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Main and Tail Rotor Contra/s 555—3—5
CONTENTS
A
Main Rotor Controls The Swashplate Assembly
l 1
Tail Rotor Controls _ Tail Rotor Pitch—Change Mechanisms
12 12
Horizontal and Vertical Stabilisers
13
Stabiliser Bar and Control Rotor Systems Control Rotor System
20 29
Stabiliser Bar System
‘
21
Copyright
This material is for the sole use of enrolled students and may not
be reproduced W1thOUt the written authority of the Principai, TOPNZ
555/3/5
AIRCRAFT ENGINEERING
HELICOPTERS
ASSIGNMENT 5 Y ROTATING FLYING CONTROLS MAIN ROTOR CONTOLS
In our Basic Flying Controls assignment, we saw how the control inputs for the main rotor were brought from rotating part of a swashplate assembly. We the relatively simple step of getting these main rotor. The inputs are directly fed to
the cockpit to the non~ are now concerned with inputs to the rotating the main rotor by the
use of a swashplate, which is given different names by different manufacturers, the most common names being 1.
The swashplate assembly,
2.
The fixed and rotating star assembly, and
3.
The azimuth star assembly.
The Swashplate Assembly In simple terms, a swashplate is a circular plate mounted obliquely on a shaft.
The swashplate assembly that is fitted to
a helicopter consists of two plates, one on top of the other, separated by and running on a heavy—duty ball- or roller-bearing. The plates are mounted on a gimbal or large universal ball, which enables the assembly to be tilted in any direction. The gimbal encircles the main rotor drive—shaft of mast. The lower plate is fixed to a stationary part of the helicopter and to it are attached the control rods bringing the cyclic and collective control inputs from the pilot.
The upper plate is attached to the main rotor assembly and thus rotates with the rotors. To it are attached the push~pull rods taking the control inputs to the individual rotor blades. 9/88
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Figure l shows schematically a swashplate assembly and its controls In Fig. l (a), the rotor—blade dampers and the two lateral control rods have been omitted. In Pig. l (b), the rotating half of the swashplate is positioned directly over the fixed half, and a pitch-change horn is shown on only one blade.
This type of
swashplate is used in Sikorsky and Hughes helicopters. The rotating scissors provide the drive to the rotating half, and the fixed scissors axially restrain the fixed half of the swashplate assembly. Movement of the cyclic pitch control will tilt the swashplate about the universal ball, and movement of the collective pitch control will raise or lower the whole assembly, with the universal ball sliding on the rotor drive shaft. Because the two control systems are mifed before they arrive at the swashplate, it can be both raised and tilted at the same time. If it is already tilted, it may be raised or lowered without any change in the angle of tilt, and the tilt may be changed without affecting the height setting.
In Fig. l (b), blade A is positioned immediately above the foreeand-aft control rod attachment. If the cyclic pitch control is moved forward to give forward flight, the fore—and~aft control rod will move down and the swashplate will tilt about the axis XX, the fixed scissors will expand, and the rotating scissors will contract.
This position will decrease the angle of attack of
blade A and increase that of blade C. Because of gyroscopic effect, blade A will be fully flapped down and blade C will be fully flapped up when they reach a position 90° later in the plane of rotation.
A lateral movement of the cyclic control would tilt
the assembly about the axis YY and, as before, the change in blade angles is made 90° early. The swashplate assembly is thus offset about the centre line of the helicopter to correct for the ggroscqpig effect discussed in Assignment 3 of this course.
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555/ 3/ 5
_ q _ A gimbal—mounted swashplate is u se d on the Bell H7 series h . . . elicopters. Besides transferring collective and cyclic—pitch co n t rol movements to the rotor-head assembly, it also mixes these controls. Fi gure 2 shows this swashplate assembl Y -
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The outer ring (l) turns on the swashplate bearings (2), which are mounted on the stationary swashplate (3).
This assembly
is attached to the gimbal ring (Q) by two pivot pins (5), and the gimbal ring is attached to the swashplate support (6) by two pivot pins (7). The collective~pitch sleeve (8) is splined to the mast and lies in the centre of the swashplate and support assembly. At its lower end, it is attached to the collectivepitch lever (9) by the yoke and bearing assembly (10). The collective-pitch lever (9) pivots about the pivot shaft (ll) and is connected by a system of push~pull rods and bellcranks to the collective-pitch control column. Operation of this control will raise or lower the rotating collective~pitch sleeve, which is driven by, and slides on, splines cut on the mast. Mounted at the top of the collective»pitch sleeve assembly (8) are two scissor levers (l2), which pivot about centrally positioned bolts (13). At one end of each scissor lever, at position D, is attached a control rod going upward to the stabiliser bar assembly, and at the other end is attached a swivel fork (1%), which is connected by a swivel (15) to the outer ring (1).
The complete swashplate
assembly is secured to the top of the transmission by studs passing through holes on the mount flange of the swashplate support (6). Fore—and—aft movement of the cyclic—pitch control column is transmitted to horn A, and the lateral control movement to horn B. In both cases, a system of push-pull rods and bellcranks is used to convey these movements. The swashplate (3) and outer ring (1) can tilt in any direction through the action of the gimbal ring (Q). with a swivel fork lying above the lateral horn B and the swashplate tilted laterally, one scissor~lever output end moves down while the other scissor output end lifts up. This movement is conducted through pushupull rods and levers to the two rotor blades, increasing the pitch on one blade and decreasing the pitch on the other blade. Because the rotor blades are installed with their spanwise axis at right~angles to the scissor levers, the pitch change is thus made 90° before the pitch~change effect is to be felt If collective pitch is increased, the collective»pitch sleeve is
555/3/5
_ 5 _
lifted up raising the pivot points of both scissor levers by the same amount, thus lifting the push-pull rods at D by the same amount. In this manner, any input from the collective control is superimposed upon, or mixed with, any movement of the cyclic control. Alternatively, any cyclic control movement is superimposed upon any collective control movement. The same series of events follows with a swivel fork (6) lying above horn A. In this position, a change in cyclic pitch is transmitted to the rotor head to produce forward or backward flight. Because the swashplate can be tilted in any direction, the rotor head can be controlled to give flight in any direction. Figure 3 shows a ball—mounted swashplate.
This swashplate
has its inner~ring (l) and outer-ring (2) assembly mounted on and attached to the pivot sleeve (3), which incorporates a large spherical surface at its top end.
This assembly can slide on
the bearings (Q) up and down on the support assembly (5).
The
collective lever (6) is pivoted and secured to the support assembly by the idler link (7), and the inner end of the lever is attached to the pivot sleeve by two pins (8) (only one pin is shown) ‘The drive to the rotating outer ring is through a collar set (9) and drive linkage (l0). The collar set is splined and clamped to the mast (ll), and the drive linkage is secured to the outer ring by a nut.
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Movement of the cyclic-pitch control column is transmitted by a system of linkages to the intermixing bellcrank, to two hydraulic servo actuators, and then to the control horns C on the inner ring (l). Prom the intermixing bellcrank onward, the cyclic control movements cannot be considered in terms of separate fore—and-aft or lateral movements, and so the horns at C cannot be identified separately as the fore-and~aft horn or the lateral horn. Movement of the collective-pitch control column is transmitted by a system of linkages through an intermixing bellcrank, where cyclic movements are superimposed, and a hydraulic servo actuator to position A on the collective lever (6). As the collectivepitch control column is raised, the inner and outer ring and the pivot sleeve are lifted up, and the push—pull rods connecting control horns B of the outer ring to the rotor head transmit the movement to the main rotor blades. Due to the intermixing bellcrank, as the collective pitch is increased, the two cyclic control rods to the horns C are raised by the same amount, the whole assembly being raised by the collective lever (6). Thus, a cyclic pitch change can be superimposed upon a collective pitch setting, and a collective pitch change can be superimposed upon a cyclic pitch setting. Another type of ball~mounted swashplate is shown in Fig. M. The rotating swashplate is mounted on a heavy—duty ball race, which is itself attached to the stationary swashplate.
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assembly is mounted on a spherical ball bearing and the Complete assembly can slide up and down on the main rotor mast.
555/3/5
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Immediately below the swashplate assembly is the control mixer assembly shown in Fig. 5, which is connected to the swashplate assembly by the two mixer links and the longitudinal link and is attached to the mast base at the mixe r support bracket. In this installation, the longitudinal link and the longitudinal—pitch mixer bellcrank also act as the fixed scissors and prevent the stationary star from rotating.
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The scissors crank and scissors link, or rotating scissors WhlCh provide the drive from the rotor hub to the rotating swashplate; and
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One of the four pitch~control rods that finally transmit control movement to each main rotor blade.
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_ 12 _
TAIL ROTOR CONTROLS
We have seen in Assignment 3 that, for control about the vertical axis, the pitch angle of all tail rotor blades is simultaneously changed by the same amount and in the same direction Because of this fact, all that is needed to control the tail rotor is a simple mechanical arrangement to transfer control movements from the fixed airframe to the rotating tail rotor. Two main types of tail rotor pitch—change mechanisms are used. One type uses a simple pitch-control assembly mounted inboard of the tail rotor on the tail—rotor gearbox output shaft. The other type has a control rod passing through the hollow tail rotor gearbox output shaft to a pitch—change head outboard of the tail rotor.
Tail Rotor Pitch-Change Mechanisms Figure 7 shows a layout of a tail rotor and its pitch—change mechanism. The tail rotor assembly is located on and driven by splines on the transmission output shaft, being held on the shaft by a retaining nut and centralised by two split, matched cone halves. Immediately inboard of the tail rotor assembly and free to slide on the output shaft splines is the pitch control assembly. This assembly is moved back and forth on the splines by movement of the station 282 bellcrank, which is itself moved through a system of push—pull rods by the tail rotor pedals.
The swashplate
of the pitch—control assembly is connected to the tail rotor blade pitch-change horns by two fixed length pitch-control links.
555/3/5
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The pitch—control assembly of Fig. 7 is shown as an exploded view in Fig. 8.
Figure 9 shows a side view of the assemblies,
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Tail rotor and pitch~control assemblies
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_ 15 _
Figure l0 shows a tail rotor and tail rotor gearbox assembly with the pitch~change head situated outboard of the tail rotor. NOTE:
Figures 10, ll, and 12 are all different views of the same gearbox and tail rotor assembly.
Thus, the numbered parts shown are common to all three figures.
Control movement from the tail rotor pedals is transmitted from tube assembly (l) to bellcrank (2) and then to the tail rotor pitch-change mechanism (5) at the back of the tail rotor gearbox (3) o
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by a short rod assembly (6). At the pitch—change mechanism in Fig. ll, the control movement is fed into the control tube (7), which passes through, and is turned by, a pin and key (8) in the hollow tail rotor shaft. The change from non»rotating to a rotating motion is effected by the trunnion (9), the bearing (10), the levers (ll) and the idler link (12). The control tube (7) always turns with the shaft, but it can also move axially the length of its keyway. 555/3/5
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The crosshead (13) is located by a pin-(1%) and secured by a nut (15) to the control tube (7). Two pitch links (16) connect the crosshead with the blade-pitch horns (17). The trunnion (21) of the tail rotor assembly (Q) is located and I
driven by external splines on the tail rotor shaft (18). The tail rotor assembly is restrained by the static stop (19) and secured by the retaining nut (20).
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Movement of the tail rotor pedals will move the control tube axially inside the tail rotor shaft and will alter the pitch of the tail rotor blades, each one by the same amount, to give the desired control of yaw. HORIZONTAL AND VERTICAL STABILISERS
Many helicopters have horizontal and vertical stabilising surfaces, which give extra stability in normal cruise flight and so permit the main and tail rotors to be relieved of some of their directional control duties. The horizontal stabiliser may be interconnected with the fore~and—aft cyclic control
so that
forward movement of the control moves the trailing edge of the surface down, and vice versa. The surfaces usually consist of an airfoil section, often of an unusual shape. Their "neutral" position on the helicopter can look to be anything but neutral. These surfaces must be installed, rigged, and maintained to the manufacturers‘ instructions if the helicopter is to achieve safety and reach its design performance.
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SUMMARY The swashplate transfers control movements from the non-rotating cyclic and collective pitch controls to the rotating rotor head. Mixing of the collective and cyclic pitch controls may
take place at the swashplate. To allow for the gyroscopic effect, discussed in Assignment 3 of this course,the fixed or stationary swashplate is positioned so that control changes to the main rotor blades are made 90° of rotation before they are to take effect.
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The tail rotor pitch~change mechanism transfers control movement from the non—rotating tail rotor (rudder) controls to the rotating tail rotor. A swashplate or a bearing ‘ and trunnion assembly is used to make the transfer.
555/3/5
_ 19 _
PRACTICE EXERCISE A
State whether each of the following statements is true or false: 1.
Another name for a swashplate assembly is an azimuth star_§§§emblg,
2.
A swashplate can be tilted in only one direction.
3.
A swashplate assembly must be mounted on a spherical ball.
4.
The fixed scissor prevents vertical movement of the swashplate.
5.
The cyclic and collective control movements may be mixed at the swashplate.
6.
Mixing of the control movements superimposes cyclic control on to collective control, and vice versa.
7.
The rotating scissor drives and locates the rotating half of the swashplate.
8.
A tail rotor pitch—change head is, in effect, the rotating half of a swashplate.
9.
10.
The pitch of a tail rotor blade is controlled cyclically and collectively. The rotating part of the tail rotor pitch—change mechanism can be driven by a key or by splines.
(Answers on page
555/3/5
25)
_ 29 _
STABILISER BAR AND CONTROL ROTOR SYSTEMS
The control rotor system used by the Hiller UH 12 helicopter and the stabiliser bar system used by the older Bell helicopters, help to control the main rotor. Both of these types of aircraft will possibly remain in use in New Zealand for some years to come, and so we shall briefly look at their main rotor~control systems.
Control Rotor System In the control rotor system shown in Fig. 13, the rotor head is underslung and gimbal~mounted. As a result, the rotor can teeter spanwise and rock chordwise. The control rotor, which is an integral part of the rotor head, consists of two small controllable in pitch aerofoils or paddles mounted at 90° to the main rotor blades.
Cyclic control»column movements are fed from the rotating
part of the wobble Elate (another name for swashplate) to each paddle. The forces generated by the rotating paddles tilt the main rotor in the desired direction. In effect, the pilot controls the paddles, and the paddles control the main rotor. Collective control is from the collective pitch~control lever to the yoke assembly at the top of the gearbox, where movement is transferred to a rod inside the rotating main—rotor driveshaft.
R55/R/5
_ 21 _
1. Cuff and trunnion 2. Contra! rotor 3. Push rod assembiy 4. Wobbie plate $11090" 5. Fore-and-aft control rod 6. Collective yoke assembly 7. Wobble plate 8. Lower scissor arm 9. Upper scissor arm 10. Ballast tube
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Control rotor system
On the top of the rotor head is mounted a cross arm, which carries a push rod to each blade incidence arm and two ballast tube assemblies, which balance the collective control forces. Movement of the collective pitch lever increases the incidence of both blades by the same amount and in the same sense.
Stabiliser Bar System In the stabiliser bar system, the rotor head is gimbalmounted and free to teeter spanwise and rock chordwise. Each blade grip is mechanically connected to the other by an equaliser beam so that the angular positions of the blades on the yoke will always be equal to each other. Mounted, usually below and always at 90° to the span of the main rotor assembly, and splined to the mast assembly, is the stabiliser bar and frame. quote the manufacturer,
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The inertia effect of the stabiliser bar tends to stabilise the helicopter and to provide an absolute horizon in reference to which the rotor is controlled independently of the body.
In Fig. 14 the stabiliser bar assembly (10) is splined to the mast (13) and is pivoted at its centre. Its frame is connected by two short links to the blade dampers (9) of the
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1. Collective pitch lever 2. Lateral horn 3. Fore-and-aft horn 4. Swash plate assembly 5. Collective pitch sleeve 6. Dust boot 7. Control rods B.Damperfrarneassembly 9. Damper 10. Stabiliser bar assembly 11. Control links 12. Mixing lever 13. Mast assembly
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FIG. 14
Stabiliser bar system
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damper frame assembly (5), which is splined and rigidly attached to the mast. The outputs from the cyclic and collective~pitch control columns are brought to the lever and to horns (1), (2), and (3). These controls are mixed in the swashplate assembly (H) andare carried by control rods (7) to the mixing levers (12) on the stabiliser bar assembly. Despite their name, the mixing levers (12) do not mix the fore—and-aft, lateral, and collective controls but mix the inputs to the rotor head from the control rods (7) and the stabiliser bar. From the levers, the control outputs arecarried to each main rotor—blade horn by a control link (ll). The hydraulic dampers restrict the pivot rate of the stabiliser bar frame. Refer to Fig. 1H and consider the events when the cyclic pitch-control column is moved to the right. l.
The horn (2) is lifted up.
2.
The swashplate is tilted to the right.
3.
The red rod (7) goes up and the white rod (7) goes down.
Q.
The inertia forces of the rotor head assembly will try to resist a change in blade~pitch angles.
5.
Because of the resistance set up, the control links will not move, and their attachments to the mixing levers (12) become pivot points to allow rods (7) to move.
6.
As rods (7) move, the red end of the stabliser bar is pushed down, and the white end is lifted up.
7.
The stabiliser bar has now had its plane of rotation displaced.
8.
with the cyclic pitch—control column held to the right, the stabiliser bar will try to return to its proper plane of rotation.
9.
The pivot point of each mixing lever is now the attachment point of rod (7).
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10.
The red rod (ll) is now lifted up and the white rod (ll) pulled down.
ll.
action in (10) rocks the rotor head assembly about its spanwise axis, lifting the leading edge of the red blade up and that of the white blade down.
The
REMEMBER
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The blades are mechanically connected together with equaliser beams.
12.
The red blade now has a greater angle of attack and will generate more lift than the white blade.
13.
Due to gyroscopic forces or phase lag, the increase in lift takes effect 90° later in the plane of rotation, and the rotor disc tilts to the right.
1%.
As the rotor turns, the white blade comes to where the red blade was, and in turn gets the increase in angle of attack.
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These events all take place together.
The same sequence of events takes place for a fore—andaft movement of the cyclic pitch control. However, a collectiv control movement moves the rods (7) equally and in the same direction, and the stabiliser bar is not displaced at all. The rate of response to the cyclic pitch-control is governed by the stiffness of the dampers (9). Stiff or hard dampers give a very quick and sensitive response. The damper rate is decided by the helicopter manufacturer and is checked and adjusted during normal routine servicing.
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SUMMARY A control rotor provides a form of power assistance to the cyclic control. A stabiliser bar gives a reference horizon base for the control of the rotor. Its rate of movement is governed by two hydraulic dampers.
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PRACTICE EXERCISE B
State whether each of the following statements is true or false: l.
A control rotor assists collective pitch changes in the control rotor system.
2.
The control rotor‘s two small aerofoils produce the force needed forcyclic control of the main rotor.
3.
A stabiliserbar is rigidly attached to the main rotor drive shaft.
4.
In the stabiliser bar system, the cyclic and collective
controls are mixed before they reach the stabiliser bar. 5.
Two ballasted tubes balance the collective forces in the controlerotor system.
6.
Two hydraulic dampers are fitted in the control~rotor system to control the rate of response of the helicopter.
7.
Wobble plate is another name for a swashplate.
8.
The mixing levers of the stabiliser bar mix the collective- and cyclic-control inputs.
9.
The timing rate of hydraulic dampers in a flight~control system is important to the handling qualities of the helicopter.
l0.
The control rotor forms an integral part of the rotor head.
(Answers on page 25)
ANSWERS TO PRACTICE EXERCISES
EXERCISE A
Statements 1, 5, 6, 7, 8 and 10 are true. 2.
False. A swashplate may or may not be tiltable. The type used in the main rotor controls can be tilted in any direction.
3.
False. A swashplate assembly may be ball-mounted, but it may also be gimbal-mounted.
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4.
False. The fixed scissor stops the fixed half of the swashplate from turning.
9.
False. The pitch of the tail rotor blades is changed collectively only. 1
EXERCISE B
E
Statements 2, H, 5, 7, 9 and l0 are true. l.
False. The control rotor produces the force for cyclic control of the main rotor.
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8.
False. The stabiliser bar is pivoted about a core, which is splined to the mast.
6.
False. No hydraulic dampers are needed or in the control-rotor system.
8.
False. The collective and cyclic controls are mixed at the swashplate assembly. The stabiliser— bar mixing levers mix the inputs from the swashplate with the movement of the stabiliser bar.
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TEST PAPER 5 l.
Describe the purpose and operation of a main rotor swashplate assembly.
2.
What are the functions of the fixed and rotating scissors?
3.
Why is correct timing necessary for the dampers fitted to a stabiliser—bar assembly?
H.
With the aid of a simple sketch, describe a tail rotor pitch—change mechanism, from the non-rotating input at the tail rotor gearbox to the rotating pitch-change beam/head of the tail rotor.
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