Rotor System

ROTOR SYSTEM

CONTENTS

1 INTRODUCTION 1

2 TYPES OF ROTOR 1

2.1 Articulated Rotor 12.2 Gimbled Rotor 22.3 Teetering Rotor 22.4 Hingeless Rotor 22.5 Bearingless Rotor 3

3 ROTOR CONTROL 3

4 BLADE RETENTION 3

4.1 Articulated Rotor 34.2 Hingeless Rotor 44.3 Bearingless Rotor 54.3.1 Bearingless Main Rotor 54.3.2 Bearingless Tail Rotor 5

5 ROTOR COMPONENTS 6

5.1 Rolling Element Bearings 65.2 Elastomeric Bearings 65.2.1 Conical Bearing 65.2.2 Radial Bearing 75.2.3 Snubber Bearing 75.2.4 Spherical Bearing 75.2.5 Thrust Bearing 75.3 Lead-Lag Damper 75.4 Teflon Lined Bearing 8

CONTENTS JAN-2001 PAGE 1

ROTOR SYSTEM

6 ROTOR CONTROL SYSTEM 9

6.1 Main Rotor Upper Control System 96.2 Main Rotor Control Mechanism 96.3 Tail Rotor Control Mechanism 106.4 Materials 11

7 DESIGN AND CONSTRUCTION OF ROTOR BLADES 11

7.1 Articulated Blades 127.2 Hingeless Blades 127.3 Blades for Bearingless Rotor 127.4 Structures of Rotor Blades 137.4.1 Metal Rotor Blades 137.4.2 Composite Main Rotor Blades 137.4.3 Composite Tail Rotor Blade 14

8 BLADE FOLDING 14

8.1 Manual Blade Folding 158.2 Automatic Blade Folding 16

9 FIGURE 1 TO 36 17 TO 51

CONTENTS JAN-2001 PAGE 2

ROTOR SYSTEM DESIGN

Dr. K. Manjunatha UdupaDeputy General Manager (Design)

Rotary Wing Research & Design CentreHindustan Aeronautics Limited

Bangalore - 560017

INTRODUCTION

A conventional rotorcraft is provided with main and tail rotors. The main rotor produces thrust, the vertical components of which lifts the helicopter and the horizontal component causes lateral movement of the helicopter. The tail rotor thrust is required to counter the reaction torque of the main rotor. By changing the tail rotor thrust, the helicopter can be yawed about the main rotor axis.

The other three types of rotors are contra-rotating, tandem and side by side. In contra-rotating system, two identical rotors rotating in opposite directions are placed one above the other along the same axis. In tandem arrangement, two main rotors are placed one behind the other. In side by side arrangement, the two rotors are positioned along the lateral axis. These three configurations do not need tail rotors.

A rotor system consists of rotor hub, rotor blades and upper control system. These systems are explained in the subsequent chapters.

2 TYPES OF ROTOR

Rotor system can be described as articulated, gimbled, teetering, hingeless and bearingless.

The blades of an articulated rotor system are attached to the hub with mechanical hinges, allowing the blade to flap up and down and swing back and forth (lead and lag) in the plane of the rotor disc. The blades of the hingeless rotor are attached to the hub without mechanical hinges for flap or lead-lag motion. Bearingless rotor employs a flexible structural attachment of the blade. In a teetering rotor, the two opposite blades are connected together rigidly and hinged at the rotor centre. In a gimbled rotor, each blade is attached to a yoke which is gimble mounted to the rotor mast.

2.1 Articulated Rotor

Vertical motion of the pitch link in response to swash plate tilt produces pitching rotation at the pitch bearing corresponding to cyclic pitch of the rotor. The lag hinge allows the blade to move in the disc plane. Due to this hinge, the steady lag bending moment at the blade root is reduced. Individual blade lag dampers are required to provide energy dissipation to control the mechanical instability associated with coupled rotor-airframe system. The rigid body lag natural frequency of this rotor blade is usually 0.2 to 0.4 times the rotor speed.

CHAPTER 1JAN 01 PAGE 1

ROTOR SYSTEM

Blade flapping freedom is provided by the horizontal flapping hinge located close to the rotor hub to minimise the flap bending of the rotor hub. The natural frequency of articulated blade is near resonance with rotor shaft speed. Aerodynamic damping of the flapping blade provides an acceptable design.

The rotor should be designed such that as the blade flaps up, the pitch angle of the blade remain the same or reduce. The kinematic coupling ratio between pitch angle and flap angle is defined as delta 3 . Negative delta 3 is required to improve stability of the rotor. General arrangement of an articulated rotor is flap hinge first and next lag hinge and feathering bearings.

Articulated rotors are used in Alouette-III, S-76, Mi-8, Kamov-25. Figure-1 shows the arrangement of this rotor.

2.2 Gimbled Rotor

The blade root moments are reacted at the yoke, leaving the hub undisturbed. Yoke should be stiff to achieve blade cantilever mode frequency greater than the rotor speed. General arrangement is given in figure-2.

2.3 Teetering Rotor

The teetering hinge allows only seesaw flap motion of the blades. For rotor tilt, the blades are forced by the trunion out of their ideal position by special mechanisms connected to swash plate. Examples for teetering rotor are Bell-206,212 & 222. General arrangement of this rotor is shown in figure-3.

2.4 Hingeless Rotor

In this type of rotor, no mechanical means are provided to allow chordwise or flapwise displacement of the blades. The blades are cantilevered from the rotor hub which is attached rigidly to rotor shaft. The pitch angle of the blade is changed by rotation of the blade about the feathering axis. Cyclic pitch input provides control moment as result of both tilting of the resultant lift vector and a moment acting at the hub.

The natural frequency of the first flapwise bending mode fixes the offset of the equivalent flap hinge. This is generally 1.1 to 1.15. This provides an equivalent flap hinge location of 0.12R. The first lead-lag bending natural frequency is adjusted to around 0.7 of rotor speed to eliminate ground resonance. This provides an equivalent lead-lag hinge location of 0.15R.

Precone of the blades permit cancellation of steady lift moment due to centrifugal force acting through the vertical displacement of the blade above disc plane.

Examples of hingeless rotor are found in BO-105, BK-117, Lynx, SA-365 Dauphin, Bell - 412 and ALH.

ROTOR SYSTEM


2.5 Bearingless Rotor

In the bearingless rotor, there exist no mechanical hinges or feathering bearings. The blade retention member is made flexible to allow flapping, lead-lag bending and feathering of the blade.Examples for bearingless rotors are EC-135 and MD Explorer.

3 ROTOR CONTROL

In-flight control of the helicopter using rotors explained in Section-2 are provided by

moment acting upon the rotor hub tilting the rotor lift vector a combination of the two

In the articulated rotor, control moment is generated as a result of lift vector tilt and hub shear force acting at the flap hinge. In gimbled and teetering rotors, rotor tilt produces corresponding tilt of the lift vector which produces control moment about helicopter C.G. In case of hingeless and bearingless rotors, the structural spring at the equivalent hinge provides an additional component of control moment at hub. Figure-4 shows control moment mechanisms in different rotors.

The conventional method of achieving rotor control is through collective and cyclic pitch changes at the blade roots. These changes are accomplished through control linkages between rotating blades and swash plate. Collective pitch of the blades is introduced by raising or lowering the swash plate. Cyclic pitch required to produce a tilt of the rotor disc plane is accomplished by tilting the swash plate.

4 BLADE RETENTION

The rotor blades are required to be adequately connected to the rotor hub to transfer the centrifugal force shear forces and moments. The connections should allow the intended blade motions. Typical retention systems are briefly described below.

4.1 Articulated Rotor

The flap, lag and pitch hinges of the articulated rotors are oil or grease lubricated. The flap hinge is offset both radially and in the direction of rotation. The radial offset is as close to the rotor centre as the shaft and hinge sizes permit. The offset of flap hinge in the direction of rotation is called torque offset is chosen to equalise load on flap hinge bearings and avoid reversing of axial motion due to blade lead-lag motion.

The cylindrical roller bearings are provided at flap and lead-lag hinges. At both hinges, thrust bearings or bronze thrust washers are also provided. Flap hinge bearings withstand the centrifugal force, alternating loads due to blade lag oscillations and blade vertical shear forces and experience one-per-rev oscillations. Side loads on flap hinge are carried by thrust bearings.


ROTOR SYSTEM

Blade flap oscillation is of the order +5 deg. The roller bearings of the lag hinge are subjected to centrifugal force plus reactions from blade steady and alternating bending moments. Vertical shear loads at lag hinge are carried by the thrust washers. Blade lag oscillation is of the order +1 deg.

The roller bearings of the feathering hinge react blade bending moments and shears at blade root. Tension-torsion strap consisting of many layers of St-steel sheet react centrifugal force. Due to the low torsional stiffness, these strips twist easily, providing freedom for blade pitch change with small efforts on pitch link. The pitch link connection on blade is nearly along the flap hinge axis. Blade is attached to the outboard end of the pitch sleeve through two bolts.

The lag damper connected across the lead-lag hinge meets the requirement of stability of lag motion and functions as lead-lag stop. Permanent stops prevent excessive blade flap or droop in parked condition. They are preset not to come in contact in normal flight. Figure-5 shows the articulated rotor head of Alouette-III

4.2 Hingeless Rotor

Hingeless blades are attached firmly to the hub which in turn attached rigidly to the rotor mast. The blade retention system must be capable of transferring axial force, shear forces and moments in both flapwise and lead-lag directions. The blade centrifugal force may be reacted by conventional tention-torsion strap or alternatively by elastomeric bearings. The retention must provide pitch change capability and the centrifugal force must be reacted across the feathering hinge. Flap and lead-lag movement of the blade occurs due to structural deflection. The geometry of retention may also include sweep, precone or droop of the span wise axis of the blade. The lag damper if required is connected across the flexible region of blade or blade sleeve.

Figure-6 shows the hingeless rotor head of Westland Lynx. Here, the blade sleeve is made of titanium and its stiffness is tailored to achieve the required natural frequencies of the blade. The sleeve contains feathering bearings and TT bar.

Figure-7 shows BO-105 rotor system. In this system, stiffness of glass fibre composite blade collar is tailored for required structural deformations. The titanium hub carries feathering bearings and TT bar.

Hingeless rotor head of ALH is shown in figure-8. In ALH, the rotor head and upper control system are integrated with the main gear box. The upper controls are enclosed within the drive shaft and titanium centre piece to which carbon composite hub plates are connected. This concept is called integrated dynamic system. Blade is retained over outboard conical elastomeric bearings and radial elastomeric bearing positioned at the centre piece. The collar of the composite blade is tailored to achieve the required blade structural deformations. Pitch change is accomplished by twisting the conical and radial elastomeric bearings which are soft in torsion.

Hingeless rotor head of SA-365 Dauphin is shown in figure-9. This concept is called starflex due to star shaped glass composite rotor hub. The flexible star allows flapwise bending deformation. The centrifugal force is transferred to the spherical bearing through composite

ROTOR SYSTEM


arms. Lead-lag motion is accomplished by the cocking motion of the spherical bearing and inplane shear deformation of the elastomeric damper. Blade pitch is changed by twisting the spherical bearing through pitch link. The damper does not offer resistance for pitch change since it is mounted on the star through a spherical bearing.

4.3 Bearingless Rotor

4.3.1 Bearingless Main Rotor

Here, the blade is connected to the outboard end of a flexible beam which can undergo structural deformation in flapwise, lead-lag and pitch directions. The flex-beam connected rigidly to the rotor hub at the inboard end transfers the centrifugal force, shears and bending moments to the hub. Pitch change of blade is achieved by twisting the flex-beam through a torsionally stiff pitch case which is attached to the flex-beam at outboard end and connected to the pitch change links at the inboard end. To prevent lateral deflection of pitch case due to pitch link loads, a shear restraint is provided between the two. This can be a mechanism or an elastomeric pad designed to offer only a small resistance to pitching motion. Lag damping is achieved through a compact elastomeric damper mounted on the pitch case. Dampers subjected to shear deformation due to relative lead-lag displacement between flex-beam and pitch case.

Schematic layout of a bearingless rotor is shown in figure-10. The concept is called bearingless since the feathering bearings either metallic or elastomeric are also eliminated. The three elements of the bearingless rotor are generally made of fibre composites. In EC-135, flexbeam is integrated with blade spar and pitch case is integrated with blade shell, thus eliminating the outboard joint between the three elements.

4.3.2 Bearingless Tail Rotor

Bearingless concept was developed first for the tail rotor. After successful use, the concept was further developed for main rotors. Typical bearingless tail rotor in operation is described below and shown in figures 11,12 and 13.

The blade consists of glass/epoxy flex-beam and pitch case integrated with the aerofoil region. Blades are made in opposite pairs so that hub attachment need not resist centrifugal force. Two blade pairs are placed perpendicular to each other and clamped together by metal top and bottom hub plates (figure 11) which are connected to the tail rotor shaft through splines. Flex-beam lugs are attached to the hub plates through four bolts (figure-12).

Pitch case and flex-beam are separated by a pair of snubber bearings located on flex-beam shoe and connected to pitch case through special bolts (figure-13). Seating on flex-beam is such that the bearings can slide freely in centrifugal direction. Bolts ensure defined precompession of the bearings during assembly. The bearing pair reacts the pitch link load.

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5 ROTOR COMPONENTS

Features of specific components such as rolling element bearings, bushes, elastomeric bearings and lag dampers are reviewed in this section.

5.1 Rolling Element Bearings

These bearings are widely used in rotor hinges. They are classified as

cylindrical roller bearings tapered roller bearings angular contact ball bearings

Lubrication of the bearing is affected by oil or grease. Installation details must be designed to take care of adequate lubrication. Smaller bearings are also available in grease filled and sealed condition.

5.2 Elastomeric Bearings

A bearing is a mechanical device that accommodates dynamic motions. Elastomeric bearings allow rotational motion but are limited to oscillatory motion from 10 deg to 90 deg. They offer some clear advantages over hard metallic bearings.

Elastomer layers add a spring back effect, tending to restore the angular motion back to undeflected position, acting like torsion spring.

Elastomeric bearings have no clearance and hence there exist no fretting or wear. This results in smooth performance through out the life of the part.

Degradation of elastomeric bearing is gradual and fail-safe.

They are maintenance free.

They are unaffected by air borne particles of sand and dirt.

On-condition removal criteria helps avoid premature removal of adequately functioning parts.

The elastomer inherently reduces vibration of the dynamic system.

Types of elastomeric bearings widely used in rotor system and briefly explained below.

5.2.1 Conical Bearing

Conical bearing consists of alternate layers of conical shaped elastomer and metal shims bonded together between the two metal housings. The housings are connected to the load acting

ROTOR SYSTEM


and reacting members (figure- 14). This is capable of carrying loads in axial and radial directions. Since it is torsionally soft, it can be twisted by applying moderate torque.

5.2.2 Radial Bearing

This consists of concentric cylindrical shims of elastomer and metal bonded between inner and outer members. It can take only radial loads. It is not intended to be loaded in axial direction. Radial bearing is also soft in torsion about the longitudinal axis XX (figure -15).

5.2.3 Snubber Bearing

Snubber bearing pair is used to separate the pitch case and flex beam of a bearingless rotor. Snubber bearing is stiff in axial and radial directions but soft in cocking motion (figure-16). It consists of spherical elastomeric and metal shims. 5.2.4 Spherical Bearing

This consists of elastomer and metal shims of spherical shape. It can resist axial compression and radial loads and can undergo cocking deformation about the spherical centre.

5.2.5 Thrust Bearing This consists of alternate flat shims of elastomer and metal. It is designed to withstand axial thrust loads.

Combination of conical bearing and radial bearing would form ideal retention for main rotor blades. Conical bearing is positioned to carry all the centrifugal force in axial direction. Radial loads due to blade bending moments are reacted by the two bearings placed at appropriate distance.

Elastomeric bearings are designed to meet the specification requirements of stiffness, environmental conditions and life under spectrum loading. Modified natural rubber is generally used as elastomer shims. Metal shims are made of aluminium alloy, titanium or corrosion resistant steel. The shim package is bonded to metal housings which have hole and location features required for the assembly.

The loading pattern on elastomeric bearings should be such that elastomer shims are not subjected to tensile strains.

5.3 Lead-lag Damper

The lead-lag dampers are provided on the main rotor head to meet blade stability requirements in ground resonance and in flight. They attenuate the dynamic motion in the rotor lead-lag plane. This is generally required on all types of rotors. There are a few hingeless rotor with high blade structural damping due to which external lag damping is formed not needed.


ROTOR SYSTEM

Three common means of energy absorption in lag dampers are hydraulic, friction and visco-elastic. Although simple, friction dampers are less reliable. They are not used in present day helicopters.

In a hydraulic damper, the damping fluid is forced to pass through the orifices due to relative movement between the ends. This action provides energy dissipation and dynamic stiffness. Some of the hydraulic dampers are of sealed construction and do not require frequent maintenance. Figure-17 shows a typical hydraulic damper.

Visco-elastic dampers are designed to utilise the good damping characteristics of certain types of elastomers when subjected to shearing strains. Here, elastomeric layers are sandwiched between metal plates. Thickness of layers are determined to operate with in the displacement and shearing strain limitations. Inner and outer plates are attached to the points between which the motion is required to be damped. Figures-18 and 19 show typical elastomeric dampers.

5.4 Teflon Lined Bearing

Teflon lined spherical bearings and bushes are widely used in the control system of modern helicopters. The backing material could be aluminium alloy, corrosion resistant steel, titanium alloy or helically wound glass/epoxy composite. These members are suitable for applications wherein oscillatory motions and high vibratory loads are encountered.

Teflon bearings with trade names such as fibreglide and fibreslip are proprietary self lubricating bearing materials of woven teflon (PTFE) fibres applied to rigid backing. To assure good bond between PTFE fibres and backing material, a secondary, more readily bondable fibre is interwoven with PTFE fibres so that teflon is predominantly presented on the bearing side of the fabric.

These bushes and bearings are unique in their ability to resist teflon col-flow under extreme high loads because the fibres have a tensile strength approximately 15 times greater than straight PTFE material. Advantages of teflon lined bearings and bushes are given below.

Operation without lubrication while tolerating common lubricating and other fluids. High lubricating capability Low coefficient of friction Freedom from stick-slip Absence of cold flow tendency High resistance to fatigue under shock loads Eliminates fretting corrosion High wear resistance Inherent damping qualities Wide range of mating material acceptability Electrically non-conducting Non magnetic

Typical teflon lined spherical bearing and bush are shown in figure-20


ROTOR SYSTEM

Typical applications of the above teflon lined parts are given below.

Bushes for blade bolt retention Bushes of mixing unit Spherical bearing of swash plate Spherical bearings of control rods and pitch links

6 ROTOR CONTROL SYSTEM The main rotor requires collective and cyclic controls. For helicopter to manoeuvre in longitudinal and lateral directions, cyclic controls are provided in these two channels. By operating collective and cyclic sticks, pilot actuates the three hydraulic boosters to obtain linear displacements in collective, longitudinal and lateral cyclic channels. The upper control mechanism converts the above inputs into collective and cyclic pitch variations of the blades. Similarly pedal control of the pilot actuates tail rotor hydraulic booster to produce axial movement which is converted into collective pitch variation of tail rotor blades. Main and tail rotor control systems are briefly explained below.

Figures 21 and 22 show the schematic diagram of typical main and tail rotor control systems. Isometric view of main rotor control system is shown in figure 23.

6.1 Main Rotor Upper Control System

The upper control system transfers pilot operated displacements from actuators into collective and cyclic variation of blade pitch angles. In collective pitch, angles are increased or decreased equally on all the blades. Collective pitch varies magnitude of helicopter lift.

In longitudinal cyclic pitch, blade angle in one blade is increased or decreased at a particular azimuth position so that the aft blade flaps up to provide forward tilt of tip plane. The forward component of the lift vector provides propulsive force.

In lateral cyclic pitch, blade angle on one blade is increased or decreased at a particular azimuth position to provide sideboard tilt of the rotor plane which results in sideboard propulsive force.

6.2 Main Rotor Control Mechanism The components of upper control system are connecting rods, mixing unit, non-rotating control rods, swash plate assembly, swash plate mast, rotating control rods, scissors and pitch horn. Their functions are explained below.

The three connecting rods in collective and cyclic channels transfer control inputs from actuator to mixing unit. Mixing unit comprises of collective bell crank, longitudinal bell crank

ROTOR SYSTEM


and lateral bell crank. Longitudinal lever assembled on collective bell crank acts as hinge axis for both longitudinal and lateral bell cranks.

Non-rotating control rods connect longitudinal bell crank, longitudinal lever and lateral bell crank to non-rotating swash plate at three points.

Swash plate mast provides the vertical axis for sliding motion of swash plate which depends upon collective input. Mixing unit is also structurally supported on swashplate mast.

Swash plate assembly consists of rotating and non-rotating plates separated by a ball bearing. The spherical bearing at the centre allows up and down sliding motion on mast and tilt in all planes.

Rotating control rods are attached between rotating swash plate and blade pitch change horn. Rotating scissors connect the rotor hub to rotating swash plate and impart rotary motion to the later. Non-rotating scissors are provided between swash plate mast and non rotating swash plate to prevent the later from rotation due to frictional torque.

Due to collective input, the collective bell crank rotates about its hinge, This causes the cyclic hinge to move up or down, driving the non-rotating control rods to move equally. Consequently, swash plate moves up or down along with rotating control rods causing equal pitch change in all blades.

When longitudinal input is given, the longitudinal bell crank rotates about cyclic hinge causing movement of non-rotating longitudinal control rods and swash plate tilt about longitudinal axis. Since rotating control rods follow swash plate plane, blade pitch angle varies over the azimuth from maximum to minimum.

Due to lateral inputs, lateral bell crank rotates about cyclic hinge causing lateral non-rotating control rod to move and the corresponding swash plate tilt about lateral axis. Blade pitch angle would vary accordingly over the azimuth.

6.3 Tail Rotor Control Mechanism

Control of tail rotor thrust is provided by collective variation of the blade pitch angle. The control mechanism of tail rotor consists of the following.

Control tube:

This is positioned inside hallow tail rotor shaft and is driven by the shaft through spline connection. This can move axially with respect to the shaft. Inboard end of the tube is connected to the actuator output through an angular contact ball bearing which can carry axial loads. This bearing separates the rotating tube from non-rotating actuator output end and transfers the axial motion of the actuator.

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Spider:

Spider is rigidly assembled at the end of control tube. It is provided with four arms corresponding to four blades.

Pitch link:

Pitch link connects spider arm to pitch horn and transfer the axial motion of control tube to the pitch horn. Movement of pitch horn causes pitch change of the blade. Pitch link is provided with turnbuckle to adjust the length for pre-setting the blade angle.

6.4 Materials

Upper control system components are dynamically loaded. They should have adequate stiffness to limit deformation under operating loads. They must have high fatigue strength and damage tolerance features. The design life of the components under operating spectrum loading is of the order 5000 hours. Components with smaller endurance life if any are to be retired and replaced in service.

Aluminium alloys are used for the parts which are subjected to moderate loads. For highly stressed parts such as control rods, pitch horn, shafts, control tube, titanium alloy is chosen due to its high specific strength. Corrosion resistant steel is used for bolts, bearings and bushes. Molybdenum di-sulphide coating is applied over the interfaces of joints to prevent fretting wear. Teflon fabric lined bushes and spherical bearings are suitable for joints undergoing oscillatory relative movements.

7 DESIGN AND CONSTRUCTION OF ROTOR BLADES

Main rotor blades produce thrust to balance the inertia of the helicopter and the propulsive force for the translational motion. The tail rotor produces thrust to counter main rotor reaction torque and to produce yawing acceleration. The aerodynamic and dynamic design parameters of the blade are the following.

Rotational speed Sense of rotation, clockwise or anticlockwise Aerofoil distribution over the span Twist distribution along the span Chord Tip shape Precore or preflap angle Mass per unit length Flapwise, lead-lag and torsional stifnesses Flap and lag hinge offset

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Placement of natural frequencies affect helicopter vibration and blade loads. On the basis blade fixity conditions, blades are classified as articulated, hingeless and bearingless. Feature of these blades are briefly explained below.

7.1 Articulated Blades

In an articulated rotor, the blade is free to articulate about flap and lead-lag hinges. Pitching of blade about longitudinal axis is achieved over two feathering bearings. The blade is generally of constant sectional properties over the length. The root end of the blade is tailored to match with the rotor sleeve.

7.2 Hingeless Blades

Flap and lead lag hinges are absent in a hingeless rotor. However the blade is designed to pitch about feathering bearings. In modern hingeless rotors, metallic feathering bearings are replaced by elastomeric bearings. In hingeless blade, the root end of the blade (approximately 20% radius) is designed for lower flapwise and lead-lag stiffnesses, with the result, flapping and lead-lag articulation takes place due to elastic bending of this flexible region. Effective flap and lead-lag hinge offsets are computed on basis of deformed blade shape. Depending on the first bending natural frequency in lead-lag plane, the blades are classified as soft-in-plane or stiff-in-plane. The first helicopter to use hingeless rotor with all composite blade was BO 105.

7.3 Blades for Bearingless Rotor

Blade of a bearingless rotor will have flex-beam, pitch case and aerofoil blade. Flex-beam is made of unidirectional glass or carbon/epoxy composites. Its cross section are tailored to achieve certain flapwise and lead-lag stiffness distribution required to satisfy strength and dynamic characteristics of the rotor. Inboard end of the flexbeam is attached to rotor head. Out board end is integrated with blade by mechanical attachment or by moulding into blade spar. The pitch case is a closed cell made of fiber/epoxy layers oriented to achieve maximum torsional stiffness.

Different types of blades are shown in the following figures.

Figure 24 Articulated metal blade (Aloutte-III) Figure 25 Teetering metal blade (Bell G-47) Figure 26 Articulated composite blade (KA-28) Figure 27 Hinge less composite blade (BO105) Figure 28 Hinge less metal blade (Lynx) Figure 29 Hinge less composite blade (ALH) Figure 30 Hinge less composite blade (Dauphin) Figure 31 Bearingless main rotor blade (EC 135)

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7.4 Structures of Rotor Blades

Rotor blades can be broadly classified as metallic and composite. Fibre reinforced plastics (FRP) such as glass, carbon and kevlar-epoxy combinations are being extensively used as structural materials in modern helicopter rotor blades and hubs. These materials offer several advantages over conventional metallic materials. With regard to mechanical properties, some of them are superior in terms of specific strength and stiffness. Composite materials are resistant to corrosion induced by hot, humid and marine environments. The important basic advantage are their higher fatigue strength, good damage tolerance and soft failure modes. If properly maintained, composite blades are capable of operating four to five times longer than metal blades. It is realised by experience that these blades are much easier to repair than metal blades. Lower radar signature of fibre/epoxy is of significance in military operations. Further, stringent requirements such as aerodynamic contour smoothness, dimensional tolerance, incorporation of aerodynamically tailored, non uniform blade geometry and twist etc. can be achieved easily due to the mouldability of FRP materials. Also these blades can be made individually interchangeable a plus point in reducing operating costs.

There are some problem areas concerning the composite blades. They need meticulous protection against erosion, moisture absorption and degradation due to ultra violet rays. However, effective measures have been found to overcome these limitations.

Several successful helicopters which were designed during 1950-70 used metal blades. Metal blades require scheduled maintenance at short intervals. They are to be retired after about 2500 hours, a relatively short service life in comparison to approximately 5000-10000 hours attainable by composite blade.

7.4.1 Metal Rotor Blades

Main structural elements of a metal blade are spar and skin contributing predominantly to bending stiffness and torsional rigidity respectively. The spar could be a machined aluminium alloy open section of semi-lunar shape or closed D-section. Titanium and stainless steel oval section tubes are also used as spars. The aft portion of the skin is stabilised by machined honeycomb or rigid foam fillers. Leading edge of the blade is protected against sand and rain erosion by a cap of stainless steel or titanium sheet. Components of the blade are bonded together by heat curing structural adhesives. The major drawbacks of a metal blade are the susceptibility to structural failures due to crack propagation and sensitivity to inadequate maintenance. All metal blades can be found in Allouette-III with C-spar (figure-24), Bell G-47 with D-spar (figure-25) and S-76 with tubular spar.

7.4.2 Composite Main Rotor Blades

Composite rotor blade cross-section consist of the following details.

C-shaped or D-shaped spar made of glass/epoxy or hybrid composite which contributes to flapwise stiffness, lead-lag stiffness and carries the centrifugal force.

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Carbon/epoxy or glass/epoxy skin with fibers oriented at 45 deg to longitudinal axis which provides torsional stiffness.

Rigid structural foam or honeycomb core which stabilise the skin in the lateral direction. This is required to keep the shape of the aerofoil.

Metallic leading edge erosion protection either in single piece or in multiple pieces.

Lead balancing mass at the leading edge for placing section centre of gravity close to 0.25 Chord axis.

Channel which is bonded between upper and lower skins to provide shear rigidity in the plane of cross-section.

BO 105, EC 135, and ALH blades are examples for C-spar. KA-28 blade consists of composites D-spar and aft honeycomb sandwich pockets bonded to spar.

Carbon/epoxy or glass/epoxy unidirectional trailing edge which increases lead-lag stiffness.

Anti node mass at 0.5R to tune second flapwise frequency (optional)

Tipmass for tuning the lead-lag frequencies (optional)

Balancing chamber at tip for static and dynamic balancing of blades.

Metallic trim tab at the trailing edge for blade track adjustment.

Electrically heated anti-icing or de-icing mat at the leading edge if the helicopter is intended to operate in icing condition (optional).

7.4.3 Composite Tail Rotor Blade

Design principles for composite tail rotor blade are similar to that followed for the main rotor blade. Tail rotor blades are smallar in size. Spar is generally C-shaped. They are not generally provided with the balancing chamber. Static and dynamic balancing is carried out by adding weights on adapters provided at the rotor head. Nose balance mass and mid chord channel are generally not needed. Erosion protection is made in one piece. On bigger helicopters, operating in icing conditions, tail rotor blades are also provided with anti-icing heater mats.

8 BLADE FOLDING

Folding of main rotor blades is needed to reduce the parking area. It is of significance for Naval helicopters which have to stowed inside hangars. After folding the blades are secured to prevent flapwise and lead-lag movements of the blades. On some helicopters, tailboom is also folded about a hinge to reduce the length and height in folded position. Blade folding can be achieved manually or automatically

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by means of actuators. Blade folding should be possible in rough sea states and high wind speeds up to 45 knots.

Blades are folded and secured when the helicopter is on the landing deck. Small helicopters are moved into the ship hangar manually. In case of a large helicopter, specially designed traversing cross bar is engaged with the main wheels and the traversing carriage is power operated along the track fixed on the floor. The system thus pulls the helicopter to a predetermined position inside the hangar.

8.1 Manual Blade Folding

To facilitate manual folding, rotor blade is attached to the hub through two bolts. For folding, one of the bolts per blade is removed and blade is moved to the folded position using the second bolt as hinge. Blades are moved to the folded position by ground crew using folding poles, the upper end of which are designed to clamp the blade. After folding, the blades are moored to the fuselage through suitable mooring fixture.

Figures 32, 33 and 34 show the folding of blades on Chetak SA 365and Lynx helicopters.

8.2 Automatic Blade Folding

Automatic blade folding systems are employed on large helicopters due to the following reasons.

Blade folding and spreading operations are quick and effortless.

There exists no risks to personnel or helicopter parts in high wind and sea state conditions.

Additional weight involved will be a small compared to the weight of the helicopter.

Due to the large width of rotor head, size of bolts and weight of blades, manualfolding involoves too much efforts.

In automatic blade folding system, rotor indexing, blade folding and spreading operations are carried out sequentially by hydraulic or electro-mechanical actuators.

Typically, Westland Sea-King helicopter is equipped with hydraulically actuated automatic blade folding system and SH-60B Sea Hawk is provided with electro-mechanically actuated automatic blade folding system.

Hydraulic fold system consists of hydraulic cylinder at each blade arm. Linear output of the cylinder is converted to rotation of the rotor blade about the folding pin.

Electro - mechanical blade folding system is much simpler and compact than the hydraulic system. This system employs a rotary output at the blade fold pin.

ROTOR SYSTEM


ROTOR SYSTEM

Fully automatic blade folding system consists of the following subsystems.

Rotor indexing actuator and sensors which position the rotor arms to predetermined position prior to folding.

Pitch lock actuators which provides locking against blade pitching.

Blade folding actuators, one per blade which fold the blades to the folded position. This actuator also contains mechanism to remove one bolt of the folding joint.

Sensors and micro switches to limit the rotation.

Folding control unit positioned in the cockpit which operates monitors folding and spreading operations.

Slip ring assembly at rotor head to supply power to the electro-mechanical folding actuators and also to connect folding control unit to the actuators. Hydraulic rotary coupling is needed in case of hydraulically actuated folding system.

Figure-35 shows the rotor head of Sea-King equipped with hydraulically operated automatic folding system. Electro mechanically operated KA-25 semi-automatic folding system is shown in figure-36.

Automatic blade folding system are not adopted on light or medium size helicopters due to implications such as cost, weight and system complexities.

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Rotor System