N N {T _ T0‘ < N V Main and Tail Rotor Canstructions He Wharekura-tml Kaihautu 0 Aotearoa T H E O P E N P O |.Y T E (H N I C OF NEW ZEALAND 555—3—6
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N N {T _
T0‘< N
V
Main and TailRotor Canstructions
He Wharekura-tmlKaihautu 0 Aotearoa
T H E O P E NP O |.Y T E (H N I COF NEW ZEALAND
555—3—6
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CONTENTS
Main Rotor Heads 1
Semierigid Rotor Head 1
Articulated Rotor Head 5
Hingeless LRigidl-Rotor Head 9
Articulated Rotor with One Hinge 1H
Tail Rotors 19
Two-bladed Deltaehinge Tail Rotor 19
Two-bladed Flapping—hinge Tail Rotor 22
Main Rotor Damper 2H
Mechanical or Friction Type 25
Hydraulic Type 27
Ancillary Devices 28
Counterweights 28
Flapping Restrainers 29
Droop Stops 29
Vibration Absorbers 29
Main and Tail Rotor Blades 31
Copyright
This material is for the sole use of enrolled students and may notbe reproduced without the written authority of the Principal, TOPNZ.
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AIRCRAFT ENGINEERING
HELICOPTERS ASSIGNMENT 6
MAIN AND TAIL ROTOR HEADS AND BLADES
MAIN ROTOR HEADS
In Assignment 3, we discussed the operation of the rotorhead and some of the forces acting on it. We shall now discussthe three types of rotor head:
l. The semi~rigid,
2. The articulated, and
3. The hingeless or rigid.
One manufacturer uses a rotor head that is a combination ofarticulated and hingeless, and so we shall also consider thistype of head.
Semi-rigid Rotor Head
The semi-rigid or teetering rotor head shown in Fig. 1is located on and driven by external splines on the mast (5)through the internal splines in the trunnion (3). It is securedand centralised on the mast by the retaining nut (1) and the coneset (#1. Two pitch~link assemblies (9) connect the trunnions (10)to the horns (ll) on the rotating half of the swashplate andsupport assembly (6). Each main rotor blade (8) is retained inthe main rotor hub assembly (12) by a blade bolt (7).
The main rotor hub assembly is free to pivot about thetrunnion C31, and the pitch of each blade can be changedcollectively by raising the swashplate assembly or changedcyclically by tilting the swashplate assembly.
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1. Mast nut2. Flapping restrainer T3 "trunnion4 Cone set
. _ Mast
. Swashpiate and support assembly
. Blade boilMain rotor blade
9. Pitch link assembly10 Trunnion
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FIG. l Semi~rigid main rotor hub and blade assembly
The flapping restrainer (2) is supplied by the manufactureras an optional extra. Its purpose is to limit the flapping ofth 'e hub and blade assembly when it is turning through the lowrev/min range during start~up and shut~down. During this timea gust of wind could flap a blade down so that it could hit thetail cone. Furthermore, and more importantly, the flappingrestrainer makes it safer for people to approach and leave thehelicopter when the rotor is turning.
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The top half of each restrainer is a boh~weight, and thelower half is a wedge. when the rotor is stationary, a springholds each restrainer upright, and the lower half interposesbetween the trunnion (3) and the teetering hub assembly (12).when the rotor turns above a predetermined rev/min, the bob—weightsmove outboard, the wedges withdraw, and the hub assembly becomes
free to flap. As the rotor slows down, the weights resume theirupright position under the action of their springs.
Figure 2 shows an exploded view of this type of rotor head.The massive steel yoke (1) is supported on the trunnion (3), whichis carried on bearings and sleeves (H) and (5) in the pillowblocks (6). Each grip (2) is located on an arm of the yoke byneedle bearings (l0) and is retained by the strap bolt (ll), thestrap (7), the pin (8), and the strap fitting (9). The strap isa tension~torsion assembly, which carries the rotor blade'scentrifugal forces. The bearings (S) and (10) are lubricatedwith a light mineral oil contained in four reservoirs, onereservoir to each grip and pillow block. Each reservoir hasits own sight glass.
The latches (13) on the strap bolt (ll) are used to locatethe blade in the grip, and they provide an adjustment for chordwisebalance of the rotor blade and hub assembly.
This rotor is underslung and preconed. That is, the centreof mass of the assembly lies below its pivoting point, and thetrunnion (3), and each arm of the yoke (1) is angled slightlyupward.
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. Piliow block $2, Pitch horn13. Latch
FIG. 2 Trunnion, yoke, and grip
Ogeration: The semi—rigid rotor head affects blade operationfollows:
1. Flapging takes place by the whole rotor head seesawingfreely about the trunnion (3).
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2. Leading and lagging are not necessary for theoperation of this type of rotor.
3. Pitch change is by the rotation of each bladegrip E2) about its feathering axis.
M. Position at rest is governed by a static stopassembly or by the flapping restrainer wherefitted.
5. Qggtrifugal forces are carried through the bladegrip (2), the strap bolt (ll), the strap C7),and the pin (8) into the yoke (ll.
Articulated Rotor Head
Figure 8 shows a three~bladed articulated rotor head. Eachrotor blade is retained in the pitch—ohange case (2) by, and pivotsabout, a lead~lag hinge bolt (6) and is connected to a damper (H)by a damper arm (5). To change the pitch of the blade, thepitch~change case, blade, and damper assembly can be turnedabout the pitch-change shaft (3) by the rod (8) from the rotatingswashplate (9). The complete pitch—bearing shaft, case, blade,and damper assembly is free to move up and down on a flappinghinge (7), which also holds the assembly to the hub (1). Theaction of the two hinges (6) and (7) means that the blade isattached to the hub by a universal joint.
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Figure H shows the same articulated rotor shown in Fig. 3without the blade and pitch—bearing case assemblies. The steelhub (2) is located and secured to the main rotor drive shaft (5)by washer plates (3), bolts (4), and the hoisting eye (1).Immediately below the hub are the droop—stop components (5), (7)and (8), the main rotor upper scissors support (10), the upperscissors assembly (ll), and the dust boot (l2)!
A lug on each pitch~bearing shaft projects into a closed slotin the droop-stop retaining ring (7) so that the length of theslot determines the maximum coning angle of the blade in flightand the droop angle when the rotor is at rest.
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Hoésting eyeHubWasher plateBoltsMain rotor drive shaflDroop-slop retainer plateDroop-stop retainer ringDroop-stop retainerRetaining ring nutUpper scissor supportUpper scissorDust boot
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Figure 5 shows the pitch—bearing case and shaft assembly insome detail. The pitchechange shaft (2) is a substantial alloysteel forging, and the pitch-bearing case (1) is an aluminiumalloy forging. The lug (5) engages in the slot in the droop~stopretainer ring (7) of Fig. H, and the stack of selected andmatched bearings (3) absorb the blade's centrifugal forces. Comparethis bearing stack with the simple strapeand-pin assembly shownin Fig. 2. ‘
This rotor head is lubricated by grease, which is applied witha hand-operated grease gun at regular intervals, although it isusual, and prudent,to regrease immediately after flying throughheavy rain. The grease used is an oscillating bearing grease tospecification MILHG-25537. No other type of grease should be usedin its place. pF r
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FIG. 5 Pitch—bearing case and shaft assembly
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Operation; The articulated rotor head affects blade operationas follows:
l. Flagging takes place by each blade and pitch—bearingcase assembly moving up and down unrestrained aboutits flapping hinge bolt (7) of Fig. 3._
2. Qeading and lagging are caused by the blade pivotingabout its leadélag hinge bolt C6) of Fig. 3,. Therate of the leading and lagging is controlled anddamped by the damper (51 of Fig. 3.
Pitch.chan'e is by the rotation of each pitchebearingcase —e (2% of Fig, 3 - about its feathering axis.
is governed by a droop-stopPosition at restassembly that serves all three blades. See Fig. M,items (6), (7), and (8).
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5. gentrifqgal forces pass into the pitch—bearing casefl) of Fig. Sithrough a matched stack of heavy dutyball bearings and a retaining nut (6) into thepitch~bearing shaft (2). The forces then go via theflapping hinge bolt into the main rotor hub (1) ofFig. 3.
Hingeless (Rigid) Rotor Head
The articulated rotor head, with its many parts and bearingspresented the designer with a problem that grew in difficulty asthe helicopter grew larger. For many years, it was known thatblade flapping, leading and lagging, and pitch changing could allbe done by twisting or bending the blades or the component partsin the rotor head, However, with the conventional materials thenavailable, this was not practical. The advent and proving of thetitanium alloys and the soecalled plastic materials completelychanged this, and the hingeless, or rigid,rotor is now apractical proposition to manufacture and operate.
The term rigid rotor is not the best one to use because thistype of head is anything but rigid. The preferred name ishingeless rotor.
Figure 6 shows a three—bladed hingeless rotor. A comparisonbetween this rotor and that shown in Fig. 3, H, and 5, showsthe simplicity of the hingeless rotor.
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FIG. 6 Hingeless rotor head assembly
The manufacturers of the rotor head shown in Fig. 6 claimthat it has a total of 70 parts in its construction compared withthe 377 parts of an articulated rotor of the same size, a weightreduction of H5%, and a cost reduction of about 75%. Add to thisimproved flight handling, reliability and much easier and reducedmaintenance, and you can see that the hingeless rotor is a veryattractive proposition.
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Figure 7 shows schematically the rotor head of Fig. 6. Therotor blade (#1 is attached to the blade sleeves (3) by twolarge quick-release bolts. The blade sleeves are located and
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and supported in a cutout in the star arm (2) by an §la§§g@e££3spherical thrust bearing (6) and a Qigh hgsterggis bladestiffener and damper (7) at the extremity of the star arm. Thewhole assembly is bolted to the rotor hub (1), which isattached in turn to the transmission main drive shaft. lncorpor-ated in the rotor hub is a lifting eye for use in ground handling
Elastomer A rubber—like substance
lHigb hysteresis: Having less than normal bounce
The star arm is made of epoxy resin—impregnated glass fabricwhich is compressed,moulded, and oven—cured. The blade sleevesare built up from wound glass fibres impregnated with epoxyresin and oven~cured.
The two sleeves are separated at their inboard end by theelastomeric thrust~bearing assembly and at their outboard endby the high—hysteresis elastomeric and spherical bearing assemblyThe complete sleeve assembly is held firmly together by longthrough bolts, of which the inboard ones also secure the pitch-change horn to the lower sleeve.
Figure 8 shows a Starflex star, with the star and oneblade—sleeve assembly.
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Rotor hub (not shown)Star armBlade sleeveRotor blade (not shown)Pitch change hornElastomeric spherical thrust bearmg
assembly7O '7. High hysleresls elaslomer layers and
spherical housing assemblyNote: Numbering is lhe same as in Fig. 7'.
FIG. 8 Exploded part view of Starflex hingeless rotor
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Operation; The hingeless rotor head affects blade operation @as follows: '
l. Flapping is accommodated by vertical movement of thestar arms, which bend, and the rotation of theblade»s1eeve assembly about the spherical thrustbearing (Gl.
2. Leading and lagging (dragging) is achieved by shearloading of the highehysteresis layers and compressionof the spherical bearing assembly (7), with rotationof the blade sleeve assembly about the sphericalthrust bearing (6)- The movement is damped by thehighehysteresis layers.
3. Pitch is changed by deflection of the sphericalthrust bearing about an axis passing through itscentre.
-H. Position at rest is governed by the rigidity of thestar arm and the stiffness of the spherical bearingat the outboard end of the arm.
5. Centrifugal forces are carried through the bladesleeves to the elastomeric thrust~bearing assemblyQfil and into the star central section.
The functional diagram of this rotor head, Fig. 9, shows thatit can be compared, in flapping, to an articulated head with alarge offset flapping hinge and an elastic return to a neutralposition and, in dragging, to a hinged head with damping and anelastic return to a neutral position.
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(a) FlappingC_L
(b) DraggingFIG. 9 Functional diagram of hingeless rotor head
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The Starflex rotor head uses plastics for all its load-carrying members. We'll now briefly consider a hingeless rotorhead using metal throughout
The Westland Lynx is a military helicopter with a developingcivilian model counterpart Both models use a hingeless rotorhead See Fig. 10.
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Flexible extension arm andcuflet
Dog bonePitch change armOil resenroirTie barRetention pinsDamper
Each rotor blade is carried at the outboard end of a dog bone(2), which is supported by needle rollers and retained by aninternal tie bar (5) to the flexible extension arm and outlet lThe needle rollers are lubricated with oil from the reservoirand the flexible extension arm and outlet are rigidly attachedto the main transmission A damper (7) lS fitted between theextremities of the dog bone
fFIG 10 Lynx rotor head
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The flexible extension arm and outlet (l) and the dog bone(2) are forged from titanium. The tie bar consists of two high-tensile steel fittings wound longitudinally and laterally withhigh-tensile resinecoated steel wire to form a dumb bell.
operation: The all-metal hingeless rotor head affects bladeoperation as follows:
l. Flagging takes place through bending of the taperedplanform outlet of the flexible extension arm andoutlet.
2. heading and laggiqgris achieved by the bending of thedog bone and is controlled by a damper.
3. Pitch is changed by the rotation of the dog bone onthe needle rollers around the outlet arm.
H. Centrifugal fogces are carried from the dog bone,through the tie bar, and into the flexible extensionarm and cutlet.
5. The static positipn of the rotor is determined bythe stiffness of the outlet.
Articulated Rotor with One Hinge
The construction of the articulated rotor head with onehinge is an intermediate stage between the articulated rotor headand the hingeless rotor head. An example is the rotor head fittedto the Hughes 369 series helicopters, which we shall now brieflystudy. 4
Figure ll shows a general view and a sectioned view of thishead, which rotates between the upper and lower bearings (l2) and(13) and is retained by the locknut (1) on the main rotor mast(l8). The head is driven by the main rotor drive shaft (17).Dust and foreign objects are kept from the hearings by the flexibleboot (19), whose lower end is attached to the rotating starassembly, which is driven by the scissor crank through two lugs (lH)
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Operation; The blade operation of the Hughes 369 series rotorhead is as follows:
1. Flapping takes place by the bending of the laminatedstrap assembly (7) between the hub and its outboardattachment on the pitch housing (3).
2. Qeading and lagging are facilitated by the movementof the blade about the lead~lag pivot bolt (H).The movement is not free but is progressively slowedby the blade damper (6).
~ 3. Pitch change is effected by loading the laminatedstrap assembly (7) in torsion. The strap is twistedbetween the hub and its outboard attachment on thepitch housing.
H. Position at rest is controlled by the droop restrainerand roller Z115 and the droop»stop ring (l5).
5. gentrifugal gorges are contained by the laminatedstrap assembly.
Thus, the laminated strap assembly is subject to torsional,bending, and centrifugal forces. The only hinge used is thelead-lag pivot bolt.
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SUMMARY
The three types of rotor head in general use are
1. The semi—rigid,
3 3. The rigid, or hingeless,rotor head.
‘ The rigid, or hingeless, rotor head is, in fact,very flexible.
The modern trend in rotor—head design is towards the7 hingeless rotor.
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_ 19 _
PRACTICE EXERCISE A
State whether each of the following statements istrue or false;
l. A rigid rotor should more correctly be called ahingeless rotor.
2. The drag hinge of a semierigid rotor doubles asa blade retention bolt.
3. An elastomer is a rubber~like compound.
4. A hinqeless rotor does not give any damping actionto the rotor blade leading and lagging.
5. Blade flapping in an articulated rotor is hydraulicallydmged.
6. A semi—rigid rotor flaps as a complete rotor about acentral trunnion.
7. A hingeless rotor contains no bearings.
8. High hysteresis means having less than normal bounce,
9. Blade centrifugal forces may be contained by tension-torsion springapacks.
10. Any general—purpose (GP) “aviation grease may be usedto lubricate a rotor head.
(Answers on page 37)
TAIL ROTORS
In Assignment 555~3~3, we examined the operation of a tailrotor and some of the forces acting on it. We shall now look attwo types of tail rotor in common use.
l. The two~bladed delta~hinge type, and
2. The flapping—hinge type.
lhe Two-bladed De1ta—hinge Tail Rotor
Figure 13 shows an exploded view of this type of tail rotorThe yoke (1) is free to flap about the trunnion (2), which issplined and rigidly attached to the tail rotor shaft by aretaining nut. The angle between the axis of the trunnionand the centre line of the yoke gives
565/3/6
the §§1ta—hinge effect, which we discussed in Assignment 555~3-3.
Each tail rotor blade (3) is mounted on two spherical bearings(H) and is held between the ears of the yoke by two bolts. Bladepitch is changed by moving the blades on the spherical bearings.The direction of rotation LDOR) of this tail rotor is clockwisewhen viewed from the side on which it is mounted, and the directionof the normal air flow in forward flight is from left to right onthe diagram.
when blade A is in the position shown in Fig. 13, it will tryto generate more lift than blade B due to the differing airvelocities felt by the blades. As a result, the whole tail rotorassembly flaps about the trunnion axis, with blade A movingoutboard and blade B moving inboard. This flapping reduces theangle of attack of blade A and increases that of blade B. Thus,symmetry of lift across the disc area is maintained.
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The yoke Q1), which is an aluminium alloy forging, issupported on the alloy steel trunnion (2). The bearings arelubricated by grease and are the only parts of the tail rotor thatneed lubrication.
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Operation; The delta~hinge tail rotor affects bladeoperation as follows:
l. Flapping takes place about the trunnion longitudinalaxis.
2. Centrifugal forces are passed into the yoke throughthe spherical bearings (H) and the retaining bolts (5).
3. Drag forces are absorbed in the blades and yoke. Nolead~lag hinge is used or needed.
A static stop, fitted to limit the amount of flapping, isnecessary because, when the rotors are stationary or turningslowly,a gusting wind could cause the tail rotor to flap violently.Without the static stop, damage to the tail rotor and the helicopterstructure could occur.
Two-bladed Flapping-hinge Tail Rotor
In this type of tail rotor, each blade is attached to a yoke,which is mounted in turn on a hub that is splined and rigidlyattached to the tail rotor drive shaft. Each blade and yokeassembly is hinged on an axis parallel to the plane of rotationand is free to flap independently. As a blade flaps, its pitchangle is changed because of the mechanical relationship betweenthe pitchechange arm, the pitchechange rod, and the arm on theblade.
In normal forward flight, when blade B is in the positionshown in Fig. 1n, it will try to generate more lift than blade Abecause of the different air velocities felt by the blades. Asa result, blade B will flap outboard and blade A will flap inboard.This flapping reduces the angle of attack of blade B and increasesthat of blade A, and thus symmetry of lift across the disc areais maintained. See Fig. ls.
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HubOutboard stop/retaining nutPitch change armPitch change rod assemblyinboard stopBlade and yoke assembly
FIG. 14 Two--bladed flapping—hinge tail rotor
“Operation: The flapping-hinge tail rotor affects bladeoperation as follows:
l. Flapping takes place about the hub.
2. ‘Centrifugaltforcei are passed from the blade, througha tensionetorsion bar into the yoke, and then intothe hub.
Drag forces are absorbed in the blades, the yokes,and the huhi No leadelag hinge is used or needed.
Blade flapping is limited by the inboard and outboard stops,
555/8/6
...2Ll_
which prevent excessive flapping during start up and run duringgusty or side-wind conditions.
A The blade bearings are greased, and a light mineral oil isused in the yokes.
Figure 15 shows the complete assembly mounted on the tailrotor gearbox.
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Oulboard stop/retaining nutPitch change armPitch change rodBlade and yoke assemblyTension/Torsion bar boilYoke cover plug
. Blade attachment tension/lorsion bolt
. Tail-rotor gearbox
FIG. l5 Tail rotor installation
fifil Rfilfifi assets.02.’.i
The main ,@rer damper controls the leading and lefiylhfi Ydifi
oi a main rotor blade in an articulated rotor head. ii ma? dlhfi, .._ . = ‘he used for the same putrose in a hinfielesh rotor head. Fwd
.types are in use?
l. The meehanical or frictien t¥?*~ dfifi
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Mechanical or Friction Type
Figure l6 shows a mechanical rotor damper, which consists ofa stack of steel plates interspersed with bronze plates in ahousing full of hydraulic fluid. Each steel plate is splined toa central operating shaft, which is connected to the trailing edgeof the rotor blade by a link arm. The bronze plates are splinedto the outer housing, which is attached to the main rotor blade-pitch bearing assembly. The stack of discs is preloaded by anadjustable nut tensioning a spring. The bronze discs (1), (2),(3), and (H) have splines of different widths.
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555/3/6
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Bronze plate (splme 0.494")Bronze plate (spline 0.324")Bronze plate (spline 0.259")Bronze plate (spline 0.140")Steel plate (14)Lock sealAdjusting nutTensionlng springActuating armShaft assemblyWindowHousingCover assemblyBolt assemblyFiller plug
-9» _
When the shaft assembly (10) is turned on an assembled unitby a small amount, the bronze discs Cl), whose splines are thesame width as those in the housing,will be held stationary, anddiscs (2), (3), and (4) will turn. The damping action will beeffected by bronze discs (l) and their steel discs (5). With alittle more movement of the shaft assembly, the splines on discs(2)will engage, and the damping action will be given by bronze discs(l) and (2) and their steel discs (5). With a little more movementagain, bronze discs (8) engage and then, finally, bronze discs(H). This arrangement allows a progressively increasing damperaction the further the shaft assembly is turned.
On installation'of the damper, the bronze discs must bealigned so that, for example, the splines of discs (3) engage asa unit and not before discs (2) or after discs (H). Aligning thediscs, called phasing, is done by moving the shaft assembly fromthe lag stop to the lead stop and then back to the lag stop. Theshaft assembly is then moved slowly towards the lead stop untila neutral position is reached. In this position, all-discs aredamping when movement toward the lead stop is made, but onlydiscs (1) are damping for the first small movement toward thelag stop. The adjusting nut (7) is then tightened until astipulated torque is needed to turn the shaft assembly backand forth in the range from the neutral position to where discs(2) start to act.
Phasing is done whenever a damper is disturbed and wheneverthe behaviour of the rotor head indicates that alignment of thediscs has been lost. The torque of the first stage [discs (l)]is checked and reechecked at routine inspections and wheneverthe behaviour of the rotor head shows a check to be necessary.
The discs operate in a bath of hydraulic fluid, which iskept at a specified level by fluid added through the filler plug(15) until the correct level is seen through the sight glass (ll).A numbered scale cast on the housing (12) is used to position theshaft assembly (10) when phasing the damper.
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¢ 27 -
Hydraulic Type
Figure 17 shows a hydraulic rotor damper which consists ofa piston-and-shaft assembly moving in a closed cylinder filled
' an adjustable timingwith hydraulic fluid. A passageway housing' "h 'des ofvalve, a refilling valve, and a reservoir connects bot S1
h i ton. The piston is fitted with a relief valve,relievingt e p sdirectly to the other side of the piston,that prevents excessivepressures building up when the blade is leading.
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FIG. 17 Hydraulic damper
is fitted at each end of the pistonA rubber shock absorberth damper should theshaft to prevent internal damage to e
‘ ‘ ' l thec linder reach the end of its travel. In this examp e,Ycylinder is attached to the main rotor blade at the anchoring
spigot, and the piston rod is attached to the rotor head at theattachment fork.
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— 28 —
The rate of the damper is set in a workshop by filling itwith the correct hydraulic fluid, applying a prescribed load tothe piston rod, and timing its travel over a certain distance.Adjustment is made by turning the tapered timing valve.
To function correctly, hydraulic dampers must be absolutelyfree of entrapped air, and so bleeding valves are fitted. Howevermodern dampers are usually selfebleeding to the extent that, afterthe reservoir has been filled with fluid, the damper needs onlyto be slowly operated through its complete travel three or fourtimes for all air to be expelled.
On some installations, instead of each damper having its ownreservoir, a common reservoir is fitted and connected to thedampers by flexible hydraulic hoses.
ANCILLARY DEVICES
Besides the essential items of hinges, bearings, dampers,and so on, many rotors have extra devices fitted to give smootheroperation or increased safety. we'll now briefly discuss themore common of these devices.
Counterweights
Counterweights, often called Chinese weights, are used on theBell 47-series rotor head. They are adjustable weights mountedon top of a long, sturdy bolt at the inboard trailing edge ofeach main rotor blade. Their purpose is to relieve the loads inthe collectiveepitch control system. Due to centrifugal force,an increase in weight tends to lift, and a decrease tends to lowerthe collective—pitch lever. The weights are adjusted so that,in cruise flight with the collective—pitch lever-friction controloff, the lever will stay where it is put, with perhaps a slighttendency to creep down.
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_ 29 i
Flapping Restrainers
Flapping restrainers are sometimes fitted to articulatedhinged rotors and to semi~rigid rotors, one to each blade assemblyTheir purpose is to "lock" the flapping hinge or yoke assembly andprevent the blades from flapping violently during gusting windconditions at very low rotor rev/min when the rotor is beingstopped or started. The restrainer is centrifugally operatedto unlock the hinge,and spring~returned to lock it.
Droop Stops
Droop stops are fitted to prevent the main rotor blades frompassing close to the ground during start—up and shut—down of therotor. They contribute greatly to the safety of people approach-ing the rotating rotor. In operation, the droop stop reducesthe static droop angle of the blade and automatically disengagesat a very low rotor rev/min. Do not confuse its operation withthat of the flapping restrainer, which stops the blade fromflapping.
Vibration Absorbers
Vibration absorbers are installed on the main rotor head,one to each blade, and are designed to cancel certain naturalharmonic vibrations from the blades. One type commonly used iscalled a Bifilar damper. The name is taken from a vibration-absorbing pendulum, which is supported on two parallel verticalwires. These dampers make the rotor head much smoother inoperation and help to prolong its working life. A similardevice can be fitted to the tail—rotor assembly of medium-sizedhelicopters to prevent or reduce vibration.
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s 30 _
SUMMARY
A damper is used to control the lead/lag rate of amain rotor blade.
The two main types of damper used are
l Hydraulic, and
2 Friction dampers.
Correct damper timing is important for smooth rotor~head operation.
Droop stops and flapping restrainers contribute toground safety and to damage—free shut—downs andstart—ups of the rotor head.
PRACTICE EXERCISE B
State whether each of the following is true or false:
Phasing is a term used for setting a friction-type damper.
Air entrapped in a hydraulic lead/lag damper willslow the damping rate.
A flapping restrainer prevents the blades frommoving up and down about the horizontal hinge atlow rotor rev/min.
Droop stops prevent the blades from lagging at lowrotor rev/min.
Correct timing of lead/lag dampers is importantfor smooth_operation of the rotor head.
A friction—type lead/lag damper must be bled ofair to ensure its smooth operation.
All tail rotors pivot about a central delta hinge.
A tail rotor must turn in a clockwise direction.
Tail rotors do not have lead/lag hinges.
A static stop is fitted to a tail rotor to limitthe amount of flapping when the tail rotor is notturning. .
(Answers on page 37)
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1 31 —
MAIN AND TAIL ROTOR BLADES
Nearly all of the power developed by the power plant isabsorbed by these blades, with the main rotor blades getting thelion's share. All rotor blades, although very strong for thejob they are designed to do, can be easily damaged duringground handling and routine maintenance work.
when main rotor blades are removed from the helicopter,they should be either placed in padded storage racks designed
for that type of blade or stored in their blade boxes. A tailrotor is usually removed as a complete unit and should also beeither placed on a rack designed for it or stowed in its ownbox. Tailerotor blades, when separate from the hub, should bekept in their special box.
when repair work is to be done on main rotor blades, theyshould be taken off the aircraft and placed on padded trestlesfor support. For the smaller tail»rotor blades, a smooth,wooden—topped workbench should be used for support.
Main Rotor Blades
The main rotor blades of early helicopters were made of ametal spar, ribs attached to the spar, a wire trailing edge wasadded, and the assembly was covered with doped fabric. The bladewas virtually a long, thin, fragile aeroplane wing. Blade
design and manufacture has progressed through the metal—sparredwooden blade to the modern all~metal blade and the glassefibreblade.
A metal rotor blade consists of an extruded hollow aluminiumalloy spar section, which may include the leading»edge section—— see Fig. 18,
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FIG. 18 Main rotor blade spar extrusion
Two aluminium alloy sheets form the top and bottom skins,meeting at a shaped trailing-edge strip. The cavity betweenthe two skins aft of the spar section is filled with aluminium-alloy honeycomb. At the inboard (root) end of the blade,aluminium alloy doublers and a steel forging transfer the bladeloads to the rotor head. A fairing is fitted to the blade tipto seal off and streamline the blade and to provide a removableaccess plate to the blade spanwise balance weights attached tothe spar. A trailing edge tab may be fitted near the outboardend of the blade to give fine adjustments to the blade'sbehaviour.
All parts are bonded together, and the complete blade isbalanced statically and dynamically during manufacture. Figurel9 shows two kinds of allemetal blade.
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_ 33 _
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FIG, 19 Metal main rotor blades
The planform of most main rotor blades is rectangular. Thatis, the blade has a constant chord and thickness throughout itsspan. washout is provided in an attempt to evenly distribute thelift generated along the span of the blade —— the blade mainspar is twisted during manufacture so that its pitch decreasesfrom the root end to the tip.
NOTE I
washout: A decrease of the angleof incidence towards a wing tip.
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-. -
Blade airfoil sections are usually symmetrical, althoughasymmetrical sections specially developed for helicopter useare being investigated,
Metal main rotor blades vary in construction from onemanufacturer to another. For example, one manufacturer may use acomplete wraparound skin to enfold an extruded spar section.One type of blade may be very light, and another type may beheavily built and have tip weights fitted to increase the inertiaof the blade. Generally, the blades fitted to an articulatedrotor are more lightly built than those fitted to a semierigidrotor, as we discussed in the assignment Basic Rotors.
After manufacture, all blades are balanced and referenced toa master blade or blades, Each blade is assigned its own serialnumber and, possibly, other identifying marks. Using thesenumbers, you can get together a set of matched blades that willensure a smooth and efficiently operating rotor head. Informationon blade numbers and blades is given in the helicopter maintenancemanual, which must be consulted before you replace a blade orblades. D
Main rotor blades have a limited service life. The manufact-urer, on the basis of calculations and tests, has decided on asafe life of just so many flying hours for his rotor blade. Whena blade has nearly reached the end of this safe life, it mustbe retired from service.
A safety feature of one type of metal main rotor blade is thesealed and inert—gas pressurised hollow-extruded main spar sectionA pressure te1l—ta1e,or gauge, is fitted at the root end of theblade, where it can be easily seen and is not susceptible todamage. A pressure loss is an indication of serious damage orcracking of the spar, thus further flight will be hazardous.A refinement of this system is an electrical monitor maintainedon the pressure in all blade spars during flight. A loss ofpressure is shown as a warning light on an instrument panel infull view of the pilot.
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+35.-
To protect the blade from abrasion by dust, sand, andwater, hard anodising or a special hard—wearing plastic tape maybe applied to the leading edge. The plastic tape covering iswidely used and has the advantage of being easily replaced asit becomes worn. Some manufacturers form the leading edge froma corrosion-resistant steel and then use the plastic tape asa further abrasion barrier.
Tail Rotor Blades
Early tail rotor blades were made of wood, with metal root-end fittings and leading edges, These blades were light andresilient, but they could absorb moisture from the air and becomedistorted and unbalanced. Modern tail rotor blades are made frommetal or fibreglass or a combination of both materials.
The construction of the tail rotor blade is similar to thatof the main rotor blade, the metal blade being bonded togetherand few, if any, rivets being used. Each manufacturer has hisown method of construction, and some of the construction detailswill be found in the maintenance manual of the helicopterconcerned.
Tail rotor blades are usually supplied as a matched set sothat, when one blade becomes unserviceable, all blades are thenreplaced. The oid blades can be returned to the manufacturerfor repair and/or rematching.
Nearly all blades have provision for sparwise balancing,and some for chordwire balancing so as to make possible the finalbalancing of the complete tail rotor assembly. The blade'sleading edge may have a layer of special plastic tape forabrasion resistance, and the entire blade will have specialpaint markings so that, when turning, it can be easily seen.
A tail rotor blade, like a main rotor blade, has a limitedservice life, The blade must be retired from service before orwhen the limit is reached. This service life must not be
exceeded.__H“wwww
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SUMMARY
Main and tail rotor blades are easily damaged. Theymust be handled with care both off and on thehelicopter.
Both main and tail rotor blades have limited servicelives, which must not be exceeded.
PRACTICE EXERCISE C
State whether each of the following is true or false:
l.
2.
3.
4.
S.
6.
7.
8.
9!
10,
The angle of incidence of a rotor blade decreasestowards the blade tip.
when a main rotor blade has been removed from thehelicopter, it must be laid flat on the hangerfloor for safety,
An extruded hollow spar section is filled withaluminium honeycomb to give stiffness.
Metalvtoemetal bonding is used in the constructionof metal rotor blades.
Main rotor blades usually have provision at theirtips for chordwire balance weights.
washout is the decrease of the angle of incidencetowards a wing tip.
Weights may be fitted to the tips of a main rotorblade to increase its inertia.
Blades are balanced and referenced to a master blade
All blades in a set have the same serial number.
The service life o£»a rotor blade may be exceededby 10%.
(Answers on page 38)
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_ 37 _
ANSWERS TO PRACTICE EXERCISES
EXERCISE A
Statements 1, 3, 6, 8, and 9 are true.
2. False. The blades on a semi-rigid rotor do notlead and lag. Thus, there is no need for a drag hinge
M. False. Some type of blade damping is needed to slowdown the lead/lag rate and can be provided by layersof elastomeric material or by hydraulic dampers.
S. False. The blades are unrestrained in their movementabout the flapping hinge.
7. False, although the bearings used are not theconventional ball, roller, or metal type.
10. False. The lubricants to use in a rotor head arespecified by the helicopter manufacturer. If thesespecifications are not followed, the result will beincreased wear and decreased reliability of the rotorhead.
EXERCISE B
Statements 1, 3, 5, 9, and 10 are true.
2. False. Air can be compressed, and so any airtrapped in the damper will compress and expandas the blade leads and lags. The result is aspongy damper, that is, a damper with afast and erratic timing rate.
4. False. The blade dampers control leading and1aS8}n8- DPOOP $YOps prevent the blades frompassing close to the ground at low rotor rev/min,
5. False._ The frictionetype lead/lag dampers usethe friction betmeen flX€d and moving plates toprovide the damping force.
7. False. _Some tail rotors pivot about a centraldelta hinge. Other tail rotors use a fixedcentral hub carrying individual blades, each onits own flapping hinge.
8. False. The direction of rotation of a tail rotorvaries from one type of helicopter to another.
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e 38 —
EXERCISE C
Statements 1, H, 6, 7, and 8 are true.
2. False. when a main rotor blade is removed from ahelicopter, it should be placed on a shapedstorage rack or a padded trestle for safe-keeping.
3. False. An extruded hollow spar may be pressurisedwith an inert gas. Aluminium honeycomb may beused to stiffen the top and bottom skinning aftof the spar.
5. False. The small weights at a blade tip areused for adjusting the spanwire balance.
9. False. Each blade has its own serial number,which is not duplicated on any other blade.
10. False. *NeVer‘exceed the service life on anyaircraft part or component,
TEST PAPER 6
1. How do the functions of a flapping restrainer and' va droop stop differ.
2. In this assignment, two types of lead/lag damperand a third form of lead/lag damping have been discussedName and briefly describe each type of damping.
3. List the advantages of the hingeless rotor overarticulated and semierigid rotors.
4. What is the main difference between the two typesof tail rotor? What great advantage has one typeover the other?
5. Make a schematic sketch.of a main rotor head lead/lagdamper.
The damper must haveCa) A reservoir,Cb)- A timing valve, andCc) A replenishment valve or valves.
‘ -\.?-4%?-A
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