Aircraft Materials and Processes PDF

Aircraft Materials and Processes By Titterton PHYSICAL TERMS Hardness - resisting penetration or permanent distortion - Can be increased by: hammering, rolling, working - If aluminum alloy, hardness can be increased by heat treatment *Annealing - modified he...

Aircraft Materials and Processes By Titterton PHYSICAL TERMS Hardness - resisting penetration or permanent distortion - Can be increased by: hammering, rolling, working - If aluminum alloy, hardness can be increased by heat treatment *Annealing - modified heat treatment that will soften the metals Brittleness - resisting a change in the relative position of molecules - Tendency to fracture without change of shape - The harder the material, the more brittle it is Malleability - property of metals which allows them to be bent or permanently distorted without rupture - Opposite of brittleness - Process: Bending and hammering - Products: sheets, bar stocks, forgings, and fabrication Ductility - property of metals to be drawn out without breaking - Products: Wire and tubing - Process: drawing - Ease of forming and resistance to failure under shock loads Elasticity - property of returning to the original shape when the force is removed - All aircraft structural design is based on this property *Elastic limit - point beyond which it cannot be loaded without causing permanent distortion Density - weight of a unit volume of a material Fusibility - property of being liquefied by heat - Process: Welding - Fuse Temperature a. Steel - 200F b. Aluminum alloys - 1100F Conductivity - property of transmitting heat or electricity - In welding, it affects the amount of heat used and the design of welding jig - In aircraft, it is important in connection with the bonding of planes to eliminate radio interference *Welding jig - a large brace that holds a welding project in place while welding, keeping it stable and allowing the workpiece to move while the tool remains stationary. Contraction and Expansion - caused by cooling or heating of metals - Affects welding jigs, castings, and tolerances for hot rolled material HEAT TREATMENT TERMS Critical Range - range of temperature between 1300F to 1600F - When steel passes through this temp, its internal structure is altered - Heat treatment of steel is based on this phenomenon Annealing - process of heating steel above the critical range, holding it at that temperature until uniform, then cooling. - Relieves internal strains Normalizing - similar to annealing but steel is allowed to *cool in still air - * a method faster than annealing-cooling - Applies only to steel - Relieves internal strains and softens the metal - Increases the strength of steel about 20% above that of annealed material Heat treatment - series of operations to improve the physical properties of a material - For steels, operations are hardening and tempering Hardening - done by heating the metal to a temperature above the critical range and then quenching it in brine, water, or oil (slower and more gentle cooling) - Heat temp of aluminum alloy - above 900F Quenching - immersion of the heated metal in a liquid (oil or water) to accelerate cooling Tempering - reheating of hardened steel to a temperature below the critical range, then cooled/quenched in oil or ail. - AKA drawing Carburizing - addition of carbon to steel by heating it at high temperature while in contact with a carbonaceous material (solid, liquid, gas) - Best formed on steels with less than 0.25% carbon content Case hardening - consists of carburizing, then suitable heat treatment to harden the metal Stabilizing - used exclusively to dissolve precipitated carbides and prevent intergranular corrosion. PHYSICAL-TEST TERMS Strain - deformation of material caused by an applied load Stress - load acting on a material Tensile Strength - ultimate tensile strength - Maximum tensile load per square inch - Formula: Max load obtained/ Cross-sec Area - PSI Elastic Limit - greatest load per square inch of area which a material can withstand without permanent deformation Proportional Limit - load per square inch beyond which the increases in strain cease to be directly proportional to the increase in stress *Hooke’s Law - proportionality between stress and strain Proof Stress - load per square inch a material can withstand without permanent elongation of more than 0.0001 inch of gage length - Total permissible elongation = 0.0002 inch Yield Strength - load per square inch at which a material exhibits a special limiting permanent set or a specified elongation under load Elongation (Percentage) - difference in gage length being subjected to any stress and after rupture Reduction of Area (Percentage) - difference between the original cross-sectional area and the least cross-sectional area after rupture Modulus of Elasticity - ratio of stress to strain within the elastic limit TESTING AIRCRAFT MATERIALS For Hardness: Brinell Hardness Test - pressing a hardened steel ball under a known pressure, into a flat surface of the specimen to be tested. Rockwell Hardness Test - determined by measuring the penetration of a diamond cone or hardened steel ball under definite loads - Difference between the depth of penetration at major and minor loads - The greater the difference, the lower the hardness (softer) STEEL AND ITS ALLOYS Plain Carbon Steels - the classification of iron and steel is based on the percentage of carbon present - Contain small amounts of: silicon, sulfur, phosphorus, and manganese *Silicon and Manganese are beneficial elements *Sulfur and Phosphorus are harmful impurities EFFECT OF INDIVIDUAL ELEMENTS Carbon - combines readily with iron to form iron carbide (Fe3C) known as cementite - The higher the carbon content of steel, the harder (greater ultimate strength) it will be - The higher carbon content, the lower the ductility, malleability, toughness, impact resistance, and weldability *Low carbon steel - deep drawing, excessive mechanical working - Formed fittings and welded parts *High carbon steel - great hardness is required  - Springs Manganese - primary purpose is to deoxidize and desulfurize the steel to produce a clean metal - Presence of this element will improve forging qualities by reducing brittleness Silicon - 0.3% of Si is present in steel - Excellent deoxidizer - Small amount can improve ductility - Main purpose is to produce a sound metal *Silico-manganese steels - good impact resistance Sulfur - very undesirable impurity - Must be limited to 0.06% - Renders steel brittle at rolling or forging temp (hot short) - Iron sulfide breaks up the cohesion of crystals, hence cracking and breaking Phosphorus - undesirable impurity limited to 0.05% - Responsible for cold shortness (brittleness when cold) - Evidence that very small amounts of phosphorus increase strength slightly Nickel - white metal almost as bright as silver - Pure state: malleable, ductile, weldable - Does not corrode quickly - Commonly used nickel steels contain 3% - 5% nickel *Addition of nickel: - Increases strength, yield point, hardness - In heat treatment, slows down critical rate of hardening, which produces finer grain - Less warpage and scaling - Increases corrosion-resistance of steel Chromium - hard gray metal with high melting point - Imparts hardness, strength, wear resistance, and corrosion resistance of steel - Used for magnets - Corrosion resistant steels contain large amounts of chromium (18-8 steel, 18% Cr and 8% Ni - nonmagnetic) *Chromium alloys are used for great wear resistance *Chrome-vanadium alloy is used for ball bearings *Tungsten-chromium is used for high-speed cutting tools Molybdenum - very effective alloying element - Improves homogeneity of the metal and reduces grain size - Increases elastic limit, wear resistance, fatigue strength Vanadium - most expensive - Seldom used in amounts over 0.20% - Intensive deoxidizing agent - Improves grain structure and fatigue strength - Increases resistance to impact, vibration, and stress reversal *Vanadium alloys for propeller hubs and engine bolts Tungsten - no direct application in aircraft construction - Red hardness - High speed steel: tungsten-chromium steel used for tools which will retain their cutting edge even when heated to dull redness by working Titanium - often added in small quantities to 18-8-corrosion-resisting steel to reduce embrittlement at the operation temperatures of exhaust stacks and collectors STEEL NUMBERING SYSTEM Ca-Ni-NiChro-Mo-Chro-CV-Tu-Na-SM 1AERO Titterton Carbon 1XXX Carbon Nickel 2XXX Nickel Nickel-Chromium 3XXX Nickel-Chromium Molybdenum 4XXX Molybdenum Chromium 5XXX Chromium Chromium-Vanadium 6XXX Chromium-Vanadium  Tungsten 7XXX – National Emergency 8XXX Nickel-Chrom-Molyb Silicon-Manganese 9XXX Silicon-Manganese HEAT TREATMENT OF STEEL Pure Iron is allotropic in three (3) states: - Alpha (stable up to 1400 F) - Beta (1400 F - 1652 F) - Gamma (1652 F and more) *Allotropic - when a material possesses the property that permits it to exist in various forms without a change in chemical composition Ex. Carbon (diamond, graphite, charcoal) INTERNAL STRUCTURE OF STEEL Pure iron - Ferrite Iron carbide - Cementite 1 part Cementite + 6 parts Ferrite = Pearlite Pearlite - usually a laminate structure - Sometimes has a granular appearance (granular pearlite) - Strong, hard, and ductile Ferrite - pure alpha iron in carbon steels - Very ductile - Does not have any hardening properties Cementite - very hard and brittle and produces hardening quality on steels Austenite - consists of a solid solution of cementite gamma iron - steel when it is heated above the critical range, melted and solidified Martensite - intermediate form of cementite in alpha iron obtained when the transition from austenite to pearlite is arrested - Main constituent of hardened steel - Hardest structure obtained in steel (extremely hard and brittle) ANNEALING Annealed steel is fine grained, soft, ductile, and without internal stresses or strains. It is readily machinable and workable Several modifications of the full annealing treatment: 1. Process annealing - commonly used in the sheet and wire industries to resolve ductility - Temp: 1020 - 1200F 2. Spheroidizing - applied to high-carbon steels to improve machinability. - Operation consists in prolonged heating just slightly below critical range, followed by slow cooling 3. Shop annealing - the practice of heating steel with a welding torch to 900F to 1000F and dropping it into a pail of ashes or lime to restrict the cooling rate. - Never used in aircraft works unless followed by regular treatment NORMALIZING Steel is harder and stronger but less ductile than annealed material Forgings are generally normalized to relieve internal stresses. Because of better physical properties, aircraft steels are often used in normalized condition but seldom if annealed. All welded parts should be normalized after fabrication to reduce cracks and fatigue failures. Low-carbon steels are often normalized to improve the machining qualities and to reduce distortion in subsequent heating operations Sorbite makes the steel stronger but also more brittle. Medium and High-carbon steels should be normalized then annealed before machining and or fabrication (double annealing) HARDENING Hardening is the first of two operations required for the development of high-strength steels by heat treatment. Produces fine grain, maximum hardness (brittle), and tensile strength, minimum ductility and internal strains. Quench cracking is a result of nonuniform or too rapid cooling of the steel. DRAWING (TEMPERING) It is the second operation required to develop high-strength, heat-treated steel. When hardened steel is reheated as in tempering, the transition from austenite to pearlite is continued further, and martensite is converted to troostite and then sorbite. Tempered steel is composed largely of sorbite (tough) PRACTICAL HEAT TREATMENT 1. Heating - aim is to transform pearlite to austenite as the critical range is passed through. - When reheating (tempering), the furnace should not be above 800F to 1000F Types of furnace: a. Dry heat - fired by oil, gas, or electricity. - Uniform temperature must be maintained - Work must not be placed too close to the wall (uneven heating) - Maintain a neutral atmosphere so heated steel is neither oxidized nor decarburized - Electric furnace is the most satisfactory - Galvo Anti-scale (paint coating) is used to minimize scaling b. Liquid heat - used for parts which have been finished-machined before what treatment. - Parts are heated in molten salt bath - Complete elimination of scaling - Better temp regulation and uniform heating - Faster heating than dry heat 2. Soaking - rearrangement of internal structure is completed - Heavier parts require longer soaking  - Hardening temperature: 50F to 70F within the material must be soaked - For steels and sizes normally used in aircraft construction, soaking period of 30-45 mins - When tempering, soaking is 30 mins to 1 hour 3. Quenching - air cooling is a very mild form of quenching - Mediums: brine, water, oil (most severe to least severe) - In aircraft work, a. High-carbon steels - oil quenched b. Medium-carbon steels - water quenched c. Mild-carbon steels - brine or water quenched d. Low-carbon steels - sever quench - Oil quenching is preferred due to reduced strain, warpage, and cracking - Quenching oil is maintained between 80F - 150F - Water is below 65F - Chrome-Nickel steels: oil quenched to avoid temper brittleness INTERRUPTED QUENCHING Procedures to attain special characteristics 1. Cycle annealing - better control of the final annealed structure and can be accomplished quickly for full annealing and spheroidizing. - Austenite is isothermally transformed to pearlite (high temp) 2. Austempering - limited to small sizes and deep-hardening steels, but increases ductility and toughness. - Austenite is isothermally transformed bainite (moderate temp) 3. Martempering - applicable only to small sizes of deep-hardening steels but minimizes distortion and cracking due to quenching - Austenite is uniformly transformed to martensite (low temp) Surface Hardening CASE HARDENING 1. Carburizing a. Solid carburizing - oldest and most commonly used method of carburizing - Usually bone, charred leather, wood charcoal, coke b. Liquid Carburizing - applicable to small parts where a depth of case not greater than 0.040 inch is satisfactory - Liquid salt bath, then amorphous carbon  - Faster than solid because laborious packing is eliminated c. Gas Carburizing - becoming more generally used - Exposing small parts in a rotating retort to gas as a medium - Solid carburizer is sometimes added d. Refining the Core - it is necessary to reheat the steel to just above the upper critical point to obtain a fine, ductile grain in core CYANIDING Surface hardening of steel obtained by heating it in contact with a cyanide salt, followed by quenching It is seldom used in aircraft work. Advantage: Speed and cheapness NITRIDING Surface hardening of special alloy steels by heating the metal in contact with ammonia gas or other nitrogenous material Applicable only to special steels, the most common are nitroalloys Gas welding of nitriding steels is not practical since a large part of the aluminum is burnt away and the remaining metals will not nitride properly. Spot welding after nitriding has been successful. INDUCTION HARDENING Induction Hardening is one application of induction heating which is finding numerous applications in aircraft and automotive work It is the process of heating metallic substances by means of a powerful, rapidly alternating electromagnetic field. SHOT PEENING AKA shot blasting, different from sand blasting. It improves the fatigue and abrasion resistance of metal parts Applicable to ferrous and nonferrous parts, but mostly on steel surfaces. Reported to increase the life of parts subject to repeated stress (springs) from 3 to 13 times Consists of throwing hardened steel balls at the surface of the work to be peened. They are thrown by either compressed air or centrifugal force as it is fired from a rotating wheel. *Fractures usually start at a point of localized stress concentration Shaping of Metals 1. Hot Working - done by either rolling or forging - Bloom: rolled shape with 6x6in dimension - Billet: approximately square, but less than 6x6 - Slabs: rectangular, width is greater than twice the thickness a. Hot rolling - More common of these rolled shapes are sheet, bar, channels, angles, I-beams, railroads rails, etc. b. Forging - complicated sections which cannot be rolled. - Done by either pressing or hammering the heated steel until desired shape is obtained - Pressing: parts to be forged are large and heavy - Hammering: used on relatively small pieces Upsetting: a hot piece of metal is increased in thickness and decreased in length by hammering the end Swaging: reducing the cross section and shaping a bar, rod, tube - Done by subjecting a revolving die to a large number of repeated blows c. Drop forging - two dies are used, one is attached to the hammer and the other to the anvil - Used for the production of individual pieces in large quantities - Aircraft fittings are drop forged quite extensively - Chrome-molybdenum and Cr-Ni-Mo are used for aircraft forgings 2. Cold working - done at atmospheric temperatures - Cold rolling or cold drawing - Rolled: Sheet steel, bars ¾ in diameter or larger - Drawn: smaller bars, wire, tubings - Cold-worked material increases in strength, elastic limit, and hardness but loses its ductility  a. Cold rolling - the material is actually hot rolled to near the required size, pickled to remove the oxidized scale, then passed through chilled finishing rolls to impart a smooth surface and reduce to accurate dimension b. Cold drawing - to reduce the cross section of a rod, it is drawn cold through a die shaped as shown in the figure - It increases tensile strength but reduces ductility - In aircraft work, large quantities of tubings must be accurate in outside diameter, and thin wall must be uniform in thickness 3. Casting - steel castings are more general used in aircraft construction due to improved quality and the high-strength heat treatments - Steel castings are used for tail-wheel forks, landing gear axles, landing gear yolks, turbo supercharger buckets, and miscellaneous fittings a. Static Casting - standard method of manufacturing casting - Pattern, mold from pattern, pour the molten metal, remove when solidified b. Centrifugal casting - applying pressure to the molten metal during casting operation, obtained by whirling the mold. c. Precision casting - “lost wax” process used for intricate parts that must be held to high accuracy in size and shape at a reasonable cost INTERGRANULAR CORROSION Occurs when steel is heated as in welding. Results in embrittlement and subsequent cracking in the vicinity of weld *Polishing - rarely required nowadays - Performed on surfaces which have been sandblasted lightly to remove the scale - Series of buffing operations using cloth and cotton wheels WELDING AND SOLDERING 1. Gas Welding - oxyacetalyne welding equipment is always available for welding chrome-molybdenum steel - Type of welding flame used is important - Oxygen: metal will bubble - Too much acetylene (reducing flame): metal will absorb carbon, brittle - Neutral flame is best - Welding rod = same material and thickness of the material being welded  2. Electric Arc Welding - Gives better welds than gas welding on heavier material - Not practical to weld metals less than 1/16 in thick - Subject to carbide precipitation and intercrystalline corrosion 3. Spot Welding - AKA shot welding, holding two materials in close contact between two electrodes and passing a low voltage, high amperage current through them for a short period of time - Advantages: a. Spot-welded joints can be designed to attain 100% of the strength of the material b. Faster than riveting c. Pitch of spot welds are closer than rivets d. Drag of rivet heads is eliminated in exterior covering *heat energy generated in a weld = resistance x (current)2 x time 4. Soldering - repairs to tanks can be made by soldering a patching plate - Will not cause carbide precipitation on corrosion-resisting steels - Silver brazing alloy (50% Ag, 15.5% Cu, 16.5% Zn, 18% Cd): excellent properties for solder Nickel Alloys Nickel is the chief constituent of a number of nonferrous alloys which are used in special applications in aircraft work Inconel - Nickel-chromium alloy with good corrosion resistance and high strength at normal and elevated temperatures - Engine exhaust collectors, turbine engines, heat exchangers, jet tail pipes, exhaust manifolds - 79.5% Ni, 13% Chromium (General: 80% Ni, 14% Cr) Monel - Nickel copper alloy with high corrosion resistance, good strength, good working properties - Cannot be hardened by heat treatment, only cold working - Not used generally in aircraft - 67% Ni, 30% Cu (General: 68% Ni, 29% Cu) - Manufacture of oil coolers, stainers, rivets K Monel - Nickel-copper-aluminum alloy with high corrosion resistance, exceptionally good strength, and nonmagnetic - Structural members in the vicinity of compasses - Can be hardened by heat treatment - Gears, chains, structural members in aircraft subject to corrosive attacks - 66% Ni, 29% Cu, 2.75% Al - Instrument parts (compass) and structural parts, retractable landing gears (amphibians) Copper and its Alloys Copper, brass, and bronze have limited use in aircraft, but have special applications such as bearings and fuel/oil lines. COPPER Copper Tubing - fuel and oil lines Copper-Silicon-Bronze Tubing - fuel, oil, water, and air lines Copper wire - locking wire Beryllium Copper - bar, rod, sheet, strip, and wire BRASS (Cu + Zn) Muntz Metal - corrosion resistant in contact with salt water - Bolts and nuts Manganese Bronze - used in rod form for machined parts when used in aircraft construction Hy-Ten-Sl-Bronze - very high strength copper alloy - Bearings and bushings subject to heavy loads Naval Brass (Tobin Bronze) - Not as strong as manganese bronze, but stronger than commercial brass - Turn-buckle barrels, bolts, studs, nuts, and parts in contact with sea water Red Brass - sometimes classified as bronze due to Tin content - Fuel and oil line fittings BRONZE (Cu + Sn) Bronze are copper alloys containing tin Gun Metal - hard bronze casting material, shrinking, fair machinability - Gears and bearings (should not be annealed) Phosphor Bronze - used for manufacture of bolts, valve discs, electric contacts, and small springs Phosphor Bronze Casting Alloy - AKA leaded phosphor bronze/ leaded gun metal - Bearings, bushings, gears Aluminum Bronze - good bearing qualities and strength Aluminum Bronze Casting Alloy - great strength, resistant to corrosion, shock, and fatigue - Worm gears, valve seats, bearings, propeller hub cones Bronze Cable - extra flexible cable (7x9 strands) is manufactured for aircraft use. *Season cracking - spontaneous cracking of metals after being in service for a period of time due to internal stresses left by cold working Wrought Aluminum Alloys The general structure of the airplane as built today is aluminum alloy. It is ⅓ as heavy as steel. Source of aluminum: ore bauxite Classification of wrought alloys: 1. Strain-hardened alloys (cold working) 2. Heat-treatable alloys (heat treatment) Pure aluminum (2S) is very resistant to atmospheric corrosion, but when alloying elements are added, the corrosion resistance is decreased Aluminum Alloy Castings It is a general rule that the casting must have a 100% margin of strength when used in aircraft. Three ways of casting aluminum alloys: 1. Sand casting - most common and used for complicated shapes or when few parts are required 2. Permanent-mold casting - similar to sand casting but a metal mold is used which permits making several parts with accuracy than sand casting 3. Die casting - used when several small parts must be made and held to close tolerance Magnesium Alloys Magnesium is the lightest of the structural metals available for aircraft construction. In pure form, it is used for flashlight powder. Magnesium alloy are ideal for fairings, ducts, doors, brackets, bulkheads and partitions Source of magnesium: Magnesite, Dolomite, Carnallite, In magnesium alloys, the greatest tensile strength and elongation can be found at right angles to the direction of rolling, or across the grain. Poorest properties are parallel Magnesium Alloy Castings Magnesium alloy castings are used extensively in aircraft wheels, brake pedals, control columns, bell cranks, instrument housings, engine housings, bomb-rack supports, gearbox housings, and other brackets Microporosity may occur in sections of magnesium alloy castings caused by intergranular shrinkage to voids. Riveting is the most commonly used method of assembling magnesium-alloy structures Metal-Joining Processes Gas Welding - Engine mounts, landing gears, and entire fuselages constructed of steel tubing - Fuel and oil tanks, air scoops, cowling - Two types: a. Oxyacetylene - much hotter, produced by combustion of acetylene gas with oxygen (6700F) - Neutral flame - right amount of acetylene and oxygen, clear and white cone tip - Carbonizing/Reducing flame - excess acetylene, feathery edge on white cone, with carbon - Oxidizing flame - excess oxygen, small pointed white cone, welding brass and bronze b. Oxyhydrogen - may be neutral, reducing, or oxidizing - Neutral - well-defined cone in center of large flame - Reducing - long and ragged, no well-defined cone at center - Oxidizing - small and short cone at tip of torch Electric Arc Welding a. Metallic Arc Welding - metal electrode is used which furnishes the filler metal for the weld as it melts. - Heat at 6000F causes less buckling and warping of work than gas weld b. Carbon Arc Welding - carbon electrode is used and a filler rod is held in the arc and fused into the joint - Not much used in aircraft c. Atomic-hydrogen Welding - a stream of hydrogen is directed into an arc drawn between two tungsten electrodes d. Inert-arc Welding (heliarc) - a tungsten or carbon electrode surrounded by helium or argon gas is used e. Multiarc Welding - new and no widespread use yet. A combination of alternating and direct current together with both metallic and carbon electrodes Electric Resistance Welding a. Butt welding - generally used commercially to weld together long sheets, bars, tubes, rods, and wires. Used to weld terminals to control rods. b. Spot welding - frequently used in aircraft construction. Only welding method used for joining structural corrosion-resistant steel. c. Seam Welding - identical with spot welding, except for the use of power-driven rollers as electrodes. Continuous airtight weld can be obtained by this method BRAZING It refers to a group of metal-joining processes in which the filler metal is nonferrous metal or alloy whose melting point is higher than 1000F, but lower than that of the metals/alloys being joined. No fusion (welding) a. Brazing (Copper) b. Silver Brazing c. Aluminum Brazing SOFT SOLDERING It is never used in aircraft work for joints requiring strength. It is used for making electrical connections,and to solder the wrapped or spliced ends of flexible aircraft control cables. Common soft-soldering alloys are composed of tin and lead ADHESIVE BONDING Thermosetting synthetic adhesives are available that permit “gluing” of metal to metal, wood, glass, rubber, plastic, etc, and of these latter materials to each other. Adhesive-bonded seams are air and liquid tight (aromatic fuels excepted) and can be used for pressurized cabins and tanks. Also applied to wing leading edges, wing flaps, stabilizers, floors, doors, and smaller parts. Corrosion and its Prevention Two types of corrosion to which metals used in aircraft construction are subject:  1. Eating away/Pitting of the surface (rusting of steel and iron) - practically all metals are subject to this corrosion when oxidized in air. - Can be prevented or retarded with plating or paint 2. Intergranular or Intercrystalline Corrosion - not visible and very dangerous because it eats its way internally through the metal around the grain or crystal boundaries PLATING OPERATIONS Cadmium Plating - used more generally on aircraft parts than any other plating method. - NOT CAD PLATED DUE TO IMPRACTICALITY: Welded tubular fuselages, engine mounts, and landing gears Galvanizing (Zinc plating) - dipping them in molten zinc maintained at a temperature between 800-925F. - Seldom used on aircraft PAINTS Lacquer - a nitrocellulose lacquer is often used for the finishing coats on airplanes. It dries almost instantly Varnish - a solution and not a mixture. Resins dissolved in oil or mineral spirits. - Aircraft spar varnish: used for outside exposed surfaces of wood, metal, and doped fabric Enamel - mixture of a pigment and varnish Wood and Glue GENERAL USES OF WOOD Wood is ideal for the construction of the first experimental model of an airplane. Classification of Trees and Woods: 1. Conifers - softwoods, needleleaf, evergreen 2. Hardwoods - deciduous, broadleaf, dicotyledons, non coniferous STRUCTURE OF WOOD Trunk a. Pith - soft central cone b. Heartwood - concentric rings surrounding the pith c. Sapwood - surrounds the heartwood d. Bark - husk or outer cover SAWING WOOD Two ways to saw wood: 1. Along any of the radii of the annual rings, which will expose the vertical grain surface 2. Tangent to the annual rings (Plain-sawed/ Flat-grain surface) *a modified sawing because there’s too much waste in 1 - Quarter-sawing - more expensive than tangential STRENGTH OF WOOD Compression wood should never be used in aircraft parts. Decay in any form is not permissible in aircraft wood. It reduced the shock-resisting qualities of wood AIRCRAFT WOODS AND THEIR USES 1. Ash, white - door jambs and sills 2. Basswood - used for webs and plywood cores 3. Beech - takes very fine polish, not durable when exposed 4. Birch (Sweet and Yellow) - used to protect other woods, best propeller wood 5. Cherry, Black - sometimes used in propellers 6. Elm, Cork - tough, elastic, and difficult to split 7. Gum, Red - manufacture of plywood for semihard faces 8. Hickory - heaviest and hardest, seldom used in aircraft 9. Mahogany, African (khaya senegalensis) - works and glues well, durable, used for semihard plywood faces 10. Mahogany, True (honduran mahogany) - strong, durable, but brittle. Manufacture of propellers and semihard faces 11. Maple, Sugar - used for hard faces in plywood, occasionally for propellers 12. Oak - used for propeller construction (seaplanes) or bent parts in aircraft. 13. Poplar, yellow - substitute for spruce, used as a core for plywood  14. Walnut, Black - manufacture of propellers, next best propeller wood after birch 15. Cedar, Port orford - sub to spruce, semihard faces of plywood 16. Cypress, Bald - no application to aircraft 17. Douglas Fir - moderately heavy and strong, but splits easily and difficult to work 18. Pine, white - general purpose wood commercially 19. Spruce - standard structural wood for aircraft, light, soft, and moderate strength (good strength/weight ratio) Fabrics and Dope A handful of aircraft still use fabric for covering wings, fuselage, and control surfaces. A mercerized cotton cloth is universally used for fabric covering of wings, fuselage, and tail surfaces Surface tape - finishing tape that is doped over each rib or seam to cover the stitching Reinforcing tape - used over fabric and under the rib stitching to prevent the stitching cord from cutting through the fabric Rib lacing cord - used to sew the fabric to the ribs Wing Covering - wings may be covered with fabric by envelope, blanket, or combination method Fuselage covering - fuselages are covered by either sleeve or blanket method DOPE AND DOPING Dope protects the fabric from deterioration by weather or sunlight, and when polished, reduced skin friction Plastics Classification of plastics: 1. Synthetic Resin Plastics - largest and thermoplastics (acrylic, vinyl, styrene) 2. Natural Resins - used in thermoplastic type molding compounds 3. Cellulose - widely used and known 4. Protein plastics - very hygroscopic and will warp 5. Thermoplastics - will repeatedly soften when heated  6. Thermosetting plastics - chemically changed by the first application, therefore infusible The use of thermoplastic sheeting materials for cabin enclosure is of course universal Transparent Materials Used in aircraft for windshields and general cabin glazing. Tempered glass - exceptionally strong glass used for large windshields Laminated plate glass - 3/15 and ¼ inch glass are used for aircraft windshields Rubber and Synthetic Rubber Synthetic Rubbers a. Buna S - most nearly like natural rubber - Used for tires and tubes b. Buna N - used for oil and gasoline hose, tank linings, hydraulic accumulator bags, gaskets, and seals c. Neoprene - oil-resistant hose, carburetor diaphragms, gaskets, caulking d. Thiokol - oil hose, tank linings for aromatic aviation gasolines, paint spray hose, gasket, and seals. Selection of Materials Specific material applications: Propeller blades - made from aluminum alloy, wood, steel, magnesium, and pressed wood - 25ST aluminum alloy forgings: commonly used for high quality blades - Wood: birch, oak, walnut, mahogany Propeller hubs - forgings of chrome-vanadium steel or Cr-Ni-Mo steel Cowl ring - aluminum alloy Engine mount - chrome-molybdenum and mild-carbon steel tubing Firewall - sheet of aluminum alloy 0.020 inch thick, corrosion-resisting steel, inconel, and terneplate Landing gears - welded chrome-molybdenum tubing, some are aluminum alloy forgings Fuselage - either welded steel (Cr-Mo) tubing or aluminum alloy monocoque construction Wing flaps - aluminum alloy sheet backed with stiffeners Windshields - non shatterable glass (Min. 3/16 in., preferably ¼ in for clear vision) Bolts - AN standard bolts made from nickel steel (2330) are used for all structural connections Rivets - 17ST/A17S (general) and 24ST (seldom) aluminum alloy rivets. Springs - high-carbon steel