Ply skins FIcURE 5-104. Cross sectional view of a stressed-skin structure.
FIGURE 5-105. Single ply structure.
wood and spruce structures is indicative of deterioration, and careful internal inspection will have to be made for security of these parts to the main wing structure, and for general deterioration of the light plywood and spruce members. If deterioration is found in these components, the main wing structure may also be affected.
Splits in the fabric covering on plywood surfaces should not be repaired by doping on another piece of fabric over the affected area. In all cases, the defective fabric should be removed to ascertain whether the plywood skin beneath is serviceable, since it is common for a split in the plywood skin to be responsible for initiating a similar defect in the protective fabric covering.
Although a preliminary inspection of the external structure can be useful in assessing the general condition of the aircraft, it should be noted that wood and glue deterioration can often take place inside a structure without any external indications.
Where moisture can enter a structure, it will seek the lowest point where it will stagnate and promote rapid deterioration. It should also be noted that glue deterioration can take place through other causes without the presence of water.
Glue failure and wood deterioration are often closely allied, and the inspection of glued joints must include an examination of the adjacent wood structure.
The inspection of a complete aircraft for glue or wood deterioration will necessitate checks on parts of the structure which may be known or suspected trouble spots and which are, in many instances, boxed in or otherwise inaccessible. In such instances, considerable dismantling is required, and it may be necessary to cut access holes in ply structures to facilitate the inspection. Such work must be done only in accordance with approved drawings or the repair manual for the aircraft concerned.
Glued Joint Inspection The inspection of glued joints in aircraft structures presents considerable difficulties. Even where access to the joint exists, it is still difficult to positively assess the integrity of the joint, Keep this in mind when inspecting wooden structures.
Some of the more common factors which may cause glue deterioration are: (1) Chemical reactions of the glue caused by aging or moisture, extreme temperatures, or a combination of these factors, (2) mechanical forces caused mainly by wood shrinkage, and (3) development of fungus growths.
Aircraft exposed to large cyclic changes of temperature and humidity are especially prone to wood shrinkage which may lead to glue deterioration.
The amount of movement of wooden members due to these changes varies with the size of each member, the rate of growth of the tree from which the wood was cut and the way in which the wood was converted. Thus, two major members in an aircraft 225
structure, secured to each other by glue, are unlikely to have identical characteristics. Differential loads will, therefore, be transmitted across the glue film since the two members will not react in an identical manner relative to each other. This will impose stresses in the glued joint which can normally be accommodated when the aircraft is new and for some years afterwards. However, glue tends to deteriorate with age, and stresses at the glued joints may cause failure of the joints. This is true even when the aircraft is maintained under ideal conditions.
When checking a glue line (the edge of the glued joint) for condition, all protective coatings of paint should be removed by careful scraping. It is important to ensure that the wood is not damaged during the scraping operation. Scraping should cease immcdiately when the wood is revealed in its natural state and the glue line is clearly discernible.
The glue line is often inspected by the use of a magnifying glass. Where the glue line tends to part or where the presence of glue cannot be detected or is suspect, the glue line should be probed with a thin feeler gage. If any penetration is possible, the joint should be regarded as defective. It is impor tant to ensure that the surrounding wood is dry;
otherwise, a false impression of the glue line would be obtained due to closing of the joint by swelling.
In instances where pressure is exerted on a joint, either by the surrounding structure or by metal Metal fitting Bolt attachment devices such as bolts or screws, a false impression of the glue condition could be obtained unless the joint is relieved of this pressure before the glue line inspection is carried out.
The choice of feeler gage thickness will vary with the type of structure, but a rough guide is that the thinnest possible gage should be used. Figure 5-106 indicates the points where checks with a feeler gage should be made.
Wood Condition Dry rot and wood decay are not usually difficult to detect. Dry rot is indicated by small patches of crumbling wood. A dark discoloration of the wood surface or gray stains running along the grain are indicative of water penetration. If such discoloration cannot be removed by light scraping, the part should be replaced. Local staining of the wood by the dye from a synthetic adhesive hardener can be disregarded.
In some instances where water penetration is sus.
pected, the removal of a few screws from the area in question will reveal, by their degree of corrosion, the condition of the surrounding joint (figure 5-107).
The adhesive will cause slight corrosion of the screw following the original construction; there fore, the condition of the screw should be compared with that of a similar screw removed from another Ply spar web Laminated spar
• Shrinkage Inspection hole in web At all points marked"A", check Shrinkage for glue condition and separation.
FIGURE 5-106. Laminated joint.
Fuselage inner and outer ply skins Position to check Reinforced laminated for separation fuselage member Bulkhead frame Screw hole member Woodscrew Bulkhead ply web Expansion gap (not to be confused with joint separation) Corrosion indicating failure of bulkhead glued joint to fuselage side FicuRE 5-107. Checking a wooden structure for water penetration.
part of the structure known to be free from water soakage.
Plain brass screws are normally used for reirforcing glued wooden members, although zinccoated brass is sometimes used. For hardwoods such as mahogany or ash, steel screws are sometimes used. Unless otherwise specified by the aircraft manufacturer, it is usual to replace screws with new screws of identical length but one gage larger.
Another means of detecting water penetration is to remove the bolts holding the fittings at spar root-end joints, aileron hinge brackets, etc. (figure 5-107). Corrosion on the surface of such bolts and wood discoloration will provide a useful indication of water penetration.
Experience with a particular aircraft will indicate those portions of the structure most prone to water penetration and moisture entrapment, e.g., at windows or the bottom lower structure of entrance doors. However, this is not necessarily indicative of the condition of the complete aircraft.
The condition of the fabric covering on ply surfaces is of great importance. If any doubt exists regarding its protective qualities or if there are any signs of poor adhesion, cracks, or other damage, it should be removed to reveal the ply akin.
The condition of the exposed ply surface should be examined. Water penetration will be shown by dark gray streaks along the grain and a dark discoloration at ply joints or screw holes. If these marks cannot be removed by light scraping or, in the case 227
of advanced deterioration, where there are small surface cracks or separation of the ply laminations, the ply should be replaced. Where evidence of water penetration is found, sufficient areas of the ply surface should be uncovered to determine its extent.
During the inspection, the structure should be examined for other defects of a mechanical nature.
Information regarding such defects is presented in the following paragraphs.
Where bolts secure fittings which take load-carry.
ing members, or where the bolts are subject to landing or shear loads, the bolt holes should be examined for elongation or surface crushing of the wood fibers. The bolts should be removed to facilitate the inspection. It is important to ensure that the bolts are a good fit in the holes.
Check for evidence of damage such as bruises or crushing of structural members which can be caused, for example, by overtightening bolts. Repair techniques for such damage are governed by the extent and depth of the defect.
Compression failures, often wrongly referred to as compression "shakes," are caused by rupture across the wood fibers. This is a serious defect which, at times, is difficult to detect. Special care is necessary when inspecting any wooden member which has been subjected to abnormal bending or compression loads during a hard landing. In the case of a member having been subjected to an excessive bending load, the failure will appear on the surface which has been compressed. The surface subjected to tension will normally show no defects.
In the case of a member taking an excessive direct compression load, the failure will usually be apparent on all surfaces.
If a compression failure is suspected, a flashlight shone along the member, with the beam of light running parallel to the grain, will assist in revealing this type of failure.
A glued joint may fail in service as a result of an accident or because of excessive mechanical loads having been imposed upon it, either in tension or in shear. It is often difficult to decide the nature of the load which caused the failure, but remember that glued joints are generally designed to take shear loads.
If a glued joint is known to have failed in tension, it is difficult to assess the quality of the joint, since these joints may often show an apparent lack of adhesion. Tension failures often appear to strip the glue from one surface leaving the bare wood. In such cases, the glue should be examined with a magnifying glass, which should reveal a fine layer of wood fibers on the glued surface. The presence of fibers indicates that the joint itself is not at fault.
If examination of the glue under magnification does not reveal any wood fibers but shows an imprint of the wood grain, this is caused by pre-drying of the glue before applying pressure during the manufacture of the joints. If the glue exhibits an irregular appearance with star-shaped patterns, this is an indication that pre-curing has occurred before pressure was applied or that pressure has been incorrectly applied or maintained. In all such instances other joints in the aircraft should be suspect.
If a joint is expected to take tension loads, it will be secured by a number of bolts or screws (or both) in the area of tension loading. If a failure occurs in this area, it is usually very difficult to form an opinion of the actual reasons for it, because of considerable breakup of the wood close to the bolts.
In all cases of glued joint failure, whatever the direction of loading, there should be a fine layer of wood fibers adhering to the glue, whether or not the glue has come away completely from one section of the wood member. If there is no evidence of fiber adhesion, this may indicate glue deterioration.
SERVICE AND REPAIR OF WOODEN STRUCTURES Damage to wooden structures such as wing ribs, spars, and skin frequently requires repair. Whenever major wood parts have been damaged, a detailed inspection must be made. Secondary cracks sometimes start some distance away from the main damage and proceed in unrelated directions.
The purpose of repairing all wooden structural parts is to obtain a structure as strong as the original. Severe damage will require replacement of the entire damaged assembly, but minor damage can be repaired by cutting away the damaged members and replacing them with new sections. This replacement is accomplished by glued, or sometimes glued and nailed, or glued and screw-reinforced splicing.
Materials Several forms of wood are commonly used in aircraft. Solid wood or the adjective "solid" used with such nouns as beam or spar refers to a member consisting of one piece of wood.
Laminated wood is an assembly of two or more layers of wood which have been glued together with the grain of all layers or laminations approximately parallel. Plywood is an assembled product of wood and glue that is usually made of an odd number of 228
thin plies (veneers) with the grain of each layer at an angle of 90° with the adjacent ply or plies.
High-density material includes compreg, impreg, or similar commercial products, heat stabilized wood, or any of the hardwood plywoods commonly used as bearing or reinforcement plates. The woods listed in figure 5-108 are those used for structural purposes. For interior trim, any of the decorative woods such as maple or walnut can be used since strength is of little consideration in this situation.
Strength Maximum properties as permissible Species of wood compared to grain devi- Remarks spruce ation (slope of grain) Spruce 100% 1:15 Excellent for all uses. Considered as standard for this table.
Douglas Fir Exceeds spruce 1:15 May be used as substitute for spruce in same sizes or in slightly reduced sizes providing reductions are substantiated. Diticult to work with hand tools. Some tendency to split and splinter during fabrication. Large solid pieces should be avoided due to inspection difficulties.
Gluing satisfactory:
Noble Fir Slightly exceeds spruce 1:15 Satisfactory characteristics with respect to except & percent de- workability, warping, and splitting. May De ficient in shear. used as direct substitute for spruce in same sizes providing shear does not become critical.
Hardness somewhat less than spruce. Gluing satisfactory.
Western Hemlock Slightly exceeds spruce 1:15 Less unitorn in texture than spruce. May t used as direct substitute for spruce. Gluing satisfactory.
Pine, Northern White Properties between 85 1:15 Excellent working qualities and uniform in percent and 96 per- properties but somewhat low in hardness and cent those of spruce. shock-resisting capacity. Cannot be used as substitute for spruce without increase in sizes to compensate for lesser strength. Gluing satisfactory.
White Cedar, Port Orford Exceeds spruce 1:15 May be used as substitute for spruce in same sizes or in slightly reduced sizes providing reductions are substantiated. Easy to work with hand tools. Gluing difficult but satisfactory joints can be obtained if suitable precautions are taken.
Poplar, Yellow Slightly less than spruce 1:15 Excellent working qualities. Should not be except in compression used as a direct substitute for spruce without (crushing) and shear. carefully accounting for slightly reduced strength properties. Somewhat low in shock-resisting capacity. Gluing satisfactory.
FIGURE 5-108. Woods for aircraft use.
All wood and plywood used in the repair of aircraft structures must be of aircraft quality. The species used to repair a part should be the same as that of the original whenever possible. If it is necessary to substitute a different species, follow the recommendations of the aircraft manufacturer in selecting a different species.
Defects Permitted.
a. Cross grain. Spiral grain, diagonal grain, or a combination of the two is acceptable providing the grain does not diverge from the longitudinal axis of the material more than specified in column 3 of figure 108. A check of all four faces of the board is necessary to determine the amount of 229
divergence. The direction of free-flowing ink will frequently assist in determining grain direction.
b. Wavy, curly, and interlocked grain. Acceptable, if local irregularities do not exceed limitations specified for spiral and diagonal grain.
c. Hard knots. Sound hard knots up to g-inch in maximum diameter are acceptable providing:
(1) they are not in projecting portions of I-beams, along the edges of rectangular or beveled unrouted beams, or along the edges of flanges of box beams (except in lowly stressed portions); (2) they do not cause grain divergence at the edges of the board or in the flanges of a beam more than specified in column 3; and (3) they are in the center third of the beam and are not closer than 20 inches to another knot or other defect (pertains to ⅜-inch knots-smaller knots may be proportionately closer). Knots greater than ¼-inch must be used with caution.
d. Pin knot clusters. Small clusters are acceptable providing they produce only a small effect on grain direction.
e. Pitch pockets. Acceptable, in center portion of a beam providing they are at least 14 inches apart when they lie in the same growth ring and do not exceed 1½ inch length by ⅛-inch width by g-inch depth and providing they are not along the projecting portions of l-beams, along the edges of rectangular or beveled unrouted beams, or along the edges of the flanges of box beams.
f. Mineral streaks. Acceptable, providing careful inspection fails to reveal any decay.
Defects Not Permitted.
g. Cross grain. Not acceptable, unless within limitations noted in a. above.
h. Wavy, curly, and interlocked grain. Not acceptable, unless within limitations noted in b.
i. Hard knots. Not acceptable, unless within limitations noted in c. above.
i. Pin knot clusters. Not acceptable, if they produce large effect on grain direction.
k. Spike knots. These are knots running com:
pletely through the depth of a beam perpendicular to the annual rings and appear most frequently in quartersawed lumber. Reject wood containing this defect.
I. Pitch pockets. Not acceptable, unless within limitations noted in e. above.
m. Mineral streaks. Not acceptable, if accompanied by decay (see f.).
n. Checks, shakes; and splits. Checks are longitudinal cracks extending, in general, across the annual rings. Shakes are longitudinal cracks usually between two annual rings. Splits are longitudinal cracks induced by artificially induced stress. Reject wood containing these defects.
o. Compression wood. This defect is very detrimental to strength and is difficult to recognize readily. It is characterized by high specific gravity; has the appearance of an excessive growth of summer wood; and in most species shows but little contrast in color between spring wood and summer In doubtful cases reject the material, or subject samples to a toughness machine test to establish the quality of the wood. Reject all material containing compression wood.
p. Compression failures. This defect is caused from the wood being overstressed in compression due to natural forces during the growth of the tree, felling trees on rough or irregular ground, or rough handling of logs or lumber. Compression failures are characterized by a buckling of the fibers that appear as streaks on the surface of the piece substantially at right angles to the grain, and vary from pronounced failures to very fine hairlines that require close inspection to detect. Reject wood containing obvious failures. In doubtful cases reject the wood, or make a further inspection in the form of microscopic examination or toughness test, the latter means being the more reliable.
q. Decay. Examine all stains and discolorations carefully to determine whether or not they are harmless, or in a stage of preliminary or advanced decay. All pieces must be free from all forms of Glues used in aircraft repair fall into two general groups: (1) Casein and (2) resin glues. Any glue that meets the performance requirements of applicable U.S. Military Specifications or has previously been accepted by the FAA is satisfactory for use in certificated civil aircraft. In all cases, glues are to be used strictly in accordance with the glue manufacturer's recommendations.
Casein glues have been widely used in wood aircraft repair work. The forms, characteristics, and properties of water-resistant casein glues have remained substantially the same for many yeara, 230
except for the addition of preservatives. Casein glues for use in aircraft should contain suitable preservatives, such as the chlorinated phenols and their sodium salts, to increase their resistance to organic deterioration under high-humidity exposures. Most casein glues are sold in powder form ready to be mixed with water at ordinary room temperatures.
Synthetic resin glues for wood are outstanding in that they retain their strength and durability under moist conditions and after exposure to water. The best known and most commonly used synthetic resin glues are the phenol-formaldehyde, resorcinol-formaldehyde, and urea-formaldehyde types. The resorcinol-formaldehyde type glue is recommended for wood aircraft applications. Materials, such as walnut shell flour, are often added by the glue manufacturer to the resin glues to give better working characteristics and joint-forming properties. The suitable curing temperatures for both urea-formaldehyde and resorcinol glues are from 70° F. up. At the 70° F. minimum temperature, it may take as long as 1 week for the glue line in a spar splice to cure to full strength. Thinner pieces of wood and/ or higher curing temperatures shorten curing time considerably. The strength of a joint cannot be depended upon if assembled and cured at temperatures below 70° F.
For those not familiar with the terms used relating to synthetic resin adhesives and their application, a glossary follows.
(1) Cold setting adhesive. An adhesive which sets and hardens satisfactorily at ordinary room temperature, i.e., 50° F.
to 86° F. (10° C. to 32° C.) within a reasonable period.
(2) Close contact adhesive. A nongap-filling adhesive suitable for use only in those joints where the surfaces to be joined can be brought into close contact by means of adequate pressure, and where glue lines exceeding 0.005 in. can be avoided with certainty.
(3) Closed assembly time. The elapsing between the assembly of the joints and the application of pressure.
(4) Double spread. The spread of adhesive equally divided between the two surfaces to be joined.
(5) Gap-filling adhesive. An adhesive suitable for use in those joints where the surfaces to be joined may or may not be in close or continuous contact, owing either to the impossibility of applying adequate pressure or to slight inaccuracies of machining. Unless otherwise stated by the manufacturer, gap-filling adhesives are not suitable for use where the glue line exceeds 0.050 in. in thickness.
(6) Glue line. The resultant layer of adhesive joining any two adjacent wood lay.
ers in the assembly.
(7) Hardener. A material used to promote the setting of the glue. It may be supplied separataly in either liquid or powder form, or it may have been incorporated with the resin by the manufacturer.
It is an essential part of the adhesive, the properties of which depend upon using the resin and hardener as directed.
(8) Open assembly time. The period of time between the application of the adhesive and the assembly of the joint components.
(9) Single sproad. The spread of adhesive to one surface only.
(10) Spread of adhesive. The amount of adhesive applied to join two surfaces.
(11) Synthetic resin. A synthetic resin (phenolic) is derived from the reaction of a phenol with an aldehyde. A synthetic resin (amino plastic) is derived from the reaction of urea, thiourea, melamine, or allied compounds with formaldehyde.
(12) Synthetic resin adhesive. A composition substantially consisting of a synthetic resin, either the phenolic or amino type, but including any hardening agent or modifier which may have been added by the manufacturer or which must be added before use, according to manufacturer's instructions.
Synthetic resin adhesives are used extensively for joining wooden structures to avoid the localized stresses and strains which may be set up by the use of mechanical methods of attachment, The strength of such structures depends largely on the efficiency of the glued joints, and cannot be verified by means other than the destruction of the joints. Acceptance must be governed by adequate precautions throughout the gluing process and by the results obtained by representative test pieces.
Synthetic resin adhesives usually consist of two 231
separate parts, the resin and the hardener. The resin develops its adhesive properties only as a result of a chemical reaction between it and the hardener. With some adhesives, an inert filler is added to increase viscosity and to improve the gap-filling properties of the mixed adhesive.
Synthetic resins can be obtained either in liquid or powder form. In general, powder resins have the longest storage life, since they are less susceptible to deterioration from high ambient temperatures.
Powder resins must be mixed with water in accordance with the manufacturer's instructions before they can be used in conjunction with a hardener. To obtain satisfactory results, it is essential that they be properly mixed. Once mixed, the adhesive must not be diluted unless this is permitted by the manufacturer's instructions. In many instances, manufacturers specify a definite period of time which must elapse between the mixing and the application of the adhesive. During this period, the adhesive should be covered to prevent contamination. When resins are supplied in liquid form, they are ready for immediate use in conjunction with the hardener. Liquid resin must not be diluted unless this is permitted by the manufacturer's instructions.
When mixing the hardener with the resin, the proportions must be in accordance with the manufacturer's instructions. Hardeners should not be permitted to come into contact with the resin except when the adhesive is mixed prior to use.
GLUING The surface to be joined must be clean, dry, and free from grease, oil, wax, paint, etc. It is important that the parts to be joined have approximately the same moisture content, since variations will cause stresses to be set up because of swelling or shrinkage which may lead to the failure of the joint.
The moisture content of wood can be determined by taking a sample of the wood to be glued, weighing it, and then drying it in an oven at a temperature of 100° C. to 105° C. Calculate the moisture content by using the formula:
• × 100 where, W: = the weight of the sample prior to drying.
W›= the weight of the sample after drying.
Example Substitution and solution of above formula:
2 - 15 x 100 = 33 or 33%.
The approximate moisture content can also be determined by using a moisture meter. When this instrument is used, its accuracy should be checked periodically.
The wood to be glued should be at room temperature. The surfaces to be joined should not be overheated since this affects the surface of the wood and reduces the efficiency of most synthetic resin adhe- Synthetic resin adhesives are very sensitive to variations in temperature. The usable (pot) life of the adhesive, proportion of hardener to use, and clamping time all depend largely on the temperature of the glue room at the time of gluing.
It is generally desirable to apply adhesive to both surfaces of the material. This applies particularly where the glue line is likely to be variable or when The adhesive can be applied by a brush, glue spreaders, or rubber rollers that are slightly grooved. The amount of adhesive required depends largely on the type of wood and the accuracy of machining. Dense wood requires less adhesive than soft or porous types. Adhesive should be applied generously to an end grain. Smooth, side-grained surfaces may be satisfactorily glued with a thin spread. The general rule is that the adhesive should completely cover the surfaces to be glued and remain tacky until pressure is applied to the joint.
Difficult gluing conditions may occur when a soft wood is to be glued to one much denser because the adhesive tends to flow into the more porous wood.
In such instances, unless otherwise specified by the manufacturer of the adhesive, precoating and partial drying of the softer surface, before normal spreading, is suggested.
Care should be taken before applying the adhesive to ensure that the surfaces will make good contact and that the joint will be positioned correctly. The interval between the application of the adhesive and assembly of the joint should be kept as short as possible. Some adhesives contain solvents which should be allowed to evaporate before the joint is assembled. If this is not done, bubbles.
may be created and result in a weak joint. For adhesives of this type, the manufacturer will specify a time interval which should elapse before the joint is closed.
To ensure that the two surfaces bind properly, pressure must be applied to the joint. The strength of the joint will depend to a great extent upon how evenly the force is applied. The results of evenly 232
and unevenly applied pressure are illustrated in figure 5-109.
Pressure is used to squeeze the glue out into a thin continuous film between the wood layers, to force air from the joint, to bring the wood surfaces into intimate contact with the glue, and to hold them in this position during the setting of the glue.
Pressure should be applied to the joint before the glue becomes too thick to fow and is accomplished by means of clamps, presses, or other mechanical devices.
Non-uniform gluing pressure commonly results in weak and strong areas in the same joint. The amount of pressure required to produce strong joints in aircraft assembly operations may vary from 125 to 150 p.s.i. for softwoods and 150 to 200
p.s.i. for hardwoods. Insuficient pressure to poorly machined wood surfaces usually results in thick glue lines, which indicates a weak joint, and should be carefully guarded against.
The methods used in applying pressure to joints in aircraft gluing operations range from the use of brads, nails, screws, or clamps to the use of hydraulic and electrical power presses. Hand-nailing is used rather extensively in the gluing of ribs and in the application of plywood skins to the wing, control surfaces, and fuselage frames.
On small joints, such as those found in wood ribs, the pressure is usually applied only by nailing the joint gussets in place after spreading the glue.
Since small nails must be used to avoid splitting, the gussets should be comparatively large in area to compensate for the relative lack of pressure. At least four nails (cement-coated or galvanized and barbed) per sq, in, are to be used, and in no event Arrows indicate pressure gap Pressure block FIGURE 5-109. Results of uneven and even pressure.
must nails be more than ¾ in, apart. Small brass screws may also be used advantageoulsy when theparticular parts to be glued are relatively small and do not allow application of pressure by means of clamps.
Apply pressure using cabinet maker clamps, parallel clamps, or similar types. Use handspring clamps in conjunction with softwood only. Because of their limited pressure area, they should be applied with a pressure-distributing strip or block at least twice as thick as the member to be pressed.
High clamping pressures are neither essential nor desirable, provided good contact between the surfaces being joined is obtained. When pressure is applied, a small quantity of glue should be squeezed from the joint. This should be wiped off before it dries. The pressure must be maintained during the full setting time. This is important since the adhesive will not reunite if disturbed before it is fully set.
The setting time depends on the temperature at which the operation is carried out. An increase in temperature results in a decrease in the setting period. Conversely, a decrease in temperature causes an increase in the setting time.
Full joint strength and resistance to moisture will develop only after conditioning for at least 2 days.
Again, this depends on the ambient temperature and the type of hardener used. Usually when repairs are made, the joint will be of sufficient strength after 1 day Testing Glued Joints Frequent tests should be made to ensure that the joints are satisfactory. Whenever possible, tests should be carried out on pieces cut from the actual component. The test samples should be 1 in. wide and at least 2 in. long. The pieces should be joined with one member overlapping the other by ⅜ in.
The glued test sample should be placed in a vice and the joint broken by exerting pressure on the overlapping member. The fractured glue faces should show at least 75% of the wood fibers evenly distributed over the fractured glue surfaces. A typical broken test piece is shown in figure 5-110.
Where repairs are to be made on old aircraft in which the wooden structure is joined with a casein cement, all traces of the casein cement must be removed from the joint, since this material is alkaline and is liable to affect the setting of a synthetic resin adhesive. Local staining of the wood by the casein cement can, however, be disregarded.
FIGURE 5-110. Typical broken test piece.
SPLICED JOINTS The scarf joint is generally used in splicing structural members in aircraft. As illustrated in figure 5-111, the two pieces to be joined are beveled and glued. The slope of the bevel should be not less than 10 to 1 in solid wood and 12 to 1 in plywood. Make the scarf cut in the general direction of the grain slope as shown in figure 5-111.
The chief difficulty encountered in making this type of joint is that of obtaining the same bevel on each piece. The strength of the joint will depend upon the accuracy of the two beveled surfaces, because an inaccurate bevel will reduce the amount of effective glue area. (See figure 5-111.)
New section to be spliced in
- Slope 10 to I in solid wood 297013 A - Correctly beveled pieces Cap B - Incorrecthy beveled pieces FIGURE 5-111. Beveling scarf joint.
One method of producing an accurate scarf joint is illustrated in figure 5-112. After the two bevels are cut, the pieces are clamped to a 2X4 backboard or some similar material. A fine-tooth saw is then run all the way through the joint. One of the pieces is then tapped on the end until it will move no farther, and the saw is again passed through the joint. This procedure is continued until the joint is perfect. A light cut of a plane is then used to smooth the surfaces of the joint.
Undamaged section Guide board FIcURE 5-112. Making a scarf joint.
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.