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Builders guide to safe aircraft materials

Wood joints and adhesives


Rev. 7 — page content was last changed 6 January 2010
  
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5.1 Adhesive requirements
Aircraft wood structural joints are not the complex mechanically interlocking joints used in joinery and cabinet making, nor are they as difficult in preparation and joining as the 'fish-mouthed' tubes in welded steel truss fabrication. They are very simple in construction, relying only on the cohesive strength of the bond between the wood and the adhesive to transmit flight loads between structural members. In forming joints, the adhesive does not bind one wood surface to the other; rather' each surface is individually bound to the glue. In wooden structures the strength of the glue line must be greater than that of the wood, whereas in adhesive bonded metal structures the glue line will most likely be weaker than the metal.

Adhesive bonding operations are much simpler than incorporating metal fasteners, and provide a lighter structure and a more uniform stress distribution.

Airframes require proven adhesives that provide waterproof glue lines without any degradation in extreme environments. The glue lines must also be highly resistant to cracking with age, attack from fungi, other micro-organisms, fuel, oil and other chemicals. The adhesives must be tough enough to resist vibratory and cyclical stressing in an exposed environment over perhaps 40+ years.

The adhesive must also have these workshop capabilities:
  • be easy to work with; i.e. readily applied to properly prepared surfaces and allowing ample time (around 15 minutes) for the joint to remain 'open' for final positioning of the members

  • bonds and cures in normal home workshop environments: temperatures 15–35° C with relative humidities up to around 70%

  • optimal results achieved when moisture content of the wood is in the range 6% to 18%

  • fills gaps up to 1.5 mm to cater for less accurate joint preparation.
In addition, two-part glues should be easily measured out (for example a 1:1 by volume mix ratio rather than 2:1 by weight), mix readily, have a reasonable pot life (perhaps 30 minutes), a reasonable curing time, not be a risk to health — where the user takes reasonable precautions in a ventilated workshop — and be easy to clean up.
5.2 Suitable adhesives
Two-part epoxy resin adhesives.These are probably the first choice for aircraft structural bonds. Epoxy resin adhesives require the precise and thorough mixing of a resin and a hardening agent immediately before use. There are quite a number of different resin and hardener combinations available offering ample choice for a particular job and/or a particular environment. Some epoxy systems allow a selected adhesive filler to be added to the resin/hardener mix to achieve a particular viscosity or gap filling requirement. Epoxy adhesives contain no carrier liquids so generally require only sufficient clamping pressure to remove voids, squeeze out excess glue and to hold the pieces immobile during curing at normal room temperatures.

Reaction commences once the two parts have been mixed together. The reaction rate roughly doubles or halves for every 10° C change in temperature from 20° C. For example, if initial cure time is 60 minutes at 20° C then it will approximate 30 minutes at 30° C or 120 minutes at 10° C. The cure times at room temperature to achieve maximum cohesion might require several days. Shrinkage during cure is slight (probably under 1%) so that machined accuracy in mating joint surfaces is not necessary but still desirable, so as to produce a maximum quality bond.

Epoxy adhesives will bond very well when the MC of the wood ranges from perhaps 6% to 20% and will exceed the shear strength of all woods so that, in a properly prepared joint, the wood will fail before the glue line. Of course it is absolutely necessary to follow the manufacturer's instructions regarding application and usage. Be particularly careful about protection from exposure to ultraviolet radiation. Heat-soaked joints will lose considerable strength but regain it as the airframe cools to normal temperatures.

System Three T-88 epoxy and West System epoxy, both from USA, seem to be highly regarded by homebuilders. The West System use guides provide information on adding fillers to the resin/hardener mix.

ATL Composites manufacture West System products in Australia under licence.

Resorcinol-formaldehyde resin [RF]. This requires MC in the range of 12% to 15%, and higher clamping pressures for good bonding. Temperatures during cure should be above 22° C. The glue shrinks while curing so gap-filling capabilities are not so good, thus accuracy of fit is vital for a high-strength joint. RF was the adhesive of choice for several decades and produces a very long-life bond. Mix is by weight — perhaps one part of the hardening powder to four parts of the resin liquid. About 50% of the liquid is water so a consistent clamping pressure is required while the water is being evaporated/removed during the cure period.

Phenol-formaldehyde [PF]. This requires MC in the range of 8% to 12%, and controlled heat and pressure to set permanently; thus PF is not appropriate to the home workshop. Gap-filling capabilities are poor and there may be age hardening/cracking problems if not used under controlled conditions. The Type A marine plywood bond is produced from a phenol-formaldehyde resin.
5.3 Unsuitable adhesives
Urea-formaldehyde resin [UF] glues were used extensively in aircraft but it is now considered that they may deteriorate badly over time in hot, humid climates such as found in northern Australia. Tests conducted by the Forest Products Laboratory (USA) indicate that polyurethane glue joints subject to shear tend to fail at the glue line rather than within the wood. Other glues that must not be used in aircraft structural joints are melamine fortified urea-formaldehyde resin [MF], acid catalysed phenolic [ACP], polyvinyl acetate [PVA], casein and animal glues.
5.4 Wood bonding joints
Adhesive joints are designed to provide a continuous bond over as much surface as feasible, whereas bolted or screwed joints apply pressure over a smaller area — and the associated holes tend to weaken the structure. Modern adhesives work very well in tension, compression and shear, so airframe joints should be designed to take advantage of that.

Lap joint layoutFlat grain joints: in such jointing the grain of all members should be close to parallel and the strength of the joint is then dependent on the shear strength of the timber species. Such joints are the strongest and generally associated with lap joints as in the layout image; or adding strengthening/stiffening blocks either side of a structural member where a particular load will be applied; or in fabricating lengthy components by lamination.

End grain to end grain joints: in basic end-grain jointing, the squared ends of two members are butted together, end grain against end grain, to produce a longer length from two shorter pieces. The surface area bonded is minimal and the joint is very weak, so the normal practice is to bond a piece of the same dimension material centred over the butt joint or plywood strengthening plates over opposing faces; much the same as the lap joint above.

Gussetted nodeEnd grain to surface grain joints: the square/angle-cut end grain of one piece is butted against the surface grain of the edge/face of another to create a T-joint or an angled joint — a node connection. End-grain joints are weak so strength is usually provided by bonding rectangular plywood gussets on one side (or both sides) of the node and/or adding solid wood corner blocks within the interior angle(s). It is important that all corners of the plywood gusset are supported on solid wood and the surface grain is appropriately oriented. The same grain orientation requirement also applies to corner blocks.

Scarf joint layoutsSimple scarf joints: used to produce (splice) a longer length from two shorter pieces or to insert a repair. The matching ends are cut at an angle, providing a much greater bonding area where the grain is as close to purely surface grain as possible. When splicing solid wood the length of the scarf should be 12 to 15 times the thickness of the piece; a 1 in 12 slope will provide a joint strength around 90% of the natural wood, and a 1 in 15 slope is the minimum in spars. The slope of the scarf should match the grain slope and as wood in aircraft structures should have a grain slope better than 1:15, then a 1 in 12 scarf slope should have no application. Smooth, accurately mated, grain-slope matched surfaces and good adhesive penetration are necessary (see scarf slope. The bottom image shows the layout for a squinted scarf joint, which avoids the feathered ends of a through slope; the squint is cut at an angle so that the butted areas are not solely end grain; however, a squinted joint is unlikely to be applicable in an airframe primary structure.

Plywood may also be edge-joined using a scarf technique (see 'Joints in plywood'). In this case the width of the scarf should not be less than 12 times the plywood thickness; i.e. 3 mm plywood, scarf width = 36 mm. If you recall the 1-in-60 rule a 1 in 12 slope equals a 5° angle, while 1 in 15 is a 4° angle.

The term scarf is derived from an Old Swedish word meaning 'to join together'. There are many glueless scarf joint designs, particularly as splices in beams of old buildings or replications.

Extract from ANC–18

The following few paragraphs and images are an extract from chapter 4 of 'ANC–18 Design of wood aircraft structures', second edition, issued June 1951 by the Subcommittee on Air Force–Navy–Civil Aircraft Design Criteria of the Munitions Board Aircraft Committee (US).

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.

Figure 4.33 shell structure joint

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.

Figure 4.34 Reinforcements

[Extract from ANC–18 ends.]
5.5 Factors affecting joint strength
The shearing strength of epoxy adhesives is greater than that of the strongest woods if accurately mixed, correctly applied to prepared surfaces and allowed to cure in appropriate environmental conditions. Other factors that ensure maximum strength of a glued joint are:
  • minimum glue line — the joint surfaces of the structural members should make close contact throughout the joint when dry mated, although gaps up to one millimetre or more may be okay, depending on the viscosity of the epoxy mix

  • continuous and even glue distribution on all surfaces throughout the joint with no trapped air. (Careless mixing of the resin and hardener will introduce air bubbles to the adhesive)

  • maximum contact surface particularly when end grain is included in the joint. Some schools of thought say planed surfaces should be slightly roughened by sandpapering 30° to the grain direction, particularly if the surface has been burnished by dulled cutting tools. Others say the surfaces should be kept smooth, which is probably correct if the surface has been freshly cut with a very sharp plane blade. (Dulled cutting edges on any woodworking tool are incompatible with quality work.) One problem with sandpapering is that imprecise action results in rounding/bevelling of edges which, in effect, reduces the contact area

  • joint cleanliness — no dust or other foreign material included

  • evenly distributed application of the necessary clamping pressure over the appropriate time relative to the type of wood and the type of adhesive

  • where end grain is involved additional glue (and time) must be allowed for capillary action (assisted by 'working in') to fill the cut cells to avoid a glue-starved joint. This also applies to scarf and edge joints in plywood where there are alternate layers of face and end grain in the joint. With some epoxy systems the manufacturer may recommend that when end grain is involved the surfaces should be pre-coated with the resin/hardener mix without fillers.


Pressure is applied using various types of clamps, but staples or nails may be used to hold three-ply gussets, and then removed after curing — particularly if likely to be in contact with fabric. It is important that excessive pressure is avoided; otherwise it may lead to joint starvation and/or crushing of wood fibres.

A 'break piece' should be made with each pot batch from scrap pieces of the timber and, when fully cured, tested to destruction to ascertain that the wood fails and not the glue line. Obviously it is necessary to take the same care with the test piece joint as with a structural joint.

Further information on wood bonding is contained in the FAA advisory circular chapter 1-1 AC 43.13-1B sections 1.4 to 1.11.



5.6 Joints in plywood
The following section is an extract as is from chapter 4 of 'ANC–18 Design of wood aircraft structures', second edition, issued June 1951 by the Subcommittee on Air Force–Navy–Civil Aircraft Design Criteria of the Munitions Board Aircraft Committee (US).

4.10 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.

Figure 4.1 Use of splice plate or apron strip

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.

Figure 4.2 Scarf splices

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.

Figure 4.3 Butt splices

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.

Figure 4.4  lap splices

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.)

Figure 4.5  tapering plywood in thickness

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.

Figure 4.6  scarfing different thicknessess

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.

Figure 4.35 bevelling of plywood webs and gussets splices

Extract from ANC–18 ends.



5.7 Glued truss structures
The structure of a simple wooden aircraft must cope with the basic stresses mentioned in the 'Properties of wood' module: compression, tension and shear. These are always present throughout the airframe in normal cruising flight (because of the forces generated by thrust, drag, lift and weight) but greatly increase when manoeuvring or gust loads are applied.

We saw that bending under load applies compression, tension and shear stresses to a structural member. Airframes are also subject to twisting forces and require torsional strength to resist those forces. One such force is the engine/propeller torque, which tries to twist the forward fuselage structure bearing the engine mountings. This torque results in compression, tension and shear stresses, so the forward fuselage of a wooden aircraft must be constructed as a form of torsion box. Another twisting force is that applied to the aft end of the fuselage by rudder action.

Matt Paxton's Pietenpol fuselage Strong, rigid, light-weight truss or framework wooden structures have been in use since the 1920s and are probably the easiest structural type to build. The fuselage depicted is that of a Pietenpol Air Camper, first built about 1930 and still a favourite with home-builders, possibly because it can be constructed from wood or from steel tube. This photograph and the one below are from Matt Paxton.

The Pietenpol four-longeron (longitudinal member) fuselage structure is representative of that employed for most light wooden (or steel tube) aircraft. The fuselage primary structure is built as two side trusses, each with a top and bottom longeron 25 mm × 25 mm in cross-section with vertical triangulated bracing in the form of struts and diagonals bonded to the longerons at calculated intervals. The struts and braces are 25 mm × 18 mm in cross-section. As can be seen, the node connections are end-grain to surface-grain joints with strengthening rectangular plywood gussets.

To ensure equal strength, it is desirable that each pair of bottom longerons is cut from the same board; the same with the top pair.

The two side trusses are connected with lateral triangulated bracing (cross-members and diagonal braces) at the top and bottom to form the box structure. Note the direction of the top and bottom diagonals are reversed; also the diagonal seen at the lower centre is glued to the inside gusset rather than the longeron. (It is possible that the internal gusset is a well-crafted saddle gusset; i.e. a cutout in the plywood straddles the horizontal diagonal). The sides are parallel at the cockpit area then taper in to the tail, providing the mounting for the empennage and tailwheel.

Each pair of vertical struts makes a node connection with a pair of horizontal cross-members to form a rectangular frame or girder; it is normal practice to number each such frame starting from the fuselage firewall as frame #1. Hardwood cross-members will also be included where extra strength or hard-points are necessary, to carry undercarriage loads and carry through wing strut loads for example. Some internal wire tension bracing, between the corners of a forward frame and the diagonally opposite corners of an aft frame, may be desirable.

The plywood bows on the top of the fuselage are bonded to cross members and are the formers for the turtle deck; a secondary structure providing a rounded top to the fuselage behind the cockpit. The slots in the bows are to accommodate stringers. These are 6 mm × 25 mm strips running the length of the aft fuselage and provide support and stiffening for a fabric or plywood turtle deck cover. Similar secondary (i.e. non-load bearing) structures could be added to each fuselage side thus presenting a rounded fuselage, if that is considered aesthetically pleasing.

Six millimetre thick plywood is bonded to the front end or firewall, to the sides of the fuselage frame from the front end to the rear of the cockpit, and to the entire underside — forming, in effect, a torsion box.

Ribs on spars - Matt Paxton's PietenpolThe easiest wing to build is rectangular with a constant aerofoil section, constant thickness and constant chord; commonly known as a 'Hershey bar' wing. Two solid wood spars braced with external fuselage struts under tension carry the flight-bending loads. Twelve or so identical rib trusses attach to the spars to provide the aerodynamic shape to each wing. Half-span length shaped leading edge and trailing edge strips are glued to the ribs (not yet added at the stage the photo was taken). Internal diagonal inter-spar wire bracing is necessary to counter drag forces; consequently compression struts are placed between the spars. The wing is totally covered with fabric or perhaps with thin plywood from the top of the front spar around the leading edge to the bottom of the front spar, thus fixing the aerofoil shape over perhaps 25% of the chord and strengthening the wing. The fabric covering is usually glued over such plywood as a protective covering and prevents checking of the formed plywood. Flight loads are transmitted from the wing skin to the ribs, and then to the spars.

Constructing 24 or so wing ribs is time consuming but not difficult if an accurate bench top jig is made. Truss-type ribs are generally one-piece and consist of a top and bottom cap strip bonded together with a number of struts/braces [perhaps 12]. All nodes are end-grain to surface-grain reinforced with small plywood gussets on both sides. The Pietenpol cap strips and braces are built from 6 mm by 12 mm spruce and the gussets from 1.5 mm plywood. The finished ribs slide onto the solid spars (25 mm × 120 mm cross-section) for glueing in place. Very simple.

The horizontal and vertical stabilisers, rudder, elevators and ailerons all have the same general structure — spars, ribs, shaped leading and trailing edge strips, and fabric skins.

5.8 Laminations in beams
Lengthy structural members — as found in box beams, for example — can be constructed by lamination (in which the laminae have much the same grain orientation). If made carefully (and without any internal splices), such members would have much the same strength as a solid counterpart.

For example if a long length of 40 × 75 mm timber is required it can be fabricated with three 25 mm-wide sections sawn from board with suitable grain slope. In fact it may be better to build such members from laminations, as defects hidden within large cross-section boards are probably revealed when re-sawn and can be eliminated from the lamination. If necessary, scarf joint splicing can be used to obtain the necessary length of clear, straight-grained timber. The scarf joint slopes must be no steeper than 1 in 15 (i.e. the slope angle must be 4° or less) and care must be taken to ensure grain direction is maintained through the splice.

There is some advantage in increased rigidity, and improvements in stability and warping, if the lamination is designed correctly. Methods of laminating beams and beam flanges are discussed in section 2.4 of ANC Bulletin 19, Wood Aircraft Inspection and Fabrication (ref. 2 – 24).

The following is another extract from ANC–18:

4.25 SCARF JOINTS IN BEAMS. The following requirements should be observed in specifying scarf joints in solid or laminated beams and beam flanges:

The slope of all scarfs should not be steeper than 1 in 15. The proportion of end grain appearing on a scarfed surface is undesirably increased if the material to be spliced is somewhat cross-grained, and the scarf is made "across" rather than in the general direction of the grain (fig. 4-17). For this reason it is very desirable that the following note be added to all beam drawings showing scarf joints:
Note: where cross grain within the specified acceptble limits is present, all scarf cuts should be made in the general direction of the grain slope.

Figure 4.17 relationship between grain slope and scarf slope

In laminated members the longitudinal distance between the nearest scarf tips in adjacent laminations shall not be less than 10 times the thickness of the thicker lamination (fig. 4-18).


Figure 4.18 Minimum permissable longitudinal separation of scarf joints in adjacent laminations

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).

Figure 4.19 Solid wing spar at tip


[Extract from ANC–18 ends.]

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5.9 FAA Advisory Circular 43.13-1B extract

The following is an extract of sections 1.4 to 1.11 from the FAA advisory circular AC 43.13-1B, chapter 1-1. The complete volume 'Acceptable methods, techniques, and practices — aircraft inspection and repair' (~ 650 pages and incorporating the 2001 changes) is available from the RA-Aus on-line shop. It is bound together with the FAA advisory circular AC 43.13-2A 'Acceptable methods, techniques, and practices — aircraft alterations' (~ 100 pages).

1-4. ADHESIVES.

Because of the critical role played by adhesives in aircraft structure, the mechanic must employ only those types of adhesives that meet all of the performance requirements necessary for use in certificated civil aircraft. Use each product strictly in accordance with the aircraft and adhesive manufacturer's instructions.

a. Adhesives acceptable to the FAA can be identified in the following ways:

    (1) Refer to the aircraft maintenance or repair manual for specific instructions on acceptable adhesive selection for use on that type aircraft.

    (2) Adhesives meeting the requirements of a Military Specification (Mil Spec), Aerospace Material Specification (AMS), or Technical Standard Order (TSO) for wooden aircraft structures are satisfactory providing they are found to be compatible with existing structural materials in the aircraft and the fabrication methods to be used in the repair.

b. Common types of adhesives that are or have been used in aircraft structure fall into two general groups: casein and synthetic resins. Adhesive technology continues to evolve, and new types (meeting the requirements of paragraph 1-4a) may become available in the future.

    (1) Casein adhesive performance is generally considered inferior to other products available today, modern adhesives should be considered first.

    (2) Synthetic-resin adhesives comprise a broad family which includes plastic resin glue, resorcinol, hot-pressed phenol, and epoxy.

    (3) Plastic resin glue (urea-formaldehyde resin glue) has been used in wood aircraft for many years. Caution should be used due to possible rapid deterioration (more rapidly than wood) of plastic resin glue in hot, moist environments and under cyclic swell-shrink stress. For these reasons, urea-formaldehyde should be considered obsolete for all repairs.

    (4) Resorcinol adhesive (resorcinol-formaldehyde resin) is a two-part synthetic resin adhesive consisting of resin and a hardener. Resorcinol is widely used in wooden aircraft structure and fully meets necessary strength and durability requirements. The appropriate amount of hardener (per manufacturer's instruction) is added to the resin, and it is stirred until it is uniformly mixed; the adhesive is now ready for immediate use. Quality of fit and proper clamping pressure are both critical to the achievement of full joint strength. The adhesive bond lines must be very thin and uniform in order to achieve full joint strength.

    CAUTION: Read and observe material safety data. Be sure to follow the manufacturer's instructions regarding mixing, open assembly and close assembly times, and usable temperature ranges.

    (5) Phenol-formaldehyde adhesive is commonly used in the manufacturing of aircraft grade plywood. This product is cured at elevated temperature and pressure; therefore, it is not practical for use in structural repair.

    (6) Epoxy adhesives are a two-part synthetic resin product, and are acceptable providing they meet the requirements of paragraph 1-4a. Many new epoxy resin systems appear to have excellent working properties. They have been found to be much less critical of joint quality and clamping pressure. They penetrate well into wood and plywood. However, joint durability in the presence of elevated temperature or moisture is inadequate in many epoxies. The epoxy adhesives generally consist of a resin and a hardener that are mixed together in the proportions specified by the manufacturer. Depending on the type of epoxy, pot life may vary from a few minutes to an hour. Cure times vary between products.

    CAUTION: Some epoxies may have unacceptable thermal or other hidden characteristics not obvious in a shop test. It is essential that only those products meeting the requirements of paragraph 1-4a be used in aircraft repair. Do not vary the resin-to-hardener ratio in an attempt to alter the cure time. Strength, thermal, and chemical resistance will be adversely affected. Read and observe material safety data. Be sure to follow the adhesive manufacturer's instructions regarding mixing, open and closed curing time, and usable temperature ranges.

1-5. BONDING PRECAUTIONS.

Satisfactory bond joints in aircraft will develop the full strength of wood under all conditions of stress. To produce this result, the bonding operation must be carefully controlled to obtain a continuous thin and uniform film of solid adhesive in the joint with adequate adhesion and penetration to both surfaces of the wood.

Some of the more important conditions involve:

a. Properly prepared wood surfaces.

b. Adhesive of good quality, properly prepared, and properly selected for the task at hand.

c. Good bonding technique, consistent with the adhesive manufacturer?s instructions for the specific application.

1-6. PREPARATION OF WOOD SURFACES FOR BONDING.

It is recommended that no more time than necessary be permitted to elapse between final surfacing and bonding. Keep prepared surfaces covered with a clean plastic sheet or other material to maintain cleanliness prior to the bonding operation. The mating surfaces should be machined smooth and true with planers, joiners, 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 bonded.

Sawn surfaces must approach well-planed surfaces in uniformity, smoothness, and freedom from crushed fibers. It is advisable to clean both joint surfaces with a vacuum cleaner just prior to adhesive application. Wood surfaces ready for bonding must be free from oil, wax, varnish, shellac, lacquer, enamel, dope, sealers, paint, dust, dirt, adhesive, crayon marks, and other extraneous materials.

a. Roughening smooth, well-planed surfaces of normal wood before bonding is not recommended. Such treatment of well-planed wood surfaces may result in local irregularities and objectionable rounding of edges. When surfaces cannot be freshly machined before bonding, such as plywood or inaccessible members, very slight sanding of the surface with a fine grit such as 220, greatly improves penetration by the adhesive of aged or polished surfaces. Sanding should never be continued to the extent that it alters the flatness of the surface. Very light sanding may also improve the wetting of the adhesive to very hard or resinous materials.

b. Wetting tests are useful as a means of detecting the presence of wax, old adhesive, and finish. A drop of water placed on a surface that is difficult to wet and thus difficult to bond will not spread or wet the wood rapidly (in seconds or minutes). The surface may be difficult to wet due to the presence of wax, exposure of the surface to heat and pressure as in the manufacture of hot press bonded plywood, the presence of synthetic resins or wood extractives, or simply chemical or physical changes in the wood surface with time. Good wettability is only an indication that a surface can be bonded satisfactorily. After performing wetting tests, allow adequate time for wood to dry before bonding. Preliminary bonding tests and tests for bond strength are the only positive means of actually determining the bonding characteristics of the adhesive and material combinations. (See paragraph 1-29h.)

1-7. APPLYING THE ADHESIVE.

To make a satisfactory bonded joint, spread the adhesive in a thin, even layer on both surfaces to be joined. It is recommended that a clean brush be used and care taken to see that all surfaces are covered. Spreading of adhesive on only one of the two surfaces is not recommended. Be sure to read and follow the adhesive manufacturer?s application instructions.

1-8. ASSEMBLY TIME IN BONDING.

Resorcinol, epoxy, and other adhesives cure as a result of a chemical reaction. Time is an important consideration in the bonding process. Specific time constraints are as follows:

a. Pot life is the usable life of the adhesive from the time that it is mixed until it must be spread onto the wood surface. Once pot life has expired, the remaining adhesive must be discarded. Do not add thinning agents to the adhesive to extend the life of the batch.

b. Open assembly time is the period from the moment the adhesive is spread until the parts are clamped together. Where surfaces are coated and exposed freely to the air, some adhesives experience a much more rapid change in consistency than when the parts are laid together as soon as the spreading has been completed.

c. Closed assembly time is the period from the moment that the structure parts are placed together until clamping pressure is applied. The consistency of the adhesive does not change as rapidly when the parts are laid together.

d. Pressing (or clamping) time is the period during which the parts are pressed tightly together and the adhesive cures. The pressing time must be sufficient to ensure that joint strength is adequate before handling or machining the bonded structure.

    NOTE: Follow the adhesive manufacturer?s instructions for all time limits in the bonding process. If the recommended open or closed assembly periods are exceeded, the bond process should not be continued. Discard the parts if feasible. If the parts cannot be discarded, remove the partially cured adhesive and clean the bond line per adhesive manufacturer?s instructions before application of new adhesive.

1-9. BONDING TEMPERATURE.

Temperature of the bond line affects the cure rate of the adhesive. Some adhesive types, such as resorcinol, require a minimum temperature which must be maintained throughout the curing process. Each type of adhesive requires a specific temperature during the cure cycle, and the manufacturer's recommendations should be followed.

1-10. CLAMPING PRESSURE.

a. Use the recommended pressure to squeeze adhesive out into a thin, continuous film between the wood layers. This forces air from the joint and brings the wood surfaces into intimate contact. Pressure should be applied to the joint before the adhesive becomes too thick to flow and is accomplished by means of clamps, presses, or other mechanical devices.

b. Nonuniform clamping 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 varies with the type of adhesive used and the type of wood to be bonded. Typical pressures when using resorcinol may vary from 125 to 150 pounds per square inch for softwoods and 150 to 200 pounds per square inch for hardwoods. (Note - December 21, 2005: DAP Products of the US advise that their resorcinol product requires clamping pressures of only 25 to 75 psi). Insufficient pressure or poorly machined wood surfaces usually result in thick bond lines, which indicate a weak joint, and should be carefully guarded against. Some epoxy adhesives require much less clamping pressure to produce acceptable joint strength. Be sure to read and follow the manufacturer's instructions in all cases.

1-11. METHOD OF APPLYING PRESSURE.

The methods of applying pressure to joints in aircraft bonding operations range from the use of brads, nails, small screws, and clamps; to the use of hydraulic and electrical power presses. The selection of appropriate clamping means is important to achieving sound bond joints.

a. Hand nailing is used rather extensively in the bonding of ribs and in the application of plywood skins to the wing, control surfaces, and fuselage frames. Small brass screws may also be used advantageously when the particular parts to be bonded are relatively small and do not allow application of pressure by means of clamps. Both nails and screws produce adverse after effects. There is considerable risk of splitting small parts when installing nails or screws. Metal fasteners also provide vulnerable points for moisture to enter during service.

b. On small joints using thin plywood for gussets or where plywood is used as an outer skin, the pressure is usually applied by nailing or stapling. Thin plywood nailing strips are often used to spread the nailing pressure over a larger area and to facilitate removal of the nails after the adhesive has cured.

c. The size of the nails must vary with the size of the members. If multiple rows of nails are required, the nails should be 1 inch apart in rows spaced 1/2 inch apart. The nails in adjacent rows should be staggered. In no case should the nails in adjacent rows be more than 3/4 inch from the nearest nail. The length of the nails should be such that they penetrate the wood below the joint at least 3/8 inch. In the case of small members, the end of the nail should not protrude through the member below the joint. Hit the nails with several light strokes, just seating the head into the surface of the gusset. Be careful not to crush the wood with a heavy hammer blow.

d. In some cases the nails are removed after adhesive cure, while in others the nails are left in place. The nails are employed for clamping pressure during adhesive cure and must not be expected to hold members together in service. In deciding whether to remove nails after assembly, the mechanic should examine adjacent structure to see whether nails remain from original manufacture.

e. On larger members (spar repairs for example), apply pressure by means of screw clamps, such as a cabinet-maker's bar or C-clamps. Strips or blocks should be used to distribute clamping pressure and protect members from local crushing due to the limited pressure area of the clamps, especially when one member is thin (such as plywood). The strip or block should be at least twice as thick as the thinner member being bonded.

f. Immediately after clamping or nailing a member, the mechanic must examine the entire joint to assure uniform part contact and adhesive squeeze-out. Wipe away excess adhesive.

[Extract from Advisory Circular 43.13-1B ends]

The next module in this group is 'Wood beams in aircraft'

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Builders guide to aircraft materials – wood, plywood and adhesives modules

| Guide contents | Properties of wood | Properties of plywoods |

| [Wood joints and adhesives] | Wood beams in aircraft | Selecting aircraft timber |

| Basic strength and elastic properties of wood |



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