• Saddle gusset Nailing strips
- Maximum diameter 25T Minimum . distance.
~ 15T Temporary backing 30T Patch -12T Backing Plywood saddle gusset (·" minimum ) Minimum thickness = T Nail-glue in place Section A-A 121 Patch • Patch Backing Framing member - Backing Clamp and glue backing to frame and skin Section B-B Section C-C Nailing strips
- 127.
Temporary backing block, shape to fit skin Section D-D Cellophane or waxpaper.
FIGURE 5-118. Scarf patches, back of skin accessible.
Front spar Ribs Saddle gusset Rear spar
- 30T -12T- f Plywood skin 81 3T (Ke minimum) - L Rib cap Plywood saddle gusset (1" minimum ) Section A-A Nail and glue in place (minimum thickness = T) Plywood skin 121- Patch -12T - Patch T -T Rib cap ЗТ (¼ minimum ) Plywood skin Plywood or spruce Section B-B Section C-C FICURE 5-119. Scarf patches, back of skin not accessible.
shaped from solid wood and fitted to the inside surface of the skin, and it is temporarily held in place with nails. A hole, the exact size of the inside circle of the scarf patch, is made in the block and is centered over the trimmed area of damage. The block is removed after the glue on the patch has set, leaving a flush surface to the repaired skin.
When the back of a damaged plywood skin is not accessible, it should be repaired as outlined in figure 5-119. After removing damaged sections, install backing strips along all edges that are not fully backed by a rib or a spar. To prevent warping of the skin, backing strips should be made of a softtextured plywood, such as yellow poplar or spruce rather than solid wood. All junctions between backing strips and ribs or spars should have the end of the backing strip supported by a saddle gusset of plywood.
If needed, nail and glue the new gusset plate to the rib. It may be necessary to remove and replace the old gusset plate with a new saddle gusset, or it may be necessary to nail a saddle gusset over the original.
Attach nailing strips to hold backing strips in place while the glue sets. Use a bucking bar where necessary to provide support for nailing. Unlike the smaller patches made in a continuous process, work on the aircraft must wait while the glue, holding the backing strips, sets. After the glue sets, fill and finish to match the original skin.
Fabric Patch Small holes that do not exceed 1 in. in diameter, after being trimmed to a smooth outline, can be repaired by doping a fabric patch on the outside of the plywood skin. The edges of the trimmed hole should first be sealed, and the fabric patch should overlap the plywood skin by at least 1 in. Holes nearer than 1 in. to any frame member, or in the wing leading edge or frontal area of the fuselage, should not be repaired with fabric patches.
SPAR AND RIB REPAIR The web members of a spar or rib can be repaired by applying an external or flush patch, provided the damaged area is small. Plates of spruce or plywood of sufficient thickness to develop the longitudinal shear strength can be glued to both sides of the spar. Extend the plates well beyond the termination of the crack as shown in figure 5-120.
If more extensive damage has occurred, the web should be cut back to structural members and repaired with a scarf patch. Not more than two splices should be made in any one spar.
A spar may be spliced at any point except under wing attachment fittings, landing-gear fittings, engine-mount fittings, or lift-and-interplane strut fittings. Do not permit these fittings to overlap any part of the splice. Splicing under minor fittings such as drag wire, antidrag wire, or compression No fittings within these limits 5 to 1 slope SA Longitudinal crack Direction of grain of spruce or face grain of plywood FIGURE 5-120. Reinforcing a longitudinal crack in a solid or internally routed spar.
strut fittings is acceptable under the following conditions:
(1) The reinforcement plates of the splice should not interfere with the proper attachment or alignment of the fittings. The locations of pulley support brackets, bellcrank support brackets, or control surface support brackets should not be altered.
(2) The reinforcement plate may overlap drag or antidrag wire or compression strut fittings if the reinforcement plates are on the front-face of the front spar or on the 10A
• No fitting within these limits
SPLICING OF SOLID RECTA GULAR SPAR BA-
- 10A No fitting within these
SPLICING OF SOLID ROUTED SPAR 6B 10B
- 10A
SPLICING OF BOX SPAR FLANGES FICURE 5-121. Spar splicing.
rear-face of the rear spar. In such cases it will be necessary to install slightly longer bolts. The inside reinforcement plate should not overlap drag strut fittings, un- • 10A Inserted web section less such overlapping does not require suf- 10A ficient shortening of compression struts or + 1" (1/6 flange adjustment for proper rigging. Even changes in drag-truss geometry to prevent - ЗВ Plywood cover strip same thickness as A 4B depth) B~ though takeup is sufficient, it may be necessary to change the angles on the fittings.
Space the splices so that they do not overlap. Reinforcement plates must be used as indicated on all scarf repairs to spars. The 4B desired slope for the scarf is 12 to 1, but a slope of not less than 10 to 1 is accepta- Filler ble. The plates are held in place by glue blocks Spar and must not be nailed. web Figure 5-121 illustrates the general method for splicing common types of wooden spars.
Always splice and reinforce plywood webs with the same type of plywood as the original. Do not use solid wood to replace plywood webs. Plywood is stronger in shear than solid wood of the same thickness because of the variation in grain direction of the individual plies. The face-grain of plywood replacement webs and reinforcement plates must be in the same direction as that of the original member to ensure that the new web will have the required strength. One method of splicing plywood webs is shown in figure 5-122.
BOLT AND BUSHING HOLES All bolts and bushings used in aircraft structures must fit snugly into the holes. Looseness allows the bolt or fitting to work back and forth, enlarging the hole. In cases of elongated bolt holes in a spar or cracks in the vicinity of bolt holes, splice in a new section of spar, or replace the spar entirely.
Holes drilled to receive bolts should be of such size that the bolt can be inserted by light tapping with a wood or rawhide mallet. If the hole is so tight that heavy blows are necessary to insert the bolt, deformation of the wood may cause splitting or unequal load distribution.
Rough holes are often caused by using dull bits or trying to bore too rapidly. Well-sharpened twist drills produce smooth holes in both solid wood and plywood. The twist drill should be sharpened to approximately a 60° angle. All holes bored for bolts which are to hold fittings in place should match exactly the hole in the fitting.
FIGURE 5-122. Method of splicing a box-spar web.
Bushings made of plastic or light metal provide additional bearing surface without any great increase in weight. Sometimes light steel bushings are used to prevent crushing the wood when bolts are tightened. The holes for bushings should be of such size that the bushing can be inserted by tapping lightly with a wood or rawhide mallet.
A J 1-31-0te - 10A • Face grain of plywood side plates
- Spruce block FIcURE 5-123. A rib cap-strip repair.
RIB REPAIRS A cap strip of a wood rib can be repaired using a scarf splice. The repair is reinforced on the side 10A opposite the wing covering by a spruce block which extends beyond the scarf joint not less than three times the thickness of the strips being repaired. The entire splice, including the reinforcing block, is reinforced on each side by a plywood side plate as shown in figure 5-123.
When the cap strip is to be repaired at a point where there is a joint between it and cross members Face grair of plywood of the rib, the repair is made by reinforcing the scarf joint with plywood gussets, as shown in figure
5-124.
A, B, C, D, and E are
•ЗАТ - 5A + ЗА- original dimensions
- E Direction of face grain of plywood A, B, C, D and E are original dimensions. FIcURE 5-125. A rib repair at a spar.
Reinforcement plates shall be plywood glued and nailed.
- No fittings within these limits Splice plate Edge damage repaired Spruce block Bevel 5 to 1 . ЗА - FIcURE 5-124. A rib repair at a joint.
When it is necessary to repair a cap strip at a spar, the joint should be reinforced by a continuous gusset entending over the spar as shown in figure
5-125.
Edge damage, cracks, or other local damage to a spar can be repaired by removing the damaged portion and gluing in a properly fitted block, as shown in figure 5-126, reinforcing the joint by means of plywood or spruce blocks glued into place.
The trailing edge of a rib can be replaced and repaired by removing the damaged portion of the cap strip and inserting a softwood block of white pine, spruce, or basswood. The entire repair is then reinforced with plywood gussets and nailed and glued as shown in figure 5-127.
- Longitudinal crack FicURE 5-126. The repair of cracks and edge damage on & solid spar.
Compression ribs are of many different types, and the proper method of repairing any part of this type of rib is specified by the manufacturer. Figure 5-128 shows a typical repair made to a compression rib built up of a plywood web and three longitudinal members, the center one of which has been 245
it is possible to replace a damaged member, it is always better to do so than to attempt a repair.
FICURE 5-127. A rib trailing-edge repair.
repaired by a properly reinforced scarf joint and an outside layer of plywood. pression members, braces, and rib diagonals should Such members as glue blocka, filler blocks, cora- FIGURE 5-128. A compression-rib repair.
not be repaired, but should be replaced. Wherever 246
GENERAL Metals can be joined by mechanical means (bolting or riveting, or by welding, brazing, soldering or adhesive bonding). All of these methods are used in aircraft construction. This chapter will discuss the methods used to join metals by welding, brazing, Welding Welding is the process of joining metal by fusing the materials while they are in a plastic or molten state. There are three general types of welding: (1) Gas, (2) electric arc, and (3) electric resistance welding. Lach of these types of welding has several variations which are used in aircraft construction.
Welding is used extensively in the repair and manufacture of aircraft. Such parts as engine mounts and landing gear are often fabricated in this manner, and many fuselages, control surfaces, fittings, tanks, etc., are also of welded construction.
Structures that have been welded in manufacture may generally be repaired economically by using the same welding process. Careful workmanship, both in preparation and actual welding, is of utmost importance.
Welding is one of the most practical of the many metal-joining processes available. The welded joint offers rigidity, simplicity, low weight, and high strength. Consequently, welding has been universally adopted in the manufacture and repair of all types of aircraft. Many structural parts as well as nonstructural parts are joined by some form of welding, and the repair of many of these parts is an indispensable part of aircraft maintenance.
It is equally important to know when not to weld, as it is to know when. Many of the alloy steels or high-carbon steel parts that have been hardened or strengthened by heat treatment cannot be restored to 100% of their former hardness and strength after they have been welded.
Gas Welaing Gas welding is accomplished by heating the ends or edges of metal parts to a molten state with a high- CHAPTER 6
AIRCRAFT WELDING temperature flame. This flame is produced with a torch burning a special gas such as acetylene or hydrogen with pure oxygen. The metals, when in a molten state, flow together to form a union without the application of mechanical pressure or blows.
Aircraft parts fabricated from chrome-molybdenum or mild carbon steel are often gas welded.
There are two types of gas welding in common use:
(1) Oxyacetylene and (2) oxyhydrogen. Nearly all gas welding in aircraft construction is done with an oxyacetylene flame, although some manufacturers prefer an oxyhydrogen flame for welding aluminum alloys.
Electric Arc Welding Electric arc welding is used extensively in both the manufacture and repair of aircraft, and can be satistactorily used in the joining of all weldable metals. The process is based on using the heat generated by an electric arc. Variations of the process are: (1) Metallic arc welding, (2) carbon arc welding, (3) atomic hydrogen welding, (4) inert-gas (helium) welding, and (5) multi-arc welding. Metallic arc and inert-gas welding are the two electric arc welding processes most widely used in aircraft construction.
Electric Resistance Welding Electric resistance welding is a welding process in which a low-voltage, high-amperage current is applied to the metals to be welded through a heavy, low-resistance copper conductor. The materials to be welded offer a high resistance to the flow of current, and the heat generated by this resistance fuses (welds) the parts together at their point of contact.
Three commonly used types of electric resistance welding are butt, spot, and seam welding. Butt welding is used in aircraft work to weld terminals to control rods. Spot welding is frequently used in airframe construction. It is the only welding method used for joining structural corrosion-resistant steel.
Seam welding is similar to spot welding, except that power-driven rollers are used as electrodes. A con- 247
tinuous airtight weld can be obtained using seam welding.
OXYACETYLENE WELDING EQUIPMENT Oxyacetylene welding equipment may be either stationary or portable. A portable equipment rig consists of the following:
(1) Two cylinders, one containing oxygen and one acetylene.
(2) Acetylene and oxygen pressure regulators, complete with pressure gages and connec tions.
(3) A welding torch, with a mixing head, extra tips and connections.
(4) Two lengths of colored hose, with adapter connections for the torch and regulators.
(5) A special wrench.
(6) A pair of welding goggles.
(7) A fint lighter.
(8) A fire extinguisher.
Figure 6-1 shows some of the equipment in a typical portable acetylene welding rig.
Oxygen pressure regulator Acetylene pressure Oxygen cylinder regulator Acetylene cylinder Torch - FicuRs 6-1. Typical portable acetylene welding rig.
Stationary oxyacetylene welding equipment is similar to portable equipment, except that acetylene and oxygen are piped to one or several welding stations from a central supply. The central supply mually consists of several cylinders connected to a common manifold. A master regulator controls the pressure in each manifold to ensure a constant pressure to the welding torch.
Acetylene Gas Acetylene gas is a flammable, colorless gas which has a distinctive, disagreeable odor, readily detectable even when the gas is heavily diluted with air.
Unlike oxygen, acetylene does not exist free in the atmosphere; it must be manufactured. The process is neither difficult nor expensive. Calcium carbide is made to react chemically with water to produce acetylene.
Acetylene is either used directly in a manifold system or stored in cylinders. If ignited, the result is a yellow, smoky flame with a low temperature.
When the gas is mixed with oxygen in the proper proportions and ignited, the result is a blue-white flame with temperatures which range from approximately 5,700° to 6,300° F.
Under low pressure at normal temperatures, acetylene is a stable compound. But when compressed in a container to pressures greater than 15 p.s.i., it becomes dangerously unstable. For this reason, manufacturers fill the acetylene storage cylinders with a porous substance (generally a mixture of asbestos and charcoal) and saturate this substance with acetone. Since acetone is capable of absorbing approximately 25 times its own volume of acetylene gas, a cylinder containing the correct amount of acetone can be pressurized to 250 p.s.i.
Acetyione Cylinders The acetylene cylinder is usually a seamless steel shell with welded ends, approximately 12 in. in diameter and 36 in. long. It is usually painted a distinctive color, and the name of the gas is stenciled or painted on the sides of the cylinder. A fully charged acetylene cylinder of this size contains approximately 225 cu. ft. of gas at pressures up to 250 p.s.i. In the event of fire or any excessive temperature rise, special safety fuse plugs installed in the cylinder will melt, allowing the excess gas to escape or burn, thus minimizing the chances of an explosion. The holes in the safety plugs are made small to prevent the flames from burning back into the cylinder. Acetylene cylinders should not be completely emptied, or a loss of filler material may result.
Oxygen Cylinders The oxygen cylinders used in welding operations are made of seamless steel of different sizes. A typi- 248
cal small cylinder holds 200 cu. ft. of oxygen at 1,800 p.s.i, pressure. A large size holds 250 cu. ft.
of oxygen at 2,265 p.s.i. pressure. Oxygen cylinders are usually painted green for identification The cylinder has a high-pressure valve located at the top of the cylinder. This valve is protected by a metal safety cap which should always be in place when the cylinder is not in use.
Oxygen should never come in contact with oil or grease. In the presence of pure oxygen, these substances become highly combustible. Oxygen hose and valve fittings should never be oiled or greased, or handled with oily or greasy hands. Even grease spots on clothing may flare up or explode if struck by a stream of oxygen. Beeswax is a commonly used lubricant for oxygen equipment and fittings.
Pressure Regulators Acetylene and oxygen regulators reduce pressures and control the flow of gases from the cylinders to the torch. Acetylene and oxygen regulators are of the same general type, although those designed for acetylene are not made to withstand such high pressures as those designed for use with oxygen. To prevent interchange of oxygen and acetylene hoses, the regulators are built with different threads on the outlet fitting. The oxygen regulator has a righthand thread, and the acetylene regulator has a lefthand thread.
On most portable welding units, each regulator is equipped with two pressure gages, a high-pressure gage which indicates the cylinder pressure and a low-pressure gage which indicates the pressure in the hose leading to the torch (working pressure).
In a stationary installation, where the gases are piped to individual welding stations, only one gage for oxygen and one for acetylene are required for each welding station, since it is necessary to indicate only the working pressure of the gases flowing through the hose to the welding torch.
A typical regulator, complete with pressure gages and connections, is shown in figure 6-2. The adjusting screw shown on the front of the regulator is for adjusting the working pressure. When this adjusting screw is turned to the left (counterclockwise) until it turns easily, the valve mechanism inside the regulator is closed. No gas can then flow to the torch. As the handle is turned to the right (clockwise), the screw presses against the regulating mechanism, the valve opens, and gas passes to the torch at the pressure shown on the working pressure gage. Changes in the working pressure can Low or torch High or cylinder pressure gage pressure gage outer scale indicates pounds ( pounds per per sq. in.pressure sq. in.) inner scale indicates cu-ft cylinder contents.
Double stage pressure regulator Cylinder connection Hose connection Adjusting screw handle FIcURE 6-2. Typical oxygen pressure regulator.
be made by adjusting the handle until the desired pressure is registered.
Before opening the high-pressure valve on a cylinder, the adjusting screw on the regulator should be fully released by turning it counterclockwise.
This closes the valve inside the regulator, protecting the mechanism against possible damage.
Welding Torch The welding torch is the unit used to mix the oxygen and acetylene together in correct proportions. The torch also provides a means of directing and controlling the size and quality of the flame produced. The torches are designed with two needle valves, one for adjusting the flow of acetylene and the for other adjusting the flow of oxygen.
Welding torches are manufactured in different sizes and styles, thereby providing a suitable type for different applications. They are also available with several different sizes of interchangeable tips in order that a suitable amount of heat can be obtained for welding the various kinds and thicknesses of metals.
Welding torches can be divided into two classes:
(1) The injector type and (2) the balanced-pressure type. The injector-type torch (figure 6-3A) is designed to operate with very low acetylene pressure as compared to the oxygen pressure.
A narrow passageway or nozzle within the torch, called the injector, through which the oxygen 249
Oxygen Acetylene A Injector-type welding torch.
Oxygen shutoff valve X Oxygen Acetylene shutoff valve Acetylene Mixed oxygen and acetylene B Balanced-pressure welding torch.
FICURE 6-3. Welding torches.
passes, causes the speed of oxygen flow to increase to a high velocity with a corresponding drop in pressure. This pressure drop across the injector creates a pressure differential which acts to draw the required amount of acetylene into the mixing chamber in the torch head.
In the balanced-pressure torch, the oxygen and acetylene are both fed to the torch at the same pressure (figure 6-3B). The openings to the mixing chamber for each gas are equal in size, and the delivery of each gas is independently controlled.
This type of torch is generally better suited for aircraft welding than the injector type because of the ease of adjustment.
Welding Torch Tips The torch tip delivers and controls the final flow of gases. It is important that the correct tip be selected and used with the proper gas pressures if a job is to be welded satisfactorily. The nature of the weld, the material, the experience of the welder, and the position in which the weld is to be made, all determine the correct size of the tip opening, The size of tip opening, in turn, determines the amount of heat (not the temperature) applied to the work.
If a tip which is too small is used, the heat provided will be insufficient to produce penetration to the proper depth. If the tip is too large, the heat will be too great, and holes will be burned in the metal.
The torch tip sizes are designated by numbers, and each manufacturer has his own arrangement for classifying them. As an example, a number two tip is made with an orifice of approximately 0.010 in. diameter. The diameter of the tip orifice is related to the volume of heat it will deliver.
Torch tips are made of copper or copper alloy and are made so that they seat well when tightened handtight. Torch tips should not be rubbed across fire brick or used as tongs to position work.
With use, the torch tip will become clogged with carbon deposits and, if it is brought in contact with the molten pool, particles of slag may lodge in the opening. A split or distorted flame is an indication of a clogged tip. Tips should be cleaned with the proper size tip cleaners or with a piece of copper or soft brass wire. Fine steel wool may be used to remove oxides from the outside of the tip. These oxides hinder the heat dissipation and cause the tip to overheat.
A flint lighter is provided for igniting the torch.
The lighter consists of a file-shaped piece of steel, usually recessed in a cuplike device, and a piece of flint that can be drawn across the steel, producing the sparks required to light the torch. Matches should never be used to ignite a torch since their length requires bringing the hand in close to the tip to ignite the gas. Accumulated gas may envelop the hand and, when ignited, cause a severe burn.
Goggles Welding goggles, fitted with colored lenses, are worn to protect the eyes from heat, light rays, sparks, and molten metal. A shade or density of color that is best suited for the particular situation should be selected. The darkest shade of lens which will show a clear definition of the work without eyestrain is the most desirable. Goggles should fit closely around the eyes and should be worn at all times during welding and cutting operations.
Welding (Filler) Rods The use of the proper type filler rod is very important in oxyacetylene welding operations. This material not only adds reinforcement to the weld Brea, but also adds desired properties to the finished weld. By selecting the proper rod, either tensile strength or ductility can be secured in a weld, or both can be secured to a reasonably high degree.
Similarly, rods can be selected which will help retain the desired amount of corrosion resistance. In some cases, a suitable rod with a lower melting point will eliminate possible cracks caused by expansion and contraction.
Welding rods may be classified as ferrous or nonferrous. The ferrous rods include carbon and alloy steel rods as well as cast-iron rods. Nonferrous rods include brazing and bronze rods, aluminum and aluminum alloy rods, magnesium and magnesium alloy rods, copper rods, and silver rods.
Welding rods are manufactured in standard 36- in. lengths and in diameters from 16 in. to ⅛ in.
The diameter of the rod to be used is governed by the thickness of the metals being joined. If the rod is too small, it will not conduct heat away from the puddle rapidly enough, and a burned weld will result. A rod that is too large will chill the puddle. As in selecting the proper size welding torch tip, experience enables the welder to select the proper diameter welding rod.
Setting Up Acetylene Welding Equipment Setting up acetylene welding equipment and preparing for welding should be done systematically and in a definite order to avoid costly mistakes. The following procedures and instructions are typical of those used to assure safety of equipment and personnel:
(1) Secure the cylinders so they cannot be upset, and remove the protective caps from the cylinders.
(2) Open each cylinder shutoff valve for an instant to blow out any foreign matter that may be lodged in the outlet. Close the valves and wipe off the connections with a clean cloth.
(3) Connect the acetylene pressure regulator to the acetylene cylinder and the oxygen regulator to the oxygen cylinder. Use a regulator wrench and tighten the connecting nuts enough to prevent leakage.
(4) Connect the red (or maroon) hose to the acetylene pressure regulator and the green (or black) hose to the oxygen regulator.
Tighten the connecting nuts enough to prevent leakage. Do not force these connections, since these threads are made of brass and are easily damaged.
(5) Release both pressure regulator adjusting screws by turning the adjusting screw handle on each regulator counterclockwise until it turns freely. This is to avoid damage to the regulators and pressure gages when the cylinder valves are opened.
(6) Open the cylinder valves slowly and read each of the cylinder pressure gages to check the contents in each cylinder. The oxygen cylinder shutoff valve should be opened fully and the acetylene cylinder 251