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CHAPTER 1

PRINCIPLES OF HELICOPTER FLIGHT

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Basic flight theory and aerodynamics are consideredin full detail when an aircraft is designed. The rotorrepairer must understand these principles in order tomaintain aircraft safely and to make repairs that arestructurally sound and aerodynamically smooth.AERODYNAMICS Aerodynamics deals with the motion of air and withthe forces acting on objects moving through air orremaining stationary in a current of air. The sameprinciples of aerodynamics apply to both rotary-wingand fixed-wing aircraft. Four forces that affect anaircraft at all times are weight, lift, thrust, and drag:

Weight is the force exerted on an aircraft bygravity. The pull of gravity acts through theaircraft’s center of gravity, which is the pointat which an aircraft would balance ifsuspended. The magnitude of this forcechanges only with a change in aircraft weight.Lift is produced by air passing over the wingof an airplane or over the rotor blades of ahelicopter. Lift is the force that overcomesthe weight of an aircraft so that it can rise inthe air.Thrust is the force that moves an aircraftthrough the air. In a conventional fixed-wingaircraft, thrust provided by the propellermoves the plane forward while the wingssupply the lift. In a helicopter both thrust andlift are supplied by the main rotor blades.Drag is the force of resistance by the air to thepassage of an aircraft through it. Thrust forcesets an aircraft in motion and keeps it in mo-tion against drag force.

Any device designed to produce lift or thrust whenpassed through air is an airfoil. Airplane wings,propeller blades, and helicopter main and tail rotorblades are all airfoils (Figure 1-1).

Chord is the distance, or imaginary line, between theleading and the trailing edge of an airfoil. Theamount of curve, or departure of the airfoil surfacefrom the chord line, is known as the camber. Uppercamber refers to the upper surface; lower camber

refers to the lower surface. If the surface is flat, thecamber is zero. The camber is positive if the surfaceis convex (curves outward from the chord line). Thecamber is negative if the surface is concave (curvesinward toward the chord line). The upper surface ofan airfoil always has positive camber, but the lowersurface may have positive, negative, or zero camber(Figure 1-2).

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BERNOULLI’S PRINCIPLE •

Bernoulli, an eighteenth century physicist, dis-covered that air moving over a surface decreases airpressure on the surface (Figure 1-3). As air speedincreases, surface air pressure decreases according-ly. This is directly related to the flight of an aircraft.As an airfoil starts moving through the air, it divides the mass of air molecules at its leading edge. Thedistance across the curved top surface is greater thanthat across the relatively flat bottom surface. Airmolecules that pass over the top must therefore movefaster than those passing under the bottom in orderto meet at the same time along the trailing edge. Thefaster airflow across the top surface creates a low-pressure area above the airfoil. Air pressure belowthe airfoil is greater than the pressure above it andtends to push the airfoil up into the area of lowerpressure. As long as air passes over the airfoil, thiscondition will exist. It is the difference in pressurethat causes lift. When air movement is fast enoughover a wing or rotor blade, the lift produced matchesthe weight of the airfoil and its attached parts. Thislift is able to support the entire aircraft. As airspeedacross the wing or rotor increases further, the liftexceeds the weight of the aircraft and the aircraftrises. Not all of the air met by an airfoil is used in lift.Some of it creates resistance, or drag, that hindersforward motion. Lift and drag increase and decreasetogether. They are therefore affected by the airfoil’sangle of attack into the air, the speed of airflow, theair density, and the shape of the airfoil or wing.

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LIFT AND THRUSTThe amount of lift that an airfoil can develop dependson five major factors:

Area (size or surface area of the airfoil).

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Shape (shape or design of the airfoil sec-tions).Speed (velocity of the air passing over theairfoil).Angle of attack (angle at which the air strikesthe airfoil).Air density (amount of air in a given space).

Area and ShapeThe specific shape and surface area of an airfoil aredetermined by the aircraft manufacturer. An airfoilmay be symmetrical or unsymmetrical, depending onspecific requirements. A symmetrical airfoil isdesigned with an equal amount of camber above andbelow the airfoil chord line. An unsymmetrical air-foil has a greater amount of camber above the chordline. An airfoil with a smooth surface produces morelift than one with a rough surface. A rough surfacecreates turbulence, which reduces lift and increasesdrag.SpeedThe speed of an airfoil can be changed by the speedof the engine or by the angle of the blade. The liftdeveloped by an airfoil increases as speed increases.However, there is a limit to the amount of lift becausethe drag (resistance) of the airfoil also increases asspeed increases.Angle of AttackThe angle of attack is the angle between the airfoilchord and the direction of relative wind. Directionof airflow in relation to the airfoil is called relativewind. Lift increases as the angle of attack increasesup to a certain point. If the angle of attack becomestoo great, airflow over the top of the airfoil tends tolose its streamlined path and break away from thecontoured surface to form eddies (burbles) near thetrailing edge. When this happens, the airfoil loses itslift, and it stalls. The angle of attack at which burblingtakes place is called the critical angle of attack.Air DensityThe density (thickness) of the air plays an importantpart in the amount of lift an airfoil is able to make.The air nearest the earth’s surface is much denserthan air at higher altitudes. Therefore, an aircraft orhelicopter can achieve more lift near the ground thanat a high altitude. While keeping at the same speedand angle of attack, an airfoil will slowly make less liftas it climbs higher and higher.

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AIRFOIL STABILITYCenter of PressureThe resultant lift produced by an airfoil is the dif-ference between the drag and lift pressures of itsupper and lower surfaces. The point on the airfoilchord line where the resultant lift is effectively con-centrated is called the center of pressure. The centerof pressure of a symmetrical airfoil remains in oneposition at all angles of attack. When the angle ofattack of an unsymmetrical airfoil changes, the centerof pressure changes accordingly: the center of pres-sure moves forward with an increase in angle ofattack, and the center of pressure moves backwardwith a decrease in angle of attack.Airfoil Aerodynamic CenterThe aerodynamic center of an airfoil is the pointalong the chord line about which the airfoil tends torotate when the center of pressure moves forward orbackward between the leading and trailing edges.TorqueAccording to Newton’s third law of motion, for everyaction there is an equal and opposite reaction. As ahelicopter main rotor or an airplane propeller turnsin one direction, the aircraft fuselage tends to rotatein the opposite direction. This effect is called torque.Solutions must be found to counteract and controltorque during flight. In helicopters torque is appliedin a horizontal rather than a vertical plane. Thereaction is therefore greater because the rotor is longand heavy relative to the fuselage, and forward speedis not always present to correct the twisting effect.

Gyroscopic Precession If a force is applied against a rotating body, thereaction will be about 90° from the point of applica-tion, in the direction of rotation. This unusual fact isknown as gyroscopic precession. It pertains to allrotating bodies. For example, if you push the 3-o’clock point on a clockwise rotating wheel, the wheelwould move as if it had been pushed at the 6-o’clockpoint. The rotors on helicopters act as gyroscopesand are therefore subject to the action of gyroscopicprecession.

STRESSStress is a force placed on a body measured interms of force (pounds) per unit area (squareinches). Aircraft design engineers design aircraft

to meet – even to exceed – strength require-ments of military service. Since Army aircraft areoperated under combat conditions, they might ex-ceed these design limits. Therefore, maintenancepersonnel must check constantly for failures and forsigns of approaching failure in aircraft structuralunits. Stress may take the form of compression, tor-sion, tension, bending, or shear or may be a combina-tion of two or more of these forces (Figure 1-4):

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Compression is resistance to being pushedtogether or crushed. Compression isproduced by two forces pushing toward eachother in the same straight line. The landingstruts of an aircraft are under compressionafter landing.Torsion is resistance to twisting. A torsionalforce is produced when an engine turns acrankshaft. Torque is the force that producestorsion.Tension is resistance to being pulled apart orstretched. Tension is produced by two forcespulling in opposite directions along the samestraight line. Pilots put the cables of a controlsystem under tension when they operate thecontrols.Bending is a combination of tension and com-pression. The inside curve of the bend isunder compression, and the outside curve isunder tension. Helicopter main rotor bladesare subjected to bending.Shear is the stress exerted when two pieces of

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• metal fastened together are separated by slid-ing one over the other in opposite directions.When force is applied to two pieces of metalfastened together by rivets or bolts, slidingthem across each other, the rivets or bolts aresubjected to shear. Stress will cut off the boltor rivet like a pair of shears. Generally, rivetsare subjected to shear only, but bolts may bestressed by shear and tension. There is inter-nal shear in all parts being bent such as theskin of sheet metal structures.

LEVERS AND MOMENT OF FORCEA lever is a useful device found in tools such as jacks,shears, wrenches, and pliers. To use tools andbalancing procedures correctly, the repairer needs tounderstand moment of force (amount of leverage).

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LeversLevers are classified as three types according to theposition of the applied force (effort), the resistance,and the fulcrum (the pivot point) (Figure 1-5). InType 1 the fulcrum is located between the appliedeffort and the resistance. In Type 2 the resistance islocated between the fulcrum and the applied effort.In Type 3 the applied effort is located between the

RMA =

E–

MA = mechanical advantage

R = resisting force (weight moved)

E = effort (applied force) resistance and the fulcrum.

R 4 MA = – = – = 4

E 1

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Mechanical advantage is the ratio between the resis-tance and the effort applied to a lever. This is ex-pressed in the following formula:

Proper use of mechanical advantage enables a rela-tively small force to overcome a larger resisting forceby applying the effort through a longer distance thanthe resistance is moved. For example, to lift a 4-poundweight (R) which is 2 inches from the fulcrum of a Type1 lever requires 1 pound of effort (E) applied 8 inchesfrom the fulcrum. The mechanical advantage of thislever would be as follows:

Thus, the applied effort in the example would movethrough a distance that is four times greater than thedistance the resistance would move.Moment of Force A moment of force is the product of a force or weightand a distance. To find a lever’s moment of force,multiply the applied effort by the distance betweenthe point of effort application and the pivot point

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(fulcrum). If the moment of force of the appliedeffort equals the moment of force of the resistance,the lever will balance. If an object to be balanced ona Type 1 lever weighs 4 pounds and is located 2 inchesfrom the fulcrum, it could be balanced by a 2-poundeffort applied 4 inches from the fulcrum on the op-posite side or by a l-pound effort applied 8 inchesfrom the fulcrum.

VIBRATIONAny type of machine vibrates. However, greater thannormal vibration usually means that there is a mal-function. Malfunctions can be caused by worn bear-ings, out-of-balance conditions, or loose hardware.If allowed to continue unchecked, vibrations cancause material failure or machine destruction.Aircraft – particularly helicopters – have a highvibration level due to their many moving parts.Designers have been forced to use many differentdampening and counteracting methods to keepvibrations at acceptable levels. Some examples are —

Driving secondary parts at different speeds toreduce harmonic vibrations; this methodremoves much of the vibration buildup.Mounting high-level vibration parts such asdrive shafting on shock-absorbent mounts.Installing vibration absorbers in high-levelvibration areas of the airframe.

LateralLateral vibrations are evident in side-to-side swing-ing rhythms. An out-of-balance rotor blade causesthis type of vibration. Lateral vibrations in helicopterrotor systems are quite common.VerticalVertical vibrations are evident in up-and-down move-ment that produces a thumping effect. An out-of-track rotor blade causes this type vibration.High-FrequencyHigh-frequency vibrations are evident in buzzing anda numbing effect on the feet and fingers of crewmembers. High-frequency vibrations are caused byan out-of-balance condition or a high-speed, movingpart that has been torqued incorrectly. The balanc-ing of high-speed parts is very important. Any build-up of dirt, grease, or fluid on or inside such a part (driveshafting for example) causes a high-frequency vibration.This type vibration is more dangerous than a lateral

or vertical one because it causes crystallization ofmetal, which weakens it. This vibration must be cor-rected before the equipment can be operated.

Ground Resonance Ground resonance is the most dangerous anddestructive of the vibrations discussed here. Groundresonance can destroy a helicopter in a matter ofseconds. It is present in helicopters with articulatedrotor heads. Ground resonance occurs while thehelicopter is on the ground with rotors turning it willnot happen in flight. Ground resonance results whenunbalanced forces in the rotor system cause thehelicopter to rock on the landing gear at or near itsnatural frequency. Correcting this problem is dif-ficult because the natural frequency of the helicopterchanges as lift is applied to the rotors. With all partsworking properly, the design of the helicopter land-ing gear, shock struts, and rotor blade lag dampenerswill prevent the resonance building up to dangerouslevels. Improper adjustment of the landing gearshock struts, incorrect tire pressure, and defectiverotor blade lag dampeners may cause groundresonance. The quickest way to remove groundresonance is to hover the helicopter clear of theground.NONDESTRUCTIVE INSPECTION Nondestructive inspection (NDI) methods determineintegrity, composition, physical/electrical/thermalproperties, and dimensions without causing a changein any of these characteristics in the item being in-spected. NDI includes —

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Liquid penetrant methods.Magnetic particle methods.Electromagnetic methods.Ultrasonic methods.Penetrating radiation.Harmonic bond testing.

NDI in the hands of a trained and experienced tech-nician is capable of detecting flaws or defects with ahigh degree of accuracy and reliability. Maintenanceengineering personnel should know the capabilitiesof each method. Equally important, they shouldrecognize the limitations of each method. NDI is nota panacea for inspection ills – it is merely a means ofextending the human senses. No NDI method shouldever be considered conclusive. A defect indicated byone method must be confirmed by some other

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method to be reliable. Further, NDI equipment is example, ultrasonic inspection equipment is fullyhighly sensitive and capable of detecting discon- capable of detecting normal grain boundaries intinuities and anomalies which may be of no conse- some cast alloys. Inspection criteria must bequence to the particular service a component is used designed to overlook these “normal” returns and tofor. Limits for acceptance and rejection are thus as discriminate in favor of those discontinuities that willmuch apart of an inspection as the method itself. For affect the component in service.

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CHAPTER 2

FUNDAMENTALS OF ROTORS

Of all airfoils the rotor blade on a helicopter isunique. Like most airfoils it provides lift, but it alsoprovides thrust and directional control. The rotorsystem produces the lift, thrust, and directional con-trol needed for helicopter flight.ROTOR SYSTEMThe rotor system includes a rotor head, rotor blades,and control systems that drive and control the pitchangles of the blade. The rotor head is the mainassembly of the rotor system; it contains the rotorhub, blade attachment fittings, and blade controllingmechanisms. Currently, all helicopters in the Armyinventory use a hub drive system (Figure 2-l). In thehub drive, blades are attached to a rotor hub that issplined to the mast, which, in turn, rotates the rotorhub and blades.

FORCES ACTING ON ROTORSSince the rotor system of a helicopter provides bothlift and thrust, it is exposed to all of the forces that acton aircraft wings and propellers. When applied torotor blades, the thrust-bending force that acts onpropellers is called coning. Because of the largemass and weight of the rotating heads, the amount ofcentrifugal force (Figure 2-2) that acts on the rotorblades must be considered.

TERMINOLOGYAngle of IncidenceThe angular connection between a reference line ona rotor blade cuff, socket, or attachment point andthe blade chord line at a specific blade station iscalled the angle of incidence (Figure 2-3). On mostblades, this angle is determined during design and isnot adjustable.

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Plane of Rotation Area equals 3.14159 multiplied by the radius, then

A plane formed by the average tip path of the blades squared (multiplied by itself). The span length of oneblade is used as the radius. The area of the hub in theis known as the plane of rotation. The plane of disc area is not included since it doesn’t make any lift.rotation is at a right angle to the axis of rotation. Disc loading is the ratio of aircraft gross weight to the

Axis of Rotation disc area:

An imaginary line that passes through a point onwhich a body rotates is called the axis of rotation(Figure 2-4). Its rotation is at a right angle to theplane of rotation.

Disc loading = gross weight of aircraftdisc area

Disc Area and Loading Symmetry and Dissymmetry of LiftThe disc area (Figure 2-5) is the total space in the Lift varies according to the square of the velocityarea of the circle formed by the rotating rotor blades. (speed of blade and forward airspeed of aircraft).The following formula is used to figure disc area: Symmetry and dissymmetry of lift are shown in

A = TTR2 Figure 2-6.

A = area The example in the figure uses a blade tip speed of300 MPH. The blade speed varies from 300 MPH at

TT = total the tip station to 0 at the center of blade rotation on

R = radius the hub. When a helicopter is hovering in a no-windcondition, there is symmetry of lift. The lift is equalon advancing and retreating halves of the rotor disc

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area because speed is the same on both halves. Dis-symmetry of lift is the difference in lift that existsbetween the advancing half of a rotor disc and theretreating half. Dissymmetry is created by forwardmovement of the helicopter. When the helicopter ismoving forward, the speed of the advancing blade isthe sum of the indicated airspeed of the helicopterplus the rotational speed of the blade. The speed ofthe retreating blade is the rotational speed of theblade minus the forward speed of the helicopter. Theadvancing half of the disc area has a blade tip speedof 300 MPH plus the indicated helicopter speed of100 MPH – a total blade tip speed of 400 MPH. Thetotal speed squared is 160,000. The retreating half ofthe disc has a blade tip speed of 300 MPH minus the100 MPH indicated forward speed of 200 MPH, andvelocity squared is 40,000. In this example the ad-vancing blade creates four times as much lift as theretreating blade.

Horsepower LoadingAlso called power loading, horsepower loading is theratio of aircraft gross weight to maximum horse-power (gross weight divided by available horse-power). The horsepower loading factor is used indetermining rotor system design and testing.FlappingThe up-and-down movement of rotor blades posi-tioned at a right angle to the plane of rotation is

referred to as flapping (Figures 2-7 and 2-8). Thispermits the rotor disc to tilt, providing directionalcontrol in flight. It also controls the required lift oneach blade when in contact with dissymmetry of lift.Up-and-down flapping is limited by the centrifugalforce acting against a smaller lifting force. Somehubs have droop stops tO limit downward movementat low rotor speed.

Coning and PreconingThe upward flexing of a rotor blade due to lift forcesacting on it is called coning (Figure 2-9). Coning isthe result of lift and centrifugal force acting on ablade in flight. The lift force is almost 7 percent asgreat as the centrifugal force, which causes the bladeto deflect upward about 3° to 4°. Coning is oftenexpressed as an angle. Helicopter manufacturersdetermine the coning angle mathemically and builda precone angle into the rotor hub that is similar tothe coning effect in normal flight. The preconed hublets the blades operate at normal coning angles

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without bending, which reduces stress. It is notnecessary to precone the articulated rotor hub be-cause the blade can flap up on horizontal hinges tothe correct coning angle.Lead and Lag of BladesThe horizontal movement of the blades around avertical pin is called leading and lagging (hunting)(Figure 2-10). This is found only on fully articulatedrotor heads. During starting the blades will resistrotational movement and will lag behind their (trueradial) position. As centrifugal force reacts on theblade, the blade will gain momentum and find its ownposition of rotation. The blade will hunt about the

vertical hinge close to a 5° range during normaloperation. The movement of the blades about thevertical hinge is restricted by a hydraulic damper.Feathering Axis The spanwise axis about which a rotor bladerotates to change pitch is known as the featheringaxis (Figure 2-11). Feathering action varies accord-ing to the position of the cyclic control in forwardflight, the dissymmetry of lift, and the collective pitchcontrol when the helicopter hovers.HoverThe versatility of a helicopter is due to its ability tohover at a point above the ground. This lets thehelicopter vertically rise from and descend to small,unimproved landing areas. When main rotor angleof attack and engine power are adjusted so that liftequals weight, the helicopter will hover. Hover isconsidered an element of vertical flight. Assuming ano-wind condition exists during hover, the tip pathplane of the rotor will remain horizontal with theearth. When the angle of attack of both blades isincreased equally while blade speed remains con-stant, more thrust will result and the helicopter willrise. By upsetting the lift-gravity balance, thehelicopter will rise or come down depending onwhich force is greater. Hovering takes a great dealof power because a large mass of air must be drawnthrough the rotor blades at high speeds.Ground EffectWhen hovering near ground or water surfaces at aheight no more than one-half of the rotor diameter,

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the helicopter encounters a condition referred to as the rotor tip vortex and the flattening out of the rotorground effect. This condition is more pronounced downwash. The benefit of ground effect is lowernearer the ground. Helicopter operations within blade angle of attack, which results in a reduction ofground effect are more efficient due to reduction of power requirements for a given load.

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CHAPTER 3

ROTOR SYSTEM OPERATION

An understanding of the rotor system is necessary tobe able to troubleshoot it analogical manner. It isimportant to know and understand the operation ofrotor heads and how rotor blades are driven.Remember that if the components of the rotor systemare not properly maintained, a malfunction mayoccur while in flight causing possible loss of life andequipment. For a complete detailed description of aspecific helicopter rotor system, refer to the ap-plicable aircraft multipart maintenance manual.

SINGLE AND TANDEM ROTORSHelicopter configurations are classified as single,tandem, coaxial, and side by side. The single- andtandem-rotor configurations are the only ones usedin Army helicopters.

Single Rotor Helicopters designed to use a main and tail rotorsystem are referred to as single-rotor helicopters.The main rotor provides lift and thrust while the tailrotor counteracts the torque made by the main rotor.This keeps the aircraft from rotating in the oppositedirection of the main rotor. The tail rotor alsoprovides the directional control for the helicopterduring hovering and engine power changes. Powerto operate the main and tail rotors is supplied by thepower train system. The single-rotor configurationhas the advantage of being simpler and lighter thanthe tandem-rotor system, and it requires less main-tenance. Since the tail rotor uses a portion of theavailable power, the single-rotor system has a smallercenter-of-gravity range.Tandem RotorNormally used on large cargo helicopters, thetandem-rotor configuration has two main rotorsystems, one mounted on each end of thefuselage. Each rotor operates the same as the mainrotor on the single-rotor helicopter, except for thedirection of rotation of the aft rotor and the methodof keeping directional control. The forward rotorturns in a counterclockwise direction viewed frombelow, and the aft rotor rotates in a clockwisedirection. A separate antitorque system is not

needed because the rotor systems rotate in oppositedirections (counteract each other’s torque). Ad-vantages of the tandem-rotor system are a largercenter-of-gravity range and good longitudinalstability also, the counter-rotating rotors do awaywith the need for an antitorque rotor. Because thereis no antitorque rotor, full engine power can be ap-plied to load lifting. Disadvantages of the tandem-rotor system are a complex transmission and moredrag due to its shape and excessive weight.FLIGHT CONTROLS As a helicopter maneuvers through the air, its at-titude in relation to the ground changes. Thesechanges are described with reference to three axesof flight: lateral, vertical, and longitudinal. Move-ment about the lateral axis produces a nose-up ornose-down attitude; this is accomplished by movingthe cyclic pitch control fore and aft. Movementabout the vertical axis produces a nose swing (orchange in direction) to the right or left; this move-ment is called yaw. This is controlled by the direc-tional control pedals. These pedals are used toincrease or decrease thrust in the tail rotor of asingle-rotor helicopter and to tilt the rotor discs inopposite directions on a tandem-rotor helicopter.Movement about the longitudinal axis is called roll.This produces a tilt to the right or left. The move-ment is accomplished by moving the cyclic pitchcontrol to the right or left. Some other helicopterflight controls are discussed below.

Cyclic Pitch Control The cyclic pitch control looks like the control stick ofa common aircraft. It acts through a mechanicallinkage to cause the pitch of each main rotor blade tochange during a cycle of rotation. To move a helicop-ter forward from a hovering height, the rotor discmust be tilted forward so that the main rotor providesforward thrust. This change from hovering to flyingis called transition and is done by moving the cycliccontrol stick. Moving the cyclic control stick changesthe angle of attack of the blades this change tilts therotor disc. The rapidly rotating rotor blades create adisc area that can be tilted in any direction relative to

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the supporting rotor mast. Horizontal movement iscontrolled by changing the direction of tilt of themain rotor to produce a force in the desired direc-tion.Collective Pitch Control Collective pitch control varies the lift of the mainrotor by increasing or decreasing the pitch of allblades at the same time. Raising the collective pitchcontrol increases the pitch of the main rotor blades.This increases the lift and causes the helicopter torise. Lowering the control decreases the pitch of theblades, causing a loss of lift. This produces a cor-responding rate of descent. Collective pitch controlis also used in coordination with cyclic pitch controlto regulate the airspeed. For example, to increaseairspeed in level flight, the cyclic is moved forwardand the collective is raised at the same time.

Control PlateForces from the cyclic and collective pitch sticks arecarried to the rotor by a control plate usually locatednear the bottom of the rotor drive. Control platesused by various builders are different in appearanceand name, but they perform the same function. Thecontrol plate is attached to the rotor blades by push-pull rods and bell cranks. The collective pitch stickchanges the pitch of the blades at the same time by avertical deflection of the entire control plate. Thecyclic pitch stick allows angular shifting of the controlplate to be sent to a single blade. This causes flappingand small angles of pitch change to make up forunequal lift across the rotor disc. The direction of tiltof the control plate decides the direction of flight:forward, backward, left, or right.

Throttle Control By working the throttle control, pilots can keep thesame engine and rotor speed, even if a change inblade pitch causes them to increase or decrease en-gine power. When the main rotor pitch angle isincreased, it makes more lift but it also makes moredrag. To overcome the drag and keep the same rotorRPM, more power is needed from the engine. Thisadded power is obtained by advancing the throttle.The opposite is true for a decrease in main rotor pitchangle. The decreased angle reduces drag, and areduction in throttle is needed to prevent rotor over-speed. The throttle is mounted on the collectivepitch grip and is operated by rotating the grip, as ona motorcycle throttle. The collective pitch stick is

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synchronized with the control of the carburetor sothat changes of collective pitch will automaticallymake small increases or decreases in throttle settings.On turbine engine helicopters, the collective pitchstick is synchronized with the fuel control unit, whichcontrols the power and rotor RPM automatically.Torque Control In tandem-rotor and coaxial helicopter designs themain rotors turn in opposite directions and therebyneutralize or eliminate torque effect. In single-rotorhelicopters torque is counteracted by an antitorquerotor called the tail rotor. It is driven by a powertakeoff from the main transmission. The antitorquerotor runs at a speed in direct ratio to the speed ofthe main rotor. For this reason, the amount of thrustdeveloped by the antitorque rotor must be changedas the power is increased or decreased. This is doneby the two directional control pedals (antitorquepedals), which are connected to a pitch-changingdevice on the antitorque rotor. Pushing the left pedalincreases the thrust of the tail rotor blades, swingingthe nose of the helicopter to the left. The right pedaldecreases the thrust, allowing the main rotor torqueto swing the nose to the right.MAIN ROTOR HEAD ASSEMBLIESThe main rotor head assembly is attached to andsupported by the main gearbox shaft. This assemblysupports the main rotor blades and is rotated bytorque from the main gearbox. It provides the meansof transmitting the movements of the flight controlsto the blades. Two types of rotor heads used on Armyhelicopters are semirigid and fully articulated.

SemirigidThe semirigid rotor head gets its name from the factthat the two blades are rigidly interconnected andpivoted about a point slightly above their center (Fig-ure 3-1). There are no flapping or drag hinges likethose on the articulating head. Since the blades areinterconnected, when one blade moves upward theother moves downward a corresponding distance.The main rotor hub is of a semirigid, underslungdesign consisting basically of the —

Yoke (1).Trunnion (2).Elastomeric bearing (3).Yoke extensions.Pitch horns (4).

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• Drag braces (5). each acting as a single unit and capable of flapping, • Grips (6).

The yoke is mounted to the trunnion by elastomericbearings which permit rotor flapping. Cyclic andcollective pitch-change inputs are received throughpitch horns mounted on the trailing edge of the grips.The grips in turn are permitted to rotate about theyoke extensions on Teflon-impregnated fabric frict-ion bearings, resulting in the desired blade pitch.Adjustable drag braces are attached to the grips andmain rotor blades to maintain alignment. Bladecentrifugal loads are transferred from the blade gripsto the extensions by wire-wound, urethane-coated,tension-torsion straps.

feathering, and leading and lagging. The assembly ismade up primarily of —

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An internally splined hub.Horizontal and vertical hinge pins.Extension links.Pitch shafts.Pitch housing.

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Dampers.Pitch arms.

• Bearing surfaces.Connecting parts.•

Fully Articulated The extension links are attached to the hub by thehorizontal pins and to the forked end of the extensionA folly articulated rotor head gets its name from the

fact that it is jointed (Figure 3-2). Jointing is made link. The pitch shafts are attached by the vertical

with vertical and horizontal pins. The fully articu- pins. The pitch housing is fitted over and fastened to

lated rotor head assembly has three or more blades, the pitch shaft by the tension-torsion straps, whichare pinned at the inboard end of the pitch shaft and

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the outboard end of the pitch-varying housing. One provide automatic equalization of thrust on the ad-end of the dampers is attached to a bracket on thehorizontal pins; the other end is fastened to the pitchhousing.

FlappingFlapping of the rotor blades is permitted by thehorizontal pin, which is the hinge or pivot point.Centrifugal force on the blades and stops on the headprevent excessive flapping.Feathering

Feathering is the controlled rotation about the lon-gitudinal axis of the blades that permits the pilot toachieve directional control in either the horizontal orvertical plane. Feathering is permitted by a pitch-change assembly on some helicopters and by a sleeve-and-spindle assembly on other types of helicopters.Leading and LaggingLeading and lagging is permitted by the vertical pin,which serves as a hinge or pivot point for the action.Excessive leading and lagging is prevented by the useof a two-way hydraulic damper in the system.

TAIL ROTOR HUBSThe tail rotor hub (antitorque rotor) is used as acentering fixture to attach the tail rotor blades so thatthey rotate about a common axis. It keeps the blocksagainst centrifugal, bending and thrust forces. Itaccepts the necessary pitch-change mechanism to

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vancing and retreating blade, or equal and simul-taneous pitch change to counteract torque made bythe main rotor system. Hub design varies with themanufacturer. Typical configurations are the hinge-mounted, flex-beamed, and fully articulated types.Hinge-Mounted TypeA single two-blade, controllable-pitch tail rotor islocated on the left side of the tail rotor gearbox(Figure 3-3). It is composed of the blades and thehub and is driven through the tail rotor gearbox.Blades are of all-metal construction and attached bybolts in blade grips, which are mounted throughbearings to spindles of the hub yoke. The tail rotorhub is hinge-mounted to provide automatic equaliza-tion of thrust on advancing and retreating blades.Control links provide equal and simultaneous pitchchange to both blades. The tail rotor counteracts thetorque of the main rotor and provides directionalcontrol.Flex-Beamed TypeThe tail rotor hub and blade assembly counteractstorque of the main rotor and provides directionalcontrol. It consists of the hub and two blades (Figure3-4). The hub assembly has a preconed, flex-beamed-type yoke and a two-piece trunnion con-nected to the yoke by self-lubricating, sphericalflapping bearings. The trunnion, which is splined tothe tail rotor gearbox shaft, drives the blades and

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Fully Articulated TypeThe articulated tail rotor system (Figure 3-5)counterbalances disturbing forces in the same waythat the hinge-type rotor does. The major differenceis that the blades can lead and lag individually duringrotation.MAIN ROTOR BLADES

The rotor blade is an airfoil designed to rotate abouta common axis to produce lift and provide directionalcontrol for a helicopter. It is often referred to as arotary wing. The design and construction of a rotorblade vary with the manufacturer, although they allstrive to manufacture the most efficient andeconomical lifting device. The particular helicopterdesign places certain requirements on the main rotorblades, which influence their design and construc-tion. Most rotor blades are designed as symmetricalairfoils to produce a stable aerodynamic pitchingcharacteristic. Aerodynamic stability is achievedwhen the center of gravity, center of pressure, andblade-feathering axis all act at the same point. Theblade is more stable in flight because these forcescontinue to act at almost the same point as the blade

serves as a flapping stop for the tail rotor. The yoke changes pitch. At present only one Army helicopterhas two self-lubricating, spherical bearings as attach- is equipped with an unsymmetrical airfoil. This un-ing points for each rotor blade. Rotor pitch change symmetrical airfoil blade is capable of producingis accomplished at these bearings. greater lift than a symmetrical airfoil blade of similar

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dimensions. Aerodynamic stability is achieved bybuilding a 3° upward angle into the trailing edge sectionof the blade. This prevents excessive center-of-pressure travel when the rotor blade angle of attackis changed. A variety of material is used in the con-struction of rotor blades; aluminum, steel, brass, andfiberglass are most common.

Types of Rotor Blades

Metal

A typical metal blade has a hollow, extrudedaluminum spar which forms the leading edge of theblade (Figure 3-6). Aluminum pockets bonded tothe trailing edge of the spar assembly provide stream-lining. An aluminum tip cap is fastened with screwsto the spar and tip pocket. A steel cuff bolted to theroot end of the spar provides a means of attachingthe blade to the rotor head. A stainless steel abrasionstrip is adhesive-bonded to the leading edge.

FiberglassThe main load-carrying member of a fiberglass bladeis a hollow, extruded steel spar (Figure 3-7). The

fairing or pockets are fiberglass covers bonded overeither aluminum ribs or aluminum foil honeycomb.The fairing assembly is then bonded to the trailingedge of the spar. The trailing edge of the fairing isbonded to a stainless steel strip forming the bladetrailing edge. Rubber chafing strips are bonded be-tween the fairings to prevent fairing chafing andprovide a weather seal for the blade fairings. A steelsocket threaded to the blade spar shank provides anattaching point to the rotor head. A stainless steeltip cap is fastened by screws to the blade spar andblade tip pocket.

Blade Nomenclature

PlanformThe blade planform is the shape of the rotor bladewhen viewed from above (Figure 3-8). It can beuniform (parallel) or tapered. Uniform planformsare most often selected by the manufacturer be-cause, with all the ribs and other internal bladeparts the same size, they are easier to make. Theuniform blade requires only one stamping die forall ribs, which reduces blade cost. This design hasa large blade surface area at the tip; it must there-fore incorporate negative tip twists to make a moreuniform lift along the blade span. If the bladeangle is the same for the length of the blade, theblade will produce more lift toward the tip becauseit moves at a higher speed than the blade root. Thisunequal lift will cause the blade to cone too muchor bend up on the end. The tapered planformblade makes a more uniform lift throughout itslength. Few blade manufacturers use it, however,because the manufacturing cost is too high due tothe many different-shaped parts required to fit thetapered airfoil interior.

Twist

The blade-element theory applies to a rotor bladeas well as to a propeller. Therefore, most rotorblades are twisted negatively from root to tip to getmore even distribution of lift.

SkinThe skin may be fiberglass or aluminum and mayconsist of single or multiple layers. The thin skincan easily be damaged by careless handling on theground. Three types of blade coverings areused: one-piece wraparound aluminum alloy,single pocket (or fairing), and multiple pocket

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(or fairing). Most main rotor blades are of single-pocket or multiple-pocket construction.RootThe blade root is the section nearest the center ofrotation that provides a means of attachment to therotor head (Figure 3-9). It is heavier and thickerthan the rest of the blade to resist centrifugal forces.

TipThe tip is located furthest from the center of rotationand travels at the highest speed during operation(Figure 3-10). The blade tip cap also has a means forattaching balance weights.

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Leading Edge The part of the blade that meets the air first is theleading edge (Figure 3-11). For the edge to workefficiently, airfoils must have a leading edge that isthicker than the trailing edge. The leading edge ofall blades is covered with a hard, abrasion-resistantcap or coating to protect against erosion caused bysand and dust.

Trailing Edge

Trailing edge is that part of the blade that follows ortrails the leading edge and is the thinnest section ofthe airfoil (Figure 3-12). The trailing edge isstrengthened to resist damage, which most often hap-pens during ground handling.

Span and Span LineThe span of a rotor blade is its length from root to tip(Figure 3-13). The span line is an imaginary linerunning parallel to the leading edge from the root ofthe blade to the tip. Span line is important to theblade repairer because damages are often locatedand classified according to their relation to it.Defects paralleling the span line are usually less

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serious because stress lines move parallel to the spanline and would therefore pass the damage withoutinterruption. Chordwise damage interrupts lines ofstress.

Chord and Chord LineThe chord of a rotor blade is its width measured atthe widest point (Figure 3-14). The chord line of arotor blade is an imaginary line from the leading edgeto the trailing edge, perpendicular to the span line.Blade chord line is used as a reference line to makeangular measurements.SparThe main supporting part of a rotor blade is the spar(Figure 3-15). Spars are usually made of aluminum,steel, or fiberglass; they always extend along the spanline of the blade. Often the spar is D-shaped andforms the leading edge of the airfoil. Spars are of

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BottomThe high-pressure side of the blade is the bottom.The bottom is the blade surface which is viewed fromthe ground. It is always painted a lusterless black toprevent glare from reflecting off the blade and intocrew compartments during flight.Blade StationsRotor blade stations are numbered in inches and aremeasured from one of two starting points. Somerotor blades are numbered from the center ofrotation (center of the mast), which is designated

different shapes, depending on the blade material station zero, and outward to the blade tip. Othersand on how they fit into the blade airfoil. are numbered from the root end of the blade, stationDoublers zero, and outward to the blade tip (Figure 3-16).

Doublers are flat plates that are bonded to both sides Blade Construction of the root end of some rotor blades to provide more Single Pocket or Fairing strength. Not all blades use doublers since some The single-pocket or fairing blade is made with aspars are made thick enough to provide the neededstrength at the root end. one-piece skin on top and bottom (Figure 3-17).

Top Each skin extends across the entire span and chord,

The low-pressure side of the blade is the top. The behind the spar. This style is simple and easy to makebecause of the minimum number of pockets or fair-top is the blade surface which is viewed from above

the helicopter. It is usually painted olive drab when ings that need positioning and clamping during thebonding process. However, minor damage to the

the blade skin is plastic or metal.

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skin often results in the blade being thrown awaysince replacing the skin costs more than replacing theblade.

Multiple Pockets or FairingsMost large rotor blades built with the multiple-pocket or fairing shape behind the spar are costly(Figure 3-18). This type of blade is selected sincedamage to the skin cover requires that only the pock-et (or fairing) be replaced. The high-cost blade canthen be used over and over. This type of blade ismore flexible across the span, which cuts down onblade vibrations.Internal Structural Components

Rotor blades have internal structural parts that helpto support the blade skin – ribs, I-beams, spanwisechannels, and aluminum honeycomb foil.Bonds and BondingBonding is a method of putting two or more partstogether with an adhesive compound. Bonding helps

reduce the use of hardware like bolts, rivets, andscrews that need holes and therefore weaken thestrength of the bond. To ensure full strength,manufacturers never drill holes in load-carryingparts of the blade except at the inboard and outboardends. However, bonds react to the chemical actionof paint thinners and many cleaning solvents.Careless use of these solvents will dissolve bondedjoints. The surface area where two objects arebonded together is known as the faying surface(Figure 3-19).

Blade Balance

Three types of weights to balance the blade are masschordwise, spanwise, and tracking (Figure 3-20).Mass balance weights (bars) are placed into the lead-ing edge of a blade while the blade is being made(Figure 3-21). This is to ensure that correctchordwise balance is about 25 percent of chord. Thetype of metal and its shape and location vary with the

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manufacturer. The repairer is not allowed to movethe weights in most Army helicopter blades. Whenmoving of weights is allowed, however, the repairermust remember that changing weights will move thecenter of gravity forward or backward.

• • •

Spanwise balance weights are at the tip of the blade,usually where they can be attached securely to thespar (Figure 3-22). They are normally installed in the

blade during manufacture. The repairer is notalways permitted to move these weights. Whenmovement is necessary, the repairer should alwaysremember that adding spanwise weight moves thecenter of gravity outward. Subtracting weight movesthe center of gravity inward. When moving the span-wise weight is permitted, the weight change is com-puted by the repairer mathematically after the bladehas been weighed.

To be efficient and vibration-free, all rotating bladesshould track on about the same level or plane ofrotation. Failure of blades to track correctly causesvibrations which can —

Damage parts of the helicopter.Reduce riding comfort.Cause a loss in blade performance due to airturbulence made by the rotating blades.

One way of retaining track is to attach trackingweights in front of and behind the feathering axis atthe blade tips (Figure 3-23). By adding removing orshifting tracking weights, the repairer can move ablade track up or down to match the track of the otherblade or blades. This causes all blades to move in thesame tip path plane.

Trim Tabs

Another method used to align the rotor blade on thesame plane of rotation is the use of trim tabs (Figure3-24). Using tracking weights adds to building costs,but the same results may be achieved by cheapermethods; for example, putting a sheet metal trim tabon the trailing edge of the blade. The trim tab isusually located near the tip of the blade where thespeed is great enough to get the needed aerodynamicreaction. In tracking operations the trim tab is bentup to make the leading edge of the rotor blade flyhigher in the plane of rotation. Or it is bent down tomake it fly lower. The trim tabs are adjusted until therotor blades are all flying in the same plane of rota-tion.

TAIL ROTOR BLADESTail rotor blades are used to provide directional con-trol only. Made of metal or fiberglass, they are builtsimilarly to main rotor blades. Metal tail rotor bladesare made of aluminum; the spars are made of solidaluminum extrusions, hollow aluminum extrusions,and aluminum sheet channels. Fiberglass rotor

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blades are made of fiberglass sheets; the sparsare made of solid titanium extrusions. Refer toFigure 3-25.Metal BladesThe blade skins are formed around and bonded tothe spars, which in most cases form the leading edgeof the blades. Metal blade skins are supported fromthe inside with aluminum honeycomb, ribs, and somesmaller blades which have no bracing or supportinside themselves.

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Fiberglass BladesThe blade skins are formed around and bondedto H-shaped titanium spars. The blade skins aresupported inside with aluminum honeycomb.The space around the spar is filled with foamplastic.

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Blade BalanceSpanwiseOn some models spanwise balance is accomplishedby adding or subtracting washers on the blade tip.On others the washers are added to the blade-cuffattaching bolts.

On some models blades are balanced chordwisc byadding weights to the tips behind the spanwise

balance screw. Other models are balanced by addingweights to the trailing edge of the blades near the cuffend.Trammeling

Fully articulated tail rotor systems must be tram-meled before they are balanced. Trammeling con-sists of aligning the tail rotor blades an equal distanceto one another with a 2° angle of lead to the blades.

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CHAPTER 4

ALIGNMENT AND TRACKING PROCEDURES

This chapter discusses procedures required foralignment and balancing of rotor blades. Specificprocedures for aligning blades vary with differenttypes of helicopters. Prior to aligning blades, consultthe appropriate technical manual for specific instruc-tions and maximum allowable tolerance. This chap-ter also includes a description of the Vibrex balancingkit.

MAIN ROTOR BLADE ALIGNMENTMain rotor blade alignment is the centering of themass (distribution of weight) of the main rotor as-sembly across the center of rotation to balance it.The alignment of the rotor system has a distinct effecton balance because of the great weight and long aiminvolved. A greater weight on one side of the centerof rotation will cause a lateral vibration. The require-ment for manually aligning the main rotor bladesapplies to rigid and semirigid rotor systems only. Thefully articulated rotor system automatically alignsitself as centrifugal force increases and pulls theblades into a pure radial position. The most commonmethod of manually aligning main rotor blades is thetelescope method.A small bore rifle telescope is the basic tool used toalign the main rotor assembly (Figure 4-1). A fixtureto hold the telescope is fitted onto the hub directlyover the center of rotation. A repairer should beconcerned with the vertical cross hair only. Ignorethe horizontal cross hair. Place a zeroed telescope inthe holding fixture and sight the vertical cross hair ona reference point of the blade. The reference pointnormally used is a rivet in the skin at the tip of theblade in line with the feathering axis. Adjust amisaligned rivet by moving the blade in the the hubto bring the rivet into alignment. Align the other(opposite) blade using the same procedure. Align-ment of the main rotor assembly has been achievedwhen both blades have been adjusted so that thevertical cross hair of the telescope is positioned at thecenter of both rivets.

UNBALANCED SEMIRIGID ROTOR SYSTEMS LateralWhen troubleshooting a semirigid main rotor system,the repairer must understand the basics of alignmentand balance to act quickly yet skillfully. An un-balanced system causes the most problems in thefield. The trouble that results is called lateral vibra-tion. A few indicators of lateral vibration are wornparts and bearings, broken parts and bearings, looseparts and fittings, and cracked parts and fittings. Therepairer must determine if the unbalanced conditionis caused by chordwise or spanwise torque before itcan be corrected.ChordwiseTo differentiate between a chordwise and a spanwiseunbalanced condition, apply a strip of tape to the tipof one blade and hover the helicopter. If the lateralvibration decreases and then increases, this indicatesthat spanwise balance is okay but chordwise un-balance exists. To balance a rotor system chordwise,select a blade and sweep it to the rear by shorteningthe drag brace. Before making adjustments,matchmark all drag brace parts so that they can bereturned to the same setting to regain alignment.Hover the helicopter. Should the lateral vibrationincrease, you have selected the wrong blade. Returnthe drag brace to the original alignment asmatchmarked. Repeat drag brace sweeping on theopposite blade. Make small sweep corrections untilthe vibration stops. Secure and safety the dragbraces.SpanwiseTo isolate spanwise balance, apply a strip of 2-inchtape to the tip of one blade. Hover the helicopter.Should the vibration increase, the wrong blade hasbeen selected. Remove the tape and apply it to theopposite blade. Add the tape one strip at a time untilthe vibration is gone. Then replace the tape with

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equal weight or secure weight approximating equal VERTICAL VIBRATIONSmoments at a specific location on the system. Forexample, apply a 3.1-ounce, lead-in retention bolt foreach wrap of tape at the tip or as authorized in theapplicable maintenance manual.

CombinedIt is not unusual for a combined chordwise-spanwiseunbalance to exist in a main rotor system. When theunit is balanced spanwise, the chordwise unbalancebecomes evident. In this case, each unbalance mustbe corrected separately. If the system cannot bebalanced by the above operations, inspect it for loose,worn, or cracked parts and for frozen Teflon bearingsor ratcheting roller or ball bearings.

Vertical vibrations – the bouncing of the helicopterup and down – are caused by a blade being out oftrack. Vibration is caused by a blade lifting thehelicopter in one quadrant of rotation and suddenlylosing lift in the remaining quadrant during cyclictravel. When present once during each revolution,this force is referred to as a one-per-revolution or1-to-1 vibration. Two bounces of the fuselage isknown as a two-per-revolution or 2-to-1 vibration.

Someone not familiar with the helicopter can deter-mine vertical vibration by looking at the tips of theskids. A vertical vibration will cause the tip of theskid to bounce vertically against the ground.Depending on the shape of the main rotor and the

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helicopter manufacturer, the vibration may be cor-rected. This is done by rolling the blade grips up ordown using the pitch-change links or adjusting thetrim tab to get a blade track that will stop the vibra-tion. One method is to blade-track at low RPM usingpitch-change links and at high RPM using trim tabsand links with power applied. The blades are trackedusing a tracking flag or trackometer. The helicopteris then flown at cruise airspeed to see if blade cross-over exists. Blade crossover occurs when blades arealmost perfectly in track. During forward flight(cruise) dissymmetry of lift causes a blade to fly highthrough 180° of rotation and low in the remaining180°. Corrections are made by adjusting either trimtab up or down to cause the blade to track high or lowwithin limits. Crossover is corrected by the pressureexerted by the trim tab, which forces the blade up ordown throughout 360° of rotation.Extreme Low FrequencyExtreme low-frequency vibration is essentiallylimited to pylon rock. Pylon rocking of two or threecycles per second is inherent with the rotor, mast, andtransmission systems. To keep the vibration fromreaching noticeable levels, transmission mountdampers are installed to absorb the rocking. Thedamper system may be checked by the pilot while ata hover. Moving the cyclic control forward and back-ward at about one movement per second will causethe pylon to start rocking. How long it takes for therocking to die out after the motion of the cyclic isstopped indicates the condition of the damper sys-tem.

Low FrequencyOne-revolution and two-revolution vibrations arecaused by the rotor. One-revolution vibrations are oftwo basic types: lateral and vertical. Low-frequency

vibration is started by a gust effect that causes amomentary increase of lift in one blade giving aone-revolution vibration. The momentary vibrationis normal. However, if picked up by the rotatingcollective controls and fed back to the rotor causingcycles of one revolution, then it is undesirable. Thiscondition is usually caused by too much differentialtab in the blades. It can be corrected by rolling oneblade at the grip and changing angular adjustment ofthe tab. Two-per-revolution (2/rev) vibrations areinherent with a two-bladed rotor system, and a lowlevel of vibration is always present. When the 2/revvibration rises to an unacceptable level, it is due tofaulty vibration dampers or loose and worn hardwarein the rotor system.Medium FrequencyMedium-frequency vibrations at four to six perrevolution are inherent with most rotor systems. Anincrease in the level of vibration is caused by a changein the capability of the fuselage to absorb vibrationdue to loose hardware, structural damage, or load.Normally this vibration is caused by loose parts –either a regular part of the aircraft or the externalload.High FrequencyHigh-frequency vibrations can be caused by anythingin the ship that rotates or vibrates at a speed equal toor greater than that of the tail rotor. Unless thevibration is isolated to one part of the aircraft – undera shaft bearing, for example – the first step generallyis checking the tail rotor track.

MAIN ROTOR BLADE TRACKINGBlade tracking (Figure 4-2) is the procedure formeasuring, recording, and adjusting the tip pathplane of the rotor blades. The measurements taken

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•

while the blades are turning show the vertical posi-tion of the rotor blade tips in relation to each other.The positions of the blade tips must be kept within acertain tolerance, usually ±.25 inch. Tolerance foreach helicopter will be listed in the applicable main-tenance manual. Several methods used to trackblades are —

Electronic blade tracker.Reflector tracking.Strobe light.

ElectronicRotor blade assemblies may also be tracked with anelectronic blade-tracking unit (Figure 4-3). The unitis made up of three major components:

••

• A phase detector with a magnetic pickup at-tached to the swash plate’s stationary ring anda sweep attached to its rotating ring.

••

• A computer containing the electronic cir-cuits, adjusting knobs, and meter.An electronic eye unit.•

The electronic blade tracker unit permits bladetracking during adverse weather and at night. Theelectronic blade tracker is operated when the rotat-ing rotor blades interrupt the electronic eye beam,sending a signal into the computer in conjunctionwith a signal from the phase detector. The computerthen determines the blade tip path plane above anautomatically selected reference plane. The metershows the height in fractions of an inch of the rotor

blades in the set relative to the predetermined refer-ence rotor blade. Refer to Figure 4-4 for an example.

ReflectorThe reflector tracking method uses the principle ofpersistency of vision, which occurs when looking at abeam that is being intercepted by two light reflectors.One reflector is installed at the tip of each main rotorblade. The surface of one reflector is plain white, andthe surface of the other is white with a horizontalblack stripe painted across the center of the face. Asthe blades rotate and the light beam is intercepted bythe reflectors, the observer will see two white bandsand one black band. One white band will be aboveand the other below the black band. A perfect in-track condition exists when both white bands are thesame width. If one reflector image moves verticallyrelative to the other, one white band will becomelarger than the other. All tracking should be done atengine-rated RPM to obtain the best track (or asspecified in the applicable repair manual). Refer toFigure 4-5 for an example.

Strobe LightThe strobe light blade-tracking system includes—

A portable power supply.A hand-held strobe lamp.

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• • •

Blade tip targets.Magnetic phase pickup.Pickup plates.

A concentrated parallel light beam from the strobelight is manually directed toward a predeterminedspot on the rotor blade disc to strike the blade tiptargets. The strobe light trigger switch is thendepressed to allow strobing of the blade tip targets.The pulse signal for strobe effect is provided by themagnetic pickup unit mounted on the stationaryswash plate ring. This strobe effect sends a pulseeach time one of the pickup plates passes over themagnetic pickup unit. The pickup is mounted oneach rotor blade pitch-change link lower attachingbolt. The strobe light targets are attached to theblade tips with patterns facing inboard. The targets

vertically relative to the others, the colored lines ofthe affected blade target will become visible to theoperator. Refer to Figure 4-6 for an example.

VIBREX BALANCING KITThe Vibrex balancing kit (hereinafter referred toas Vibrex) is used to measure and indicate the levelof vibrations induced by the main and tail rotors of ahelicopter. The Vibrex analyzes the vibration in-duced by out-of-track or out-of-balance rotors.Then by plotting vibration amplitude and clock angleon a chart, it determines the amount and location ofrotor track or weight changes. The Vibrex is alsoused in troubleshooting to measure the RPM or fre-quency of unknown disturbances. DA Pam 738-751prescribes forms, records, and reports to be used bymaintenance personnel at all levels.

have silver reflective tape with an identifying pattern The Vibrex is housed in a carrying case; it consists offor each blade (a straight line pattern on the redblade, a right-slanting pattern on the yellow blade, a the components detailed in Figure 4-7. The main

left-slanting pattern on the green blade). When the units of the Vibrex are the Balancer/Phazor,177M6A; the Strobex tracker, 135M11; and the

single line of the master blade target is aligned axially Vibrex tester 11. Three accelerometers, 4177B, andwith the centerline of the other blade targets, the two magnetic pickups, 303AN, are the primarysystem is in track. If one target image is displayed airframe-mounted components.

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Balancer/Phazor, 177M6A The key feature of the Balancer/Phazor (hereinafterreferred to as a balancer) is a tunable, electronichand-pass filter which is tuned to reject all but theone frequency or vibration under study (Figure 4-8).The meter reads the level of vibration at the rate(RPM) of concern, which indicates the amount of therequired change (track or balance). The Phazor

section contains a phase meter that reads clock angle,or phase angle, between a one-per-revolution mag-netic pickup azimuth signal from the rotor and avibration signal from the accelerometer.Strobex Tracker, 135M11 The Strobex tracker (hereinafter referred to as aStrobex) is a small hand-held, lightweight, combina-tion power supply and strobe flash tube (Figure 4-9).It illuminates reflective targets on the tail rotor tomeasure tail rotor clock angle and on the main rotorto indicate rotor track and lead-lag.Vibrex Tester 11The Vibrex tester (hereinafter referred to as a tester)provides accurate calibration and a complete func-tional check of the Vibrex (Figure 4-10). The tester

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shakes (vibrates) the accelerometer to measure • vibration amplitude in inches per second (IPS) andrate (RPM) functions of the balancer. Phase or clockangle functions of the Phazor section are verified bya rotating interrupter plate and the magnetic pickupto provide double and single interrupter logic signals.The RPM dial of the Strobex is accurately checkedagainst the known rotor speed of the tester motor.AccessoriesFollowing is a list of accessories that are used with thebalancer, Strobex, and tester:

•

Magnetic pickups and interrupter sets.These devices provide magnetic impulsesfrom rotor to balancer. Magnetic pickups arelocated on stationary platforms; interruptersets are located on rotating platforms.Accelerometers. Accelerometers providethe balancer with an electrical representationof the physical motion of the point to which itis attached.Reflective and tip target sets. These setsreflect Strobex flash pulses back to theStrobex operator.

•

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• Balancer and tracking charts. These chartsare used to calculate weight, sweep, pitch link,tab, and so forth to correct rotor problems.

NOTE: For further information on how to usethe Vibrex, refer to TM 55-4920-402-13&P.

PRESERVATIONPreservation is the term used for protection of equip-ment against deterioration due to exposure to atmos-pheric conditions during storage and shipment. Thefollowing items are needed for preservation:

•••

••

Mild soap and water.Solvent – PD-680.Moisture-absorbent cloth or filtered com-pressed air.Corrosion-preventive compound (CPC).Greaseproof paper.Desiccant bags.Padded contours (jute felt).Historical records.Parking and stenciling materials.

••••

Temporary Preservation and StorageWhen rotor blades are removed from an aircraft andstored for any length of time, they must be properlypreserved to remain in serviceable condition.Preservation procedures are the same for all rotorblades in the Army inventory. After the bladeshave been removed from the aircraft, cleanpainted surfaces with mild soap and water andunpainted surfaces with solvent, PD-680. Never usesolvent on painted blade surfaces because it can

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loosen the bonding. After cleaning dry rotorblades with a moisture-absorbent cloth or filteredcompressed air, make sure all surfaces and contoursare completely dry. Then apply a corrosion-preventive compound to the blades. CPC is a liq-uid; it should be spread only on the machinedsurfaces and only with a brush. Be careful to includethe inside of all retention bolt holes. The paintedsurfaces should not be coated. If the blades are to bestored for no longer than 3 months, they may beplaced in a slotted rack. Periodic inspection ofstored blades is essential to prevent corrosion ofmachined surfaces and to keep painted surfacesclean. Blades are stored in slots with the leading edgedown and the trailing edge up.

Shipment and Long-Term Storage

When preparing a rotor blade for immediate ship-ment or long-term storage, use the procedures ex-plained above to first clean, dry, and process it. Thenprepare the blade for shipment in a container. Metalcontainers are used Armywide for shipment andstorage of rotor blades.After preserving the blade with a CPC, wrap theblade, sockets, cuffs, retention plates, and machinedsurfaces with greaseproof paper and tape them. Thisfurther protects the blade at these areas from mois-ture, condensation, and wear during shipment.Place a bag of desiccant inside the container to aid indehumidifying the interior. The desiccant (silica

gel) absorbs moisture. Jute felt padding comes withblade containers from the manufacturer. The pad-ding is a plant fiber made to the shape of the bladeand used to support the blade at different pointsalong its span. Place the blade in the container withthe jute felt padding supporting it.Main rotor blades are secured inside containers atthe root ends by either of two methods: by bolting acuff fixture to the blade and its container wall, or bywelding along bolt at one end of the container, whichprotrudes through the retention bolt holes and issecured with a nut.Place the lid on the container and secure it. Makesure that the outside of the container is stenciled withthe sender, receiver, and serial number of item beingshipped. Remove the container’s old serial number.A new center-of-balance line should also be stenciledon at this time.Use DA Form 2402 (Exchange Tag) and DA Form2410 (Component Removal and Repair/OverhaulRecord) to ship rotor blades. Prepare two copiesof DA Form 2402. Tie one to the item beingshipped. Tape one on the outside of the shippingcontainer or place it in the cylinder, which is usedfor historical records. Complete one copy of DAForm 2410 and place it in the cylinder. If bladesare to be stored, the containers can be stacked ontop of each other in a warehouse. Check storedblades periodically for corrosion and for formsrequired by TM 55-1500-344-23.

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CHAPTER 5

SINGLE-ROTOR POWER TRAIN SYSTEM

A typical single-rotor power tram system (Figure 5-1)consists of a main transmission (main gearbox), amain drive shaft, and a series of tail rotor drive shaftswith two gearboxes. The main transmission includesinput drive with freewheeling provisions if no clutchassembly is required, output drive, and main rotormast. The main drive shaft between the engine andmain transmission drives the main transmission. Aseries of tail rotor drive shafts with two gearboxes(transmissions) – intermediate and tail rotor – be-tween the main transmission and tail rotor drive thetail rotor.

transmission input drive on the other end. The clutchassembly provides freewheeling (Figure 5-3). Onsystems not requiring a clutch assembly, the shaft isattached to an adapter on the engine output shaft onone end and to the freewheel coupling of the trans-mission input drive assembly on the other end.

Clutch AssemblyThe clutch assembly allows for a smooth engagementof the engine to the power train system. The clutchis used to stop possible blade damage and shaftshearing due to sudden torque loading. Some

MAIN DRIVE SHAFT clutches are designed to let the engine start and run

The main drive shaft (Figure 5-2) transmits torquefrom the engine to the main transmission. The shaftis a hollow, statically balanced tube. In addition torequired fittings, bolts, nuts, and washers areprovided with flexible splined or rubber couplingsfor installation between the engine and transmis-sion. On systems using a clutch assembly, the mainshaft is attached to the clutch on one end and to the

without the rotor turning. This is very useful forwarm-up and maintenance procedures. Due to thefree power system in all gas turbine engines used bythe Army, a clutch assembly is not needed on aircraftwith gas turbine engines.

The centrifugal clutch assembly is used only withengines of low horsepower output. When the enginespeed is increased, centrifugal force throws the

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clutch shoe against the inner surface of a drum,completing the drive to the rotor. This type of clutch,because of its slippage at low and medium speeds,generates heat, which is harmful to the life of clutchparts.Freewheeling UnitAll rotary-wing aircraft have a freewheel unit locatedbetween the engine and the main rotor or rotors.Three basic types of freewheel units are roller, sprag

freewheel unit is to free the power train drive systemfrom the drag made by the dead or idling engine. Bydoing this the freewheel unit makes autorotationpossible. This allows an aircraft to land safelywithout engine power. All types of freewheel unitsgenerally work in the same manner. They provide apositive lock of the power train drive system to theengine at any time engine speed equals rotor speed.When rotor speed is faster than engine speed, thefreewheel unit unlocks the power train drive system

clutch, and overrunning clutch. The purpose of the from the engine.

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MAIN TRANSMISSION A typical main transmissionfunctions (Figure 5-4). It—

• Drives the main rotor

Input Driveperforms a number of Engine torque is transmitted through the main drive

shaft to the input drive, which drives the main trans-mission gear trains. On systems not using a clutch

mast assembly. assembly, a freewheel coupling is provided in the •

•

Changes the angle of drive from the engine to input drive assembly, which automatically engages tothe main motor assembly. allow the engine to drive the rotor or disengages theProvides RPM reduction through a train of idling engine during autorotational descent. Onspiral bevel gears and planetary gears. dual-engine, single-rotor power train systems, theProvides a means of driving the tail rotor and transmission has two input drive assemblies.the transmission accessories.Supports the main rotor assembly. Tail Rotor Drive

•

•

The main transmission is mounted in a variety of waysaccording to a particular manufacturer’s design.Some transmissions contain a support case. The caseis an integral part of the transmission mounteddirectly to the transmission deck. The transmissionmay be secured to the transmission deck by a systemof tubular support assemblies. In one power trainsystem, the transmission is secured to the main rotormast support structure. In the power train system ofa reciprocating-engine-powered, observation-typehelicopter where neither shafting from engine totransmission nor drive angle change is necessary, the

The tail rotor is mounted on the end of the maintransmission and is driven by the accessory geartrain. A flexible splined coupling provides a meansof attaching the tail rotor drive shaft.

Generator DriveThe generator drive is driven by the main transmis-sion accessory gear train. The generator is drivenoff the main transmission so that, when thehelicopter goes into autorotation and the engineis idling or stopped, enough electrical power willbe left to operate instruments, radio, and electrical fuel

main transmission is mounted directly on the engine. pumps.

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Main Transmission Oil SystemMost main transmissions are lubricated by a wetsump oil system which is separate from the engine oilsystem (Figure 5-5). However, the engine oil coolerand transmission oil cooler may be mounted closetogether so that they can use the same blower systemto cool the oil. Oil supply from the transmissionsump is circulated under pressure from a gear-drivenpump through internal passages and a filter to thesump outlet. From this outlet external lines arerouted to an oil cooler with a separate thermal bypass

valve, then to a manifold on the transmission maincase. This manifold is equipped with a relief valve toregulate system pressure and distribute oil throughjets and internal passages. This lubricates bearingsand gears inside the transmission where the oil drainsback to the sump.Oil temperature and pressure gage readings areshown by a thermobulb and a pressure transmitter.Most transmission oil systems provide a heat switchand a pressure switch which will light caution lightson panels lettered XMSN OIL HOT and XMSN OIL

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PRESS (low pressure) if such conditions occur. Ser-vicing and draining provisions are provided in thetransmission oil system. Oil level sight gages areprovided on most transmissions; others use thedipstick method. Chip detectors used in the trans-mission oil system are similar to those used onengines.Rotor Tachometer-Generator DriveThe rotor tachometer RPM indications are providedby the rotor tachometer-generator. The tachometer-generator drive is driven by the main transmissionaccessory gear train.Hydraulic Pump DriveThe hydraulic pump drive is driven by the main trans-mission accessory gear train. The hydraulic pumpprovides hydraulic pressure for the flight controlservo system. Some helicopters use two separateflight control servo systems completely independentof each other. One system is the primary servo sys-tem; it gets hydraulic operating pressure from ahydraulic pump driven by the main transmission. Asecondary servo system gets hydraulic operatingpressure from a hydraulic pump driven by the engine.MAIN ROTOR MAST ASSEMBLYThe main rotor mast assembly is a tubular steel shaftfitted with two bearings which support it vertically inthe transmission (Figure 5-6). Mast driving splintsengage with transmission upper-stage planetarygear, providing counterclockwise rotation (viewedfrom above). The upper bearing retainer plate hasan oil jet fed by an external oil hose. Splints on theupper portion of the mast provide mounting for mainrotor arid control assemblies.

TAIL ROTOR DRIVE SHAFTThe purpose of the tail rotor drive shaft is to transmittorque from the main transmission to the tail rotorgearbox (Figure 5-7). The shaft is made up of a seriesof hollow tubes with provisions for statically balanc-ing and coupling attachments on each end.Flexibility in the shaft is provided by splined or rub-ber couplings. The tail rotor drive shaft is supportedby a series of support bearings and support hangerassemblies.INTERMEDIATE GEARBOXAn intermediate gearbox is located on the tail boomof the helicopter (Figure 5-8). This gearbox provides

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a specific degree change in direction of the tail rotordrive shaft with no speed change. The gearbox as-sembly consists of a case with flexible couplingprovisions for attaching onto the tail rotor shaft foreand aft. The gearbox is splash-lubricated, and thecase is fitted with an oil filter cap, a vent breather, anoil level sight gage, and a drain plug equipped with amagnetic insert. The magnetic insert collects metalparticles coming from inside the gearbox. Whenthere is a requirement, the metal particles can becollected and analyzed to determine the condition ofthe gears and bearings in the gearbox.TAIL ROTOR GEARBOXThe tail rotor gearbox is located on the extreme aftend of the tail boom – in some cases on top of the tail

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boom vertical fin (Figure 5-9). The gearbox is splash- change in tail rotor drive shaft direction and alubricated. It consists of mating input and output specific speed reduction between input shaft andgear assemblies set into a case provided with a vented output shaft on which the tail rotor assembly isoil filter cap, oil level sight gage, and a drain plug witha magnetic insert plug for collecting metal particles.By analyzing these metal particles, the condition ofthe gearbox gears and bearings can be determined.Flexible couplings are provided for attaching the tailrotor drive shaft onto the input end of the gearbox.The tail rotor gearbox provides a specific degree

mounted.

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CHAPTER 6

TANDEM-ROTOR POWER TRAIN SYSTEM

A typical tandem-rotor power train system consistsof an engine transmission for each of the two enginesand an engine-combining transmission (Figure 6-1).The system also includes a forward rotary-wing drivetransmission (containing a rotary-wing drive shaft[mast]) and an aft rotary-wing drive transmission(containing a rotary-wing drive shaft [mast]). Thedrive shaft consists of an engine drive shaft as-

the engine-combining transmission. This assembly isused by the engine to drive the engine-combiningtransmission. The drive shaft assembly also consistsof a forward synchronizing drive shaft assemblythrough which the engine-combining transmissiondrives the forward rotary-wing drive transmissionand an aft synchronizing drive shaft assembly throughwhich the engine-combining transmission drives the

sembly between each engine transmission and aft rotary-wing drive transmission.

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ENGINE TRANSMISSION transmissions provide angle of drive and RPM reduc-tion in torque. Torque from the engine is transmitted

The two engine transmissions are identical as- by the engine transmission and engine drive shaftsemblies. Minor rearrangement of transmission ex- assembly to the engine-combining transmission.ternal parts provides interchangeability between Freewheeling is provided in the output shaft of theright- and left-hand engine transmission installa- engine transmissions. This permits the drive systemtions. The transmissions are mounted directly on the to overrun the engine during failure, a sudden reduc-engine being driven by the engine output shaft. The tion of RPM, or autorotation. (See Figure 6-2.)

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Oil System Components

For each engine transmission there is a complete andseparate oil system (Figure 6-3). The oil system ismade up of the transmission sump, transmissionlubrication jets, check valve, electrical chip detector,oil temperature transmitter (bulb), oil pump, oilpressure transmitter (transducer), filter and reliefvalve assembly, oil tank, and oil cooler. Although theoil systems of the engine transmission and engine-combining transmission are not interconnected, oil

the left-hand engine transmission, and one sectionfor the engine-combining transmission. The oil filterand relief valve assembly for each of the engine trans-missions and engine-combining transmission aremounted on the aft side of the oil tank. The oil coolerfor the engine transmissions is mounted on top of thecombining transmission. It receives cooling air fromthe fan assembly mounted on top of the combiningtransmission.

Oil Circulationpressure and circulation for both types of transmis- Oil is circulated in each transmission lubrication sys-sions are provided from a six-element oil pump tem by two separate elements of the six-elementmounted onto and driven by the engine-combining lubrication pump in the combining transmission. Oiltransmission.

The engine-combining transmission is a three- flows from the oil tank through the pressure pump,section tank that is mounted onto and above the through the filter and relief valve assembly, throughengine-combining transmission: one section for the oil coder, and through the check valve. Then thethe right-hand engine transmission, one section for oil flows into the transmission where it is sprayed on

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the gears and bearings by various jets. Oil isscavenged from the sump by the scavenge section ofthe oil pump and returned to the oil tank.

ENGINE-COMBINING TRANSMISSION The combining transmission is a central collectionand distribution point for the drive system. The com-bining transmission is mounted in the lower forwardsection of the pylon. Torque from the engine trans-mission is transmitted by the combining transmissionand the forward and aft synchronizing drive shafts tothe forward and aft rotary-wing drive transmission.Speed reduction is also attained within the combin-ing transmission. The output shaft drives the lubri-cated pump. The three-section oil tank (one sectioneach for the combining transmission and each enginetransmission) forms the uppermost portion of thecombining transmission.Oil System ComponentsThe combining transmission oil system is a completeand separate oil system. The system includes the oilsump, oil temperature transmitter (bulb), oil pump,oil pressure transmitter (transducer), filter and reliefvalve, bypass valve, transmission lubricating jets,check valve, magnetic chip detector, oil tank, and oilcoolers.

NOTE: The reservoir for the oil system isthe center section of the three-section oiltank on the combining transmission. Thethree-section oil cooler and fan assembly ismounted on the top section of the transmis-sion.

Oil CirculationOil is circulated by two separate elements: onepressure element and one scavenge element of thesix-element oil pump in the combining transmis-sion (Figures 6-4 and 6-5). Oil is routed from theoil tank through the filter and bypass valve andthrough an external line to the oil cooler. The oil isthen routed by an external line through a check valveto the transmission. In the transmission oil is dis-tributed through internal passages and jets and issprayed on bearings and gears. Oil is scavenged fromthe sump through internal passages by the scavengeclement of the oil pump. The pump then pumps theoil to the tank. A sight level gage is installed on theforward end of the oil tank.

FORWARD ROTARY-WING DRIVE TRANSMIS-SIONTorque is delivered to the forward rotary-wing drivetransmission by the forward synchronizing drive shaftfrom the combining transmission (Figure 6-6). Theforward rotary-wing transmission then changes thedirection of torque from a horizontal plane to avertical plane. This reduces the input shaft speed.The forward rotary-wing transmission transmits thetorque through the rotary-wing drive shaft (mast) tothe rotor head.

Oil System ComponentThe oil system serving the forward rotary-wing drivetransmission is a complete, separate system. It is awet-sump system, which includes the oil tempera-ture transmitter (bulb), screens, oil pressurepump, oil pressure transmitter (transducer), reliefvalve, pressure filter, magnetic chip detector, oilcooler, and No. 1 flight control pump.Oil CirculationOil from the sump flows through the screen to theinlet of the pressure pump. The pump pressurizesthe system and pumps oil through the filter andcooler and then back to the transmission. Drilledand cored passages in the transmission distribute theoil to lubricators and oil jets. The lubricators and jetsspray the oil into the transmission bearings and ontothe rotating gears.AFT ROTARY-WING DRIVE TRANSMISSIONTorque is delivered to the aft rotary-wing drive trans-mission by the aft synchronizing drive shaft from thecombining transmission. The direction of torquechanges from a horizontal plane to a vertical plane,and the aft rotary-wing transmission reduces inputshaft speed (Figure 6-7). The aft rotary-wing trans-mission transmits the torque through the rotary-wingdrive shaft (mast) to the rotor head. In addition tothe reduction gearing, there are two accessory drivegears. One, driven by the input shaft, drives thetransmission oil-cooling fan shaft and an oil scavengepump. A second accessory gear drives the accessorygear section. The accessory gear section consists ofgearing for driving the lubrication pump, one flight-control hydraulic pump, two AC generators, and autility hydraulic pump. A one-way-drive clutch isunited into one body in the accessory gear driveshaft for the accessory gear section. This type ofclutch permits the accessory gears to overrun the

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transmission gears when the accessory gears are Oil System Componentsbeing driven by the hydraulic motor. In normal The oil system serving the aft rotary-wing driveoperation the accessory gears are driven by the bevel transmission is a complete, separate system (Fig-gear in the transmission through the clutch. ure 6-8). It is a wet-sump system which includes

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the temperature transmitter (bulb), magnetic chipdetector, screens, drain valve, oil level sight gage, oilpressure pump, oil pressure transmitter(transducer), relief valve, pressure filter, and oilcooler. DRIVE SHAFTS

NOTE: The sump is the lower circular cast-ing of the transmission. The fan drive shaftdrives an oil scavenger pump in the fan drivehousing. The pump returns scavenge oilwhich collects in the forward part of thetransmission when the aircraft is in a nose-down attitude.

Oil CirculationOil is drawn from the sump, through the screen to theinlet of the pressure pump. The pump pressurizesthe system and pumps the oil through the filter andcooler and back to the transmission. Oil is suppliedthrough external piping to the aft rotary-wing driveshaft (mast) thrust bearing. Drilled and cored pas-sages in the transmission unit distribute oil tolubricators and jets. The lubricators and jets sprayoil into the transmission bearings and onto the rotat-ing gears. A scavenge pump in the fan drive housing

is driven by the fan drive shaft. The pump returnsscavrnged oil which collects in the forward part ofthe transmission when the aircraft is in a nose-downattitude.

The purpose of drive shafting is to transmit torquefrom the engines to the transmissions and rotors. Italso transmits torque to the fan assembly.Engine Drive ShaftThere are two engine drive shaft assemblies in eachtandem-rotor power train system (Figure 6-9). Thepurpose of the engine drive shift assembly is to trans-mit torque from each of the engine transmissions tothe combining transmission. The shaft is a hollow,dynamically balanced tube. Each shaft is connectedto the combining transmission and engine transmis-sion by an adapter and plate assemblies, one on eachend of the shaft. The adapter and plate assembliesprovide flexibility in the shaft assembly.

Forward Synchronizing Drive ShaftThe purpose of the forward synchronizing drive shaft is to transmit torque from the engine-combiningtransmission. It also keeps the forward and aft

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rotors mechanically connected and in phase witheach other. The shafting assembly consists of aseries of shafts, each of which is a hollow, dynamic-ally balanced tube. Each shaft is interconnected tothe other, to the combining transmission on the aftend, and to the forward rotary-wing drive transmis-sion on the forward end by an adapter and plateassembly. The shaft is mounted and supported bybearings and support assemblies which contain shockmounts.

Synchronizing Drive Shaft The purpose of the aft synchronizing drive shaftis to transmit torque from the engine-combiningtransmission to the aft rotary-wing drive trans-mission (Figure 6-10). The aft synchronizing driveshaft is similar to and supported in the same manneras the forward synchronizing drive shaft assembly.The only difference is that the aft synchronizingshaft assemblv contains fewer shaft units and fewer.supports.

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REFERENCES

DOCUMENTS NEEDEDThese documents must be available to the intended users of this publication.TM 55-15W-W-23. Avionic Cleaning and Corrosion Prevention/Control. 22 April 1987TM 55-4920-402-13&P. Operator’s, Aviation Unit and Intermediate Maintenace Manual (Including Repair

Parts and Special Tools Lists) for Vibrex Balancing Kit. 15 August 1980.DA Form 2402 Exchange Tag. December 1985.DA Form 2410. Component Removal and Repair/Overhaul Record. May 1981.READINGS RECOMMENDEDThese readings contain relevant supplemental information.AR 310-25. Dictionary of United States Army Terms (Short Title: AD). 15 October 1983.AR 310-50. Authorized Abbreviations and Brevity Codes. 15 November 1985.DA Pamphlet 25-30. Consolidated Index of Army Publications and Blank Forms. 31 December 1989.DA Pamphlet 738-751. Functional Users Manual for the Army Maintenance Management System -

Aviation (TAMMS-A). 15 January 1988.TM 55-1500-335-23. Nondestructive Inspection Methods. 1 June 1984.

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