Basic construction


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Basic construction

  1. 1. CHAPTER 4 AIRCRAFT BASIC CONSTRUCTION INTRODUCTION useless. All materials used to construct an aircraft must be reliable. Reliability minimizes the possibility of Naval aircraft are built to meet certain specified dangerous and unexpected failures.requirements. These requirements must be selected sothey can be built into one aircraft. It is not possible for Many forces and structural stresses act on anone aircraft to possess all characteristics; just as it isnt aircraft when it is flying and when it is static. When it ispossible for an aircraft to have the comfort of a static, the force of gravity produces weight, which ispassenger transport and the maneuverability of a supported by the landing gear. The landing gear absorbsfighter. The type and class of the aircraft determine how the forces imposed on the aircraft by takeoffs andstrong it must be built. A Navy fighter must be fast, landings.maneuverable, and equipped for attack and defense. To During flight, any maneuver that causesmeet these requirements, the aircraft is highly powered acceleration or deceleration increases the forces andand has a very strong structure. stresses on the wings and fuselage. The airframe of a fixed-wing aircraft consists of the Stresses on the wings, fuselage, and landing gear offollowing five major units: aircraft are tension, compression, shear, bending, and 1. Fuselage torsion. These stresses are absorbed by each component of the wing structure and transmitted to the fuselage 2. Wings structure. The empennage (tail section) absorbs the 3. Stabilizers same stresses and transmits them to the fuselage. These 4. Flight controls surfaces stresses are known as loads, and the study of loads is called a stress analysis. Stresses are analyzed and 5. Landing gear considered when an aircraft is designed. The stresses A rotary-wing aircraft consists of the following acting on an aircraft are shown in figure 4-1.four major units: TENSION 1. Fuselage 2. Landing gear Tension (fig. 4-1, view A) is defined as pull. It is the stress of stretching an object or pulling at its ends. 3. Main rotor assembly Tension is the resistance to pulling apart or stretching 4. Tail rotor assembly produced by two forces pulling in opposite directions along the same straight line. For example, an elevator You need to be familiar with the terms used for control cable is in additional tension when the pilotaircraft construction to work in an aviation rating. moves the control column. STRUCTURAL STRESS COMPRESSION LEARNING OBJECTIVE: Identify the five If forces acting on an aircraft move toward each basic stresses acting on an aircraft. other to squeeze the material, the stress is called The primary factors to consider in aircraft compression. Compression (fig. 4-1, view B) is thestructures are strength, weight, and reliability. These opposite of tension. Tension is pull, and compression isfactors determine the requirements to be met by any push. Compression is the resistance to crushingmaterial used to construct or repair the aircraft. produced by two forces pushing toward each other inAirframes must be strong and light in weight. An the same straight line. For example, when an airplane isaircraft built so heavy that it couldnt support more than on the ground, the landing gear struts are under aa few hundred pounds of additional weight would be constant compression stress. 4-1
  2. 2. Figure 4-1.—Five stresses acting on an aircraft.SHEAR compression one instant and under tension the next. The strength of aircraft materials must be great enough Cutting a piece of paper with scissors is an example to withstand maximum force of varying stresses.of a shearing action. In an aircraft structure, shear (fig.4-1, view D) is a stress exerted when two pieces of SPECIFIC ACTION OF STRESSESfastened material tend to separate. Shear stress is theoutcome of sliding one part over the other in opposite You need to understand the stresses encountered ondirections. The rivets and bolts of an aircraft experience the main parts of an aircraft. A knowledge of the basicboth shear and tension stresses. stresses on aircraft structures will help you understand why aircraft are built the way they are. The fuselage ofBENDING an aircraft is subject the fives types of stress—torsion, bending, tension, shear, and compression. Bending (fig. 4-1, view E) is a combination of Torsional stress in a fuselage is created in severaltension and compression. For example, when bending a ways. For example, torsional stress is encountered inpiece of tubing, the upper portion stretches (tension) engine torque on turboprop aircraft. Engine torqueand the lower portion crushes together (compression). tends to rotate the aircraft in the direction opposite toThe wing spars of an aircraft in flight are subject to the direction the propeller is turning. This force createsbending stresses. a torsional stress in the fuselage. Figure 4-2 shows theTORSION effect of the rotating propellers. Also, torsional stress on the fuselage is created by the action of the ailerons Torsional (fig. 4-1, view C) stresses result from a when the aircraft is maneuvered.twisting force. When you wring out a chamois skin, you When an aircraft is on the ground, there is aare putting it under torsion. Torsion is produced in an bending force on the fuselage. This force occursengine crankshaft while the engine is running. Forces because of the weight of the aircraft. Bending increasesthat produce torsional stress also produce torque. when the aircraft makes a carrier landing. This bending action creates a tension stress on the lower skin of theVARYING STRESS fuselage and a compression stress on the top skin. Bending action is shown in figure 4-3. These stresses All structural members of an aircraft are subject to are transmitted to the fuselage when the aircraft is inone or more stresses. Sometimes a structural member flight. Bending occurs because of the reaction of thehas alternate stresses; for example, it is under airflow against the wings and empennage. When the 4-2
  3. 3. TORSIONAL STRESS PROPELLER ROTATION ANfO4O2 Figure 4-2.—Engine torque creates torsion stress in aircraft fuselages.aircraft is in flight, lift forces act upward against the Q4-4. Define the term bending.wings, tending to bend them upward. The wings are Q4-5. Define the term torsion.prevented from folding over the fuselage by theresisting strength of the wing structure. The bendingaction creates a tension stress on the bottom of the CONSTRUCTION MATERIALSwings and a compression stress on the top of the wings. LEARNING OBJECTIVE: Identify the Q4-1. The resistance to pulling apart or stretching various types of metallic and nonmetallic produced by two forces pulling in opposite materials used in aircraft construction. directions along the same straight lines is An aircraft must be constructed of materials that defined by what term? are both light and strong. Early aircraft were made of Q4-2. The resistance to crushing produced by two wood. Lightweight metal alloys with a strength greater forces pushing toward each other in the same than wood were developed and used on later aircraft. straight line is defined by what term? Materials currently used in aircraft construction are classified as either metallic materials or nonmetallic Q4-3. Define the term shear as it relates to an materials. aircraft structure. SSION COMPRE TENSION ANf0403 Figure 4-3.—Bending action occurring during carrier landing. 4-3
  4. 4. METALLIC MATERIALS forces that occur on todays modern aircraft. These steels contain small percentages of carbon, nickel, The most common metals used in aircraft chromium, vanadium, and molybdenum. High-tensileconstruction are aluminum, magnesium, titanium, steels will stand stress of 50 to 150 tons per square inchsteel, and their alloys. without failing. Such steels are made into tubes, rods, and wires.Alloys Another type of steel used extensively is stainless An alloy is composed of two or more metals. The steel. Stainless steel resists corrosion and is particularlymetal present in the alloy in the largest amount is called valuable for use in or near water.the base metal. All other metals added to the base metalare called alloying elements. Adding the alloying NONMETALLIC MATERIALSelements may result in a change in the properties of the In addition to metals, various types of plasticbase metal. For example, pure aluminum is relatively materials are found in aircraft construction. Some ofsoft and weak. However, adding small amounts or these plastics include transparent plastic, reinforcedcopper, manganese, and magnesium will increase plastic, composite, and carbon-fiber materials.aluminums strength many times. Heat treatment canincrease or decrease an alloys strength and hardness. Transparent PlasticAlloys are important to the aircraft industry. Theyprovide materials with properties that pure metals do Transparent plastic is used in canopies,not possess. windshields, and other transparent enclosures. You need to handle transparent plastic surfaces carefullyAluminum because they are relatively soft and scratch easily. At approximately 225°F, transparent plastic becomes soft Aluminum alloys are widely used in modern and pliable.aircraft construction. Aluminum alloys are valuablebecause they have a high strength-to-weight ratio. Reinforced PlasticAluminum alloys are corrosion resistant andcomparatively easy to fabricate. The outstanding Reinforced plastic is used in the construction ofcharacteristic of aluminum is its lightweight. radomes, wingtips, stabilizer tips, antenna covers, and flight controls. Reinforced plastic has a highMagnesium strength-to-weight ratio and is resistant to mildew and rot. Because it is easy to fabricate, it is equally suitable Magnesium is the worlds lightest structural metal. for other parts of the aircraft.It is a silvery-white material that weighs two-thirds asmuch as aluminum. Magnesium is used to make Reinforced plastic is a sandwich-type material (fig.helicopters. Magnesiums low resistance to corrosion 4-4). It is made up of two outer facings and a centerhas limited its use in conventional aircraft. layer. The facings are made up of several layers of glass cloth, bonded together with a liquid resin. The coreTitanium material (center layer) consists of a honeycomb HONEYCOMB Titanium is a lightweight, strong, corrosion- COREresistant metal. Recent developments make titaniumideal for applications where aluminum alloys are tooweak and stainless steel is too heavy. Additionally,titanium is unaffected by long exposure to seawater andmarine atmosphere.Steel Alloys Alloy steels used in aircraft construction have great Anf0404strength, more so than other fields of engineering FACINGSwould require. These materials must withstand the (MULTIPLE LAYERS OF GLASS CLOTH) Figure 4-4.—Reinforced plastic. 4-4
  5. 5. structure made of glass cloth. Reinforced plastic is Q4-8. What are the nonmetallic materials used infabricated into a variety of cell sizes. aircraft construction?Composite and Carbon Fiber FIXED-WING AIRCRAFTMaterials LEARNING OBJECTIVE: Identify the High-performance aircraft require an extra high construction features of the fixed-wing aircraftstrength-to-weight ratio material. Fabrication of and identify the primary, secondary, andcomposite materials satisfies this special requirement. auxiliary flight control surfaces.Composite materials are constructed by using several The principal structural units of a fixed-winglayers of bonding materials (graphite epoxy or boron aircraft are the fuselage, wings, stabilizers, flightepoxy). These materials are mechanically fastened to control surfaces, and landing gear. Figure 4-5 showsconventional substructures. Another type of composite these units of a naval consists of thin graphite epoxy skinsbonded to an aluminum honeycomb core. Carbon fiber NOTE: The terms left or right used in relation tois extremely strong, thin fiber made by heating any of the structural units refer to the right or left handsynthetic fibers, such as rayon, until charred, and then of the pilot seated in the cockpit.layering in cross sections. FUSELAGE Q4-6. Materials currently used in aircraft construc- tion are classified as what type of materials? The fuselage is the main structure, or body, of the Q4-7. What are the most common metallic materials aircraft. It provides space for personnel, cargo, used in aircraft construction? controls, and most of the accessories. The power plant, wings, stabilizers, and landing gear are attached to it. VERTICAL STABILIZER (FIN) HORIZONTAL AILERON STABILIZER RUDDER FLAP ENGINE EXHAUST LEADING EDGE ENGINE OF WING EXHAUST ELEVATOR CANOPY COCKPIT WING ENGINE MAIN ENGINE AIR INLET LANDING RADOME NACELLE FAIRING GEAR NOSE LANDING ANf0405 GEAR Figure 4-5.—Principal structural units on an F-14 aircraft. 4-5
  6. 6. There are two general types of fuselage considered to be of semimonocoque-typeconstruction—welded steel truss and monocoque construction.designs. The welded steel truss was used in smaller The semimonocoque fuselage is constructedNavy aircraft, and it is still being used in some primarily of aluminum alloy, although steel andhelicopters. titanium are found in high-temperature areas. Primary The monocoque design relies largely on the bending loads are taken by the longerons, whichstrength of the skin, or covering, to carry various loads. usually extend across several points of support. TheThe monocoque design may be divided into three longerons are supplemented by other longitudinalclasses—monocoque, semimonocoque, and reinforced members known as stringers. Stringers are moreshell. numerous and lightweight than longerons. · The true monocoque construction uses The vertical structural members are referred to as formers, frame assemblies, and bulkheads to bulkheads, frames, and formers. The heavier vertical give shape to the fuselage. However, the skin members are located at intervals to allow for carries the primary stresses. Since no bracing concentrated loads. These members are also found at members are present, the skin must be strong points where fittings are used to attach other units, such enough to keep the fuselage rigid. The biggest as the wings and stabilizers. problem in monocoque construction is The stringers are smaller and lighter than longerons maintaining enough strength while keeping the and serve as fill-ins. They have some rigidity but are weight within limits. chiefly used for giving shape and for attachment of · Semimonocoque design overcomes the skin. The strong, heavy longerons hold the bulkheads strength-to-weight problem of monocoque and formers. The bulkheads and formers hold the construction. See figure 4-6. In addition to stringers. All of these join together to form a rigid having formers, frame assemblies, and fuselage framework. Stringers and longerons prevent bulkheads, the semimonocoque construction tension and compression stresses from bending the has the skin reinforced by longitudinal fuselage. members. The skin is attached to the longerons, bulkheads, · The reinforced shell has the skin reinforced by and other structural members and carries part of the a complete framework of structural members. load. The fuselage skin thickness varies with the load Different portions of the same fuselage may carried and the stresses sustained at particular loca- belong to any one of the three classes. Most are tion. ANf0406 Figure 4-6.—Semimonocoque fuselage construction. 4-6
  7. 7. There are a number of advantages in using the semimonocoque fuselage can withstandsemimonocoque fuselage. damage and still be strong enough to hold together. · The bulkhead, frames, stringers, and longerons aid in the design and construction of a Points on the fuselage are located by station streamlined fuselage. They add to the strength numbers. Station 0 is usually located at or near the nose and rigidity of the structure. of the aircraft. The other stations are located at measured distances (in inches) aft of station 0. A · The main advantage of the semimonocoque typical station diagram is shown in figure 4-7. On this construction is that it depends on many particular aircraft, fuselage station (FS) 0 is located structural members for strength and rigidity. 93.0 inches forward of the nose. Because of its stressed skin construction, a WS 400 AIRCRAFT STATIONS 380 360 340 320 300 75o WING FS - FUSELAGE 280 20o WING STATION UNSWEPT OVERSWEPT 260 68o WS - WING 240 WING STATION SWEPT 220 200 180 160 140 120 100 80 60 40 20 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 STATIC GROUND ARRESTING HOOK LINE FULLY EXTENDED 0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 ANfO407 Figure 4-7.—Fuselage station diagram of an F-14 aircraft. 4-7
  8. 8. WINGS that is constructed so it can be used as a fuel cell. The wet wing is sealed with a fuel-resistant compound as it Wings develop the major portion of the lift of a is built. The wing holds fuel without the usual rubberheavier-than-air aircraft. Wing structures carry some of cells or tanks.the heavier loads found in the aircraft structure. The The wings of most naval aircraft are of all metal,particular design of a wing depends on many factors, full cantilever construction. Often, they may be foldedsuch as the size, weight, speed, rate of climb, and use of for carrier use. A full cantilever wing structure is verythe aircraft. The wing must be constructed so that it strong. The wing can be fastened to the fuselageholds its aerodynamics shape under the extreme without the use of external bracing, such as wires orstresses of combat maneuvers or wing loading. struts. Wing construction is similar in most modern A complete wing assembly consists of the surfaceaircraft. In its simplest form, the wing is a framework providing lift for the support of the aircraft. It alsomade up of spars and ribs and covered with metal. The provides the necessary flight control of an aircraft wing is shown in figure 4-8. NOTE: The flight control surfaces on a simple Spars are the main structural members of the wing. wing may include only ailerons and trailing edge flaps.They extend from the fuselage to the tip of the wing. All The more complex aircraft may have a variety ofthe load carried by the wing is taken up by the spars. devices, such as leading edge flaps, slats, spoilers, andThe spars are designed to have great bending strength. speed brakes.Ribs give the wing section its shape, and they transmitthe air load from the wing covering to the spars. Ribs Various points on the wing are located by wingextend from the leading edge to the trailing edge of the station numbers (fig. 4-7). Wing station (WS) 0 iswing. located at the centerline of the fuselage, and all wing stations are measured (right or left) from this point (in In addition to the main spars, some wings have a inches).false spar to support the ailerons and flaps. Mostaircraft wings have a removable tip, which streamlines STABILIZERSthe outer end of the wing. Most Navy aircraft are designed with a wing The stabilizing surfaces of an aircraft consist ofreferred to as a wet wing. This term describes the wing vertical and horizontal airfoils. They are called the TRAILING EDGE LEADING EDGE RIBS SPARS ANf0408 Figure 4-8.—Two-spar wing construction. 4-8
  9. 9. vertical stabilizer (or fin) and horizontal stabilizer. FLIGHT CONTROL SURFACESThese two airfoils, along with the rudder and elevators,form the tail section. For inspection and maintenance Flight control surfaces are hinged (movable)purposes, the entire tail section is considered a single airfoils designed to change the attitude of the aircraftunit called the empennage. during flight. These surfaces are divided into three groups—primary, secondary, and auxiliary. The main purpose of stabilizers is to keep theaircraft in straight-and-level flight. The vertical Primary Groupstabilizer maintains the stability of he aircraft about itsvertical axis (fig. 4-9). This is known as directional The primary group of flight control surfacesstability. The vertical stabilizer usually serves as the includes ailerons, elevators, and rudders. The aileronsbase to which the rudder is attached. The horizontal attach to the trailing edge of the wings. They control thestabilizer provides stability of the aircraft about its rolling (or banking) motion of the aircraft. This actionlateral axis. This is known as longitudinal stability. The is known as longitudinal control.horizontal stabilizer usually serves as the base to whichthe elevators are attached. On many newer, The elevators are attached to the horizontalhigh-performance aircraft, the entire vertical and/or stabilizer and control the climb or descent (pitchinghorizontal stabilizer is a movable airfoil. Without the motion) of the aircraft. This action is known as lateralmovable airfoil, the flight control surfaces would lose control.their effectiveness at extremely high altitudes. The rudder is attached to the vertical stabilizer. It Stabilizer construction is similar to wing determines the horizontal flight (turning or yawingconstruction. For greater strength, especially in the motion) of the aircraft. This action is known asthinner airfoil sections typical of trailing edges, a directional control.honeycomb-type construction is used. Some larger The ailerons and elevators are operated from thecarrier-type aircraft have vertical stabilizers that are cockpit by a control stick on single-engine aircraft. Afolded hydraulically to aid aircraft movement aboard yoke and wheel assembly operates the ailerons andaircraft carriers. elevators on multiengine aircraft, such as transport and VERTICAL AXIS LATERAL AXIS ROLL LONGITUDINAL AXIS YAW PITCH ANf0409 Figure 4-9.—Axes and fundamental movements of the aircraft. 4-9
  10. 10. patrol aircraft. The rudder is operated by foot pedals onall types of aircraft. PLAIN FLAPSecondary Group The secondary group includes the trim tabs andspring tabs. Trim tabs are small airfoils recessed intothe trailing edges of the primary control surface. Each SPLIT FLAPtrim tab hinges to its parent primary control surface, butoperates by an independent control. Trim tabs let thepilot trim out an unbalanced condition without exerting LEADING EDGE FLAPpressure on the primary controls. Spring tabs are similar in appearance to trim tabsbut serve an entirely different purpose. Spring tabs areused for the same purpose as hydraulic actuators. They FOWLER FLAPaid the pilot in moving a larger control surface, such asthe ailerons and elevators. ANf0410 Figure 4-10.—Types of flaps.Auxiliary Group The auxiliary group includes the wing flaps,spoilers, speed brakes, and slats. they are flush with the wing skin. In the raised position, WING FLAPS.—Wing flaps give the aircraft they greatly reduce wing lift by destroying the smoothextra lift. Their purpose is to reduce the landing speed. flow of air over the wing surface.Reducing the landing speed shortens the length of the SPEED BRAKES.—Speed brakes are movablelanding rollout. Flaps help the pilot land in small or control surfaces used for reducing the speed of theobstructed areas by increasing the glide angle without aircraft. Some manufacturers refer to them as divegreatly increasing the approach speed. The use of flaps brakes; others refer to them as dive flaps. On someduring takeoff serves to reduce the length of the takeoff aircraft, theyre hinged to the sides or bottom of therun. fuselage. Regardless of their location, speed brakes Some flaps hinge to the lower trailing edges of the serve the same purpose—to keep the airspeed fromwings inboard of the ailerons. Leading edge flaps are building too high when the aircraft dives. Speed brakesused on the F-14 Tomcat and F/A-18 Hornet. Four slow the aircrafts speed before it lands.types of flaps are shown in figure 4-10. The plain flap SLATS.—Slats are movable control surfaces thatforms the trailing edge of the airfoil when the flap is in attach to the leading edge of the wing. When the slat isthe up position. In the split flap, the trailing edge of the retracted, it forms the leading edge of the wing. Whenairfoil is split, and the lower half is hinged and lowers to the slat is open (extended forward), a slot is createdform the flap. The fowler flap operates on rollers and between the slat and the wing leading edge.tracks, causing the lower surface of the wing to roll out High-energy air is introduced into the boundary layerand then extend downward. The leading edge flap over the top of the wing. At low airspeeds, this actionoperates like the plain flap. It is hinged on the bottom improves the lateral control handling characteristics.side. When actuated, the leading edge of the wing This allows the aircraft to be controlled at airspeedsactually extends in a downward direction to increase below normal landing speed. The high-energy air thatthe camber of the wing. Landing flaps are used in flows over the top of the wing is known as boundaryconjunction with other types of flaps. layer control air. Boundary layer control is intended SPOILERS.—Spoilers are used to decrease wing primarily for use during operations from carriers.lift. The specific design, function, and use vary with Boundary layer control air aids in catapult takeoffs anddifferent aircraft. On some aircraft, the spoilers are long arrested landings. Boundary control air can also benarrow surfaces, hinged at their leading edge to the accomplished by directing high-pressure engine bleedupper surfaces of the wings. In the retracted position, air across the top of the wing or flap surface. 4-10
  11. 11. On all high-performance aircraft, the control surfaces have great pressure exerted on them. At high airspeed, it is physically impossible for the pilot to ANf0411 move the controls manually. As a result, power-operated control mechanisms are used. In a Figure 4-11.—Push-pull tube assembly. power-operated system, a hydraulic actuator (cylinder) is located within the linkage to assist the pilot in moving the control surface. A typical flight control mechanism is shown inFLIGHT CONTROL MECHANISMS figure 4-12. This is the elevator control of a lightweight The term flight control refers to the linkage that trainer-type aircraft. It consists of a combination ofconnects the control(s) in the cockpit with the flight push-pull tubes and cables.control surfaces. There are several types of flight The control sticks in the system shown in figurecontrols in naval aircraft; some are manually operated 4-12 are connected to the forward sector by push-pullwhile others are power operated. tubes. The forward sector is connected to the aft (rear ) Manually operated flight control mechanisms are sector by means of cable assemblies. The aft sector isfurther divided into three groups—cable operated, connected to the flight control by another push-pullpush-pull tube operated, and torque tube operated. tube assembly.Some systems may combine two or more of these types. LANDING GEAR In the manually operated cable system, cables areconnected from the control in the cockpit to a bell crank Before World War II, aircraft were made with theiror sector. The bell crank is connected to the control main landing gear located behind the center of gravity.surface. Movement of the cockpit controls transfers An auxiliary gear under the fuselage nose was added.force through the cable to the bell crank, which moves This arrangement became known as the tricycle type ofthe control surface. landing gear. Nearly all present-day Navy aircraft are equipped with tricycle landing gear. The tricycle gear In a push-pull tube system, metal push-pull tubes has the following advantages over older landing gear:(or rods) are used as a substitute for the cables (fig.4-11). Push-pull tubes get their name from the way they · More stable in motion on the groundtransmit force. · Maintains the fuselage in a level position In the torque tube system, metal tubes (rods) withgears at the ends of the tubes are used. Motion is · Increases the pilots visibility and controltransmitted by rotating the tubes and gears. · Makes landing easier, especially in cross winds ANf0412 Figure 4-12.—Typical flight control mechanism. 4-11
  12. 12. ACTUATING TO LEFT CYLINDER MAIN GEAR DOOR DOWNLOCK CYLINDER CYLINDER RETRACTING FROM CYLINDER COMBINED SYSTEM LANDING GEAR DOOR AND SELECTOR DOORLATCH UPLOCK VALVE CYLINDERS CYLINDER MAIN GEAR DOWNLOCK NOSE GEAR CYLINDER NOTE TIMER VALVES ARE USED IN MAIN GEAR SYSTEM TO CONTROL PROPER SEQUENCE. Anf0413 Figure 4-13.—Typical landing gear system. The landing gear system (fig. 4-13) consists of The hook hinges from the structure under the rearthree retractable landing gear assemblies. Each main of the aircraft. A snubber meters hydraulic fluid andlanding gear has a conventional air-oil shock strut, a works in conjunction with nitrogen pressure. Thewheel brake assembly, and a wheel and tire assembly.The nose landing gear has a conventional air-oil shockstrut, a shimmy damper, and a wheel and tire assembly. AIR VALVE The shock strut is designed to absorb the shock thatwould otherwise be transmitted to the airframe duringlanding, taxiing, and takeoff. The air-oil strut is used onall naval aircraft. This type of strut has two telescoping OUTER CYLINDERcylinders filled with hydraulic fluid and compressed airor nitrogen. Figure 4-14 shows the internal construction METERING PINof one type of air-oil shock strut. ORIFICE PLATE The main landing gear is equipped with brakes forstopping the aircraft and assisting the pilot in steeringthe aircraft on the ground. The nose gear of most aircraft can be steered from ORIFICE TORQUE ARMSthe cockpit. This provides greater ease and safety on therunway when landing and taking off and on the taxiwayin taxiing. INNERARRESTING GEAR WHEEL AXLE CYLINDER (PISTON) TOWING EYE A carrier-type aircraft is equipped with an arrestinghook for stopping the aircraft when it lands on thecarrier. The arresting gear has an extendible hook andthe mechanical, hydraulic, and pneumatic equipment ANf0414necessary for hook operation. See figure 4-15. Thearresting hook on most aircraft releases mechanically,lowers pneumatically, and raises hydraulically. Figure 4-14.—Internal construction of a shock strut. 4-12
  13. 13. Q4-11. In an aircraft, what are the main structural members of the wing? Q4-12. What does the term “wet wing” mean? Q4-13. The stabilizing surfaces of an aircraft consist of what two airfoils? Q4-14. What are the three groups of flight control surfaces? Q4-15. What is the purpose of speed brakes on an aircraft? Q4-16. Most present-day Navy aircraft are equipped with what type of landing gear? ROTARY-WING AIRCRAFT LEARNING OBJECTIVE: Identify the construction features of the rotary-wing aircraft and recognize the fundamental ANf0415 differences between rotary-wing and fixed-wing aircraft. Figure 4-15.—Arresting gear installation. Within the past 20 years, helicopters have become a reality, and are found throughout the world. They perform countless tasks suited to their uniquesnubber holds the hook down and prevents it from capabilities.bouncing when it strikes the carrier deck. A helicopter has one or more power-drivenCATAPULT EQUIPMENT horizontal airscrews (rotors) to develop lift and propulsion. If a single main rotor is used, it is necessary Carrier aircraft have built-in equipment for to employ a means to counteract torque. If more thancatapulting off the aircraft carrier. Older aircraft had one main rotor (or tandem) is used, torque is eliminatedhooks on the airframe that attached to the cable bridle. by turning each main rotor in opposite directions.The bridle hooks the aircraft to the ships catapult. The fundamental advantage the helicopter has overNewer aircraft have a launch bar built into the nose fixed-wing aircraft is that lift and control arelanding gear assembly. See figure 4-16. The holdback independent of forward speed. A helicopter can flyassembly allows the aircraft to be secured to the carrier forward, backward, or sideways, or it can remain indeck for full-power turnup of the engine prior to stationary flight (hover) above the ground. No runwaytakeoff. For nose gear equipment, a track attaches to the is required for a helicopter to take off or land. Fordeck to guide the nosewheel into position. The track has example, the roof of an office building is an adequateprovisions for attaching the nose gear to the catapult landing area. The helicopter is considered a safe aircraftshuttle and for holdback. because the takeoff and landing speed is zero, and it has NOTE: The holdback tension bar separates when autorotational capabilities. This allows a controlledthe catapult is fired, allowing the aircraft to be launched descent with rotors turning in case of engine failure inwith the engine at full power. flight. FUSELAGE Q4-9. In fuselage construction, what are the three Like the fuselage of a fixed-wing aircraft, the classes of monocoque design? helicopter fuselage may be welded truss or some formQ4-10. Points on the fuselage are located by what of monocoque construction. Many Navy helicopters are method? of the monocoque design. 4-13
  15. 15. A typical Navy helicopter, the H-60, is shown in rings, drag braces, and safety switches. They are part offigure 4-17. Some of its features include a single main the lower end of the shock strut piston.rotor, twin engine, tractor-type canted tail rotor,controllable stabilizer, fixed landing gear, rescue hoist, Tail Landing Gearexternal cargo hook, and weapons pylons. The fuselageconsists of the entire airframe, sometimes known as the The H-60s tail landing gear is a nonretracting, dualbody group. wheel, 360-degree swiveling type. It is equipped with tubeless tires, tie-down ring, shimmy damper, The body group is an all-metal semimonocoque tail-wheel lock, and an air/oil shock-strut, which servesconstruction. It consists of an aluminum and titanium as an aft touchdown point for the pilots to cushion theskin over a reinforced aluminum frame. landing shock.LANDING GEAR GROUP MAIN ROTOR ASSEMBLY The landing gear group includes all the equipment The main rotor (rotor wing) and rotor head (hubnecessary to support the helicopter when it is not in assembly) are identical in theory of flight but differ inflight. There are several types of landing gear on engineering or design. They are covered here becausehelicopters—conventional fixed (skid type), their functions are closely related. The power plant,retractable, and nonretractable. transmission, drive-train, hydraulic flight control, and rotor systems all work together. Neither has a functionMain Landing Gear without the other. The H-60s nonretracting main landing gear Rotary Wingconsists of two single axle, air/oil type of shock-strutassemblies that mount to the fuselage. Each is equipped The main rotor on the H-60 (fig. 4-17) has fourwith tubeless tires, hydraulic disc brakes, tie-down identical wing blades. Other types of helicopters may Anf0417 Figure 4-17.—H-60 helicopter. 4-15
  16. 16. have two, four, five, six, or seven blades. Figure 4-18 main gearbox or transmission. The flight controls andshows some typical rotor blades. hydraulic servos transmit movements to the rotor blades. The principal components of the rotor head are Rotary-wing blades are made of titanium, the hub and swashplate assemblies (fig. 4-19). The hubaluminum alloys, fiber glass, graphite, honeycomb is one piece, made of titanium and sits on top of thecore, nickel, and steel. Each has a nitrogen-filled, rotor mast. Attaching components are the sleeve andpressurized, hollow internal spar, which runs the length spindles, blade fold components, vibration absorber,of the blade. The cuff provides the attachment of the bearings, blade dampers, pitch change horns,blade to the rotor hub. A titanium abrasion strip covers adjustable pitch control rods, blade fold hinges, balancethe entire leading edge of the spar from the cuff end to weights, antiflapping and droop stops, and faring.the removable blade tip faring. This extends the life ofthe rotor blade. The swashplate consists of a rotating disc (upper), The examples shown in figure 4-18 show other stationary (lower) portion with a scissors and sleevefeatures—trim tabs, deicing protection, balance assembly separated by a bearing. The swashplate ismarkings, and construction. permitted to slide on the main rotor vertical driveshaft and mounts on top the main transmission. The entireMain Rotor Head/Hub Assembly assembly can tilt in any direction following the motion of the flight controls. The rotor head is fully articulating and is rotated by The hydraulic servo cylinders, swashplate, andtorque from the engines through the drive train and adjustable pitch control rods permit movement of the DEICE ANTI-CHAFE CONNECTION STRIP ABRASION STRIP TIP CAP BLADE INSPECTION INDICATOR BALANCE STRIP BLADE CUFF TRIM TABS TIP CAP 1 SPAR ABRASION 2 3 STRIP 4 ICE GUARD 5 6 7 SPAR 8 9 10 11 ROOT POCKET 12 13 14 POCKET IDENTIFICATION 15 16 17 CUFF 18 19 20 21 22 23 ANf0418 Figure 4-18.—Types of main rotor blades. 4-16
  17. 17. SPINDLE ASSEMBLY FAIRING BIFILAR FOLD HINGE ROTOR HUB PITCH LOCK ACTUATOR DAMPER BLADE FOLD BLADE ACTUATOR LOCKPIN PULLERSROTOR HEAD BALANCE WEIGHTS LOWER PRESSURE PLATE PITCH CHANGE ROTATING SCISSORS HORN PITCH CONTROL SWASHPLATE ANf0119 ROD Figure 4-19.—Main rotor head/hub assembly.flight controls to be transmitted to the rotary-wing · Flap is the tendency of the blade to rise withblades. The sleeve and spindle and blade dampers allow high-lift demands as it tries to screw itselflimited movement of the blades in relation to the hub. upward into the air.These movements are known as lead, lag, and flap. Antiflapping stops and droop stops restrict flapping · Lead occurs during slowing of the drive and conning motion of the rotary-wing head and blades mechanism when the blades have a tendency to at low rotor rpm when slowing or stopping. remain in motion. TAIL ROTOR GROUP · Lag is the opposite of lead and occurs during acceleration when the blade has been at rest The directional control and antitorque action of the and tends to remain at rest. helicopter is provided by the tail rotor group. See 4-17
  18. 18. figure 4-20. These components are similar in function such items as the hub, spindle, pitch control beam, pitchto the main rotor. change links, bearings, and tail rotor blades. Change in blade pitch is accomplished through thePylon pitch change shaft that moves through the horizontal The pylon, shown in figure 4-20, attaches on the shaft of the tail gearbox, which drives the rotary rudderaircraft to the main fuselage by hinge fittings. These assembly. As the shaft moves inward toward the tailhinge fittings serve as the pivot point for the pylon to gearbox, pitch of the blade is decreased. As the shaftfold along the fuselage. Folding the pylon reduces the moves outward from the tail gearbox, pitch of the bladeoverall length of the helicopter, which helps for is increased. The pitch control beam is connected byconfined shipboard handling. links to the forked brackets on the blade sleeves. The pylon houses the intermediate and tail rotor Rotary Rudder Bladesgearboxes, tail rotor drive shaft, cover, tail bumper, Like the blades on a main rotor head, the bladesposition/anticollision lights, hydraulic servos, flight found on a rotary rudder head may differ, depending oncontrol push-pull tubes/cables/bell cranks, stabilizer/ the type of aircraft. Tail rotor blades may consist of theelevator flight control surface, some antennas, and following components:rotary rudder assembly. · Aluminum alloy, graphite composite, orRotary Rudder Head titanium spar · Aluminum pocket and skin with honeycomb The rudder head can be located on either side of the core or cross-ply fiber glass exteriorpylon, depending on the type of aircraft, and includes · Aluminum or graphite composite tip cap ROTARY RUDDER BLADE PITCH CHANGE LINK SPINDLE ROTARY RUDDER HUB TAIL ROTOR GEAR BOX PYLON PITCH CONTROL BEAM ANf0420 Figure 4-20.—Tail rotor group. 4-18
  19. 19. · Aluminum trailing edge cap · A reservoir to hold a supply of hydraulic fluid · Aluminum or polyurethane and nickel abrasion · A pump to provide a flow of fluid leading edge strip · Tubing to transmit the fluid Additionally, rotary rudder blades may havedeicing provisions, such as electrothermal blankets that · A selector valve to direct the flow of fluidare bonded into the blades leading edge. or a neoprene · An actuating unit to convert the fluid pressureanti-icing guard embedded with electrical heating into useful workelements.Q4-17. What is the main advantage of rotary-wing A simple system using these essential units is aircraft over fixed-wing aircraft? shown in figure 4-21.Q4-18. What are the three types of landing gear used You can trace the flow of fluid from the reservoir on helicopters? through the pump to the selector valve. In figure 4-21, the flow of fluid created by the pump flows through theQ4-19. The directional control and antitorque action valve to the right end of the actuating cylinder. Fluid of the helicopter is provided by what group? pressure forces the piston to the left. At the same time, the fluid that is on the left of the piston is forced out. It AIRCRAFT HYDRAULIC SYSTEMS goes up through the selector valve and back to the reservoir through the return line. LEARNING OBJECTIVE: Identify the components of aircraft hydraulic systems and When the selector valve is moved to the position recognize their functions. indicated by the dotted lines, the fluid from the pump flows to the left side of the actuating cylinder. The aircraft hydraulic systems found on most naval Movement of the piston can be stopped at any timeaircraft perform many functions. Some systems simply by moving the selector valve to neutral. Whenoperated by hydraulics are flight controls, landing gear, the selector valve is in this position, all four ports arespeed brakes, fixed-wing and rotary-wing folding closed, and pressure is trapped in both working lines.mechanisms, auxiliary systems, and wheel brakes. Hydraulics has many advantages as a power sourcefor operating these units on aircraft. RESERVOIR · Hydraulics combine the advantages of lightweight, ease of installation, simplification of inspection, and minimum maintenance requirements. PRESSURE LINE · Hydraulics operation is almost 100-percent efficient, with only a negligible loss due to fluid friction. HAND However, there are some disadvantages to using PUMPhydraulics. RETURN LINE SELECTOR VALVE · The possibility of leakage, both internal and IN "DOWN" external, may cause the complete system to POSITION become inoperative. SELECTOR VALVE IN "UP" · Contamination by foreign matter in the system POSITION can cause malfunction of any unit. Cleanliness WORKING in hydraulics cannot be overemphasized. LINESCOMPONENTS OF A BASIC HYDRAULIC ANF0421 ACTUATINGSYSTEM UNIT Basically, any hydraulic system contains the Figure 4-21.—Basic hydraulic system, hand pump operated.following units: 4-19
  20. 20. Figure 4-22 shows a basic system with the addition automatically adjusts to supply the proper volume ofof a power-driven pump and other essential fluid as needed.components. These components are the filter, pressure The accumulator serves a twofold purpose.regulator, accumulator, pressure gauge, relief valve,and two check valves. The function of these 1. It serves as a cushion or shock absorber bycomponents is described below. maintaining an even pressure in the system. The filter (fig. 4-22) removes foreign particles 2. It stores enough fluid under pressure to providefrom the fluid, preventing moisture, dust, grit, and other for emergency operation of certain actuatingundesirable matter from entering the system. units. The pressure regulator (fig. 4-22) unloads or The accumulator is designed with a compressed-airrelieves the power-driven pump when the desired chamber separated from the fluid by a flexiblepressure in the system is reached. Therefore, it is often diaphragm, or a removable piston.referred to as an unloading valve. With none of the The pressure gauge indicates the amount ofactuating units operating, the pressure in the line pressure in the system.between the pump and selector valve builds up to thedesired point. A valve in the pressure regulator The relief valve is a safety valve installed in theautomatically opens and fluid is bypassed back to the system. When fluid is bypassed through the valve to thereservoir. (The bypass line is shown in figure 4-22, return line, it returns to the reservoir. This actionleading from the pressure regulator to the return line.) prevents excessive pressure in the system. NOTE: Many aircraft hydraulic systems do not Check valves allow the flow of fluid in oneuse a pressure regulator. These systems use a pump that direction only. There are numerous check valves installed at various points in the lines of all aircraft hydraulic systems. A careful study of figure 4-22 shows why the two check valves are necessary in this system. One check valve prevents power pump pressure from entering the hand-pump line. The other valve prevents hand-pump pressure from being directed to the accumulator. HYDRAULIC CONTAMINATION Hydraulic contamination is defined as foreign material in the hydraulic system of an aircraft. Foreign material might be grit, sand, dirt, dust, rust, water, or any other substance that is not soluble in the hydraulic fluid. There are two basic ways to contaminate a hydraulic system. One is to inject particles, and the other is to intermix fluids, including water. Particle contamination in a system may be self-generated through normal wear of system components. It is the injection of contaminants from ANf0422 outside that usually causes the most trouble. Regardless of its origin, any form of contamination in the hydraulic 1. Reservoir 7. Hand pump system will slow performance. In extreme cases, it 2. Power pump 8. Pressure gauge seriously affects safety. 3. Filter 9. Relief valve 4. Pressure regulator 10. Selector valve A single grain of sand or grit can cause internal 5. Accumulator 11. Actuating unit failure of a hydraulic component. Usually, this type of 6. Check valves contamination comes from poor servicing andFigure 4-22.—Basic hydraulic system with addition of power pump. fluid-handling procedures. For this reason, the highest 4-20
  21. 21. level of cleanliness must be maintained when working brakes, emergency landing gear extension, emergencyon hydraulic components. flap extension, and for canopy release mechanisms. Only approved fill stand units are used to service When the control valve is properly positioned, thenaval aircraft hydraulic systems. By following a few compressed air in the storage bottle is routed throughbasic rules, you can service hydraulic systems safely the shuttle valve to the actuating cylinder.and keep contamination to a minimum. NOTE: The shuttle valve is a pressure-operated · Never use fluid that has been left open for an valve that separates the normal hydraulic system from undetermined period of time. Hydraulic fluid the emergency pneumatic system. When the control that is exposed to air will absorb dust and dirt. handle is returned to the normal position, the air pressure in the lines is vented overboard through the · Never pour fluid from one container into vent port of the control valve. another. The other type of pneumatic system in use has its · Use only approved servicing units for the own air compressor. It also has other equipment specific aircraft. necessary to maintain an adequate supply of · Maintain hydraulic fluid-handling equipment compressed air during flight. Most systems of this type in a high state of cleanliness. must be serviced on the ground prior to flight. The air · Always make sure you use the correct hydraulic fluid. Contamination of the hydraulic system may becaused by wear or failure of hydraulic components andseals. This type of contamination is usually foundthrough filter inspection and fluid analysis. Continuedoperation of a contaminated system may causemalfunctioning or early failure of hydrauliccomponents.Q4-20. What are two disadvantages of a hydraulic system?Q4-21. On a basic hydraulic system, what is the purpose of the selector valve?Q4-22. On a basic hydraulic system, what is the purpose of the actuating unit?Q4-23. Define hydraulic contamination. PNEUMATIC SYSTEMS LEARNING OBJECTIVE: Identify the components of aircraft pneumatic systems and recognize their functions. There are two types of pneumatic systems currentlyused in naval aircraft. One type uses storage bottles foran air source, and the other has its own air compressor. Generally, the storage bottle system is used only foremergency operation. See figure 4-23. This system hasan air bottle, a control valve in the cockpit for releasingthe contents of the cylinders, and a ground charge ANf0423(filler) valve. The storage bottle must be filled withcompressed air or nitrogen prior to flight. Air storagecylinder pneumatic systems are in use for emergency Figure 4-23.—Emergency pneumatic system. 4-21
  22. 22. compressor used in most aircraft is driven by a SUMMARYhydraulic motor. Aircraft that have an air compressor In this chapter, you have learned about aircraftuse the compressed air for normal and emergency construction and the materials used in construction.system operation. You have also learned about the features and materialsQ4-24. What are the two types of pneumatic systems used to absorb stress on both fixed-wing and currently used in naval aircraft? rotary-wing aircraft. 4-22