the purpose of carrying fitting loads (fig. 4-16), should be tapered as much as possible to avoid stress concentrations. It is desirable to include a few cross-banded laminations in all blocking in order to reduce the possibility of checking.
4.25 SCARF JOINTS IN BEAMS. The following requirements should be observed in specifying scarf joints in solid or laminated beams and beam flanges:
In addition to the previously mentioned specific requirements, it is recommended that the number of scarf joints be limited as much as possible; the location be limited to the particular portion of a member where margins of safety are most adequate and stress concentrations are not serious; and special care be exercised to employ good technique in all the preparatory gluing, and pressing operations.
4.26. REINFORCEMENT OF SLOPING GRAIN. Where necessary tapering produces an angle between the grain and edge of the piece greater than the allowable slope for the particular species, that piece should be reinforced to prevent splitting by gluing plywood reinforcing plates to the faces (fig. 4-19).
4.3 Ribs4.30. TYPES OF RIBS. Rib design has changed very little for several years. The more common types are the plywood web, the lightened plywood web, and the truss. The truss type is undoubtedly the most efficient, but lacks the simplicity of the other types.
For fabric-covered wings the ribs are usually one piece, with the cap strips continuous across the spars. When plywood covering is used the ribs are usually constructed in separate sections (figure 4-20).
Continuous gusset stiffen cap strips in the plane of the rib. This aids in preventing buckling and helps obtain better rib-skin glue joints where nail gluing is used because such a rib can resist the driving force of nails better than other types. Continuous gussets (figure 4-21) are more easily handled than the many small separate gussets otherwise required.
Any type of rib may be canted to increase the torsional rigidity of a structure such as a wood-framework, fabric-covered control surface (figure 4-22).
Diagonals loaded in compression are more satisfactory than diagonals loaded in tension since tension diagonals are more difficult to hold at the joints
4.31. SPECIAL PURPOSE RIBS. Where concentrated loads are introduced, as at landing gear or nacelle attachments, bulkhead type ribs can be used to advantage. When this is the case, the rib acts as a chordwise beam, and the principles presented in section 4.2 will apply ( figure 4-23).
4.32. ATTACHMENT OF RIBS TO THE STRUCTURE. In general, ribs are glued to the adjacent structure by means of corner blocks, plywood angles or gussets, or in special cases, by some mechanical means. These are all shown in detail in figures 4-24, 4-25, 4-26, 4-27, 4-34, and 4-39.
Although the attachment of ribs to I-beams may complicate the rib design, many engineers believe that the mechanical shear connection obtained by notching the ribs so that the end may be inserted between the I-beam flanges is an advantage since the shear connection is not dependent upon quality of the glue joint between the rib and the beam shear web. This type of connection is shown in figure 4-25. The web vertical also acts as a stiffener for the beam shear web and as a flange spreader.
The end rib verticals of plywood web type ribs are sometimes preassembled to plain rectangular spars to act as locating members for rib-to-spar assembly. This is shown in figure 4-26. Preassembled locating corner blocks might also be used to advantage in other types of rib-to-spar attachments if care is taken to provide sufficient backing for plywood webs to which corner blocks are being glued so that sufficient gluing pressure can be obtained.
Canted ribs may be attached to beam members by bevelling the ends of ribs or by using corner blocks as shown in figure 4-27.
4.4 Frames and bulkheads4.40. TYPES OF FRAMES AND BULKHEADS. No one type of frame or bulkhead seems to be the best for all types of loading, but the laminated ring is probably the best type for use as an intermediate stiffening frame. Frames or bulkheads are usually made of formed laminated wood, cut or routed from plywood, or are acombination of the two (figure 4-28).
4.41. GLUE AREA FOR ATTACHMENT OF PLYWOOD COVERING. Care must be taken when using the routed plywood type of bulkhead that the plywood edge provides sufficient gluing area for the skin. It is often necessary to glue solid wood to the face of the ring near its edge to provide additional gluing surface. This is illustrated in figure 4-29.
4.41. GLUE AREA FOR ATTACHMENT OF PLYWOOD COVERING. Care must be taken when using the routed plywood type of bulkhead that the plywood edge provides sufficient gluing area for the skin. It is often necessary to glue solid wood to the face of the ring near its edge to provide additional gluing surface. This is illustrated in figure 4-29.
4.42. REINFORCEMENTS AND CONCENTRATED LOADS. When concentrated loads are carried into a frame it may be desirable to scarf in some high density material and brace the frame with a plywood web or solid truss members.
4.5 Stiffeners4.50. GENERAL. The terms "stringer", "stiffener", and "intercostal" are often used interchangeably. In the following discussion stringer will refer to members continuous through ribs and frames and intercostal will refer to members terminating at each rib or frame. The term stiffener will not be used, since both stringers and intercostals are stiffeners.
4.51. ATTACHMENT OF STRINGERS. Ribs or frames must be notched if stringers are used. A method of reinforcing these notches and fastening the stringers to the rib or frame is illustrated in figure 4-30. Attachments may also be made one of the methods shown in figure 4-34.
4.51. ATTACHMENT OF INTERCOSTALS. All intercostals should be firmly attached to ribs or frames. Figure 4-31 illustrates the undesirable practice of terminating intercostals some distance from the rib or frame. This usually results in cleavage along the glue line starting at the free end of the intercostal. It is better to butt the stiffeners to the rib or frame and fasten them with saddle gussets as illustrated in figure 4-32 or by one of the attachments shown in figure 4-34.
4.6 Glue joints
4.60 GENERAL. Glue joints should be used for all attachments of wood to wood unless concentrated loads, cleavage loads, or other considerations necessitate the use of mechanical connections.
4.61 ECCENTRICITIES. Eccentricities and tension components should be avoided in glue joints by means of careful design. Figure 4-33 illustrates an example of an eccentricity and a method of avoiding it.
4.62 AVOIDANCE OF END GRAIN JOINTS. End grain glue joints will carry no appreciable load. Strength is given to such a joint by using corner blocks or gussets as shown in figure 4-34. These sketches are typical of joints encountered in joining rib members, in attaching ribs to beams or intercostals [stiffeners terminating at a rib or frame] to frames, or any other similar application.
4.63 GLUING OF PLYWOOD OVER WOOD-PLYWOOD COMBINATIONS. Many secondary glue joints must be made between plywood covering and wood-plywood structural members having plywood edges appearing on the surface to be glued. Wood-plywood beams or wing ribs employing continuous gussets are examples of such members. The plywood edge has a tendency to project above the surface thereby preventing contact between the plywood covering and the wood portion of the plywood of the wood-plywood surface. This condition can be the result of differential shrinkage between the wood and plywood or may be caused by the surfacing machine having a different effect cutting across the grain of the plywood from cutting parallel to the grain of the wood. Figure 4.35 shows this condition and shows and shows how it can be eliminated by beveling the edges of the plywood.
4.7 Mechanical joints4.70. GENERAL. Mechanical joints in wood are usually limited to types employing aircraft bolts. Since bolts in wood can carry a much higher load parallel to the grain of the wood than across the grain, it is generally advantageous to design a fitting and its mating wood parts so that the loads on the bolts are parallel to the grain. The use of a pair of bolts on the same grain line, carrying loads perpendicular to the grain and oppositely directed, is likely to increase the tendency to split. When a long row of bolts is used to join two parts of a structure, consideration should be given to the relative deformation of the parts, as explained in section 4.82.
4.71. USE OF BUSHINGS. Bushings are often used in wood to provide additional bearing area and to prevent crushing of the wood when bolts are tightened. See figure 4-36. When bolts of large length/diameter ratio are used, or when bolts are used through a member having high density plates on the faces, plug bushings may be used to advantage.
4.72. USE OF HIGH DENSITY MATERIAL. Wherever highly concentrated loads are introduced, greater bearing strength can be obtained by scarfing-in high-density material (section 4.63). Some high density materials are quite sensitive to stress concentrations and the possibility of the serious effects of such stress concentrations should be considered when large loads must be carried through the high-density material.
Wherever metal fittings are attached to wood members, it is generally advisable to reinforce the wood against crushing by the use of high-density bearing plates (figure 4-37) and to use a coat of bitumastic or similar material between the wood and metal to guard against corrosion. Cross banding of these plates will help to prevent splitting of the solid wood member.
4.73. MECHANICAL ATTACHMENT OF RIBS. When ribs carry heavy or concentrated loads it is sometimes desirable to insure their attachment by use of mechanical fastenings (see figure 4-39).
4.74. ATTACHMENT OF VARIOUS TYPES OF FITTINGS. Fittings should always have wide base plates to prevent crushing at edges. Wood washers have a tendency to cone under tightening loads. Where possible, it is desirable to use washer plates for bolt groups, as illustrated in figure 4-40, but if washers are used, a special type for wood, AN-970 or equivalent, are necessary to provide sufficient bearing area.
Clamps around wood members should be constructed so that they can be tightened symmetrically (figure 4-41).
4.75. USE OF WOOD SCREWS, RIVETS, NAILS, AND SELF-LOCKING NUTS. Wood screws and rivets are sometimes used for the attachment of secondary structure but should not be used in connecting primary members. Wood screws have been successfully used to prevent cleavage of plywood skin from stringers in some skin-stringer applications. Nails should never be used in aircraft to carry structural loads.
Self-locking nuts of approved types designed for use with wood and plywood structures are preferable to plate or anchor nuts. When the latter type is used, however, attachment may be made to the structure with wood screws or rivets provided that care is taken not to reduce the strength of load-carrying members. Rivetting through wood is always questionable because of the danger of crushing the rivet heads and the possibility of bending the shank while bucking the rivet. Also, there is no way of tightening the joint when dimensional changes from shrinkage occur.
4.8 Miscellaneous design details4.80. METAL TO WOOD CONNECTIONS. Metal to wood connections are complicated by an inherent weakness of all untreated wood low shear and bearing strength. Sections 4.6 and 4.7 present various methods of minimizing this drawback.
Another way of improving the efficiency of wood structures is to keep the number of joints to a minimum. For example, when other design considerations will permit, a one-piece wood wing is desirable; when this is not permissable, the wing joint should be placed as far outboard as possible so that the fitting loads will be low.
4.81. STRESS CONCENTRATIONS. Since wood in tension has practically no elongation between the proportional limit and the ultimate strength, there is little of the "internal adjustment" common to metal structures. Stress concentrations, therefore, become more critical and, for efficient design, must be held to a minimum. The fact that compreg and similar materials are very sensitive to stress concentrations should be carefully considered when these materials are used.
4.82. BEHAVIOR OF DISSIMILAR MATERIALS WORKING TOGETHER. When materials of differing rigidities, such as normal wood, cornpreg, or metal fittings, are fastened together for a considerable distance, and are under high stress, consideration should be given to the fact that the fastening causes the total deformation of all materials to be the same. A typical example is a long metal strap bolted to a wood spar flange for the purpose of taking the load out of the wood at a wing joint. In order that the load be uniformly distributed among the bolts, the ratio of the stress to the modulus of elasticity should be the same for both materials at every point. This may be approximated in practical structures by tapering the straps and the wood in such a manner that the average stress in each (over the length of the fastening) divided by its modulus of elasticity gives the same ratio.
When splicing high-density materials to wood, or in dropping off bearing plates, the slope of the scarf should be less steep than the slope allowed for normal wood.
4.83. EFFECTS OF SHRINKAGE. When the moisture content of a piece of wood is lowered its dimensions decrease. The dimensional change is greatest in a tangential direction (across the fibers and parallel to the growth rings), somewhat less in a radial direction (across the fibers and perpendicular to the growth rings), and is negligible in a longitudinal direction (parallel to the fibers). For this reason a flat-grained board will have a greater change in width for a given moisture content change than an edge-grained board. Flat-grained boards also have a greater tendency to warp than do edge-grained boards.
These dimensional changes can have several deleterious effects upon a wood structure and the designer must study each case to determine which effects are most harmful, and which are the most satisfactory methods of minimizing them. Loosening of fittings and wire bracing are common results of shrinkage. Checking or splitting of wood members frequently occurs when shrinkage takes place in members that are restrained against dimensional change. Restraint is sometimes given by metal fittings and quite often by plywood reinforcements since plywood shrinkage is roughly only 1/20 of cross grain shrinkage of solid wood.
A few of the methods of minimizing these shrinkage effects are:
1. Use bushings that are slightly short so that when the wood member shrinks the bushings do not protrude and the fittings may be tightened firmly against the member (fig. 4-36).
2. Place the wood so that the more important face, in regard to maintaining dimension, is edge-grained. For example, solid spars are required to be edge-grained on their vertical face so that the change in depth is a minimum.
3. Wood members can be reinforced against checking or splitting by means of plywood inserts or cross bolts (fig. 4-42). Care should be taken to avoid constructions that introduce cleavage (crossgrain) loads when shrinkage occurs.
4. Plywood face plates should be dropped off gradually either by feathering or by shaping so that the cleavage loads at the edge of the plywood are minimized when shrinkage occurs (fig. 4-43).
4.84. DRAINAGE AND VENTILATION. Wood structures must be adequately drained to insure a normal length of service life. This applies to box spar sections as well as all low portions of wings and fuselages. The usual method is to drain each compartment separately as illustrated in figure 4-44.
Another acceptable method is to drain from one compartment to another until the lowest compartment is reached, or structural requirements prohibit further internal drainage, before drainage holes to the exterior are bored. This method is illustrated in figure 4-45.
Service experience indicates that drainage holes for individual compartments should be not less than one-quarter inch in diameter, with three-eigths inch being preferable. Drainage holes to the exterior used with the internal drainage system should probably be somewhat larger. If the internal drainage system is used it is suggested that after the inter-compartment drainage holes be inspected after the internal finish has been applied to make sure that the finish has not clogged the internal drain holes. This will necessitate attaching the top skin last.
Drain holes are usually drilled from the external surface so that the splintering does not mar the external finish. After drilling drain holes, all splinters should be carefully removed froin the inner surface, and the edges of the holes should be sanded lightly and protected by the application of several coats of spar varnish. It is common practice, In order to avoid damage to structural members by the drill, to drill drainage holes an appreciable distance from the low corner of a compartment. This practice must be avoided and some method of ensuring proper location of drain holes at the actual low points must be developed by the aircraft manufacturer that will not only prevent damage to the framework but will also provide complete drainage of the structure.
It is, therefore, recommended that proof of the adequacy of the drainage system chosen be demonstrated by setting up the structure, with the top cover removed , in a position corresponding to its atrtitude when the airplane is resting on the ground. Water is then poured into the structure and the actual performance of the drainage system observed.
Careful design to prevent entry of water into the structure is equally important.L. Careful location of all openings and use of boots and gaskets should be considered. If interiors do happen to get wet, good ventilation will accelerate the drying. Marine grommets have been suggested for use with external drain holes in wing, tail and control surfaces. This type of grommet produces a suction or scavenging action in flight and also protects the holes themselves from direct splash during taxiing on wet or muddy fields. Periodic inspection and cleaning of drainage holes covered with marine grommets, however, may be difficult
4.85. INTERNAL FINISHING. It is recognized that applying finish to the inner surfaces of the closing panels of plywood-covered structures is a difficult problem. The usual method, other than dipping, is to mask off the locations of secondary glue areas prior to the application of finish to the surface, for wood coated with a protective finish cannot be glued. This is a time-consuming operation, and after the plywood covering is finally fitted into place, the film of finish usually stops short of the intersection lines between the plywood covering and framework. These are the very places where the finish is needed most if water does accumulate in the interior.
Wood-rotting organisms can act only if the moisture content of the wood is above approximately 20 to 25 percent. Although finishes will not prevent moisture content changes in wood, they will retard such changes so that the wood moisture content will not follow the rapid changes in atmospheric conditions but only the more gradual changes. Therefore, if wood members are finished, dangerously high moisture contents will be reached in wood aircraft structures only when parts are in contact, with standing water since, atmospheric conditions that produce high moisture contents are generally of relatively short duration, except in extreme climates such. as the tropics, and the retarding effect of the finish may be expected to prevent the wood from attaining a high moisture content within this short period.
In view of the foregoing discussion, it is suggested that consideration be given to the following method of finishing the inner surfaces of plywood-covered assemblies. Since any free water would be in contact with the lower skin almost entirely, the lower wing covering and control surface coverings should be attached to the framework prior to the upper covering. In this way, finish can be applied thoroughly to the lower covering and adjacent framework quite easily after the assembly gluing operation has been completed. Since gaps in the finish on the upper covering along framework members are not so harmful as they would be on the inner surfaces of the lower covering, wider masking strips may be used over secondary glue areas on the upper covering at the time of applying the internal finish, thereby reducing the chance of finished surfaces falling over framework members. Some method of accurately registering the covering should be used.
4.86. EXTERNAL FINISHING. Two types of external finish for plywood covered aircraft have been used successfuily, the direct-to-plywood finish and the fabric-covered plywood finish. There is little difference in weight between the two systems because the weight of the fabric is offset by thee differences in weight between the finishes used in the two systems.
Direct-to-plywood finishes have a tendency to check wherever a glue joint appears on the surface. Checking of the finish is also apt to occur when the grain of the wood tends to raise, as in those softwoods having appreciable contrast berween spring and summer wood, such as Douglas fir. Fabric-covered finishes do not check from these causes.
Light airplane fabric of the type specified in ANC-83 is the usual material used for the fabric-covered plywood finish system. The fabric provides a better protection from the abrasive action of stones, sand, and other objects kicked up while taxiing than does the direct-to-plywood finish.
Observation of wood airplanes in service has revealed that plywood or fiber plates glued over exposed end grain may act as a moisture trap rather than as a moisture barrier. Several coats of brushed-in aluminized spar varnish are believed to give a much more satisfactory protection to exposed end grain. Exposed end grain should be interpreted to include exposed feathered surfaces.
4.87. SELECTION OF SPECIES. Properties other than the usually listed strength and elastic properties should also be considered when selecting a wood for any specific purpose. For example, birch and maple are relatively difficult to glue; yellow poplar has lower resistance to shock than spruce; Douglas fir is low in cleavage strength.
4.88. USE OF STANDARD PLYWOOD. From a maintenance viewpoint it is desirable to use only standard plywoods for design so that too great a variety of types will not need to be carried in stock. Table 2-9 lists many of the more common constructions. lf one of these is used, the formulas in chapter 2 can be used with greater ease because many of the basic parameters and strength values are given in this table. Two-ply diagonal plywood is considered a special construction by most plywood manufacturers and has the disadvantage of tending to warp because of its unsymmetrical construction.
4.89. TESTS. Quite often, time and effort may be saved by the use of simple tests in the early stages of the design of complex joints.