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Main and Tail Rotor Canstructions 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 Tail Rotors
1H 19
Two-bladed Deltaehinge Tail Rotor
19
Two-bladed Flapping—hinge Tail Rotor
22
Main Rotor Damper
i if sy a
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 not be reproduced without the written authority of the Principal, TOPNZ.
555/3/6
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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 rotor head and some of the forces acting on it. We shall now discuss the 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 of articulated and hingeless, and so we shall also consider this type of head. Semi-rigid Rotor Head The semi-rigid or teetering rotor head shown in Fig. 1 is located on and driven by external splines on the mast (5) through the internal splines in the trunnion (3). It is secured and centralised on the mast by the retaining nut (1) and the cone set (#1. Two pitch~link assemblies (9) connect the trunnions (10) to the horns (ll) on the rotating half of the swashplate and support assembly (6). Each main rotor blade (8) is retained in the main rotor hub assembly (12) by a blade bolt (7). The main rotor hub assembly is free to pivot about the trunnion C31, and the pitch of each blade can be changed collectively by raising the swashplate assembly or changed cyclically by tilting the swashplate assembly.
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Mast nut
2. Flapping restrainer 3 4
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"trunnion Cone set . _ Mast
. Swashpiate and support assembly .
Blade boil Main rotor blade 9. Pitch link assembly 10 Trunnion 1 I Swashplate horn 1?. Main rolor hub assembly
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Semi~rigid main rotor hub and blade assembly
The flapping restrainer (2) is supplied by the manufacturer as an optional extra. Its purpose is to limit the flapping of th e
' hub and blade assembly when it is turning through the low rev/min range during start~up and shut~down. During this time 5 a gust of wind could flap a blade down so that it could hit the tail cone. Furthermore, and more importantly, the flapping restrainer makes it safer for people to approach and leave the
helicopter when the rotor is turning.
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_ 3 _
The top half of each restrainer is a boh~weight, and the lower half is a wedge. when the rotor is stationary, a spring holds each restrainer upright, and the lower half interposes
between the trunnion (3) and the teetering hub assembly (12). when the rotor turns above a predetermined rev/min, the bob—weights move outboard, the wedges withdraw, and the hub assembly becomes
free to flap. As the rotor slows down, the weights resume their upright position under the action of their springs. Figure 2 shows an The massive steel yoke is carried on bearings blocks (6). Each grip
exploded view of this type of rotor head. (1) is supported on the trunnion (3), which and sleeves (H) and (5) in the pillow (2) is located on an arm of the yoke by
needle bearings (l0) and is retained by the strap bolt (ll), the strap (7), the pin (8), and the strap fitting (9). The strap is a tension~torsion assembly, which carries the rotor blade's centrifugal forces.
The bearings (S) and (10) are lubricated
with a light mineral oil contained in four reservoirs, one reservoir to each grip and pillow block.
Each reservoir has
its own sight glass. The latches (13) on the strap bolt (ll) are used to locate the blade in the grip, and they provide an adjustment for chordwise balance of the rotor blade and hub assembly. This rotor is underslung and preconed. That is, the centre of mass of the assembly lies below its pivoting point, and the trunnion (3), and each arm of the yoke (1) is angled slightly upward.
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1. Yoke 2. Grip 3. Trunrfiorl Inner race
7. Strap B. Pin 9. Strap fittmg 10. Bearing
Bearing
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Piliow block
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Pitch horn
13. Latch
FIG. 2
Trunnion, yoke, and grip
Ogeration: The semi—rigid rotor head affects blade operation follows: 1.
Flapging takes place by the whole rotor head seesawing freely about the trunnion (3).
@555/3/6
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2.
Leading and lagging are not necessary for the
operation of this type of rotor. 3.
Pitch change is by the rotation of each blade grip E2) about its feathering axis.
M.
Position at rest is governed by a static stop assembly or by the flapping restrainer where fitted.
5.
Qggtrifugal forces are carried through the blade grip (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. Each rotor blade is retained in the pitch—ohange case (2) by, and pivots about, 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, the pitch~change case, blade, and damper assembly can be turned about the pitch-change shaft (3) by the rod (8) from the rotating swashplate (9).
The complete pitch—bearing shaft, case, blade,
and damper assembly is free to move up and down on a flapping hinge (7), which also holds the assembly to the hub (1). The action of the two hinges (6) and (7) means that the blade is attached to the hub by a universal joint.
555/3/6
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_ 7 _ Figure H shows the same articulated rotor shown in Fig. 3 without the blade and pitch—bearing case assemblies. The steel hub (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 upper scissors assembly (ll), and the dust boot (l2)! A lug on each pitch~bearing shaft projects into a closed slot in the droop-stop retaining ring (7) so that the length of the slot determines the maximum coning angle of the blade in flight and the droop angle when the rotor is at rest. 4
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Washer plate Bolts Main rotor drive shafl Droop-slop retainer plate
Droop-stop retainer ring ?“.“.°’P‘ Droop-stop retainer 9. 10 11 12.
Hub and dro0p—stop assembly
585/3/6
Hoésting eye Hub
Retaining ring nut Upper scissor support Upper scissor Dust boot
_ 3 _ Figure 5 shows the pitch—bearing case and shaft assembly in some detail. The pitchechange shaft (2) is a substantial alloy steel forging, and the pitch-bearing case (1) is an aluminium alloy forging. The lug (5) engages in the slot in the droop~stop retainer ring (7) of Fig. H, and the stack of selected and matched bearings (3) absorb the blade's centrifugal forces. Compare this bearing stack with the simple strapeand-pin assembly shown in Fig. 2.
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This rotor head is lubricated by grease, which is applied with a hand-operated grease gun at regular intervals, although it is usual, and prudent,to regrease immediately after flying through
heavy rain. The grease used is an oscillating bearing grease to specification MILHG-25537. No other type of grease should be used in its place.
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. Stainless steel sleeve and bushing
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assembly (sell lubricating) Lug Retaining nut
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Pitch—bearing case and shaft assembly
S55/3/6
_ 9 a Operation; The articulated rotor head affects blade operation as follows: l.
Flagging takes place by each blade and pitch—bearing case assembly moving up and down unrestrained about its flapping hinge bolt (7) of Fig. 3._
2.
Qeading and lagging are caused by the blade pivoting about its leadélag hinge bolt C6) of Fig. 3,. The rate of the leading and lagging is controlled and damped by the damper (51 of Fig. 3.
8.
Pitch.chan'e is by the rotation of each pitchebearing case —e (2% of Fig, 3 - about its feathering axis.
U.
Position at rest is governed by a droop-stop assembly that serves all three blades. See Fig. M, items (6), (7), and (8).
5.
gentrifqgal forces pass into the pitch—bearing case fl) of Fig. Sithrough a matched stack of heavy duty ball bearings and a retaining nut (6) into the pitch~bearing shaft (2). The forces then go via the flapping hinge bolt into the main rotor hub (1) of Fig.
3.
Hingeless (Rigid) Rotor Head The articulated rotor head, with its many parts and bearings presented the designer with a problem that grew in difficulty as the helicopter grew larger. For many years, it was known that blade flapping, leading and lagging, and pitch changing could all be done by twisting or bending the blades or the component parts in the rotor head, However, with the conventional materials then available, this was not practical. The advent and proving of the titanium alloys and the soecalled plastic materials completely changed this, and the hingeless, or rigid,rotor is now a practical proposition to manufacture and operate. The term rigid rotor is not the best one to use because this type of head is anything but rigid. The preferred name is hingeless rotor.
Figure 6 shows a three—bladed hingeless rotor. A comparison between this rotor and that shown in Fig. 3, H, and 5, shows the simplicity of the hingeless rotor.
555/3/6
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_ 19 _
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FIG. 6
Hingeless rotor head assembly
The manufacturers of the rotor head shown in Fig. 6 claim that it has a total of 70 parts in its construction compared with the 377 parts of an articulated rotor of the same size, a weight reduction of H5%, and a cost reduction of about 75%. Add to this improved flight handling, reliability and much easier and reduced maintenance, and you can see that the hingeless rotor is a very attractive proposition.
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. Rotor hub . Star arm Blade sieeve Rotor blade . Pitch change horn . Eiastomericsphericaithrustbearing assembly . High hysteresis elastomerlayers and spherical bearing assembly
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Schematic views of a Starflfix hingeless rotor
Figure 7 shows schematically the rotor head of Fig. 6.
rotor blade (#1 is attached to the blade sleeves (3) by two large quick-release bolts. The blade sleeves are located and
555/3/6
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~11and supported in a cutout in the star
arm (2) by an §la§§g@e££3
spherical thrust bearing (6) and a Qigh hgsterggis blade
stiffener and damper (7) at the extremity of the star arm.
The
whole assembly is bolted to the rotor hub (1), which is
attached 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 fabric which is compressed,moulded, and oven—cured. The blade sleeves are built up from wound glass fibres impregnated with epoxy resin and oven~cured. The two sleeves are separated at their inboard end by the elastomeric thrust~bearing assembly and at their outboard end by the high—hysteresis elastomeric and spherical bearing assembly The complete sleeve assembly is held firmly together by long through bolts, of which the inboard ones also secure the pitchchange horn to the lower sleeve. Figure 8 shows a Starflex star, with the star
and one
blade—sleeve assembly.
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Star arm Blade sleeve
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Rotor blade (not shown)
Pitch change horn o>o1_4>_wm- Elastomeric spherical thrust bearmg
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High hysleresls elaslomer layers and spherical housing assembly
Note: Numbering is lhe same as in Fig. 7'.
FIG. 8
Exploded part view of Starflex hingeless rotor
555/3/6
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Operation; The hingeless rotor head affects blade operation as follows: ' l.
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Flapping is accommodated by vertical movement of the star arms, which bend, and the rotation of the blade»s1eeve assembly about the spherical thrust bearing (Gl.
2.
Leading and lagging (dragging) is achieved by shear loading of the highehysteresis layers and compression of the spherical bearing assembly (7), with rotation of the blade sleeve assembly about the spherical thrust bearing (6)- The movement is damped by the highehysteresis layers.
3.
Pitch is changed by deflection of the spherical thrust bearing about an axis passing through its centre.
H.
Position at rest is governed by the rigidity of the star arm and the stiffness of the spherical bearing at the outboard end of the arm.
5.
Centrifugal forces are carried through the blade sleeves to the elastomeric thrust~bearing assembly Qfil and into the star central section.
-
The functional diagram of this rotor head, Fig. 9, shows that it can be compared, in flapping, to an articulated head with a large offset flapping hinge and an elastic return to a neutral position and, in dragging, to a hinged head with damping and an elastic return to a neutral position.
Offset
(a) Flapping C_L
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Functional diagram of hingeless rotor head
555/3/6
The Starflex rotor head uses plastics for all its load-
carrying members. We'll now briefly consider a hingeless rotor head using metal throughout The Westland Lynx is a military helicopter with a developing civilian model counterpart Both models use a hingeless rotor head
See Fig. 10.
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Dog bone Pitch change arm Oil resenroir
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10
Lynx rotor head
Each rotor blade is carried at the outboard end of a dog bone (2), which is supported by needle rollers and retained by an internal tie bar (5) to the flexible extension arm and outlet The needle rollers are lubricated with oil from the reservoir and the flexible extension arm and outlet are rigidly attached
to the main transmission A damper (7) lS fitted between the extremities of the dog bone
555/3/6
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i lu _ The flexible extension arm and outlet (l) and the dog bone (2) are forged from titanium. The tie bar consists of two hightensile steel fittings wound longitudinally and laterally with high-tensile resinecoated steel wire to form a dumb bell. operation: The all-metal hingeless rotor head affects blade operation as follows: l.
Flagging takes place through bending of the tapered planform outlet of the flexible extension arm and outlet.
2.
heading and laggiqgris achieved by the bending of the dog bone and is controlled by a damper.
3.
Pitch is changed by the rotation of the dog bone on the needle rollers around the outlet arm.
H.
Centrifugal fogces are carried from the dog bone, through the tie bar, and into the flexible extension arm and cutlet.
5.
The static positipn of the rotor is determined by the stiffness of the outlet.
Articulated Rotor with One Hinge The construction of the articulated rotor head with one hinge is an intermediate stage between the articulated rotor head and the hingeless rotor head. An example is the rotor head fitted to the Hughes 369 series helicopters, which we shall now briefly study. 4 Figure ll shows a general view and a sectioned view of this head, 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 flexible boot (19), whose lower end is attached to the rotating star assembly, which is driven by the scissor crank through two lugs (lH)
.555/d/6
_ 15 W Operation; The blade operation of the Hughes 369 series rotor head is as follows:
~
1.
Flapping takes place by the bending of the laminated strap assembly (7) between the hub and its outboard attachment on the pitch housing (3).
2.
Qeading and lagging are facilitated by the movement of the blade about the lead~lag pivot bolt (H). The movement is not free but is progressively slowed by the blade damper (6).
3.
Pitch change is effected by loading the laminated strap assembly (7) in torsion. The strap is twisted between the hub and its outboard attachment on the pitch housing.
H.
Position at rest is controlled by the droop restrainer and roller Z115 and the droop»stop ring (l5).
5.
gentrifugal gorges are contained by the laminated strap assembly.
Thus, the laminated strap assembly is subject to torsional, bending, and centrifugal forces. lead-lag pivot bolt.
The only hinge used is the
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to the rotor hub and the pitch housings.
SUMMARY The three types of rotor head in general use are 1.
The semi—rigid,
I
2.
The articulated, and
3
3.
The rigid, or hingeless,rotor head.
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The rigid, or hingeless, rotor head is, in fact, very flexible.
The modern trend in rotor—head design is towards the 7
hingeless rotor.
555/3/8
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_ 19 _ PRACTICE EXERCISE A State whether each of the following statements is true or false; l.
A rigid rotor should more correctly be called a hingeless rotor.
2.
The drag hinge of a semierigid rotor doubles as a blade retention bolt.
3.
An elastomer is a rubber~like compound.
4.
A hinqeless rotor does not give any damping action to the rotor blade leading and lagging.
5.
Blade flapping in an articulated rotor is hydraulically
dmged. 6.
A semi—rigid rotor flaps as a complete rotor about a central 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 tensiontorsion springapacks.
10.
Any general—purpose (GP) “aviation grease may be used to lubricate a rotor head. (Answers on page 37)
TAIL ROTORS
In Assignment 555~3~3, we examined the operation of a tail rotor and some of the forces acting on it.
We shall now look at
two 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 rotor The yoke (1) is free to flap about the trunnion (2), which is splined and rigidly attached to the tail rotor shaft by a retaining nut. The angle between the axis of the trunnion and 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. Blade pitch is changed by moving the blades on the spherical bearings. The direction of rotation LDOR) of this tail rotor is clockwise when viewed from the side on which it is mounted, and the direction of the normal air flow in forward flight is from left to right on the diagram. when blade A is in the position shown in Fig. 13, it will try
to generate more lift than blade B due to the differing air velocities felt by the blades. As a result, the whole tail rotor assembly flaps about the trunnion axis, with blade A moving outboard and blade B moving inboard. This flapping reduces the angle of attack of blade A and increases that of blade B.
Thus,
symmetry of lift across the disc area is maintained. 0
The yoke Q1), which is an aluminium alloy forging, is supported on the alloy steel trunnion (2). The bearings are lubricated by grease and are the only parts of the tail rotor that need lubrication.
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_ 22 _ Operation; The delta~hinge tail rotor affects blade operation as follows: l.
Flapping takes place about the trunnion longitudinal axis.
2.
Centrifugal forces are passed into the yoke through the spherical bearings (H) and the retaining bolts (5).
3.
Drag forces are absorbed in the blades and yoke.
No
lead~lag hinge is used or needed. A static stop, fitted to limit the amount of flapping, is necessary because, when the rotors are stationary or turning slowly,a gusting wind could cause the tail rotor to flap violently. Without the static stop, damage to the tail rotor and the helicopter structure 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 rigidly attached to the tail rotor drive shaft. Each blade and yoke assembly is hinged on an axis parallel to the plane of rotation and is free to flap independently. As a blade flaps, its pitch angle is changed because of the mechanical relationship between the pitchechange arm, the pitchechange rod, and the arm on the blade. In normal forward flight, when blade B is in the position shown in Fig. 1n, it will try to generate more lift than blade A because of the different air velocities felt by the blades. As a result, blade B will flap outboard and blade A will flap inboard. This flapping reduces the angle of attack of blade B and increases that of blade A, and thus symmetry of lift across the disc area is maintained. See Fig. ls.
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LOW FIG. 14
Two--bladed flapping—hinge tail rotor
“Operation: The flapping-hinge tail rotor affects blade operation as follows: l.
Flapping takes place about the hub.
2. ‘Centrifugaltforcei are passed from the blade, through a tensionetorsion bar into the yoke, and then into the hub. ¢
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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
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which prevent excessive flapping during start up and run during gusty or side-wind conditions. A
The blade bearings are greased, and a light mineral oil is
used in the yokes. Figure 15 shows the complete assembly mounted on the tail rotor gearbox.
Oulboard stop/retaining nut Pitch change arm Pitch change rod Blade and yoke assembly Tension/Torsion bar boil Yoke cover plug . Blade attachment tension/lorsion bolt m*‘9’.°‘:“f-“N.-‘ Tail-rotor gearbox
FIG. l5
Tail rotor installation
fifil i .02.’. Rfilfifi assets The main ,@rer damper controls the leading and lefiylhfi Ydifi
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Mechanical or Friction Type Figure l6 shows a mechanical rotor damper, which consists of a stack of steel plates interspersed with bronze plates in a housing full of hydraulic fluid. Each steel plate is splined to a central operating shaft, which is connected to the trailing edge of the rotor blade by a link arm. The bronze plates are splined to the outer housing, which is attached to the main rotor bladepitch bearing assembly. The stack of discs is preloaded by an adjustable 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 seal Adjusting nut Tensionlng spring Actuating arm Shaft assembly
Window Housing
Cover assembly Bolt assembly Filler plug
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When the shaft assembly (10) is turned on an assembled unit by a small amount, the bronze discs Cl), whose splines are the same width as those in the housing,will be held stationary, and discs (2), (3), and (4) will turn. The damping action will be effected by bronze discs (l) and their steel discs (5).
With a
little 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 movement again, bronze discs (8) engage and then, finally, bronze discs (H). This arrangement allows a progressively increasing damper action the further the shaft assembly is turned. On installation'of the damper, the bronze discs must be aligned so that, for example, the splines of discs (3) engage as a unit and not before discs (2) or after discs (H). Aligning the discs, called phasing, is done by moving the shaft assembly from the lag stop to the lead stop and then back to the lag stop. The shaft assembly is then moved slowly towards the lead stop until a neutral position is reached. In this position, all-discs are damping when movement toward the lead stop is made, but only discs (1) are damping for the first small movement toward the lag stop. The adjusting nut (7) is then tightened until a stipulated torque is needed to turn the shaft assembly back and forth in the range from the neutral position to where discs (2) start to act. Phasing is done whenever a damper is disturbed and whenever the behaviour of the rotor head indicates that alignment of the discs has been lost. The torque of the first stage [discs (l)] is checked and reechecked at routine inspections and whenever the behaviour of the rotor head shows a check to be necessary. The discs operate in a bath of hydraulic fluid, which is kept 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 the shaft assembly (10) when phasing the damper.
555/3/6
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Hydraulic Type Figure 17 shows a hydraulic rotor damper which consists of a piston-and-shaft assembly moving in a closed cylinder filled ' an adjustable timing with hydraulic fluid. A passageway housing ' "h 'des of valve, a refilling valve, and a reservoir connects bot S1 The piston is fitted with a relief valve,relieving t h e p i s ton. directly to the other side of the piston,that prevents excessive pressures building up when the blade is leading.
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Hydraulic damper
A rubber shock absorber is fitted at each end of the piston th e damper should the shaft to prevent internal damage to ‘ ‘ c Y linder reach the end of its travel.
' In this examp l e, the
cylinder is attached to the main rotor blade at the anchoring
spigot, and the piston rod is attached to the rotor head at the attachment fork.
555/3/6
—
28 —
The rate of the damper is set in a workshop by filling it
with the correct hydraulic fluid, applying a prescribed load to the 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 absolutely free of entrapped air, and so bleeding valves are fitted. However modern dampers are usually selfebleeding to the extent that, after the reservoir has been filled with fluid, the damper needs only to be slowly operated through its complete travel three or four times for all air to be expelled. On some installations, instead of each damper having its own reservoir, a common reservoir is fitted and connected to the dampers 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 smoother operation or increased safety. we'll now briefly discuss the more common of these devices.
Counterweights Counterweights, often called Chinese weights, are used on the Bell 47-series rotor head. They are adjustable weights mounted on top of a long, sturdy bolt at the inboard trailing edge of each main rotor blade. Their purpose is to relieve the loads in the collectiveepitch control system. Due to centrifugal force, an increase in weight tends to lift, and a decrease tends to lower the collective—pitch lever. The weights are adjusted so that, in cruise flight with the collective—pitch lever-friction control off, the lever will stay where it is put, with perhaps a slight tendency to creep down.
555/3/6
_ 29 i Flapping Restrainers Flapping restrainers are sometimes fitted to articulated hinged rotors and to semi~rigid rotors, one to each blade assembly Their purpose is to "lock" the flapping hinge or yoke assembly and prevent the blades from flapping violently during gusting wind conditions at very low rotor rev/min when the rotor is being stopped or started. The restrainer is centrifugally operated to unlock the hinge,and spring~returned to lock it. Droop Stops Droop stops are fitted to prevent the main rotor blades from passing close to the ground during start—up and shut—down of the rotor. They contribute greatly to the safety of people approaching the rotating rotor. In operation, the droop stop reduces the static droop angle of the blade and automatically disengages at a very low rotor rev/min. Do not confuse its operation with that of the flapping restrainer, which stops the blade from flapping. Vibration Absorbers Vibration absorbers are installed on the main rotor head, one to each blade, and are designed to cancel certain natural harmonic vibrations from the blades. One type commonly used is called a Bifilar damper. The name is taken from a vibrationabsorbing pendulum, which is supported on two parallel vertical wires. These dampers make the rotor head much smoother in operation and help to prolong its working life. A similar device can be fitted to the tail—rotor assembly of medium-sized helicopters to prevent or reduce vibration.
555/3/6
s 30 _
SUMMARY A damper is used to control the lead/lag rate of a main 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 to ground safety and to damage—free shut—downs and start—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 frictiontype damper. Air entrapped in a hydraulic lead/lag damper will slow the damping rate. A flapping restrainer prevents the blades from moving up and down about the horizontal hinge at low rotor rev/min. Droop stops prevent the blades from lagging at low
rotor rev/min. Correct timing of lead/lag dampers is important for smooth_operation of the rotor head. A friction—type lead/lag damper must be bled of
air 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 limit the amount of flapping when the tail rotor is not turning. . (Answers on page 37)
555/3/8
1 31 —
MAIN AND TAIL ROTOR BLADES
Nearly all of the power developed by the power plant is absorbed by these blades, with the main rotor blades getting the lion's share. All rotor blades, although very strong for the job they are designed to do,
can be easily damaged during
ground 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 tail rotor is usually removed as a complete unit and should also be either placed on a rack designed for it or stowed in its own box. Tailerotor blades, when separate from the hub, should be kept in their special box. when repair work is to be done on main rotor blades, they should be taken off the aircraft and placed on padded trestles for 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 a metal spar, ribs attached to the spar, a wire trailing edge was added, and the assembly was covered with doped fabric. The blade was virtually a long, thin, fragile aeroplane wing.
Blade
design and manufacture has progressed through the metal—sparred wooden blade to the modern all~metal blade and the glassefibre blade.
A metal rotor blade consists of an extruded hollow aluminium alloy spar section, which may include the leading»edge section —— see Fig. 18,
555/3/6
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 between the two skins aft of the spar section is filled with aluminiumalloy honeycomb. At the inboard (root) end of the blade, aluminium alloy doublers and a steel forging transfer the blade loads to the rotor head. A fairing is fitted to the blade tip to seal off and streamline the blade and to provide a removable access plate to the blade spanwise balance weights attached to the spar. A trailing edge tab may be fitted near the outboard end of the blade to give fine adjustments to the blade's behaviour.
All parts are bonded together, and the complete blade is balanced statically and dynamically during manufacture. Figure l9 shows two kinds of allemetal blade.
555/3/6
_ 33 _
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Metal main rotor blades
The planform of most main rotor blades is rectangular. That is, the blade has a constant chord and thickness throughout its span. washout is provided in an attempt to evenly distribute the lift generated along the span of the blade —— the blade main spar is twisted during manufacture so that its pitch decreases from the root end to the tip.
NOTE
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washout: A decrease of the angle of incidence towards a wing tip.
555/3/6
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Blade airfoil sections are usually symmetrical, although asymmetrical sections specially developed for helicopter use are being investigated, Metal main rotor blades vary in construction from one manufacturer to another. For example, one manufacturer may use a complete wraparound skin to enfold an extruded spar section. One type of blade may be very light, and another type may be heavily built and have tip weights fitted to increase the inertia of the blade. Generally, the blades fitted to an articulated rotor are more lightly built than those fitted to a semierigid rotor, as we discussed in the assignment Basic Rotors. After manufacture, all blades are balanced and referenced to a master blade or blades, Each blade is assigned its own serial number and, possibly, other identifying marks. Using these numbers, you can get together a set of matched blades that will ensure a smooth and efficiently operating rotor head. Information on blade numbers and blades is given in the helicopter maintenance manual, which must be consulted before you replace a blade or blades. D Main rotor blades have a limited service life. The manufacturer, on the basis of calculations and tests, has decided on a safe life of just so many flying hours for his rotor blade. When a blade has nearly reached the end of this safe life, it must be retired from service. A safety feature of one type of metal main rotor blade is the sealed and inert—gas pressurised hollow-extruded main spar section A pressure te1l—ta1e,or gauge, is fitted at the root end of the blade, where it can be easily seen and is not susceptible to damage. A pressure loss is an indication of serious damage or cracking of the spar, thus further flight will be hazardous. A refinement of this system is an electrical monitor maintained on the pressure in all blade spars during flight. A loss of pressure is shown as a warning light on an instrument panel in full view of the pilot.
555/3/6
+35.-
To protect the blade from
abrasion by dust, sand, and
water, hard anodising or a special hard—wearing plastic tape may be applied to the leading edge.
The plastic tape covering is
widely used and has the advantage of being easily replaced as it becomes worn.
Some manufacturers form the leading edge from
a corrosion-resistant steel and then use the plastic tape as a further abrasion barrier.
Tail Rotor Blades Early tail rotor blades were made of wood, with metal rootend fittings and leading edges, These blades were light and resilient, but they could absorb moisture from the air and become distorted and unbalanced. Modern tail rotor blades are made from metal or fibreglass or a combination of both materials. The construction of the tail rotor blade is similar to that of the main rotor blade, the metal blade being bonded together and few, if any, rivets being used. Each manufacturer has his own method of construction, and some of the construction details will be found in the maintenance manual of the helicopter concerned. Tail rotor blades are usually supplied as a matched set so that, when one blade becomes unserviceable, all blades are then replaced. The oid blades can be returned to the manufacturer for repair and/or rematching. Nearly all blades have provision for sparwise balancing, and some for chordwire balancing so as to make possible the final balancing of the complete tail rotor assembly. The blade's leading edge may have a layer of special plastic tape for abrasion resistance, and the entire blade will have special paint markings so that, when turning, it can be easily seen. A tail rotor blade, like a main rotor blade, has a limited service life, The blade must be retired from service before or when the limit is reached.
This service life must not be
exceeded. __H“wwww
555/3/6
SUMMARY Main and tail rotor blades are easily damaged. must be handled with care both off and on the helicopter.
They
Both main and tail rotor blades have limited service lives, which must not be exceeded.
PRACTICE EXERCISE C State whether each of the following is true or false: l.
The angle of incidence of a rotor blade decreases towards the blade tip.
2.
when a main rotor blade has been removed from the helicopter, it must be laid flat on the hanger floor for safety,
3.
An extruded hollow spar section is filled with aluminium honeycomb to give stiffness.
4.
Metalvtoemetal bonding is used in the construction of metal rotor blades.
S.
Main rotor blades usually have provision at their tips for chordwire balance weights.
6.
washout is the decrease of the angle of incidence towards a wing tip.
7.
Weights may be fitted to the tips of a main rotor blade to increase its inertia.
8.
Blades are balanced and referenced to a master blade
9!
All blades in a set have the same serial number.
10,
The service life o£»a rotor blade may be exceeded by 10%. (Answers on page 38)
555/3/6
_ 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 not lead and lag. Thus, there is no need for a drag hinge
M.
False. Some type of blade damping is needed to slow down the lead/lag rate and can be provided by layers of elastomeric material or by hydraulic dampers.
S.
False. The blades are unrestrained in their movement about the flapping hinge.
7.
False, although the bearings used are not the conventional ball, roller, or metal type.
10.
False. The lubricants to use in a rotor head are specified by the helicopter manufacturer. If these specifications are not followed, the result will be increased wear and decreased reliability of the rotor head.
EXERCISE B Statements 1, 3, 5, 9, and 10 are true. 2.
False. Air can be compressed, and so any air trapped in the damper will compress and expand as the blade leads and lags. The result is a spongy damper, that is, a damper with a fast and erratic timing rate.
4.
False. The blade dampers control leading and 1aS8}n8- DPOOP $YOps prevent the blades from passing close to the ground at low rotor rev/min,
5.
False._ The frictionetype lead/lag dampers use the friction betmeen flX€d and moving plates to provide the damping force.
7.
False. _Some tail rotors pivot about a central delta hinge. Other tail rotors use a fixed central hub carrying individual blades, each on its own flapping hinge.
8.
False. The direction of rotation of a tail rotor varies from one type of helicopter to another.
555/3/6
e 38 —
EXERCISE C
Statements 1, H, 6, 7, and 8 are true. 2.
False. when a main rotor blade is removed from a helicopter, it should be placed on a shaped storage rack or a padded trestle for safe-keeping.
3.
False. An extruded hollow spar may be pressurised with an inert gas. Aluminium honeycomb may be used to stiffen the top and bottom skinning aft of the spar.
5.
False. The small weights at a blade tip are used 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 any aircraft part or component,
TEST PAPER 6 1.
How do the functions of a flapping restrainer and ' v a droop stop differ.
2.
In this assignment, two types of lead/lag damper and a third form of lead/lag damping have been discussed
Name and briefly describe each type of damping. 3.
List the advantages of the hingeless rotor over articulated and semierigid rotors.
4.
What is the main difference between the two types of tail rotor? What great advantage has one type over the other?
5.
Make a schematic sketch.of a main rotor head lead/lag damper. The damper must have Ca)
A reservoir,
Cb)-
A timing valve, and
Cc)
A replenishment valve or valves.
‘ -\.?-4%?-A 555/3/5
