FAA Structures PDF

Chapter 1 Aircraft Structures A Brief History of Aircraft Structures The history of aircraft structures underlies the history of aviation in general. Advances in materials and processes used to construct aircraft have led to their evolution from simple wood truss structures to the sl...

Chapter 1 Aircraft Structures A Brief History of Aircraft Structures The history of aircraft structures underlies the history of aviation in general. Advances in materials and processes used to construct aircraft have led to their evolution from simple wood truss structures to the sleek aerodynamic flying machines of today. Combined with continuous powerplant development, the structures of “flying machines” have changed significantly. The key discovery that “lift” could be created by passing air over the top of a curved surface set the development of fixed and rotary-wing aircraft in motion. George Cayley developed an efficient cambered airfoil in the early 1800s, as well as successful manned gliders later in that century. He established the principles of flight, including the existence of lift, weight, thrust, and drag. It was Cayley who first stacked wings and created a tri-wing glider that flew a man in 1853. 1-1 Earlier, Cayley studied the center of gravity of flying machines, as well as the effects of wing dihedral. Furthermore, he pioneered directional control of aircraft by including the earliest form of a rudder on his gliders. [Figure 1-1] In the late 1800s, Otto Lilienthal built upon Cayley’s discoveries. He manufactured and flew his own gliders on over 2,000 flights. His willow and cloth aircraft had wings designed from extensive study of the wings of birds. Lilienthal also made standard use of vertical and horizontal fins behind the wings and pilot station. Above all, Lilienthal proved that man could fly. [Figure 1-2] Octave Chanute, a retired railroad and bridge engineer, was active in aviation during the 1890s. [Figure 1-3] His interest was so great that, among other things, he published a definitive work called “Progress in Flying Machines.” This was the culmination of his effort to gather and study all the Figure 1-2. Master of gliding and wing study, Otto Lilienthal (top) and one of his more than 2,000 glider flights (bottom) information available on aviation. With the assistance of others, he built gliders similar to Lilienthal’s and then his own. In addition to his publication, Chanute advanced aircraft structure development by building a glider with stacked wings incorporating the use of wires as wing supports. The work of all of these men was known to the Wright Brothers when they built their successful, powered airplane in 1903. The first of its kind to carry a man aloft, the Wright Flyer had thin, cloth-covered wings attached to what was primarily truss structures made of wood. The wings contained forward and rear spars and were supported with both struts Figure 1-1. George Cayley, the father of aeronautics (top) and a and wires. Stacked wings (two sets) were also part of the flying replica of his 1853 glider (bottom). Wright Flyer. [Figure 1-4] 1-2  Figure 1-5. The world’s first mono-wing by Louis Bleriot. still supported by wires, but a mast extending above the fuselage enabled the wings to be supported from above, as well as underneath. This made possible the extended wing length needed to lift an aircraft with a single set of wings. Bleriot used a Pratt truss-type fuselage frame. [Figure 1-5] Figure 1-3. Octave Chanute gathered and published all of the aeronautical knowledge known to date in the late 1890s. Many More powerful engines were developed, and airframe early aviators benefited from this knowledge. structures changed to take advantage of the benefits. As early as 1910, German Hugo Junkers was able to build an Powered heavier-than-air aviation grew from the Wright aircraft with metal truss construction and metal skin due to design. Inventors and fledgling aviators began building their the availability of stronger powerplants to thrust the plane own aircraft. Early on, many were similar to that constructed forward and into the sky. The use of metal instead of wood by the Wrights using wood and fabric with wires and struts for the primary structure eliminated the need for external to support the wing structure. In 1909, Frenchman Louis wing braces and wires. His J-1 also had a single set of wings Bleriot produced an aircraft with notable design differences. (a monoplane) instead of a stacked set. [Figure 1-6] He built a successful mono-wing aircraft. The wings were Figure 1-4. The Wright Flyer was the first successful powered aircraft. It was made primarily of wood and fabric. 1-3 Figure 1-6. The Junker J-1 all metal construction in 1910. Leading up to World War I (WWI), stronger engines also construction of the fuselage. [Figure 1-9] The fiberglass allowed designers to develop thicker wings with stronger radome was also developed during this period. spars. Wire wing bracing was no longer needed. Flatter, lower wing surfaces on high-camber wings created more lift. WWI After WWII, the development of turbine engines led to expanded the need for large quantities of reliable aircraft. higher altitude flight. The need for pressurized aircraft Used mostly for reconnaissance, stacked-wing tail draggers pervaded aviation. Semimonocoque construction needed with wood and metal truss frames with mostly fabric skin to be made even stronger as a result. Refinements to the dominated the wartime sky. [Figure 1-7] The Red Baron’s all-metal semimonocoque fuselage structure were made to Fokker DR-1 was typical. increase strength and combat metal fatigue caused by the pressurization-depressurization cycle. Rounded window In the 1920s, the use of metal in aircraft construction and door openings were developed to avoid weak areas increased. Fuselages able to carry cargo and passengers where cracks could form. Integrally machined copper were developed. The early flying boats with their hull-type alloy aluminum skin resisted cracking and allowed thicker construction from the shipbuilding industry provided the skin and controlled tapering. Chemical milling of wing blueprints for semimonocoque construction of fuselages. skin structures provided great strength and smooth high- [Figure 1-8] Truss-type designs faded. A tendency toward cleaner mono-wing designs prevailed. Into the 1930s, all-metal aircraft accompanied new lighter and more powerful engines. Larger semimonocoque fuselages were complimented with stress-skin wing designs. Fewer truss and fabric aircraft were built. World War II (WWII) brought about a myriad of aircraft designs using all metal technology. Deep fuel-carrying wings were the norm, but the desire for higher flight speeds prompted the development of thin-winged aircraft in which fuel was carried in the fuselage. The first composite structure aircraft, the De Havilland Mosquito, used a balsa wood sandwich material in the Figure 1-8. The flying boat hull was an early semimonocoque design Figure 1-7. World War I aircraft were typically stacked-wing fabric- covered aircraft like this Breguet 14 (circa 1917). like this Curtiss HS-2L. 1-4 Figure 1-9. The De Havilland Mosquito used a laminated wood construction with a balsa wood core in the fuselage. Figure 1-10. The nearly all composite Cessna Citation Mustang very light jet (VLJ). performance surfaces. Variable contour wings became easier to construct. Increases in flight speed accompanying jet travel General brought about the need for thinner wings. Wing loading also increased greatly. Multispar and box beam wing designs were An aircraft is a device that is used for, or is intended to be used developed in response. for, flight in the air. Major categories of aircraft are airplane, rotorcraft, glider, and lighter-than-air vehicles. [Figure 1-11] In the 1960s, ever larger aircraft were developed to carry Each of these may be divided further by major distinguishing passengers. As engine technology improved, the jumbo jet features of the aircraft, such as airships and balloons. Both was engineered and built. Still primarily aluminum with a are lighter-than-air aircraft but have differentiating features semimonocoque fuselage, the sheer size of the airliners of and are operated differently. the day initiated a search for lighter and stronger materials from which to build them. The use of honeycomb constructed The concentration of this handbook is on the airframe of panels in Boeing’s airline series saved weight while not aircraft; specifically, the fuselage, booms, nacelles, cowlings, compromising strength. Initially, aluminum core with fairings, airfoil surfaces, and landing gear. Also included are aluminum or fiberglass skin sandwich panels were used on the various accessories and controls that accompany these wing panels, flight control surfaces, cabin floor boards, and structures. Note that the rotors of a helicopter are considered other applications. part of the airframe since they are actually rotating wings. By contrast, propellers and rotating airfoils of an engine on A steady increase in the use of honeycomb and foam core an airplane are not considered part of the airframe. sandwich components and a wide variety of composite materials characterizes the state of aviation structures from The most common aircraft is the fixed-wing aircraft. As the 1970s to the present. Advanced techniques and material the name implies, the wings on this type of flying machine combinations have resulted in a gradual shift from aluminum are attached to the fuselage and are not intended to move to carbon fiber and other strong, lightweight materials. These independently in a fashion that results in the creation of lift. new materials are engineered to meet specific performance One, two, or three sets of wings have all been successfully requirements for various components on the aircraft. Many utilized. [Figure 1-12] Rotary-wing aircraft such as airframe structures are made of more than 50 percent helicopters are also widespread. This handbook discusses advanced composites, with some airframes approaching features and maintenance aspects common to both fixed- 100 percent. The term “very light jet” (VLJ) has come to wing and rotary-wing categories of aircraft. Also, in certain describe a new generation of jet aircraft made almost entirely cases, explanations focus on information specific to only of advanced composite materials. [Figure 1-10] It is possible one or the other. Glider airframes are very similar to fixed- that noncomposite aluminum aircraft structures will become wing aircraft. Unless otherwise noted, maintenance practices obsolete as did the methods and materials of construction described for fixed-wing aircraft also apply to gliders. The used by Cayley, Lilienthal, and the Wright Brothers. same is true for lighter-than-air aircraft, although thorough 1-5 Figure 1-11. Examples of different categories of aircraft, clockwise from top left: lighter-than-air, glider, rotorcraft, and airplane. coverage of the unique airframe structures and maintenance practices for lighter-than-air flying machines is not included in this handbook. The airframe of a fixed-wing aircraft consists of five principal units: the fuselage, wings, stabilizers, flight control surfaces, and landing gear. [Figure 1-13] Helicopter airframes consist of the fuselage, main rotor and related gearbox, tail rotor (on helicopters with a single main rotor), and the landing gear. Airframe structural components are constructed from a wide variety of materials. The earliest aircraft were constructed primarily of wood. Steel tubing and the most common material, aluminum, followed. Many newly certified aircraft are built from molded composite materials, such as carbon fiber. Structural members of an aircraft’s fuselage include stringers, longerons, ribs, bulkheads, and more. The main structural member in a wing is called the wing spar. The skin of aircraft can also be made from a variety of materials, ranging from impregnated fabric to plywood, aluminum, or composites. Under the skin and attached to the structural fuselage are the many components that support airframe function. The entire airframe and its components are joined by rivets, bolts, screws, and other fasteners. Welding, adhesives, and special bonding techniques are also used. Major Structural Stresses Aircraft structural members are designed to carry a load or to resist stress. 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 Figure 1-12. A monoplane (top), biplane (middle), and tri-wing of the material of which it is made. Every part of the aircraft aircraft (bottom). must be planned to carry the load to be imposed upon it. 1-6  Wings Flight controls Powerplant Stabilizers Fuselage Flight controls Landing gear Figure 1-13. Principal airframe units. The determi­nation of such loads is called stress analysis. forward, but air resistance tries to hold it back. The result is Although planning the design is not the function of the aircraft tension, which stretches the aircraft. The tensile strength of technician, it is, nevertheless, important that the technician a material is measured in pounds per square inch (psi) and is understand and appreciate the stresses in­volved in order to calculated by dividing the load (in pounds) re­quired to pull the avoid changes in the original design through improper repairs. material apart by its cross-sec­tional area (in square inches). The term “stress” is often used interchangeably with the Compression is the stress that res­ists a crushing force. word “strain.” While related, they are not the same thing. [Figure 1-14B] The compressive strength of a material is External loads or forces cause stress. Stress is a material’s also measured in psi. Compression is the stress that tends to internal resistance, or counterforce, that opposes deformation. shorten or squeeze aircraft parts. The degree of deformation of a material is strain. When a material is subjected to a load or force, that material is Torsion is the stress that produces twisting. [Figure 1-14C] deformed, regardless of how strong the material is or how While moving the aircraft forward, the en­gine also tends to light the load is. twist it to one side, but other aircraft components hold it on course. Thus, torsion is created. The torsion strength of a There are five major stresses [Figure 1-14] to which all material is its resistance to twisting or torque. aircraft are subjected: Tension Shear is the stress that resists the force tending to cause one layer of a material to slide over an adjacent layer. Compression [Figure 1-14D] Two riveted plates in tension subject the Torsion rivets to a shearing force. Usually, the shearing strength of a material is either equal to or less than its tensile or Shear compressive strength. Aircraft parts, especially screws, bolts, Bending and rivets, are often subject to a shearing force. Tension is the stress that resists a force that tends to pull Bending stress is a combination of compression and tension. something apart. [Figure 1-14A] The engine pulls the aircraft The rod in Figure 1-14E has been short­ened (compressed) on 1-7  A. Tension B. Compression C. Torsion D. Shear Tension outside of bend Bent structural member Shear along imaginary line (dotted) Compression inside of bend E. Bending (the combination stress) Figure 1-14. The five stresses that may act on an aircraft and its parts. the inside of the bend and stretched on the outside of the bend. Fixed-Wing Aircraft A single member of the structure may be subjected to Fuselage a combination of stresses. In most cases, the struc­tural The fuselage is the main structure or body of the fixed-wing members are designed to carry end loads rather than side aircraft. It provides space for cargo, controls, acces­sories, loads. They are designed to be subjected to tension or passengers, and other equipment. In single-engine aircraft, compression rather than bending. the fuselage houses the powerplant. In multiengine aircraft, the engines may be either in the fuselage, attached to the Strength or resistance to the external loads imposed during fuselage, or suspended from the wing structure. There are two operation may be the principal requirement in cer­tain general types of fuselage construc­tion: truss and monocoque. structures. However, there are numerous other characteristics in addition to designing to control the five major stresses that Truss-Type engineers must consider. For example, cowling, fairings, and simi­lar parts may not be subject to significant loads requiring A truss is a rigid framework made up of members, such as a high degree of strength. However, these parts must have beams, struts, and bars to resist deforma­tion by applied loads. streamlined shapes to meet aerodynamic requirements, such The truss-framed fuselage is generally covered with fabric. as reducing drag or directing airflow. The truss-type fuselage frame is usually constructed of steel 1-8 tubing welded together in such a manner that all members of the truss can carry both tension and compression loads. Skin Former [Figure 1-15] In some aircraft, principally the light, single- engine models, truss fuselage frames may be 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 loads. The design may be di­vided into two classes: 1. Monocoque 2. Semi­monocoque Different por­tions of the same fuselage may belong to either Bulkhead of the two classes, but most modern aircraft are considered to be of semimonocoque type construction. Figure 1-16. An airframe using monocoque construction. The true monocoque construction uses formers, frame members called longerons. Longerons usually extend across assemblies, and bulkheads to give shape to the fuselage. several frame members and help the skin support primary [Figure 1-16] The heaviest of these structural members are bending loads. They are typically made of aluminum alloy located at intervals to carry concentrated loads and at points either of a single piece or a built-up construction. where fittings are used to attach other units such as wings, powerplants, and stabilizers. Since no other bracing members Stringers are also used in the semimonocoque fuselage. These are present, the skin must carry the primary stresses and longitudinal members are typically more numerous and lighter keep the fuselage rigid. Thus, the biggest problem involved in weight than the longerons. They come in a variety of shapes in mono­coque construction is maintaining enough strength and are usually made from single piece aluminum alloy while keeping the weight within allowable limits. extrusions or formed aluminum. Stringers have some rigidity but are chiefly used for giving shape and for attachment of Semimonocoque Type the skin. Stringers and longerons together prevent tension To overcome the strength/weight problem of monocoque and compression from bending the fuselage. [Figure 1-17] construction, a modification called semi­m onocoque construction was devel­o ped. It also consists of frame Longeron Skin assemblies, bulkheads, and formers as used in the monocoque design but, additionally, the skin is reinforced by longitudinal Longeron Diagonal web members Stringer Bulkhead Vertical web members Figure 1-17. The most common airframe construction is Figure 1-15. A truss-type fuselage. A Warren truss uses mostly semimonocoque. diagonal bracing. 1-9 Other bracing between the longerons and stringers can also Fuselages are generally constructed in two or more sections. be used. Often referred to as web members, these additional On small aircraft, they are generally made in two or three support pieces may be installed vertically or diagonally. It sections, while larger aircraft may be made up of as many as must be noted that manufacturers use different nomenclature six sections or more before being assembled. to describe structural members. For example, there is often little difference between some rings, frames, and formers. Pressurization One manufacturer may call the same type of brace a ring or Many aircraft are pressurized. This means that air is pumped a frame. Manufacturer instructions and specifications for a into the cabin after takeoff and a difference in pressure specific aircraft are the best guides. between the air inside the cabin and the air outside the cabin is established. This differential is regulated and maintained. In The semimonocoque fuselage is constructed primarily of alloys this manner, enough oxygen is made available for passengers of aluminum and magnesium, although steel and titanium are to breathe normally and move around the cabin without sometimes found in areas of high temperatures. Individually, special equipment at high altitudes. not one of the aforementioned components is strong enough to carry the loads imposed during flight and landing. But, Pressurization causes significant stress on the fuselage when combined, those components form a strong, rigid structure and adds to the complexity of design. In addition framework. This is accomplished with gussets, rivets, nuts to withstanding the difference in pressure between the air and bolts, screws, and even friction stir welding. A gusset is inside and outside the cabin, cycling from unpressurized to a type of connection bracket that adds strength. [Figure 1-18] pressurized and back again each flight causes metal fatigue. To deal with these impacts and the other stresses of flight, To summarize, in semimonocoque fuselages, the strong, nearly all pressurized aircraft are semimonocoque in design. heavy longerons hold the bulkheads and formers, and these, Pressurized fuselage structures undergo extensive periodic in turn, hold the stringers, braces, web members, etc. All are inspections to ensure that any damage is discovered and designed to be attached together and to the skin to achieve repaired. Repeated weakness or failure in an area of structure the full-strength benefits of semimonocoque design. It is may require that section of the fuselage be modified or important to recognize that the metal skin or covering carries redesigned. part of the load. The fuselage skin thickness can vary with the load carried and the stresses sustained at a particular location. Wings Wing Configurations The advantages of the semimonocoque fuselage are many. Wings are airfoils that, when moved rapidly through the The bulkheads, frames, stringers, and longerons facilitate the air, create lift. They are built in many shapes and sizes. de­sign and construction of a streamlined fuselage that is both Wing design can vary to provide certain desirable flight rigid and strong. Spreading loads among these structures and characteristics. Control at various operating speeds, the the skin means no single piece is failure critical. This means amount of lift generated, balance, and stability all change as that a semimonocoque fuselage, because of its stressed-skin the shape of the wing is altered. Both the leading edge and construction, may with­stand considerable damage and still the trailing edge of the wing may be straight or curved, or be strong enough to hold together. one edge may be straight and the other curved. 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. The wing tip may be square, rounded, or even pointed. Figure 1-19 shows a number of typical wing leading and trailing edge shapes. The wings of an aircraft can be attached to the fuselage at the top, mid-fuselage, or at the bottom. They may extend perpendicular to the horizontal plane of the fuselage or can angle up or down slightly. This angle is known as the wing dihedral. The dihedral angle affects the lateral stability of the aircraft. Figure 1-20 shows some common wing attach points and dihedral angle. Figure 1-18. Gussets are used to increase strength. 1-10  Tapered leading edge, Tapered leading and Delta wing straight trailing edge trailing edges Straight leading and Straight leading edge, Sweptback wings trailing edges tapered trailing edge Figure 1-19. Various wing design shapes yield different performance. Wing Structure The wings of an aircraft are designed to lift it into the air. Their particular design for any given aircraft depends on a number of factors, such as size, weight, use of the aircraft, Low wing Dihedral desired speed in flight and at landing, and desired rate of climb. The wings of aircraft are designated left and right, corresponding to the left and right sides of the operator when seated in the cockpit. [Figure 1-21] High wing Mid wing Often wings are of full cantilever design. This means they are built so that no external bracing is needed. They are supported internally by structural members assisted by the skin of the aircraft. Other aircraft wings use external struts Gull wing Inverted gull or wires to assist in supporting the wing and carrying the aerodynamic and landing loads. Wing support cables and Figure 1-20. Wing attach points and wing dihedrals. struts are generally made from steel. Many struts and their 1-11  Left wing Right wing Figure 1-21. “Left” and “right” on an aircraft are oriented to the perspective of a pilot sitting in the cockpit. attach fittings have fairings to reduce drag. Short, nearly bulkheads running chordwise (leading edge to trailing edge). vertical supports called jury struts are found on struts that The spars are the principle structural members of a wing. attach to the wings a great distance from the fuselage. This They support all distributed loads, as well as concentrated serves to subdue strut movement and oscillation caused by weights such as the fuselage, landing gear, and engines. The the air flowing around the strut in flight. Figure 1-22 shows skin, which is attached to the wing structure, carries part of samples of wings using external bracing, also known as the loads imposed during flight. It also transfers the stresses semicantilever wings. Cantilever wings built with no external to the wing ribs. The ribs, in turn, transfer the loads to the bracing are also shown. wing spars. [Figure 1-23] Aluminum is the most common material from which In general, wing construction is based on one of three to construct wings, but they can be wood covered with fundamental designs: fabric, and occasionally a magnesium alloy has been used. 1. Monospar Moreover, modern aircraft are tending toward lighter and stronger materials throughout the airframe and in wing 2. Multispar construction. Wings made entirely of carbon fiber or other 3. Box beam composite materials exist, as well as wings made of a combination of materials for maximum strength to weight Modification of these basic designs may be adopted by performance. various manufacturers. The internal structures of most wings are made up of spars The monospar wing incorporates only one main spanwise or and stringers running spanwise and ribs and formers or longitudinal member in its construction. Ribs or bulkheads Wire braced biplane Full cantilever Semicantilever Long struts braced with jury struts Figure 1-22. Externally braced wings, also called semicantilever wings, have wires or struts to support the wing. Full cantilever wings have no external bracing and are supported internally. 1-12  Ribs Rear spar Stringer Nose rib Ribs Front spar Skin Figure 1-23. Wing structure nomenclature. supply the necessary contour or shape to the airfoil. Although corrugated sheets. A combination of corrugated sheets on the the strict monospar wing is not common, this type of design upper surface of the wing and stiffeners on the lower surface modified by the addition of false spars or light shear webs is sometimes used. Air transport category aircraft often utilize along the trailing edge for support of control surfaces is box beam wing construction. sometimes used. Wing Spars The multispar wing incorporates more than one main Spars are the principal structural members of the wing. They longitudinal member in its construction. To give the wing correspond to the longerons of the fuse­lage. They run parallel contour, ribs or bulkheads are often included. to the lateral axis of the aircraft, from the fuselage toward the tip of the wing, and are usually attached to the fuselage The box beam type of wing construction uses two main by wing fittings, plain beams, or a truss. longitudinal members with connecting bulkheads to furnish additional strength and to give contour to the wing. Spars may be made of metal, wood, or composite materials [Figure 1-24] A corrugated sheet may be placed between depending on the design criteria of a specific aircraft. the bulkheads and the smooth outer skin so that the wing Wooden spars are usually made from spruce. They can be can better carry tension and compression loads. In some generally classified into four different types by their cross- cases, heavy longitudinal stiffeners are substituted for the sectional configu­ration. As shown in Figure 1-25, they may upper surface of the wing and stiffeners on the lower surface be (A) solid, (B) box-shaped, (C) partly hollow, or (D) in Figure 1-24. Box beam construction. 1-13  A B C D E Figure 1-25. Typical wooden wing spar cross-sections. the form of an I-beam. Lamination of solid wood spars is the loads caused by wing bending, with the caps providing a often used to increase strength. Laminated wood can also be foundation for attaching the skin. Although the spar shapes found in box-shaped spars. The spar in Figure 1-25E has had in Figure 1-26 are typi­cal, actual wing spar configura­tions material removed to reduce weight but retains the strength assume many forms. For example, the web of a spar may be of a rectangular spar. As can be seen, most wing spars are a plate or a truss as shown in Figure 1-27. It could be built up basically rectangular in shape with the long dimension of the from lightweight materials with vertical stiffeners employed cross-section oriented up and down in the wing. for strength. [Figure 1-28] Currently, most manufactured aircraft have wing spars made of solid extruded aluminum or aluminum extrusions It could also have no stiffeners but might contain flanged riveted together to form the spar. The increased use of holes for reducing weight but maintaining strength. Some composites and the combining of materials should make metal and composite wing spars retain the I-beam concept airmen vigilant for wings spars made from a variety of but use a sine wave web. [Figure 1-29] materials. Figure 1-26 shows examples of metal wing spar cross-sections. Additionally, fail-safe spar web design exists. Fail-safe means that should one member of a complex structure fail, In an I–beam spar, the top and bottom of the I–beam are some other part of the structure assumes the load of the failed called the caps and the vertical section is called the web. member and permits continued operation. A spar with fail- The entire spar can be extruded from one piece of metal safe construction is shown in Figure 1-30. This spar is made but often it is built up from multiple extrusions or formed in two sections. The top section consists of a cap riveted to angles. The web forms the principal depth portion of the the upper web plate. The lower section is a single extrusion spar and the cap strips (extrusions, formed angles, or milled consisting of the lower cap and web plate. These two sections sections) are attached to it. Together, these members carry are spliced together to form the spar. If either section of this Figure 1-26. Examples of metal wing spar shapes. 1-14  Upper cap member Upper spar cap Diagonal tube Rivets Splice Vertical tube Up pe rs pa rw Lo eb we rs pa rw eb Lower spar cap Lower cap member Figure 1-30. A fail-safe spar with a riveted spar web. Figure 1-27. A truss wing spar. other structural members of the wing are placed under load, Upper spar cap most of the resulting stress is passed on to the wing spar. Stiffener False spars are commonly used in wing design. They are longitudinal members like spars but do not extend the entire spanwise length of the wing. Often, they are used as hinge attach points for control surfaces, such as an aileron spar. Rib attach angle Wing Ribs Lower spar cap Ribs are the structural crosspieces that combine with spars and stringers to make up 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 Figure 1-28. A plate web wing spar with vertical stiffeners. its cambered shape and transmit the load from the skin and stringers to the spars. Similar ribs are also used in ailerons, elevators, rudders, and stabilizers. Sine wave web Wing ribs are usually manufactured from either wood or metal. Aircraft with wood wing spars may have wood or Caps metal ribs while most aircraft with metal spars have metal ribs. Wood ribs are usually manufactured from spruce. The three most common types of wooden ribs are the plywood web, the lightened plywood web, and the truss types. Of these three, the truss type is the most efficient because it is strong and lightweight, but it is also the most complex to construct. Figure 1-31 shows wood truss web ribs and a lightened Figure 1-29. A sine wave wing spar can be made from aluminum plywood web rib. Wood ribs have a rib cap or cap strip or composite materials. fastened around the entire perimeter of the rib. It is usually made of the same material as the rib itself. The rib cap stiffens type of spar breaks, the other section can still carry the load. and strengthens the rib and provides an attaching surface This is the fail-safe feature. for the wing covering. In Figure 1-31A, the cross-section of a wing rib with a truss-type web is illustrated. The dark As a rule, a wing has two spars. One spar is usually located rectangular sections are the front and rear wing spars. Note that near the front of the wing, and the other about two-thirds of to reinforce the truss, gussets are used. In Figure 1-31B, a truss the distance toward the wing’s trailing edge. Regardless of web rib is shown with a continuous gusset. It provides greater type, the spar is the most important part of the wing. When support throughout the entire rib with very little additional 1-15  names that reflect their uniqueness. For example, ribs that are located entirely forward of the front spar that are used to A shape and strengthen the wing leading edge are called nose ribs or false ribs. False ribs are ribs that do not span the entire wing chord, which is the distance from the leading edge to the trailing edge of the wing. Wing butt ribs may be found B at the inboard edge of the wing where the wing attaches to the fuselage. Depending on its location and method of attachment, a butt rib may also be called a bulkhead rib or a compression rib if it is designed to receive compression C loads that tend to force the wing spars together. Since the ribs are laterally weak, they are strengthened in some Figure 1-31. Examples of wing ribs constructed of wood. wings by tapes that are woven above and below rib sections to prevent sidewise bending of the ribs. Drag and anti-drag wires may also be found in a wing. In Figure 1-32, they are weight. A continuous gusset stiffens the cap strip in the plane shown criss­crossed between the spars to form a truss to resist of the rib. This aids in preventing buckling and helps to obtain forces acting on the wing in the direction of the wing chord. better rib/skin joints where nail-gluing is used. Such a rib can These tension wires are also referred to as tie rods. The wire resist the driving force of nails better than the other types. designed to resist the back­ward forces is called a drag wire; Continuous gussets are also more easily handled than the many the anti-drag wire resists the forward forces in the chord small separate gussets otherwise required. Figure 1-31C shows direction. Figure 1-32 illustrates the structural components a rib with a lighten plywood web. It also contains gussets to of a basic wood wing. support the web/cap strip interface. The cap strip is usually laminated to the web, especially at the leading edge. At the inboard end of the wing spars is some form of wing attach fitting as illustrated in Figure 1-32. These provide A wing rib may also be referred to as a plain rib or a main rib. a strong and secure method for attaching the wing to the Wing ribs with specialized locations or functions are given fuselage. The interface between the wing and fuselage is often Leading edge strip Nose rib or false rib Wing tip Front spar Anti-drag wire or tie rod Drag wire or tie rod Wing attach fittings False spar or aileron spar Rear spar Wing rib or plain rib Aileron Aileron hinge Wing butt rib (or compression rib or bulkhead rib) Figure 1-32. Basic wood wing structure and components. 1-16  of the wing tips to damage, especially during ground handling and taxiing. Figure 1-34 shows a removable wing tip for a large aircraft wing. Others are different. The wing tip assembly is of aluminum alloy construction. The wing tip cap is secured to the tip with countersunk screws and is secured to the interspar structure at four points with ¼-inch diameter bolts. To prevent ice from forming on the leading edge of the wings of large aircraft, hot air from an engine is often channeled through the leading edge from wing root to wing tip. A louver on the top surface of the wing tip allows this warm air to be exhausted overboard. Wing position lights are located at the center of the tip 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 Figure 1-33. Wing root fairings smooth airflow and hide wing to the leading edge. attach fittings. Wing Skin covered with a fairing to achieve smooth airflow in this area. Often, the skin on a wing is designed to carry part of the The fairing(s) can be removed for access to the wing attach flight and ground loads in combination with the spars and fittings. [Figure 1-33] ribs. This is known as a stressed-skin design. The all-metal, full cantilever wing section illustrated in Figure 1-35 shows The wing tip is often a removable unit, bolted to the outboard the structure of one such design. The lack of extra internal end of the wing panel. One reason for this is the vulnerability Access panel Upper skin Points of attachment to front and rear spar fittings (2 upper, 2 lower) Louver Wing tip navigation light Leading edge outer skin Corrugated inner skin Reflector rod Heat duct Wing cap Figure 1-34. A removable metal wing tip. 1-17 Figure 1-35. The skin is an integral load carrying part of a stressed skin design. or external bracing requires that the skin share some of the milled aluminum skin can provide skin of varied thicknesses. load. Notice the skin is stiffened to aid with this function. On aircraft with stressed-skin wing design, honeycomb structured wing panels are often used as skin. A honeycomb Fuel is often carried inside the wings of a stressed-skin structure is built up from a core material resembling a bee aircraft. The joints in the wing can be sealed with a special hive’s honeycomb which is laminated or sandwiched between fuel resistant sealant enabling fuel to be stored directly inside thin outer skin sheets. Figure 1-37 illustrates honeycomb the structure. This is known as wet wing design. Alternately, panes and their components. Panels formed like this are a fuel-carrying bladder or tank can be fitted inside a wing. lightweight and very strong. They have a variety of uses Figure 1-36 shows a wing section with a box beam structural on the aircraft, such as floor panels, bulkheads, and control design such as one that might be found in a transport category surfaces, as well as wing skin panels. Figure 1-38 shows the aircraft. This structure increases strength while reducing locations of honeycomb construction wing panels on a jet weight. Proper sealing of the structure allows fuel to be stored transport aircraft. in the box sections of the wing. A honeycomb panel can be made from a wide variety of The wing skin on an aircraft may be made from a wide variety materials. Aluminum core honeycomb with an outer skin of of materials such as fabric, wood, or aluminum. But a single aluminum is common. But honeycomb in which the core is thin sheet of material is not always employed. Chemically Sealed structure fuel tank—wet wing Figure 1-36. Fuel is often carried in the wings. 1-18 an Arimid® fiber and the outer sheets are coated Phenolic® at the empennage (tail section). Occasionally, a multiengine is common as well. In fact, a myriad of other material aircraft is designed with a nacelle in line with the fuselage aft combinations such as those using fiberglass, plastic, Nomex®, of the passenger compartment. Regardless of its location, a Kevlar ®, and carbon fiber all exist. Each honeycomb nacelle contains the engine and accessories, engine mounts, structure possesses unique characteristics depending upon structural members, a firewall, and skin and cowling on the the materials, dimensions, and manufacturing techniques exterior to fare the nacelle to the wind. employed. Figure 1-39 shows an entire wing leading edge formed from honeycomb structure. Some aircraft have nacelles that are designed to house the landing gear when retracted. Retracting the gear to reduce Nacelles wind resistance is standard procedure on high-performance/ Nacelles (sometimes called “pods”) are streamlined high-speed aircraft. The wheel well is the area where the enclosures used primarily to house the engine and its landing gear is attached and stowed when retracted. Wheel components. They usually present a round or elliptical wells can be located in the wings and/or fuselage when not profile to the wind thus reducing aerodynamic drag. On part of the nacelle. Figure 1-40 shows an engine nacelle most single-engine aircraft, the engine and nacelle are at the incorporating the landing gear with the wheel well extending forward end of the fuselage. On multiengine aircraft, engine into the wing root. nacelles are built into the wings or attached to the fuselage Core Skin A Skin Constant thickness Core Skin B Skin Tapered core Figure 1-37. The honeycomb panel is a staple in aircraft construction. Cores can be either constant thickness (A) or tapered (B). Tapered core honeycomb panels are frequently used as flight control surfaces and wing trailing edges. 1-19  Trailing edge sandwich panels constant-thickness core Wing leading edge Spoiler sandwich panel tapered core, solid wedge Trailing edge sandwich panels constant-thickness core Inboard flap Spoiler sandwich panel Outboard flap tapered core, solid wedge Aileron tab sandwich panel Aileron tab sandwich panel tapered core, Phenolic wedge constant-thickness core Trailing edge wedge sandwich panel tapered core, cord wedge Figure 1-38. Honeycomb wing construction on a large jet transport aircraft. The framework of a nacelle usually consists of structural components inside. Both are usually made of sheet aluminum members similar to those of the fuselage. Lengthwise or magnesium alloy with stainless steel or titanium alloys members, such as longerons and stringers, combine with being used in high-temperature areas, such as around the horizontal/vertical members, such as rings, formers, and exhaust exit. Regardless of the material used, the skin is bulkheads, to give the nacelle its shape and structural typically attached to the framework with rivets. integrity. A firewall is incorporated to isolate the engine compartment from the rest of the aircraft. This is basically a Cowling refers to the detachable panels covering those areas stainless steel or titanium bulkhead that contains a fire in the into which access must be gained regularly, such as the engine confines of the nacelle rather than letting it spread throughout and its accessories. It is designed to provide a smooth airflow the airframe. [Figure 1-41] over the nacelle and to protect the engine from damage. Cowl panels are generally made of aluminum alloy construction. Engine mounts are also found in the nacelle. These are However, stainless steel is often used as the inner skin aft the structural assemblies to which the engine is fastened. of the power section and for cowl flaps and near cowl flap They are usually constructed from chrome/molybdenum openings. It is also used for oil cooler ducts. Cowl flaps are steel tubing in light aircraft and forged chrome/nickel/ moveable parts of the nacelle cowling that open and close molybdenum assemblies in larger aircraft. [Figure 1-42] to regulate engine temperature. The exterior of a nacelle is covered with a skin or fitted with There are many engine cowl designs. Figure 1-43 shows an a cowling which can be opened to access the engine and exploded view of the pieces of cowling for a horizontally 1-20  Metal wing spar Metal member bonded to sandwich Honeycomb sandwich core Wooden members spanwise and chordwise Glass reinforced plastics sandwich the core Figure 1-39. A wing leading edge formed from honeycomb material bonded to the aluminum spar structure. Figure 1-40. Engine nacelle incorporating the landing gear with the wheel well extending into the wing root. 1-21 Figure 1-41. An engine nacelle firewall. Figure 1-42. Various aircraft engine mounts. opposed engine on a light aircraft. It is attached to the nacelle by means of screws and/or quick release fasteners. Some large reciprocating engines are enclosed by “orange peel” cowlings which provide excellent access to components inside the nacelle. [Figure 1-44] These cowl panels are attached to the forward firewall by mounts which also serve as hinges for opening the cowl. The lower cowl mounts are secured to the hinge brackets by quick release pins. The side and top panels are held open by rods and the lower panel is retained in the open position by a spring and a cable. All of the cowling panels are locked in the closed position by over- center steel latches which are secured in the closed position by spring-loaded safety catches. An example of a turbojet engine nacelle can be seen in Figure 1-45. The cowl panels are a combination of fixed and easily removable panels which can be opened and closed during maintenance. A nose cowl is also a feature on a jet Figure 1-43. Typical cowling for a horizontally opposed engine nacelle. It guides air into the engine. reciprocating engine. Empennage The tail cone serves to close and streamline the aft end of The empennage of an aircraft is also known as the tail most fuselages. The cone is made up of structural members section. Most empennage designs consist of a tail cone, like those of the fuselage; however, cones are usually of fixed aerodynamic surfaces or stabilizers, and movable lighter con­struction since they receive less stress than the aerodynamic surfaces. fuselage. [Figure 1-46] 1-22 Figure 1-44. Orange peel cowling for large radial reciprocating engine. Figure 1-45. Cowling on a transport category turbine engine nacelle. 1-23  Frame Longeron Skin Stringer Rib Stringer Bulkhead Spars Skin Figure 1-46. The fuselage terminates at the tail cone with similar Figure 1-48. Vertical stabilizer. but more lightweight construction. the remainder on to the others. Ultimately, the spar transmits The other components of the typical empennage are of any overloads to the fuselage. A horizontal stabilizer is built heavier construction than the tail cone. These members the same way. include fixed surfaces that help stabilize the aircraft and movable surfaces that help to direct an aircraft during flight. The rudder and elevator are flight control surfaces that are The fixed surfaces are the horizon­tal stabilizer and vertical also part of the empennage discussed in the next section of stabilizer. The movable surfaces are usually a rudder located this chapter. at the aft edge of the vertical stabilizer and an elevator located at the aft edge the horizontal stabilizer. [Figure 1-47] Flight Control Surfaces The directional control of a fixed-wing aircraft takes place The structure of the stabilizers is very similar to that which around the lateral, longitudinal, and vertical axes by means is used in wing construction. Figure 1-48 shows a typical of flight control surfaces designed to create movement about vertical stabilizer. Notice the use of spars, ribs, stringers, these axes. These control devices are hinged or movable and skin like those found in a wing. They perform the surfaces through which the attitude of an aircraft is controlled same functions shaping and supporting the stabilizer and during takeoff, flight, and landing. They are usually divided transferring stresses. Bending, torsion, and shear created into two major groups: 1) pri­mary or main flight control by air loads in flight pass from one structural member to surfaces and 2) secondary or auxiliary control surfaces. another. Each member absorbs some of the stress and passes Primary Flight Control Surfaces Vertical stabilizer The primary flight control surfaces on a fixed-wing aircraft include: ailerons, elevators, and the rudder. The ailerons are attached to the trailing edge of both wings and when moved, Horizontal stabilizer Rudder rotate the aircraft around the longitudinal axis. The elevator is attached to the trailing edge of the horizontal stabilizer. When it is moved, it alters aircraft pitch, which is the attitude Trim tabs about the horizontal or lateral axis. The rudder is hinged to the trailing edge of the vertical stabilizer. When the rudder changes position, the aircraft rotates about the vertical axis (yaw). Figure 1-49 shows the primary flight controls of a light aircraft and the movement they create relative to the Elevator three axes of flight. Figure 1-47. Components of a typical empennage. 1-24  Primary control surfaces constructed from composite El Rudder—Yaw ev materials are also commonly used. These are found on many at La or— Vertical axis heavy and high-performance aircraft, as well as gliders, t (lo era Pit (directional stangi l ax ch stability) home-built, and light-sport aircraft. The weight and strength bil tud is Roll ity ina on— advantages over traditional construction can be significant. ) l Ailer al itudin A wide variety of materials and construction techniques are Long(lateral axis ity) employed. Figure 1-51 shows examples of aircraft that use stabil composite technology on primary flight control surfaces. Note that the control surfaces of fabric-covered aircraft often have fabric-covered surfaces just as aluminum-skinned (light) aircraft typically have all-aluminum control surfaces. There is a critical need for primary control surfaces to be balanced so they do not vibrate or flutter in the wind. Primary Control Airplane Axes of Type of Surface Movement Rotation Stability Aileron Roll Longitudinal Lateral Elevator/ Pitch Lateral Longitudinal Stabilator Rudder Yaw Vertical Directional Figure 1-49. Flight control surfaces move the aircraft around the three axes of flight. Primary control surfaces are usually similar in construc­tion to one another and vary only in size, shape, and methods of attachment. On aluminum light aircraft, their structure is often similar to an all-metal wing. This is appropriate because the primary control surfaces are simply smaller aerodynamic devices. They are typically made from an aluminum alloy structure built around a sin­gle spar member or torque tube to which ribs are fitted and a skin is attached. The lightweight ribs are, in many cases, stamped out from flat aluminum sheet stock. Holes in the ribs lighten the assembly. An aluminum skin is attached with rivets. Figure 1-50 illustrates this type of structure, which can be found on the primary control surfaces of light aircraft as well as on medium and heavy aircraft. Aileron hinge-pin fitting Actuating horn Figure 1-51. Composite control surfaces and some of the many Spar Lightning hole aircraft that utilize them. Figure 1-50. Typical structure of an aluminum flight control surface. 1-25 Performed to manufacturer’s instructions, balancing usually consists of assuring that the center of gravity of a particular Up aileron device is at or forward of the hinge point. Failure to properly balance a control surface could lead to catastrophic failure. Down aileron Figure 1-52 illustrates several aileron configurations with their hinge points well aft of the leading edge. This is a common design feature used to prevent flutter. Ailerons Ailerons are the primary flight control surfaces that move the aircraft about the longitudinal axis. In other words, movement of the ailerons in flight causes the aircraft to roll. Ailerons are usually located on the outboard trailing edge of each of the wings. They are built into the wing and are calculated as Figure 1-54. Differential aileron control movement. When one aileron part of the wing’s surface area. Figure 1-53 shows aileron is moved down, the aileron on the opposite wing is deflected upward. locations on various wing tip designs. Ailerons are controlled by a side-to-side motion of the control a sensitive response to the control input to roll the aircraft. stick in the cockpit or a rotation of the control yoke. When The pilot’s request for aileron movement and roll are the aileron on one wing deflects down, the aileron on the transmitted from the cockpit to the actual control surface in a opposite wing deflects upward. This amplifies the movement variety of ways depending on the aircraft. A system of control of the aircraft around the longitudinal axis. On the wing on cables and pulleys, push-pull tubes, hydraulics, electric, or a which the aileron trailing edge moves downward, camber is combination of these can be employed. [Figure 1-55] increased, and lift is increased. Conversely, on the other wing, Simple, light aircraft usually do not have hydraulic or electric the raised aileron decreases lift. [Figure 1-54] The result is fly-by-wire aileron control. These are found on heavy and high-performance aircraft. Large aircraft and some high- performance aircraft may also have a second set of ailerons located inboard on the trailing edge of the wings. These are part of a complex system of primary and secondary control surfaces used to provide lateral control and stability in flight. At low speeds, the ailerons may be augmented by the use of flaps and spoilers. At high speeds, only inboard aileron deflection is required to roll the aircraft while the Stop Figure 1-52. Aileron hinge locations are very close to but aft of the Elevator cables center of gravity to prevent flutter. Tether stop Stop To ailerons Note pivots not on center of shaft Figure 1-55. Transferring control surface inputs from the cockpit. Figure 1-53. Aileron location on various wings. 1-26 other control surfaces are locked out or remain stationary. the rudder to the right which moves the nose of the aircraft Figure 1-56 illustrates the location of the typical flight control to the right. The left pedal is rigged to simultaneously move surfaces found on a transport category aircraft. aft. When the left pedal is pushed forward, the nose of the aircraft moves to the left. Elevator The elevator is the primary flight control surface that moves As with the other primary flight controls, the transfer of the the aircraft around the horizontal or lateral axis. This causes the movement of the cockpit controls to the rudder varies with nose of the aircraft to pitch up or down. The elevator is hinged the complexity of the aircraft. Many aircraft incorporate the to the trailing edge of the horizontal stabilizer and typically directional movement of the nose or tail wheel into the rudder spans most or all of its width. It is controlled in the cockpit control system for ground operation. This allows the operator by pushing or pulling the control stick or yoke forward or aft. to steer the aircraft with the rudder pedals during taxi when the airspeed is not high enough for the control surfaces to be Light aircraft use a system of control cables and pulleys or effective. Some large aircraft have a split rudder arrangement. push-pull tubes to transfer cockpit inputs to the movement This is actually two rudders, one above the other. At low of the elevator. High-performance and large aircraft speeds, both rudders deflect in the same direction when the typically employ more complex systems. Hydraulic power pedals are pushed. At higher speeds, one of the rudders is commonly used to move the elevator on these aircraft. On becomes inoperative as the deflection of a single rudder is aircraft equipped with fly-by-wire controls, a combination aerodynamically sufficient to maneuver the aircraft. of electrical and hydraulic power is used. Dual Purpose Flight Control Surfaces Rudder The ailerons, elevators, and rudder are considered The rudder is the primary control surface that causes an conventional primary control surfaces. However, some aircraft to yaw or move about the vertical axis. This provides aircraft are designed with a control surface that may serve a directional control and thus points the nose of the aircraft dual purpose. For example, elevons perform the combined in the direction desired. Most aircraft have a single rudder functions of the ailerons and the elevator. [Figure 1-57] hinged to the trailing edge of the vertical stabilizer. It is controlled by a pair of foot-operated rudder pedals in the A movable horizontal tail section, called a stabilator, is a cockpit. When the right pedal is pushed forward, it deflects control surface that combines the action of both the horizontal Flight spoilers Outboard aileron Inboard aileron Figure 1-56. Typical flight control surfaces on a transport category aircraft. 1-27  Each contains a movable ruddervator built into the trailing Elevons edge. Movement of the ruddervators can alter the movement of the aircraft around the horizontal and/or vertical axis. Additionally, some aircraft are equipped with flaperons. [Figure 1-60] Flaperons are ailerons which can also act as flaps. Flaps are secondary control surfaces on most wings, discussed in the next section of this chapter. Secondary or Auxiliary Control Surfaces There are several secondary or auxiliary flight control surfaces. Their names, locations, and functions of those for most large aircraft are listed in Figure 1-61. Figure 1-57. Elevons. Flaps Flaps are found on most aircraft. They are usually inboard on the wings’ trailing edges adjacent to the fuselage. Leading edge flaps are also common. They extend forward and down from the inboard wing leading edge. The flaps are lowered to increase the camber of the wings and provide greater lift and control at slow speeds. They enable landing at slower speeds and shorten the amount of runway required for takeoff and landing. The amount that the flaps extend and the angle they form with the wing can be selected from the cockpit. Typically, flaps can extend up to 45–50°. Figure 1-62 shows various aircraft with flaps in the extended position. Flaps are usually constructed of materials and with techniques used on the other airfoils and control surfaces of a particular Figure 1-58. A stabilizer and index marks on a transport category aircraft. Aluminum skin and structure flaps are the norm on aircraft. light aircraft. Heavy and high-performance aircraft flaps may also be aluminum, but the use of composite structures is also common. stabilizer and the elevator. [Figure 1-58] Basically, a stabilator is a horizontal stabilizer that can also be rotated There are various kinds of flaps. Plain flaps form the trailing about the horizontal axis to affect the pitch of the aircraft. edge of the wing when the flap is in the retracted position. [Figure 1-63A] The airflow over the wing continues over the A ruddervator combines the action of the rudder and elevator. upper and lower surfaces of the flap, making the trailing edge [Figure 1-59] This is possible on aircraft with V–tail empennages where the traditional horizontal and vertical stabilizers do not exist. Instead, two stabilizers angle upward and outward from the aft fuselage in a “V” configuration. Flaperons Ruddervator Figure 1-60. Flaperons. Figure 1-59. Ruddervator. 1-28  Secondary/Auxiliary Flight Control Surfaces Name Location Function Extends the camber of the wing for greater lift and slower flight. Flaps Inboard trailing edge of wings Allows control at low speeds for short field takeoffs and landings. Trailing edge of primary Trim tabs flight control surfaces Reduces the force needed to move a primary control surface. Trailing edge of primary Balance tabs Reduces the force needed to move a primary control surface. flight control surfaces Trailing edge of primary Anti-balance tabs Increases feel and effectiveness of primary control surface. flight control surfaces Trailing edge of primary Servo tabs Assists or provides the force for moving a primary flight control. flight control surfaces Spoilers Upper and/or trailing edge of wing Decreases (spoils) lift. Can augment aileron function. Extends the camber of the wing for greater lift and slower flight. Slats Mid to outboard leading edge of wing Allows control at low speeds for short field takeoffs and landings. Outer leading edge of wing Directs air over upper surface of wing during high angle of attack. Slots Lowers stall speed and provides control during slow flight. forward of ailerons Extends the camber of the wing for greater lift and slower flight. Leading edge flap Inboard leading edge of wing Allows control at low speeds for short field takeoffs and landings. NOTE: An aircraft may possess none, one, or a combination of the above control surfaces. Figure 1-61. Secondary or auxiliary control surfaces and respective locations for larger aircraft. Figure 1-62. Various aircraft with flaps in the extended position. A B Plain flap Split flap C Fowler flap Figure 1-63. Various types of flaps. 1-29 of the flap essentially the trailing edge of the wing. The plain flap is hinged so that the trailing edge can be lowered. This increases wing camber and provides greater lift. Hinge point A split flap is normally housed under the trailing edge of the wing. [Figure 1-63B] It is usually just a braced flat metal Actuator plate hinged at several places along its leading edge. The upper surface of the wing extends to the trailing edge of the flap. When deployed, the split flap trailing edge lowers away from the trailing edge of the wing. Airflow over the top of the wing remains the same. Airflow under the wing now follows the camber created by the lowered split flap, increasing lift. Flap extended Flap retracted Fowler flaps not only lower the trailing edge of the wing when deployed but also slide aft, effectively increasing the area of the Retractable nose wing. [Figure 1-63C] This creates more lift via the increased surface area, as well as the wing camber. When stowed, the Figure 1-65. Leading edge flaps. fowler flap typically retracts up under the wing trailing edge similar to a split flap. The sliding motion of a fowler flap can The differing designs of leading edge flaps essentially be accomplished with a worm drive and flap tracks. provide the same effect. Activation of the trailing edge flaps automatically deploys the leading edge flaps, which An enhanced version of the fowler flap is a set of flaps are driven out of the leading edge and downward, extending that actually contains more than one aerodynamic surface. the camber of the wing. Figure 1-66 shows a Krueger flap, Figure 1-64 shows a triple-slotted flap. In this configuration, recognizable by its flat mid-section. the flap consists of a fore flap, a mid flap, and an aft flap. When deployed, each flap section slides aft on tracks as it lowers. The flap sections also separate leaving an open slot Slats between the wing and the fore flap, as well as between each Another leading edge device which extends wing camber is of the flap sections. Air from the underside of the wing flows a slat. Slats can be operated independently of the flaps with through these slots. The result is that the laminar flow on the their own switch in the cockpit. Slats not only extend out upper surfaces is enhanced. The greater camber and effective of the leading edge of the wing increasing camber and lift, wing area increase overall lift. but most often, when fully deployed leave a slot between their trailing edges and the leading edge of the wing. Heavy aircraft often have leading edge flaps that are used [Figure 1-67] This increases the angle of attack at which in conjunction with the trailing edge flaps. [Figure 1-65] the wing will maintain its laminar airflow, resulting in the They can be made of machined magnesium or can have an ability to fly the aircraft slower with a reduced stall speed, aluminum or composite structure. While they are not installed and still maintain control. or operate independently, their use with trailing edge flaps can greatly increase wing camber and lift. When stowed, Spoilers and Speed Brakes leading edge flaps retract into the leading edge of the wing. A spoiler is a device found on the upper surface of many heavy and high-performance aircraft. It is stowed flush to the wing’s u