FUNDAMENTALS AND PROCEDURES OF AIRFRAME

FM 1-563AIRFOIL SPEEDAs the speed of the airfoil moving through the airincreases, the difference in pressure between theupper and lower relative wind layers also increases.Thus, lift increases as speed increases but not indirect proportion to it.AIR DENSITYAir density at 18,000 feet is only about half its densityat sea level. Because air becomes thinner as altitudeincreases, any aircraft flying at high altitude mustincrease speed to maintain level flight. Aircraft flightis also affected by temperature because air density isreduced as air temperature rises.modifîcations were developed—semimonocoque,reinforced-shell construction, box beam, etc. Allpresent-day Army helicopters and airplanes use oneor the other of these classes of construction. Thesemimonocoque type (Figure 1-9) has vertical reinforcements with the skin being reinforced by longitudinalmembers called stringers. All of these variations havethe skin reinforced by a complete framework of structural members (Figure 1-10).FORMERBULKHEADAIRFOIL SHAPEThe shape of an airfoil affects lift in many ways. Upto a certain point the greater the camber or curvature, the greater the lift. An airfoil with a smoothsurface has more lift in relation to drag than one witha rough surface. A rough surface produces turbulence, which reduces lift and increases drag.SKINFigure 1-8. Monocoque-type constructionSTRUCTURAL MEMBERSSection II. Main Structural UnitsBulkheads, Frame Assemblies, and FormersAn aircraft is constructed of many parts that areeither riveted, bolted, screwed, bonded, or weldedtogether. Because these parts make up the structureof the aircraft, they are called structured members.Many of them can be grouped into several units orassemblies; these combined units are often called theaircraft structure or the airframe. Repair and maintenance of structural members is called airframemaintenance or repair. To deal with the problems ofairframe repair intelligently and effectively, airframerepairers must have a clear understanding of thelocation, construction, and purpose of the aircraft’svarious structural units.Bulkheads, frame assemblies, and formers (Figure 1-11)serve a dual purpose. They give cross-sectional shapeto the fuselage, and they add rigidity and strength to thestructure. Their shape and size vary considerablydepending on their function and position in thefuselage. Formers are the lightest. They are usedprimarily for fillings or skin attachments between thelarger members. Frame assemblies are the mostnumerous and important members in the fuselage.When frame assemblies are used to separate onesection of the fuselage from another, they are calledbulkheads. Heavier than formers, they are equippedwith doors or other means of access.STRUCTUREThe monocoque type of construction (Figure 1-8) islike a shell in which the skin carries the major stressesand functions as the main part of the airframe. Thestrength required in construction depends on thepower rating, speed, maneuverability, and design ofthe aircraft. In full-monocoque-type construction,formers, frame assemblies, and bulkheads provideshape, but the skin carries the primary stresses.However, this type is seldom used because of itslimited load-carrying capacity. To overcome thestrength-versus-weight problem inherent in fullmonocoque construction, additional classes orPFORMERSKINSTRINGERFigure 1-9. Semimonocoque-typeconstruction1-3

FM 1-563BEAM (BOX)SEMIMONOCOOUEFigure 1-10. Variations of semimonocoque KHEADS0oooIFigure 1-11. Airframe structural members1-4

FM 1-563Stringers and LongeronsStringers and longerons are the main lengthwisemembers of the fuselage structure. The longeron isfairly heavy. Severed longerons usually run the fulllength of the fuselage. They hold the bulkheads andformers which, in turn, hold the stringers. TheLONGERON(TYPICAL EXTRUSION)stringers are smaller, lighter, and weaker than thelongerons. They have some rigidity and are the structural members that “string” or join together the skin andvertical structural members. Longerons, bulkheads,formers, and stringers are all joined together to form arigid fuselage framework (Figure 1-12).HAT SECTION(TYPICAL FORMED)STRINGER(TYPICAL FORMED)FRAMESTRINGERSTRINGERFigure 1-12. Rigid fuselage framework1-5

FM 1-563PRINCIPAL AIRFRAME PARTS (ROTARYWING)FuselageThe fuselage of a typical helicopter has two mainsections, the cabin and the tail cone or tail boom(Figure 1-13). The cabin section contains passengeror cargo compartments with space for crew, passengers, cargo, fuel, oil tanks, controls, and powerplant. Multiengine helicopter are an exception; theirpower plants are mounted internally or externally inseparate engine nacelles (see discussion of nacellesbelow). The tail cone section and landing gear areattached to the cabin section in such a manner thatthey can be inspected, removed, repaired, andreplaced when necessary. The cabin is strongenough at the points of attachment to withstand theforces involved in flying and landing. Size and construction of cabin compartments vary with differenttypes of helicopters. The tail cone section attachesto the cabin and supports the tail rotor, tail rotordrive shafts, and stabilizer (Figure 1-14). CABIN SECTIONMain and Tail Rotor BladesThe rotor blades are rotary airfoil surfaces. Mainrotor blades provide the lift that the helicopter needsfor flight. Thil rotor blades compensate for thetorque created by the rotation of the main rotorblades. A typical rotor blade has a leading edge spar,a trailing edge, balance plates, root and tip fairings,and a cuff attachment point. The leading edge sparis the main supporting member. The trailing edgeconsists of ribs and pockets attached to the leadingedge spar with adhesive bonding. Figures 1-15 and1-16 illustrate typical construction features of mainand tail rotor blades.PRINCIPAL AIRFRAME PARTS (FIXED-WING)FuselageThe fuselage is the main structural unit of anyairplane. Other structural units are directly orindirectly attached to it (Figure 1-17). The outlineand general design of the fuselage are much the samein all types of airplanes. Various designs have been »oTAIL CONE SECTION (BOOM)Figure 1-13. Cabin and tail cone sectionsTAIL ROTOR BLADEFigure 1-14. Stabilizer1-6developed that successfully meet the many differentrequirements for mission performance. The bestfeatures of these designs are incorporated in thelatest airplanes. Airplanes differ in the size and construction of the different compartments. On singleengine military airplanes, the fuselage houses thepower plant, the personnel, and the cargo. On mostmultiengine airplanes, power plants are housed incompartments called nacelles, which are either builtinto the wings or suspended in pods from the wingsor fuselage. Fuselage design varies with the manufacturer and the requirements of the service for which itis intended. The basic internal structures ofairplanes are the same as for helicopters.

FM 1-563NOSE CAPCAPTIP COVER ASSEMBLYTIE-DOWN FITTING(ON BOTTOM SIDE)TUBESTIP ENDFAIRINGTUBEOR SOLIDBALANCEWEIGHTBASIC FINGER DOUBLERTRIMTABFORWARD TUBEBALANCE TUBEAFT TUBEROOT ENDFAIRINGSOCKETCORESPARTRAILING EDGETRAILING EDGE DOUBLERFigure 1-15. Main rotor blade construction (basic) (metal spar)1-7

FM 1-563ABRASION STRIPPOLYURETHANEABRASION STRIPPOLYURETHANEABRASION STRIPNICKELDEICINGBLANKETI!C-E COVERPHENOLICSPARSKINSKINSKINBRACKETTITANIUMSHADED AREA POLYURETHANEAND NICKEL EROSION/ABRASION STRIPSALUMINUMHONEYCOMBCORESPAR BOLTS,TYPICALCOVERsct- T ’f *ar*SKIN, REINFORCEDPLASTIC (COMPOSITE)FILLERNOMEXHONEYCOMBCOREFigure 1-16. Tail rotor blade construction1-8c : »

FM 1-5631.2.3.4.FUSELAGEEMPENNAGENACELLESWINGSFigure 1-17. Principal airframe partsof whether the nacelle houses a piston-typereciprocating engine, a jet engine, landing gear,cargo, or personnel. The nacelle’s construction,whether of the monocoque or semimonocoque type,must be strong; its weight must be kept to a minimum;and its exterior must be aerodynamically suited to itsposition on the airplane. As in the fuselage, airframemaintenance on the nacelle involves the skin,formers, bulkheads, rings, and longerons.NacellesNacelles (Figure 1-18) are enclosed, streamlinedstructures used on multiengine aircraft primarily tohouse the engines. Nacelle design, like that offuselages, depends partly on the manufacturer andpartly on intended use. On twin-engine airplanesnacelles also house the main landing gear and relatedequipment. Fundamentals of nacelle repair are essentially the same as for fuselage repair, regardlessFORMERSSTRINGERSX\LONGERONBULKHEADFigure 1-18. Multiengine-type nacelle with cross section1-9

FM 1-563WingsThe wings of an airplane are airfoils designed to givelifting force when the airplane moves forward rapidlythrough the air. The wing design of any airplanedepends on a number of factors, such as the plane’ssize, weight, and intended use, desired in-flight andlanding speeds, and rate of climb. The wing tip maybe square, rounded, or even pointed. Both leadingand trailing edges may be straight or curved, or oneedge may be straight and the other curved. Also, oneor both edges may be tapered so that the wing isnarrower at the tip than at the root where it joins thefuselage. Many types of modern airplanes haveswept-back wings. The wings of military airplanesare usually of cantilever construction; that is, they arebuilt so that they need no external bracing. With fewexceptions wings of this design are of the stressedskin type. This means that the skin is part of the wingstructure and carries part of the wing stresses.Types of Wing DesignIn general, wing construction is based on one of threefundamental designs—monospar, multispar, and boxbeam. Various slight modifications of these designsmay be adopted by different manufacturers.Monospar. The monospar wing (Figure 1-19) usesonly one main longitudinal member in its construction. Ribs or bulkheads provide the necessary contour or shape to the airfoil. Wings of the strictmonospar type are not in common use. However,this type of wing design is often modified by addingfalse spars or light shear webs along the trailing edgeas support for the control surfaces.FRONT SPAAREAR SPARAUXILIARY SPARRIBSFigure 1-20. Multispar wingBox beam. The box beam wing (Figure 1-21) usestwo main longitudinal members with connectingbulkheads to provide additional strength and givecontour to the wing. A corrugated sheet may beplaced between the bulkheads and the smooth outerskin to enable the wing to carry tension and compression loads better. In some cases heavy longitudinalstiffeners axe substituted for the corrugated sheets,or a combination of corugated sheets on the uppersurface of the wing and stiffeners on the lower surfaceis used.aooFigure 1-21. Box beam wingInternal ConstructionThe main structural components of a wing are thespars, the ribs or bulkheads, and the stringers orstiffeners (Figure 1-22). These structural parts areriveted or bonded together.SPARRIBFLAPFRONT SPARREAR SPARFigure 1-19. Monospar wingMultispar. The multispar wing (Figure 1-20) usesmore than one main longitudinal member in its construction. Ribs or bulkheads are often included togive contour to the wing. This type of construction,or a modification of it, is used in lighter types ofairplane.1-10AUXILIARY SPARFigure 1-22. Internal wing construction

FM 1-563Spars and stiffeners. Spars are the principal structural members of the wing. They correspond to thelongerons of the fuselage. Spars run parallel to thelateral axis or toward the wing tip. They are usuallyattached to the fuselage by wing fittings, plainbeams, or part of a truss system. The I-beam typeof spar construction consists of a web and capstrips (Figure 1-23). The web is the portion of theI-beam that is between the cap strips. Cap strips areextrusions, formed angles, or milled sections to whichthe web is attached. They carry the loads caused bythe wing’s bending and also provide a foundation forattaching the skin. Stiffeners give the spar structureadditional strength. They may be either beadspressed into the web or extrusions of formed anglesriveted vertically or diagonally to the web.with lightening holes and beads formed between theholes (Figure 1-25). Lightening holes lessen ribweight without decreasing strength. Lightening holeareas me made rigid by flanging the edges of theholes. Beads stiffen the web portion of the rib.-g; ; o ; » i 5!FORMED RIBREINFORCED RIBCAP STRIPTRUSS RIBFigure 1-24. Types of ribsSTIFFENERWEBSPARCAP STRIPFigure 1-23. Spar designRibs. Ribs are the crosspieces that make up theframework of a wing. They run from the leading edgetoward the trailing edge (front to rear). Ribs give thewing its contour or shape and transfer the load fromthe skin to the spars. Ribs are also used in ailerons,elevators, fins, and stabilizers. There me threegeneral types of rib construction: reinforced, truss,and formed (Figure 1-24). Reinforced and truss ribsare relatively heavy compared to formed ribs and arelocated only at points of greatest stress. Formed ribsare located in many places throughout the wing. Theconstruction of reinforced ribs is similar to that ofspars; it consists of upper and lower cap strips joinedtogether by a web plate. The web is reinforcedbetween the cap strips by vertical and diagonalangles. Reinforced ribs me much more widely usedthan truss ribs. The latter consist of cap strips reinforced only by vertical and diagonal cross members.Formed ribs me made of reformed sheet metal andme very lightweight. The bent portion of a formedrib is known as the flange; the vertical portion iscalled the web. The web is generally constructedFigure 1-25. Lightening holesEmpennageThe tail section of an airplane is called the empennage (Figures 1-26,1-27). It includes the aft end ofthe fuselage or booms, rudders, elevators, stabilizers,and trim tabs. The empennage includes theairplane’s stabilizing units, which comprise the vertical and horizontal airfoils located at the rem of thefuselage. The fixed vertical surface is called the verticalstabilizer or fin; the fixed horizontal surface is called thehorizontal stabilizer. The vertical stabilizer or fin servesto maintain the airplane’s directional stability in flightabout its vertical axis. On single-engine, propellerdriven airplanes the vertical fin is sometimes offset inrelation to the centerline of the fuselage. Thisprovides directional stability by compensating for thetorque or twist caused by the engine propeller. Thevertical fin is also the base or anchorage for attachingthe rudder. The horizontal stabilizer providesstability about the airplane’s lateral axis and alsoserves as a base or anchorage for attaching the1-11

FM 1-563elevators. As with the wings, there are many variations in size, shape, and number of component partsin the empennage and also in its placement in relationto the fuselage. In many respects the empennage hasthe same construction features as the wings. It isusually all-metal with a cantilever design. Bothmonospar and multispar construction are common.Ribs give shape to the cross section. Fairing streamlines the angles formed between these surfaces andthe fuselage.Flight Control SurfacesThese surfaces may be divided into three groups:primary, secondary, and auxiliary. Primary flightcontrol surfaces are directly attached to the empennage and are movable. Secondary surfaces arehinged on other movable surfaces and are themselvesmovable. Auxiliary surfaces are attached to nonmovable surfaces of the aircraft. Primary controlsurfaces include ailerons, elevators, and the rudder. They control the airplane about all threeaxes: lateral, longitudinal, and vertical.Ailerons are attached to the trailing edge of boththe right and left sections of the airplane’s twowings. Elevators are attached to the trailing edge ofthe horizontal stabilizer, and the rudder is attachedto the trailing edge of the vertical stabilizer.Secondary control surfaces include trim tabs andspring tabs. Their purpose is to trim the airplane inflight or reduce the force required to activate theprimary control surfaces. Trim tabs and spring tabsare small airfoils recessed into the trailing edges ofthe primary control surfaces.Figure 1-26. EmpennageTIP SKINSVERTICALSTABILIZERRUDDERBALANCEWEIGHT -ELEVATORTIP RIBHORIZONTALSTABILIZERTRIMTABMAIN SPARAUXILIARY SPARRIBSREARSPARDORSAL FINTRIM TABCHANNELRIBSAUXILIARY SPARDORSAL RIBS' FRONTSPARRIBSTIP SKINSLEADINGEDGE SKINBALANCE—— WEIGHTFigure 1-27. Empennage construction1-12

FM 1-563Auxiliary control surfaces include wing flaps, speedbrakes, slats, and spoilers. Their purpose is to reducelanding speed or shorten the length of the landing rolla single spar member or torque tube. Ribs are fittedto the spar at the leading and trailing edges and arejoined together with a metal strip. In most cases theribs are formed from flat sheet stock. They are notsolid but contain punched lightening holes, whichsaves weight without reducing strength.and change the airplane’s speed in flight.The construction of flight control surfaces is similarto that of the wings. They are usually made of analuminum alloy. They form a structure built aroundAILERON RIBSPARP6LIGHTENING HOLEAILERONORUDDERSOAILERON TRIM TABFLAPELEVATORSPEED.BRAKE. FLAPiAILERONOFigure 1-28. Flight control surfaces1-13

FM 1-563CHAPTER 2AIRCRAFT STRUCTURAL METALSArmy aircraft are constructed primarily of metalalloys selected for their strength-to-weight ratio. Toincrease their strength, they are heat-treated andformed to various shapes. To select materials forArmy aircraft repairs, airframe repairers must understand the properties and characteristics of metalsand alloys and metalworking processes. For additional information on metals, refer to TM 43-0106.Section I. Properties and CharacteristicsPROPERTIES OF METALSMetallurgists have been working for over 50 years toimprove metals used in aircraft construction andrepair. Each type of metal or alloy has certainproperties and characteristics that make itdesirable for a particular use; however, it may haveother undesirable qualities. The metallurgist’s jobis to build up the desirable qualities and tone downthe undesirable ones. This is done by alloying (combining) metals and by various heat-treating processes.Airframe

Airframe repairers repair, fabricate, onand modify aircraft structures. They need to know the names, locations, and purposes of all structural members and parts as well as methods weightand techniques for repairing them. Repairers should also know basic flight theory and aero