CHAPTER 4

AIRCRAFT BASIC CONSTRUCTION

INTRODUCTION

Naval aircraft are built to meet certain specifiedrequirements. These requirements must be selected sothey can be built into one aircraft. It is not possible forone aircraft to possess all characteristics; just as it isn'tpossible for an aircraft to have the comfort of apassenger transport and the maneuverability of afighter. The type and class of the aircraft determine howstrong it must be built. A Navy fighter must be fast,maneuverable, and equipped for attack and defense. Tomeet these requirements, the aircraft is highly poweredand has a very strong structure.

The airframe of a fixed-wing aircraft consists of thefollowing five major units:

1. Fuselage

2. Wings

3. Stabilizers

4. Flight controls surfaces

5. Landing gear

A rotary-wing aircraft consists of the followingfour major units:

1. Fuselage

2. Landing gear

3. Main rotor assembly

4. Tail rotor assembly

You need to be familiar with the terms used foraircraft construction to work in an aviation rating.

STRUCTURAL STRESS

LEARNING OBJECTIVE: Identify the fivebasic stresses acting on an aircraft.

The primary factors to consider in aircraftstructures are strength, weight, and reliability. Thesefactors determine the requirements to be met by anymaterial used to construct or repair the aircraft.Airframes must be strong and light in weight. Anaircraft built so heavy that it couldn't support more thana few hundred pounds of additional weight would be

useless. All materials used to construct an aircraft mustbe reliable. Reliability minimizes the possibility ofdangerous and unexpected failures.

Many forces and structural stresses act on anaircraft when it is flying and when it is static. When it isstatic, the force of gravity produces weight, which issupported by the landing gear. The landing gear absorbsthe forces imposed on the aircraft by takeoffs andlandings.

During flight, any maneuver that causesacceleration or deceleration increases the forces andstresses on the wings and fuselage.

Stresses on the wings, fuselage, and landing gear ofaircraft are tension, compression, shear, bending, andtorsion. These stresses are absorbed by each componentof the wing structure and transmitted to the fuselagestructure. The empennage (tail section) absorbs thesame stresses and transmits them to the fuselage. Thesestresses are known as loads, and the study of loads iscalled a stress analysis. Stresses are analyzed andconsidered when an aircraft is designed. The stressesacting on an aircraft are shown in figure 4-1.

TENSION

Tension (fig. 4-1, view A) is defined as pull. It is thestress of stretching an object or pulling at its ends.Tension is the resistance to pulling apart or stretchingproduced by two forces pulling in opposite directionsalong the same straight line. For example, an elevatorcontrol cable is in additional tension when the pilotmoves the control column.

COMPRESSION

If forces acting on an aircraft move toward eachother to squeeze the material, the stress is calledcompression. Compression (fig. 4-1, view B) is theopposite of tension. Tension is pull, and compression ispush. Compression is the resistance to crushingproduced by two forces pushing toward each other inthe same straight line. For example, when an airplane ison the ground, the landing gear struts are under aconstant compression stress.

4-1

SHEAR

Cutting a piece of paper with scissors is an exampleof a shearing action. In an aircraft structure, shear (fig.4-1, view D) is a stress exerted when two pieces offastened material tend to separate. Shear stress is theoutcome of sliding one part over the other in oppositedirections. The rivets and bolts of an aircraft experienceboth shear and tension stresses.

BENDING

Bending (fig. 4-1, view E) is a combination oftension and compression. For example, when bending apiece of tubing, the upper portion stretches (tension)and the lower portion crushes together (compression).The wing spars of an aircraft in flight are subject tobending stresses.

TORSION

Torsional (fig. 4-1, view C) stresses result from atwisting force. When you wring out a chamois skin, youare putting it under torsion. Torsion is produced in anengine crankshaft while the engine is running. Forcesthat produce torsional stress also produce torque.

VARYING STRESS

All structural members of an aircraft are subject toone or more stresses. Sometimes a structural memberhas alternate stresses; for example, it is under

compression one instant and under tension the next.The strength of aircraft materials must be great enoughto withstand maximum force of varying stresses.

SPECIFIC ACTION OF STRESSES

You need to understand the stresses encountered onthe main parts of an aircraft. A knowledge of the basicstresses on aircraft structures will help you understandwhy aircraft are built the way they are. The fuselage ofan aircraft is subject the fives types of stress—torsion,bending, tension, shear, and compression.

Torsional stress in a fuselage is created in severalways. For example, torsional stress is encountered inengine torque on turboprop aircraft. Engine torquetends to rotate the aircraft in the direction opposite tothe direction the propeller is turning. This force createsa torsional stress in the fuselage. Figure 4-2 shows theeffect of the rotating propellers. Also, torsional stresson the fuselage is created by the action of the aileronswhen the aircraft is maneuvered.

When an aircraft is on the ground, there is abending force on the fuselage. This force occursbecause of the weight of the aircraft. Bending increaseswhen the aircraft makes a carrier landing. This bendingaction creates a tension stress on the lower skin of thefuselage and a compression stress on the top skin.Bending action is shown in figure 4-3. These stressesare transmitted to the fuselage when the aircraft is inflight. Bending occurs because of the reaction of theairflow against the wings and empennage. When the

4-2

Figure 4-1.—Five stresses acting on an aircraft.

aircraft is in flight, lift forces act upward against thewings, tending to bend them upward. The wings areprevented from folding over the fuselage by theresisting strength of the wing structure. The bendingaction creates a tension stress on the bottom of thewings and a compression stress on the top of the wings.

Q4-1. The resistance to pulling apart or stretchingproduced by two forces pulling in oppositedirections along the same straight lines isdefined by what term?

Q4-2. The resistance to crushing produced by twoforces pushing toward each other in the samestraight line is defined by what term?

Q4-3. Define the term shear as it relates to anaircraft structure.

Q4-4. Define the term bending.

Q4-5. Define the term torsion.

CONSTRUCTION MATERIALS

LEARNING OBJECTIVE: Identify thevarious types of metallic and nonmetallicmaterials used in aircraft construction.

An aircraft must be constructed of materials thatare both light and strong. Early aircraft were made ofwood. Lightweight metal alloys with a strength greaterthan wood were developed and used on later aircraft.Materials currently used in aircraft construction areclassified as either metallic materials or nonmetallicmaterials.

4-3

TORSIONALSTRESS

PROPELLERROTATION

ANfO4O2

Figure 4-2.—Engine torque creates torsion stress in aircraft fuselages.

COMPRESSION

TENSIONANf0403

Figure 4-3.—Bending action occurring during carrier landing.

METALLIC MATERIALS

The most common metals used in aircraftconstruction are aluminum, magnesium, titanium,steel, and their alloys.

Alloys

An alloy is composed of two or more metals. Themetal present in the alloy in the largest amount is calledthe base metal. All other metals added to the base metalare called alloying elements. Adding the alloyingelements may result in a change in the properties of thebase metal. For example, pure aluminum is relativelysoft and weak. However, adding small amounts orcopper, manganese, and magnesium will increasealuminum's strength many times. Heat treatment canincrease or decrease an alloy's strength and hardness.Alloys are important to the aircraft industry. Theyprovide materials with properties that pure metals donot possess.

Aluminum

Aluminum alloys are widely used in modernaircraft construction. Aluminum alloys are valuablebecause they have a high strength-to-weight ratio.Aluminum alloys are corrosion resistant andcomparatively easy to fabricate. The outstandingcharacteristic of aluminum is its lightweight.

Magnesium

Magnesium is the world's lightest structural metal.It is a silvery-white material that weighs two-thirds asmuch as aluminum. Magnesium is used to makehelicopters. Magnesium's low resistance to corrosionhas limited its use in conventional aircraft.

Titanium

Titanium is a lightweight, strong, corrosion-resistant 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 greatstrength, more so than other fields of engineeringwould require. These materials must withstand the

forces that occur on today's modern aircraft. Thesesteels contain small percentages of carbon, nickel,chromium, vanadium, and molybdenum. High-tensilesteels will stand stress of 50 to 150 tons per square inchwithout failing. Such steels are made into tubes, rods,and wires.

Another type of steel used extensively is stainlesssteel. Stainless steel resists corrosion and is particularlyvaluable for use in or near water.

NONMETALLIC MATERIALS

In addition to metals, various types of plasticmaterials are found in aircraft construction. Some ofthese plastics include transparent plastic, reinforcedplastic, composite, and carbon-fiber materials.

Transparent Plastic

Transparent plastic is used in canopies,windshields, and other transparent enclosures. Youneed to handle transparent plastic surfaces carefullybecause they are relatively soft and scratch easily. Atapproximately 225°F, transparent plastic becomes softand pliable.

Reinforced Plastic

Reinforced plastic is used in the construction ofradomes, wingtips, stabilizer tips, antenna covers, andflight controls. Reinforced plastic has a highstrength-to-weight ratio and is resistant to mildew androt. Because it is easy to fabricate, it is equally suitablefor other parts of the aircraft.

Reinforced plastic is a sandwich-type material (fig.4-4). It is made up of two outer facings and a centerlayer. The facings are made up of several layers of glasscloth, bonded together with a liquid resin. The corematerial (center layer) consists of a honeycomb

4-4

HONEYCOMBCORE

FACINGS

(MULTIPLE LAYERS OF GLASS CLOTH)

Anf0404

Figure 4-4.—Reinforced plastic.

structure made of glass cloth. Reinforced plastic isfabricated into a variety of cell sizes.

Composite and Carbon FiberMaterials

High-performance aircraft require an extra highstrength-to-weight ratio material. Fabrication ofcomposite materials satisfies this special requirement.Composite materials are constructed by using severallayers of bonding materials (graphite epoxy or boronepoxy). These materials are mechanically fastened toconventional substructures. Another type of compositeconstruction consists of thin graphite epoxy skinsbonded to an aluminum honeycomb core. Carbon fiberis extremely strong, thin fiber made by heatingsynthetic fibers, such as rayon, until charred, and thenlayering in cross sections.

Q4-6. Materials currently used in aircraft construc-tion are classified as what type of materials?

Q4-7. What are the most common metallic materialsused in aircraft construction?

Q4-8. What are the nonmetallic materials used inaircraft construction?

FIXED-WING AIRCRAFT

LEARNING OBJECTIVE: Identify theconstruction features of the fixed-wing aircraftand identify the primary, secondary, andauxiliary flight control surfaces.

The principal structural units of a fixed-wingaircraft are the fuselage, wings, stabilizers, flightcontrol surfaces, and landing gear. Figure 4-5 showsthese units of a naval aircraft.

NOTE: The terms left or right used in relation toany of the structural units refer to the right or left handof the pilot seated in the cockpit.

FUSELAGE

The fuselage is the main structure, or body, of theaircraft. It provides space for personnel, cargo,controls, and most of the accessories. The power plant,wings, stabilizers, and landing gear are attached to it.

4-5

ENGINENACELLE

HORIZONTALSTABILIZER

MAINLANDING

GEAR

WING

NOSELANDING

GEAR

RADOME

CANOPY

AILERON

LEADINGEDGE

OF WING

FLAP

ENGINEEXHAUST

RUDDERENGINE

EXHAUST

VERTICALSTABILIZER

(FIN)

ENGINEAIR INLETFAIRING

ELEVATOR

COCKPIT

ANf0405

Figure 4-5.—Principal structural units on an F-14 aircraft.

There are two general types of fuselageconstruction—welded steel truss and monocoquedesigns. The welded steel truss was used in smallerNavy aircraft, and it is still being used in somehelicopters.

The monocoque design relies largely on thestrength of the skin, or covering, to carry various loads.The monocoque design may be divided into threeclasses—monocoque, semimonocoque, and reinforcedshell.

� The true monocoque construction usesformers, frame assemblies, and bulkheads togive shape to the fuselage. However, the skincarries the primary stresses. Since no bracingmembers are present, the skin must be strongenough to keep the fuselage rigid. The biggestproblem in monocoque construction ismaintaining enough strength while keeping theweight within limits.

� Semimonocoque design overcomes thestrength-to-weight problem of monocoqueconstruction. See figure 4-6. In addition tohaving formers, frame assemblies, andbulkheads, the semimonocoque constructionhas the skin reinforced by longitudinalmembers.

� The reinforced shell has the skin reinforced bya complete framework of structural members.Different portions of the same fuselage maybelong to any one of the three classes. Most are

considered to be of semimonocoque-typeconstruction.

The semimonocoque fuselage is constructedprimarily of aluminum alloy, although steel andtitanium are found in high-temperature areas. Primarybending loads are taken by the longerons, whichusually extend across several points of support. Thelongerons are supplemented by other longitudinalmembers known as stringers. Stringers are morenumerous and lightweight than longerons.

The vertical structural members are referred to asbulkheads, frames, and formers. The heavier verticalmembers are located at intervals to allow forconcentrated loads. These members are also found atpoints where fittings are used to attach other units, suchas the wings and stabilizers.

The stringers are smaller and lighter than longeronsand serve as fill-ins. They have some rigidity but arechiefly used for giving shape and for attachment ofskin. The strong, heavy longerons hold the bulkheadsand formers. The bulkheads and formers hold thestringers. All of these join together to form a rigidfuselage framework. Stringers and longerons preventtension and compression stresses from bending thefuselage.

The skin is attached to the longerons, bulkheads,and other structural members and carries part of theload. The fuselage skin thickness varies with the loadcarried and the stresses sustained at particular loca-tion.

4-6

ANf0406

Figure 4-6.—Semimonocoque fuselage construction.

There are a number of advantages in using thesemimonocoque fuselage.

� The bulkhead, frames, stringers, and longeronsaid in the design and construction of astreamlined fuselage. They add to the strengthand rigidity of the structure.

� The main advantage of the semimonocoqueconstruction is that it depends on manystructural members for strength and rigidity.Because of its stressed skin construction, a

semimonocoque fuselage can withstanddamage and still be strong enough to holdtogether.

Points on the fuselage are located by stationnumbers. Station 0 is usually located at or near the noseof the aircraft. The other stations are located atmeasured distances (in inches) aft of station 0. Atypical station diagram is shown in figure 4-7. On thisparticular aircraft, fuselage station (FS) 0 is located93.0 inches forward of the nose.

4-7

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0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850

ARRESTING HOOKFULLY EXTENDED

STATICGROUND

LINE

FS - FUSELAGESTATION

WS - WINGSTATION

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AIRCRAFT STATIONS

WS

20 WINGUNSWEPT

o

68WING

SWEPT

o

75 WINGOVERSWEPT

o

Figure 4-7.—Fuselage station diagram of an F-14 aircraft.

WINGS

Wings develop the major portion of the lift of aheavier-than-air aircraft. Wing structures carry some ofthe heavier loads found in the aircraft structure. Theparticular design of a wing depends on many factors,such as the size, weight, speed, rate of climb, and use ofthe aircraft. The wing must be constructed so that itholds its aerodynamics shape under the extremestresses of combat maneuvers or wing loading.

Wing construction is similar in most modernaircraft. In its simplest form, the wing is a frameworkmade up of spars and ribs and covered with metal. Theconstruction of an aircraft wing is shown in figure 4-8.

Spars are the main structural members of the wing.They extend from the fuselage to the tip of the wing. Allthe load carried by the wing is taken up by the spars.The spars are designed to have great bending strength.Ribs give the wing section its shape, and they transmitthe air load from the wing covering to the spars. Ribsextend from the leading edge to the trailing edge of thewing.

In addition to the main spars, some wings have afalse spar to support the ailerons and flaps. Mostaircraft wings have a removable tip, which streamlinesthe outer end of the wing.

Most Navy aircraft are designed with a wingreferred to as a wet wing. This term describes the wing

that is constructed so it can be used as a fuel cell. Thewet wing is sealed with a fuel-resistant compound as itis built. The wing holds fuel without the usual rubbercells or tanks.

The wings of most naval aircraft are of all metal,full cantilever construction. Often, they may be foldedfor carrier use. A full cantilever wing structure is verystrong. The wing can be fastened to the fuselagewithout the use of external bracing, such as wires orstruts.

A complete wing assembly consists of the surfaceproviding lift for the support of the aircraft. It alsoprovides the necessary flight control surfaces.

NOTE: The flight control surfaces on a simplewing may include only ailerons and trailing edge flaps.The more complex aircraft may have a variety ofdevices, such as leading edge flaps, slats, spoilers, andspeed brakes.

Various points on the wing are located by wingstation numbers (fig. 4-7). Wing station (WS) 0 islocated at the centerline of the fuselage, and all wingstations are measured (right or left) from this point (ininches).

STABILIZERS

The stabilizing surfaces of an aircraft consist ofvertical and horizontal airfoils. They are called the

4-8

TRAILING EDGE

RIBS

SPARS

LEADING EDGE

ANf0408

Figure 4-8.—Two-spar wing construction.

vertical stabilizer (or fin) and horizontal stabilizer.These two airfoils, along with the rudder and elevators,form the tail section. For inspection and maintenancepurposes, the entire tail section is considered a singleunit called the empennage.

The main purpose of stabilizers is to keep theaircraft in straight-and-level flight. The verticalstabilizer maintains the stability of he aircraft about itsvertical axis (fig. 4-9). This is known as directionalstability. The vertical stabilizer usually serves as thebase to which the rudder is attached. The horizontalstabilizer provides stability of the aircraft about itslateral axis. This is known as longitudinal stability. Thehorizontal stabilizer usually serves as the base to whichthe elevators are attached. On many newer,high-performance aircraft, the entire vertical and/orhorizontal stabilizer is a movable airfoil. Without themovable airfoil, the flight control surfaces would losetheir effectiveness at extremely high altitudes.

Stabilizer construction is similar to wingconstruction. For greater strength, especially in thethinner airfoil sections typical of trailing edges, ahoneycomb-type construction is used. Some largercarrier-type aircraft have vertical stabilizers that arefolded hydraulically to aid aircraft movement aboardaircraft carriers.

FLIGHT CONTROL SURFACES

Flight control surfaces are hinged (movable)airfoils designed to change the attitude of the aircraftduring flight. These surfaces are divided into threegroups—primary, secondary, and auxiliary.

Primary Group

The primary group of flight control surfacesincludes ailerons, elevators, and rudders. The aileronsattach to the trailing edge of the wings. They control therolling (or banking) motion of the aircraft. This actionis known as longitudinal control.

The elevators are attached to the horizontalstabilizer and control the climb or descent (pitchingmotion) of the aircraft. This action is known as lateralcontrol.

The rudder is attached to the vertical stabilizer. Itdetermines the horizontal flight (turning or yawingmotion) of the aircraft. This action is known asdirectional control.

The ailerons and elevators are operated from thecockpit by a control stick on single-engine aircraft. Ayoke and wheel assembly operates the ailerons andelevators on multiengine aircraft, such as transport and

4-9

ANf0409

YAW

PITCH

ROLL

VERTICAL AXIS

LATERAL AXIS

LONGITUDINAL AXIS

Figure 4-9.—Axes and fundamental movements of the aircraft.

patrol aircraft. The rudder is operated by foot pedals onall types of aircraft.

Secondary Group

The secondary group includes the trim tabs andspring tabs. Trim tabs are small airfoils recessed intothe trailing edges of the primary control surface. Eachtrim tab hinges to its parent primary control surface, butoperates by an independent control. Trim tabs let thepilot trim out an unbalanced condition without exertingpressure 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. Theyaid the pilot in moving a larger control surface, such asthe ailerons and elevators.

Auxiliary Group

The auxiliary group includes the wing flaps,spoilers, speed brakes, and slats.

WING FLAPS.—Wing flaps give the aircraftextra lift. Their purpose is to reduce the landing speed.Reducing the landing speed shortens the length of thelanding rollout. Flaps help the pilot land in small orobstructed areas by increasing the glide angle withoutgreatly increasing the approach speed. The use of flapsduring takeoff serves to reduce the length of the takeoffrun.

Some flaps hinge to the lower trailing edges of thewings inboard of the ailerons. Leading edge flaps areused on the F-14 Tomcat and F/A-18 Hornet. Fourtypes of flaps are shown in figure 4-10. The plain flapforms the trailing edge of the airfoil when the flap is inthe up position. In the split flap, the trailing edge of theairfoil is split, and the lower half is hinged and lowers toform the flap. The fowler flap operates on rollers andtracks, causing the lower surface of the wing to roll outand then extend downward. The leading edge flapoperates like the plain flap. It is hinged on the bottomside. When actuated, the leading edge of the wingactually extends in a downward direction to increasethe camber of the wing. Landing flaps are used inconjunction with other types of flaps.

SPOILERS.—Spoilers are used to decrease winglift. The specific design, function, and use vary withdifferent aircraft. On some aircraft, the spoilers are longnarrow surfaces, hinged at their leading edge to theupper surfaces of the wings. In the retracted position,

they are flush with the wing skin. In the raised position,they greatly reduce wing lift by destroying the smoothflow of air over the wing surface.

SPEED BRAKES.—Speed brakes are movablecontrol surfaces used for reducing the speed of theaircraft. Some manufacturers refer to them as divebrakes; others refer to them as dive flaps. On someaircraft, they're hinged to the sides or bottom of thefuselage. Regardless of their location, speed brakesserve the same purpose—to keep the airspeed frombuilding too high when the aircraft dives. Speed brakesslow the aircraft's speed before it lands.

SLATS.—Slats are movable control surfaces thatattach to the leading edge of the wing. When the slat isretracted, it forms the leading edge of the wing. Whenthe slat is open (extended forward), a slot is createdbetween the slat and the wing leading edge.High-energy air is introduced into the boundary layerover the top of the wing. At low airspeeds, this actionimproves the lateral control handling characteristics.This allows the aircraft to be controlled at airspeedsbelow normal landing speed. The high-energy air thatflows over the top of the wing is known as boundarylayer control air. Boundary layer control is intendedprimarily for use during operations from carriers.Boundary layer control air aids in catapult takeoffs andarrested landings. Boundary control air can also beaccomplished by directing high-pressure engine bleedair across the top of the wing or flap surface.

4-10

SPLIT FLAP

LEADING EDGE FLAP

PLAIN FLAP

FOWLER FLAP

ANf0410

Figure 4-10.—Types of flaps.

FLIGHT CONTROL MECHANISMS

The term flight control refers to the linkage thatconnects the control(s) in the cockpit with the flightcontrol surfaces. There are several types of flightcontrols in naval aircraft; some are manually operatedwhile others are power operated.

Manually operated flight control mechanisms arefurther divided into three groups—cable operated,push-pull tube operated, and torque tube operated.Some systems may combine two or more of these types.

In the manually operated cable system, cables areconnected from the control in the cockpit to a bell crankor sector. The bell crank is connected to the controlsurface. Movement of the cockpit controls transfersforce through the cable to the bell crank, which movesthe control surface.

In a push-pull tube system, metal push-pull tubes(or rods) are used as a substitute for the cables (fig.4-11). Push-pull tubes get their name from the way theytransmit force.

In the torque tube system, metal tubes (rods) withgears at the ends of the tubes are used. Motion istransmitted by rotating the tubes and gears.

On all high-performance aircraft, the controlsurfaces have great pressure exerted on them. At highairspeed, it is physically impossible for the pilot tomove the controls manually. As a result,power-operated control mechanisms are used. In apower-operated system, a hydraulic actuator (cylinder)is located within the linkage to assist the pilot inmoving the control surface.

A typical flight control mechanism is shown infigure 4-12. This is the elevator control of a lightweighttrainer-type aircraft. It consists of a combination ofpush-pull tubes and cables.

The control sticks in the system shown in figure4-12 are connected to the forward sector by push-pulltubes. The forward sector is connected to the aft (rear )sector by means of cable assemblies. The aft sector isconnected to the flight control by another push-pulltube assembly.

LANDING GEAR

Before World War II, aircraft were made with theirmain landing gear located behind the center of gravity.An auxiliary gear under the fuselage nose was added.This arrangement became known as the tricycle type oflanding gear. Nearly all present-day Navy aircraft areequipped with tricycle landing gear. The tricycle gearhas the following advantages over older landing gear:

� More stable in motion on the ground

� Maintains the fuselage in a level position

� Increases the pilot's visibility and control

� Makes landing easier, especially in cross winds

4-11

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Figure 4-11.—Push-pull tube assembly.

ANf0412

Figure 4-12.—Typical flight control mechanism.

The landing gear system (fig. 4-13) consists ofthree retractable landing gear assemblies. Each mainlanding gear has a conventional air-oil shock strut, awheel 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.

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 telescopingcylinders filled with hydraulic fluid and compressed airor nitrogen. Figure 4-14 shows the internal constructionof one type of air-oil shock strut.

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 fromthe cockpit. This provides greater ease and safety on therunway when landing and taking off and on the taxiwayin taxiing.

ARRESTING GEAR

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 equipmentnecessary for hook operation. See figure 4-15. Thearresting hook on most aircraft releases mechanically,lowers pneumatically, and raises hydraulically.

The hook hinges from the structure under the rearof the aircraft. A snubber meters hydraulic fluid andworks in conjunction with nitrogen pressure. The

4-12

ACTUATING

CYLINDER

DOWNLOCK

CYLINDER

FROM

COMBINED

SYSTEM

LANDING

GEAR

SELECTOR

VALVE

DOOR AND

DOORLATCH

CYLINDERS

UPLOCK

CYLINDER

DOOR

CYLINDER

DOWNLOCK

CYLINDER

RETRACTING

CYLINDER

MAIN GEAR

TO LEFT

MAIN GEAR

NOSE GEAR

NOTE

TIMER VALVES ARE USEDIN MAIN GEAR SYSTEM TOCONTROL PROPER SEQUENCE.

Anf0413

Figure 4-13.—Typical landing gear system.

ANf0414

TOWING EYE

TORQUEARMS

METERING PIN

INNERCYLINDER(PISTON)

AIR VALVE

WHEEL AXLE

ORIFICE PLATE

ORIFICE

OUTERCYLINDER

Figure 4-14.—Internal construction of a shock strut.

snubber holds the hook down and prevents it frombouncing when it strikes the carrier deck.

CATAPULT EQUIPMENT

Carrier aircraft have built-in equipment forcatapulting off the aircraft carrier. Older aircraft hadhooks on the airframe that attached to the cable bridle.The bridle hooks the aircraft to the ship's catapult.Newer aircraft have a launch bar built into the noselanding gear assembly. See figure 4-16. The holdbackassembly allows the aircraft to be secured to the carrierdeck for full-power turnup of the engine prior totakeoff. For nose gear equipment, a track attaches to thedeck to guide the nosewheel into position. The track hasprovisions for attaching the nose gear to the catapultshuttle and for holdback.

NOTE: The holdback tension bar separates whenthe catapult is fired, allowing the aircraft to be launchedwith the engine at full power.

Q4-9. In fuselage construction, what are the threeclasses of monocoque design?

Q4-10. Points on the fuselage are located by whatmethod?

Q4-11. In an aircraft, what are the main structuralmembers of the wing?

Q4-12. What does the term “wet wing” mean?

Q4-13. The stabilizing surfaces of an aircraft consistof what two airfoils?

Q4-14. What are the three groups of flight controlsurfaces?

Q4-15. What is the purpose of speed brakes on anaircraft?

Q4-16. Most present-day Navy aircraft are equippedwith what type of landing gear?

ROTARY-WING AIRCRAFT

LEARNING OBJECTIVE: Identify theconstruction features of the rotary-wingaircraft and recognize the fundamentaldifferences between rotary-wing andfixed-wing aircraft.

Within the past 20 years, helicopters have become areality, and are found throughout the world. Theyperform countless tasks suited to their uniquecapabilities.

A helicopter has one or more power-drivenhorizontal airscrews (rotors) to develop lift andpropulsion. If a single main rotor is used, it is necessaryto employ a means to counteract torque. If more thanone main rotor (or tandem) is used, torque is eliminatedby turning each main rotor in opposite directions.

The fundamental advantage the helicopter has overfixed-wing aircraft is that lift and control areindependent of forward speed. A helicopter can flyforward, backward, or sideways, or it can remain instationary flight (hover) above the ground. No runwayis required for a helicopter to take off or land. Forexample, the roof of an office building is an adequatelanding area. The helicopter is considered a safe aircraftbecause the takeoff and landing speed is zero, and it hasautorotational capabilities. This allows a controlleddescent with rotors turning in case of engine failure inflight.

FUSELAGE

Like the fuselage of a fixed-wing aircraft, thehelicopter fuselage may be welded truss or some formof monocoque construction. Many Navy helicopters areof the monocoque design.

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Figure 4-15.—Arresting gear installation.

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CATAPULTSHUTTLE

CATAPULTTRACK

CABLEGUIDE

AIRCRAFT CATAPULTBRIDLE HOOKS

BRIDLE ARRESTERLANYARD

FUSELAGE

SLIDE LANYARD

CATAPULTBRIDLE

BLASTSCREEN

CATAPULTHOLDBACKPENDANT

CATAPULTPENDANT

ARRESTERBUNGEE

SLIDELANYARD

CATAPULTBRIDLE

CATAPULTSHUTTLE BRIDLE

ARRESTERLANYARD

CLEATLINK

DECK CLEAT

CATAPULT HOLDBACKPENDANT

AIRCRAFT CATAPULTHOLDBACK FITTING

TENSIONBAR

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(A)

(B)

Figure 4-16.—Aircraft catapult equipment.

A typical Navy helicopter, the H-60, is shown infigure 4-17. Some of its features include a single mainrotor, twin engine, tractor-type canted tail rotor,controllable stabilizer, fixed landing gear, rescue hoist,external cargo hook, and weapons pylons. The fuselageconsists of the entire airframe, sometimes known as thebody group.

The body group is an all-metal semimonocoqueconstruction. It consists of an aluminum and titaniumskin over a reinforced aluminum frame.

LANDING GEAR GROUP

The landing gear group includes all the equipmentnecessary to support the helicopter when it is not inflight. There are several types of landing gear onhelicopters—conventional fixed (skid type),retractable, and nonretractable.

Main Landing Gear

The H-60's nonretracting main landing gearconsists of two single axle, air/oil type of shock-strutassemblies that mount to the fuselage. Each is equippedwith tubeless tires, hydraulic disc brakes, tie-down

rings, drag braces, and safety switches. They are part ofthe lower end of the shock strut piston.

Tail Landing Gear

The H-60's tail landing gear is a nonretracting, dualwheel, 360-degree swiveling type. It is equipped withtubeless tires, tie-down ring, shimmy damper,tail-wheel lock, and an air/oil shock-strut, which servesas an aft touchdown point for the pilots to cushion thelanding shock.

MAIN ROTOR ASSEMBLY

The main rotor (rotor wing) and rotor head (hubassembly) are identical in theory of flight but differ inengineering or design. They are covered here becausetheir functions are closely related. The power plant,transmission, drive-train, hydraulic flight control, androtor systems all work together. Neither has a functionwithout the other.

Rotary Wing

The main rotor on the H-60 (fig. 4-17) has fouridentical wing blades. Other types of helicopters may

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Figure 4-17.—H-60 helicopter.

have two, four, five, six, or seven blades. Figure 4-18shows some typical rotor blades.

Rotary-wing blades are made of titanium,aluminum alloys, fiber glass, graphite, honeycombcore, nickel, and steel. Each has a nitrogen-filled,pressurized, hollow internal spar, which runs the lengthof the blade. The cuff provides the attachment of theblade to the rotor hub. A titanium abrasion strip coversthe entire leading edge of the spar from the cuff end tothe removable blade tip faring. This extends the life ofthe rotor blade.

The examples shown in figure 4-18 show otherfeatures—trim tabs, deicing protection, balancemarkings, and construction.

Main Rotor Head/Hub Assembly

The rotor head is fully articulating and is rotated bytorque from the engines through the drive train and

main gearbox or transmission. The flight controls andhydraulic servos transmit movements to the rotorblades. The principal components of the rotor head arethe hub and swashplate assemblies (fig. 4-19). The hubis one piece, made of titanium and sits on top of therotor mast. Attaching components are the sleeve andspindles, blade fold components, vibration absorber,bearings, blade dampers, pitch change horns,adjustable pitch control rods, blade fold hinges, balanceweights, antiflapping and droop stops, and faring.

The swashplate consists of a rotating disc (upper),stationary (lower) portion with a scissors and sleeveassembly separated by a bearing. The swashplate ispermitted to slide on the main rotor vertical driveshaftand mounts on top the main transmission. The entireassembly can tilt in any direction following the motionof the flight controls.

The hydraulic servo cylinders, swashplate, andadjustable pitch control rods permit movement of the

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DEICECONNECTION ABRASION STRIP

BALANCE STRIP

TRIM TABS

TIP CAP

ANTI-CHAFESTRIP

BLADE CUFF

BLADE INSPECTIONINDICATOR

CUFF

ROOT POCKET

ICE GUARD

SPAR

SPAR ABRASIONSTRIP

TIP CAP

POCKET IDENTIFICATION

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12

34

56

78

910

1112

1314

1516

1718

1920

2122

23

Figure 4-18.—Types of main rotor blades.

flight controls to be transmitted to the rotary-wingblades. The sleeve and spindle and blade dampers allowlimited movement of the blades in relation to the hub.These movements are known as lead, lag, and flap.

� Lead occurs during slowing of the drivemechanism when the blades have a tendency toremain in motion.

� Lag is the opposite of lead and occurs duringacceleration when the blade has been at restand tends to remain at rest.

� Flap is the tendency of the blade to rise withhigh-lift demands as it tries to screw itselfupward into the air.

Antiflapping stops and droop stops restrict flappingand conning motion of the rotary-wing head and bladesat low rotor rpm when slowing or stopping.

TAIL ROTOR GROUP

The directional control and antitorque action of thehelicopter is provided by the tail rotor group. See

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BLADE FOLDACTUATOR

FAIRING BIFILAR

DAMPER

PITCH LOCKACTUATOR

ROTOR HUB

FOLDHINGE

SPINDLEASSEMBLY

BLADELOCKPINPULLERS

ROTATING SCISSORS

LOWER PRESSURE PLATE

SWASHPLATEPITCH CONTROLROD

PITCH CHANGEHORN

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ROTOR HEADBALANCEWEIGHTS

Figure 4-19.—Main rotor head/hub assembly.

figure 4-20. These components are similar in functionto the main rotor.

Pylon

The pylon, shown in figure 4-20, attaches on theaircraft to the main fuselage by hinge fittings. Thesehinge fittings serve as the pivot point for the pylon tofold along the fuselage. Folding the pylon reduces theoverall length of the helicopter, which helps forconfined shipboard handling.

The pylon houses the intermediate and tail rotorgearboxes, tail rotor drive shaft, cover, tail bumper,position/anticollision lights, hydraulic servos, flightcontrol push-pull tubes/cables/bell cranks, stabilizer/elevator flight control surface, some antennas, androtary rudder assembly.

Rotary Rudder Head

The rudder head can be located on either side of thepylon, depending on the type of aircraft, and includes

such items as the hub, spindle, pitch control beam, pitchchange links, bearings, and tail rotor blades.

Change in blade pitch is accomplished through thepitch change shaft that moves through the horizontalshaft of the tail gearbox, which drives the rotary rudderassembly. As the shaft moves inward toward the tailgearbox, pitch of the blade is decreased. As the shaftmoves outward from the tail gearbox, pitch of the bladeis increased. The pitch control beam is connected bylinks to the forked brackets on the blade sleeves.

Rotary Rudder Blades

Like the blades on a main rotor head, the bladesfound on a rotary rudder head may differ, depending onthe type of aircraft. Tail rotor blades may consist of thefollowing components:

� Aluminum alloy, graphite composite, ortitanium spar

� Aluminum pocket and skin with honeycombcore or cross-ply fiber glass exterior

� Aluminum or graphite composite tip cap

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PITCH CHANGE LINK

ROTARY RUDDER BLADE

SPINDLE

ROTARY RUDDER HUB

PITCH CONTROL BEAMPYLON

TAIL ROTORGEAR BOX

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Figure 4-20.—Tail rotor group.

� Aluminum trailing edge cap

� Aluminum or polyurethane and nickel abrasionleading edge strip

Additionally, rotary rudder blades may havedeicing provisions, such as electrothermal blankets thatare bonded into the blade's leading edge. or a neopreneanti-icing guard embedded with electrical heatingelements.Q4-17. What is the main advantage of rotary-wing

aircraft over fixed-wing aircraft?

Q4-18. What are the three types of landing gear usedon helicopters?

Q4-19. The directional control and antitorque actionof the helicopter is provided by what group?

AIRCRAFT HYDRAULIC SYSTEMS

LEARNING OBJECTIVE: Identify thecomponents of aircraft hydraulic systems andrecognize their functions.

The aircraft hydraulic systems found on most navalaircraft perform many functions. Some systemsoperated by hydraulics are flight controls, landing gear,speed brakes, fixed-wing and rotary-wing foldingmechanisms, auxiliary systems, and wheel brakes.

Hydraulics has many advantages as a power sourcefor operating these units on aircraft.

� Hydraulics combine the advantages oflightweight, ease of installation, simplificationof inspection, and minimum maintenancerequirements.

� Hydraulics operation is almost 100-percentefficient, with only a negligible loss due tofluid friction.

However, there are some disadvantages to usinghydraulics.

� The possibility of leakage, both internal andexternal, may cause the complete system tobecome inoperative.

� Contamination by foreign matter in the systemcan cause malfunction of any unit. Cleanlinessin hydraulics cannot be overemphasized.

COMPONENTS OF A BASIC HYDRAULICSYSTEM

Basically, any hydraulic system contains thefollowing units:

� A reservoir to hold a supply of hydraulic fluid

� A pump to provide a flow of fluid

� Tubing to transmit the fluid

� A selector valve to direct the flow of fluid

� An actuating unit to convert the fluid pressureinto useful work

A simple system using these essential units isshown in figure 4-21.

You can trace the flow of fluid from the reservoirthrough the pump to the selector valve. In figure 4-21,the flow of fluid created by the pump flows through thevalve to the right end of the actuating cylinder. Fluidpressure forces the piston to the left. At the same time,the fluid that is on the left of the piston is forced out. Itgoes up through the selector valve and back to thereservoir through the return line.

When the selector valve is moved to the positionindicated by the dotted lines, the fluid from the pumpflows to the left side of the actuating cylinder.Movement of the piston can be stopped at any timesimply by moving the selector valve to neutral. Whenthe selector valve is in this position, all four ports areclosed, and pressure is trapped in both working lines.

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PRESSURELINE

SELECTOR VALVEIN "DOWN"POSITION

RETURNLINE

SELECTOR VALVEIN "UP"

POSITION

WORKINGLINES

HANDPUMP

ACTUATINGUNIT

RESERVOIR

Figure 4-21.—Basic hydraulic system, hand pump operated.

Figure 4-22 shows a basic system with the additionof a power-driven pump and other essentialcomponents. These components are the filter, pressureregulator, accumulator, pressure gauge, relief valve,and two check valves. The function of thesecomponents is described below.

The filter (fig. 4-22) removes foreign particlesfrom the fluid, preventing moisture, dust, grit, and otherundesirable matter from entering the system.

The pressure regulator (fig. 4-22) unloads orrelieves the power-driven pump when the desiredpressure in the system is reached. Therefore, it is oftenreferred to as an unloading valve. With none of theactuating units operating, the pressure in the linebetween the pump and selector valve builds up to thedesired point. A valve in the pressure regulatorautomatically opens and fluid is bypassed back to thereservoir. (The bypass line is shown in figure 4-22,leading from the pressure regulator to the return line.)

NOTE: Many aircraft hydraulic systems do notuse a pressure regulator. These systems use a pump that

automatically adjusts to supply the proper volume offluid as needed.

The accumulator serves a twofold purpose.

1. It serves as a cushion or shock absorber bymaintaining an even pressure in the system.

2. It stores enough fluid under pressure to providefor emergency operation of certain actuatingunits.

The accumulator is designed with a compressed-airchamber separated from the fluid by a flexiblediaphragm, or a removable piston.

The pressure gauge indicates the amount ofpressure in the system.

The relief valve is a safety valve installed in thesystem. When fluid is bypassed through the valve to thereturn line, it returns to the reservoir. This actionprevents excessive pressure in the system.

Check valves allow the flow of fluid in onedirection only. There are numerous check valvesinstalled at various points in the lines of all aircrafthydraulic systems. A careful study of figure 4-22 showswhy the two check valves are necessary in this system.One check valve prevents power pump pressure fromentering the hand-pump line. The other valve preventshand-pump pressure from being directed to theaccumulator.

HYDRAULIC CONTAMINATION

Hydraulic contamination is defined as foreignmaterial in the hydraulic system of an aircraft. Foreignmaterial might be grit, sand, dirt, dust, rust, water, orany other substance that is not soluble in the hydraulicfluid.

There are two basic ways to contaminate ahydraulic system. One is to inject particles, and theother is to intermix fluids, including water.

Particle contamination in a system may beself-generated through normal wear of systemcomponents. It is the injection of contaminants fromoutside that usually causes the most trouble. Regardlessof its origin, any form of contamination in the hydraulicsystem will slow performance. In extreme cases, itseriously affects safety.

A single grain of sand or grit can cause internalfailure of a hydraulic component. Usually, this type ofcontamination comes from poor servicing andfluid-handling procedures. For this reason, the highest

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1. Reservoir2. Power pump3. Filter4. Pressure regulator5. Accumulator6. Check valves

7. Hand pump8. Pressure gauge9. Relief valve

10. Selector valve11. Actuating unit

Figure 4-22.—Basic hydraulic system with addition of powerpump.

level of cleanliness must be maintained when workingon hydraulic components.

Only approved fill stand units are used to servicenaval aircraft hydraulic systems. By following a fewbasic rules, you can service hydraulic systems safelyand keep contamination to a minimum.

� Never use fluid that has been left open for anundetermined period of time. Hydraulic fluidthat is exposed to air will absorb dust and dirt.

� Never pour fluid from one container intoanother.

� Use only approved servicing units for thespecific aircraft.

� Maintain hydraulic fluid-handling equipmentin a high state of cleanliness.

� Always make sure you use the correcthydraulic 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 hydraulicsystem?

Q4-21. On a basic hydraulic system, what is thepurpose of the selector valve?

Q4-22. On a basic hydraulic system, what is thepurpose of the actuating unit?

Q4-23. Define hydraulic contamination.

PNEUMATIC SYSTEMS

LEARNING OBJECTIVE: Identify thecomponents of aircraft pneumatic systems andrecognize 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(filler) valve. The storage bottle must be filled withcompressed air or nitrogen prior to flight. Air storagecylinder pneumatic systems are in use for emergency

brakes, emergency landing gear extension, emergencyflap extension, and for canopy release mechanisms.

When the control valve is properly positioned, thecompressed air in the storage bottle is routed throughthe shuttle valve to the actuating cylinder.

NOTE: The shuttle valve is a pressure-operatedvalve that separates the normal hydraulic system fromthe emergency pneumatic system. When the controlhandle is returned to the normal position, the airpressure in the lines is vented overboard through thevent port of the control valve.

The other type of pneumatic system in use has itsown air compressor. It also has other equipmentnecessary to maintain an adequate supply ofcompressed air during flight. Most systems of this typemust be serviced on the ground prior to flight. The air

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Figure 4-23.—Emergency pneumatic system.

compressor used in most aircraft is driven by ahydraulic motor. Aircraft that have an air compressoruse the compressed air for normal and emergencysystem operation.

Q4-24. What are the two types of pneumatic systemscurrently used in naval aircraft?

SUMMARY

In this chapter, you have learned about aircraftconstruction and the materials used in construction.You have also learned about the features and materialsused to absorb stress on both fixed-wing androtary-wing aircraft.

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