GENERAL The airframe of a fixed-wing aircraft is generally considered to consist of five principal units, the fuselage, wings, stabilizers, fright control surfaces, and landing gear. Helicopter airframes consist of the fuselage, main rotor and related gearbox, tail rotor (on helicopters with a single main rotor), and the landing gear.
The airframe components are constructed from a wide variety of materials and are joined by rivets, bolts, screws, and welding or adhesives. The aircraft components are composed of various parts called structural members (i.e., stringers, longerons, ribs, bulkheads, etc.). Aircraft structural members are designed to carry a load or to resist stress. A single member of the structure may be subjected to a combination of stresses. In most cases the structural members are designed to carry end loads rather than side loads: that is, to be subjected to tension or compression rather than bending.
Strength may be the principal requirement in certain structures, while others need entirely different qualities. For example, cowling, fairing, and similar parts usually are not required to carry the stresses imposed by flight or the landing loads.
However, these parts must have such properties as neat appearance and streamlined shapes.
MAJOR STRUCTURAL STRESSES In designing an aircraft, every square inch of wing and fuselage, every rib, spar, and even each metal fitting must be considered in relation to the physical characteristics of the metal of which it is made. Every part of the aircraft must be planned to carry the load to be imposed upon it. The determination of such loads is called stress analysis. Although planning the design is not the function of the aviation mechanic, it is, nevertheless, important that he understand and appreciate the stresses involved in order to avoid changes in the original design through improper repairs.
There are five major stresses to which all aircraft are subjected (figure 1-1) :
CHAPTER 1
AIRCRAFT STRUCTURES
(1) Tension.
(2) Compression.
(3) Torsion.
(4) Shear.
(5) Bending.
The term "streas" is often used interchangeably with the word "strain." Stress is an internal force of a substance which opposes or resiste deformation. Strain is the deformation of a material or substance. Stress, the internal force, can cause Tension (figure l-la) is the stress that resists a force that tenda to pull apart. The engine pulls the aircraft forward, but air resistance tries to hold it back. The result is tension, which tries to stretch the aircraft. The tensile strength of a material is measured in p.s.i. (pounds per square inch) and is calculated by dividing the load (in pounds) required to pull the material apart by its cross-sectional area (in square inches).
Compression (figure 1-1b) is the stress that resists a crushing force. The compressive strength of a material is also measured in p.s.i. Compression is the stress that tends to shorten or squeeze aircraft Torsion is the stress that produces twisting (figure 1-1c). While moving the aircraft forward, the engine also tends to twist it to one side, but other aircraft components hold it on course. Thus, torsion is created. The torsional strength of a material is ite resistance to twisting or torque.
Shear is the stress that resists the force tending to cause one layer of a material to slide over an adjacent layer. Two riveted plates in tension (figure 1-1d) subject the rivets to a shearing force.
Usually, the shearing strength of a material is either equal to or less than its tensile or compressive strength. Aircraft parts, especially screws, bolts, and rivets, are often subject to a shearing force.
Bending stress is a combination of compression and tension. The rod in figure I-le has been shortened (compressed) on the inside of the bend and stretched on the outside of the bend.
(a) Tension
(b) Compression
(c) Torsional (d) Shear Tension outside of bond Bent structural memb Shear along imaginary line (dotted) Compression inside of bend
(e) Bending (the combiuation stress) FIGURE 1-1. Five stresses acting on an aircraft.
FIXED-WING AIRCRAFT The principal components of a single-engine, propeller-driven aircraft are shown in figure 1-2.
Figure 1-3 illustrates the structural components of a typical turbine powered aircraft. One wing and the empennage assemblies are shown exploded into the many components which, when assembled, form major structural units.
FUSELAGE The fuselage is the main structure or body of the aircraft. It provides space for cargo, controls, accessories, passengers, and other equipment. In singleengine aircraft, it also houses the powerplant. In multi-engine aircraft the engines may either be in the fuselage, attached to the fuselage, or suspended from the wing structure. They vary principally in size and arrangement of the different compartments.
There are two general types of fuselage construction, the truss type, and the monocoque type. A truss is a rigid framework made up of members such as beams, struts, and bars to resist deformation by applied loads. The truss-framed fuselage is generally covered with fabric.
Truss Type The truss type fuselage frame (figure 1-4) is usually constructed of steel tubing welded together in such a manner that all members of the truss can carry both tension and compression loads. In some 2
Vertical stabilizer Horizontal Wing stabilizer -Rudder Right aileron Elevator Empennage Nacelle Fuselage Left aileron Landing gear
- Propeller Wing FIGURE 1-2. Aircraft structural componenta, aircraft, principally the light, single-engine models, truss fuselage frames are constructed of aluminum alloy and may be riveted or bolted into one piece, with cross-bracing achieved by using solid rods or tubes.
Monocoque Type The monocoque (single shell) fuselage relies largely on the strength of the skin or covering to carry the primary stresses. The design may be divided into three classes: (1) Monocoque, (2) semimonocoque, or (3) reinforced shell. The true monocoque construction (figure 1-5) uses formers, frame assemblies, and bulkheads to give shape to the fuselage, but the skin carries the primary streases. Since no bracing members are present, the skin must be strong enough to keep the fuselage rigid. Thus, the biggest problem involved in monocoque construction is maintaining enough strength while keeping the weight within allowable limits.
To overcome the strength/weight problem of monocoque construction, a modification called semimonocoque construction (figure 1-6) was developed.
In addition to formers, frame assemblies, and bulkheads, the semimonocoque construction has the skin reinforced by longitudinal members. The reinforced shell has the skin reinforced by a complete framework of structural members. Different portions of the same fuselage may belong to any one of the three classes, but most aircraft are considered to be of semimonocoque type construction.
Semimonocoque Type The semimonocoque fuselage is constructed primarily of the alloys of aluminum and magnesium, although steel and titanium are found in areas of high temperatures. Primary bending loads are taken by the longerons, which usually extend across several points of support. The longerons are supplemented by other longitudinal members, called stringers. Stringers are more numerous and lighter in weight than longerons. The vertical structural members are referred to as bulkheads, frames, and formers. The heaviest of these vertical members are located at intervals to carry concentrated loads and at points where fittings are used to attach other units, such as the wings, powerplants, and stabiliz-
Vertical stabilizer Rudder Fuselage section Tail cone Elevator Horizontal stabilizer /Ventral fin Wing flap Inboard aileron Spoiler
•Outboard aileron Nose Wing tip Nose gear Main landing gear Pylon Wing leading Wing section Wing center section or. strut edge Powerplant FreuRE 1-3. Typical structural components of a turbine powered aircraft.
Longeron (tubular steel) rons and serve as fill-ins. They have some rigidity, Diagonal web members but are chiefly used for giving shape and for attach- (tubular steel) ment of the skin. The strong, heavy longerons hold the bulkheads and formers, and these, in turn, hold the stringers. All of these joined together form a rigid fuselage framework.
There is often little difference between some rings, frames, and formers. One manufacturer may call a brace a former, whereas another may call the same type of brace a ring or frame. Manufacturers' Vertical instructions and specifications for a specific aircraft members web are the best guides.
Stringers and longerons prevent tension and com- FIGURE 1-4. Warren truss of welded tubular steel, pression from bending the fuselage. Stringers are ers. Figure 1-7 shows one form of the semimonocoque design now in use.
The stringers are smaller and lighter than longeusually of a one-piece aluminum alloy construction, and are manufactured in a variety of shapes by casting, extrusion, or forming. Longerons, like stringers, are usually made of aluminum alloy; how.
Pormer Longeron- Gussets Bulkhead Frame FICURE 1-5. Monocoque construction.
FIGURE 1-7. Fuselage structural members.
Skin Formers Stringers Bulkhead FIGURE 1-6. Semimonocoque construction.
ever, they may be of either a one-piece or a built-up construction.
By themselves, the structural members discussed do not give strength to a fuselage. They must first be joined together by such connective devices as gussets, rivets, nuts and bolts, or metal screws. A gusset (figure 1-7) is a type of connecting bracket.
The bracing between longerons is often referred to as web members. They may be installed vertically or diagonally.
The metal skin or covering is riveted to the longerons, bulkheads, and other structural members and carries part of the load. The fuselage skin thickness will vary with the load carried and the stresses sustained at a particular location.
There are a number of advantages in the use of the semimonocoque fuselage. The bulkheads, frames, stringers, and longerons facilitate the design and construction of a streamlined fuselage, and add to the strength and rigidity of the structure.
The main advantage, however, lies in the fact that it does not depend on a few members for strength and rigidity. This means that a semimonocoque fuselage, because of its stressed-skin construction, may withstand considerable damage and still be strong enough to hold together.
Fuselages are generally constructed in two or more sections. On small aircraft, they are generally made in two or three sections, while larger aircraft may be made up of as many as six sections.
Quick access to the accessories and other equipment carried in the fuselage is provided for by numerous access doors, inspection plates, landing wheel wells, and other openings. Servicing diagrams showing the arrangement of equipment and location of accees doors are supplied by the manufacturer in the aircraft maintenance manual.
Location Numbering Systems There are various numbering systems in use to facilitate location of specific wing frames, fuselage bulkheads, or any other structural members on an aircraft. Most manufacturers use some system of station marking; for example, the nose of the aircraft may be designated zero station, and all other stations are located at measured distances in inches behind the zero station. Thus, when a blueprint reads "fuselage frame station 137," that particular frame station can be located 137 in. behind the nose of the aircraft. A typical station diagram is shown in figure 1-8.
To locate structures to the right or left of the center line of an aircraft, many manufacturers consider the center line as a zero station for structural member location to its right or left. With such a 5
008 Fuselage WL = Waterline stations FICURE 1-8. Fuselage stations.
system the stabilizer frames can be designated as being so many inches right or left of the aircraft center line.
The applicable manufacturer's numbering system and abbreviated designations or symbols should always be reviewed before attempting to locate a structural member. The following list includes location designations typical of those used by many manufacturers.
(1) Fuselage stations (Fus, Sta, or 1.S.) are numbered in inches from a reference or zero point known as the reference datum.
The reference datum is an imaginary vertical plane at or near the nose of the aircraft from which all horizontal distances are measured. The distance to a given point is measured in inches parallel to a center line extending throngh the aircraft from the nose through the center of the tail cone. Some manufacturers may call the fuselage station a body station, abbreviated B.S.
(2) Buttock line or butt line (B.L.) is a width measurement left or right of, and parallel to, the vertical center line.
(3) Water line (W.L.) is the measurement of height in inches perpendicular from a horizontal plane located a fixed number of inches below the bottom of the aircraft fuselage.
(4) Aileron station (A.S.) is measured outboard from, and parallel to, the inboard edge of the aileron, perpendicular to the rear beam of the wing.
(5) Flap station (F.S.) is measured perpendicular to the rear beam of the wing and parallel to, and outboard from, the inboard edge of the flap.
(6) Nacelle station (N.C. or Nac. Sta.) is measured either forward of or behind the front spar of the wing and perpendicular to a designated water line.
In addition to the location stations listed above, other measurements are used, especially on large aircraft. Thus, there may be horizontal stabilizer stations (H.S.S.), vertical stabilizer stations (V.S.S.) or powerplant stations (P.P.S.). In every case the manufacturer's terminology and station location system should be consulted before locating a point on a particular aircraft.
WING STRUCTURE The wings of an aircraft are surfaces which are designed to produce lift when moved rapidly through the air. The particular design for any given aircraft depends on a number of factors, such as size, weight, use of the aircraft, desired speed in flight and at landing, and desired rate of climb. The wings of a fixed-wing aircraft are designated left and right, corresponding to the left and right sides of the operator when seated in the cockpit.
The wings of some aircraft are of cantilever design; that is, they are built so that no external bracing is needed. The skin is part of the wing structure and carries part of the wing stresses.
Other aircraft wings use external bracings (struts, wires, etc.) to assist in supporting the wing and carrying the aerodynamic and landing loads. Both aluminum alloy and magnesium alloy are used in wing construction. The internal structure is made up of spars and stringers running spanwise, and
ribs and formers running chordwise (leading edge to trailing edge). The spars are the principal structural members of the wing. The skin is attached to the internal members and may carry part of the wing stresses. During flight, applied loads which are imposed on the wing structure are primarily on the skin. From the skin they are transmitted to the ribs and from the ribs to the spars. The spars support all distributed loads as well as concentrated weights, such as fuselage, landing gear, and, on multi-engine aircraft, the nacelles or pylons.
The wing, like the fuselage, may be constructed in sections. One commonly used type is made up of a center section with outer panels and wing tips.
Another arrangement may have wing stubs as an integral part of the fuselage in place of the center section.
Inspection openings and access doors are provided, usually on the lower surfaces of the wing.
Drain holes are also placed in the lower surface to provide for drainage of accumulated moisture or fluids. On some aircraft built-in walkways are provided on the areas where it is safe to walk or step.
On some aircraft jacking points are provided on the underside of each wing.
Various points on the wing are located by station number. Wing station 0 (zero) is located at the center line of the fuselage, and all wing stations are measured outboard from that point, in inches.
In general, wing construction is based on one of three fundamental designs: (1) Monospar, (2) multi-spar; or (3) box beam. Modifications of these basic designs may be adopted by various manufacturers.
The monospar wing incorporates only one main longitudinal member in its construction. Ribs or bulkheads supply the necessary contour or shape to the airfoil. Although the strict monospar wing is not common, this type of design, modified by the addition of false spars or light shear webs along the trailing edge as support for the control surfaces, is sometimes used.
The multi-spar wing incorporates more than one main longitudinal member in its construction. To give the wing contour, ribs or bulkheads are often included.
The box beam type of wing construction uses two main longitudinal members with connecting bulkheads to furnish additional strength and to give contour to the wing. A corrugated sheet may be Tapered leading edge, Tapered leading straight trailing edge and trailing edges Delta wing Sweptback Straight leading and Straight leading edge, wings trailing edges tapered trailing edge FIcURE 1-9. Typical wing leading and trailing edge ahapes.
placed between the bulkheads and the smooth outer skin so that the wing can better carry tension and compression loads. In some cases, heavy longitudinal stiffeners are substituted for the corrugated sheets. A combination of corrugated sheets on the upper surface of the wing and stiffeners on the lower surface is sometimes used.
Wing Configurations Depending on the desired flight characteristics, wings are built in many shapes and sizes. Figure 1-9 shows a number of typical wing leading and trailing edge shapes.
In addition to the particular configuration of the leading and trailing edges, wings are also designed to provide certain desirable flight characteristics, such as greater lift, balance, or stability. Figure 1-10 shows some common wing forms.
Features of the wing will cause other variations in its design. The wing tip may be square, rounded, or even pointed. Both the leading edge and the trailing edge of the wing may be straight or curved, or one edge may be straight and the other curved.
In addition, one or both edges may be tapered so that the wing is narrower at the tip than at the root where it joins the fuselage. Many types of modern aircraft employ sweptback wings (figure 1-9).
Wing Spars The main structural parts of a wing are the spars, the ribs or bulkheads, and the stringers or stiffeners, as shown in figure 1-11.
Spars are the principal structural members of the wing. They correspond to the longerons of the fuselage. They run parallel to the lateral axis, or toward the tip of the wing, and are usually attached to the fuselage by wing fittings, plain beams, or a truss system.
Low wing Dihedral High wing Mid wing Gull wing Inverted gull FIGURE 1-10. Common wing forms.
Wooden spars can be generally classified into four different types by their cross sectional configuration. As shown in figure 1-12, they may be partly hollow, in the shape of a box, solid or laminated, rectangular in shape, or in the form of an I-beam.
Spars may be made of metal or wood depending on the design criteria of a specific aircraft. Most aircraft recently manufactured use spars of solid extruded aluminum or short aluminum extrusions riveted together to form a spar.
The shape of most wooden spars is usually similar to one of the shapes shown in figure 1-12. The rectangular form, figure 1-12A, can be either solid or laminated. Figure 1-12B is an I-beam spar that has been externally routed on both sides to reduce weight while retaining adequate strength. A box spar, figure 1-12C, is built up from plywood and solid spruce. The I-beam spar, figure 1-12D, may be built up of wood or manufac- Stringer
- Leading edge skin Leading edge ribs Spars - FICURE 1-11. Internal wing construction,
B D FICURE 1-12. Typical spar cross sectional configurationa.
tured by an aluminum extrusion process. The 1- a foundation for attaching the skin. An example of beam construction for a spar usually consists of a a hollow or internally routed spar is represented in web (a deep wall plate) and cap strips, which are figure 1-12E.
extrusions or formed angles. The web forms the Figure 1-13 shows the basic configuration of principal depth portion of the spar. Cap strips are extrusions, formed angles, or milled sections to which the web is attached. These members carry the loads caused by the wing bending and also provide some typical metal spars. Most metal spars are built up from extruded aluminum alloy sections, with riveted aluminum alloy web sections to provide extra strength.
FIGURE 1-13. Metal spar shapes.
Although the spar shapes of figure 1-13 are typical of most basic shapes, the actual spar configuration may assume many forms. For example, a spar may have either a plate or truss type web. The plate web (figure 1-14) consists of a solid plate with vertical stiffeners which increase the strength of the web. Some spar plate webs are constructed differently. Some have no stiffeners; others contain flanged holes for reducing weight. Figure 1-15 shows a truss spar made up of an upper cap, a lower cap, and connecting vertical and diagonal tubes.
A structure may be designed so as to be considered "fail-safe." In other words, should one member of a complex structure fail, some other member would assume the load of the failed member.
A spar with "fail-safe" construction is shown in figure 1-16. This spar is made in two sections. The top section consists of a cap, riveted to the upper web plate. The lower section is a single extrusion, consisting of the lower cap and web plate. These two sections are spliced together to form the spar.
If either section of this type of spar breaks, the other section can still carry the load, which is the "fail-safe" feature.
As a rule, a wing has two spars. One spar is usually located near the front of the wing, and the other about two-thirds of the distance toward the wing's trailing edge. Regardless of type, the spar is the most important part of the wing. When other structural members of the wing are placed under load, they pass most of the resulting stress on to the wing spars.
Upper Wing Ribs spar Ribs are the structural crosspieces that make up cap the framework of the wing. They usually extend from the wing leading edge to the rear spar or to the trailing edge of the wing. The ribs give the wing its cambered shape and transmit the load from the skin and stringers to the spars. Ribs are also used in ailerons, elevators, rudders, and stabilizers.
Ribs are manufactured from wood or metal. Either wood or metal ribs are used with wooden spars while metal ribs are usually used with metal spars.
Some typical wooden ribs, usually manufactured Stiffener from spruce, are shown in figure 1-17.
Rib attach angle Lower spar cap FICURE 1-14. Plate web wing spar.
Upper cap member Diagonal tube Vertical tube Lower cap member FIGURE 1-17. Typical wooden ribs.
FIGURE 1-15. Truss wing spar.
The most common types of wooden ribs are the plywood web, the lightened plywood web, and the Upper spar cap truss types. Of these three types, the truss type is Rivets the most efficient, but it lacks the simplicity of the other types.
The wing rib shown in Figure 1-17A is a truss type, with plywood gussets on both sides of the rib Splice- Upper spar and a continuous rib cap around the entire rib. Rib web caps, often called cap strips, are usually made of the Rivets 1 same material as the rib itself, especially when Lower ee.o using wooden ribs. They stiffen and strengthen the spar web rib and provide an attaching surface for the rib covering.
A lightened plywood web rib is illustrated in figure 1-17B. On this type the cap strip may be laminated, especially at the leading edge. Figure Lower 1-17C shows a rib using a continuous gusset, which spar cap provides extra support throughout the entire rib FICURE 1-16. Wing spar with "fail-safe" construction. with very little additional weight.
A continuous gusset stiffens cap strips in the plane of the rib. This aids in preventing buckling and helps to obtain better rib/skin glue joints where nail-gluing is used because such a rib can resist the driving force of nails better than the other types. Continuous gussets are more easily handled than the many small separate gussets otherwise required.
Figure 1-18 shows the basic rib and spar structure of a wooden wing frame, together with some of the other wing structural members. In addition to the front and rear spars, an aileron spar, or false spar, is shown in figure 1-18. This type of spar extends only part of the spanwise length of the wing and provides a hinge attachment point for the aileron.
Various types of ribs are also illustrated in figure
1-18. In addition to the wing rib, sometimes called "plain rib" or even "main rib," nose ribs and the butt rib are shown. A nose rib is also called a false rib, since it usually extends from the wing leading edge to the front spar or slightly beyond. The nose ribs give the wing leading edge area the necessary curvature and support. The wing rib, or plain rib, extends from the leading edge of the wing to the rear spar and in some cases to the trailing edge of the wing. The wing butt rib is normally the heavily stressed rib section at the inboard end of the wing near the attachment point to the fuselage. Depending on its location and method of attachment, a butt rib may be called a bulkhead rib or a compression rib, if it is designed to receive compression loads that tend to force the wing spars together.
Since the ribs are laterally weak, they are strengthened in some wings by tapes that are woven above and below rib sections to prevent sidewise bending of the ribs.
Drag and antidrag wires (figure 1-18) are crisscrossed between the spars to form a truss to resist forces acting on the wing in the direction of the wing chord. These tension wires are also referred to as tie rods. The wire designed to resist the backward forces is called a drag wire; the antidrag wire resists the forward forces in the chord direction.
The wing attachment fittings, shown in figure 1-18, provide a means of attaching the wing to the aircraft fuselage.
The wing tip is often a removable unit, bolted to the outboard end of the wing panel. One reason for this is the vulnerability of the wing tips to damage, especially during ground handling and taxiing.
Figure 1-19 shows a removable wing tip for a large aircraft wing. The wing-tip assembly is of aluminum alloy construction. The wing-tip cap is secured to the tip with countersunk screws and is Wing tip Leading edge strip Nose rib or false rib Antidrag wire Front spar or tie rod Drag wire or tie rod False spar or aileron spar Aileron Aileron Hear spar Wing rib or plain rib or compression rib (or bulkhead rib Wing butt rib hinge Wing attach fittings FIGURE 1-18. Basic rib and spar structure, 11
secured to the interspar structure at four points with ¼-in. bolts. The tip leading edge contains the heat anti-icing duct. Wing-heated air is exhausted through a louver on the top surface of the tip. Wing position lights are located at the center of the tip Points of attachment to front and rear spar fittings (2 upper, 2 lower) Louver and are not directly visible from the cockpit. As an indication that the wing tip light is operating, some wing tips are equipped with a lucite rod to transmit the light to the leading edge.
Figure 1-20 shows a cross sectional view of an Access door Upper skin Wing tip navigation light Leading edge • Anti-icing outer skin Heat duct - Wing cap exhaust air outlet Reflector rod Corrugated inner skin FICURE 1-19. Removable wing tip.
FIGURE 1-20. All-metal wing with chemically milled channels.