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Main and Tail Rotor Contra/s

He Wharekura-tiniKaihautu 0 Aotearoa

THE OPENPDLYTECHNICUFNEW ZEALAND

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CONTENTS A

Main Rotor Controls lThe Swashplate Assembly 1

Tail Rotor Controls _ 12Tail Rotor Pitch—Change Mechanisms 12

Horizontal and Vertical Stabilisers 13Stabiliser Bar and Control Rotor Systems 20

Control Rotor System 29Stabiliser Bar System 21

‘ Copyright

This material is for the sole use of enrolled students and may notbe reproduced W1thOUt the written authority of the Principai, TOPNZ

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AIRCRAFT ENGINEERING

HELICOPTERS ASSIGNMENT 5 Y

ROTATING FLYING CONTROLS

MAIN ROTOR CONTOLS

In our Basic Flying Controls assignment, we saw how the controlinputs for the main rotor were brought from the cockpit to the non~rotating part of a swashplate assembly. We are now concerned withthe relatively simple step of getting these inputs to the rotatingmain rotor. The inputs are directly fed to the main rotor by theuse of a swashplate, which is given different names by differentmanufacturers, 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 mountedobliquely on a shaft. The swashplate assembly that is fitted toa 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, whichenables the assembly to be tilted in any direction. The gimbalencircles the main rotor drive—shaft of mast.

The lower plate is fixed to a stationary part of the helicopterand to it are attached the control rods bringing the cyclic andcollective control inputs from the pilot.

The upper plate is attached to the main rotor assembly andthus rotates with the rotors. To it are attached the push~pullrods taking the control inputs to the individual rotor blades.

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Figure l shows schematically a swashplate assembly and its controlsIn Fig. l (a), the rotor—blade dampers and the two lateral controlrods have been omitted. In Pig. l (b), the rotating half of theswashplate is positioned directly over the fixed half, and apitch-change horn is shown on only one blade. This type ofswashplate is used in Sikorsky and Hughes helicopters.

The rotating scissors provide the drive to the rotatinghalf, and the fixed scissors axially restrain the fixed half ofthe swashplate assembly. Movement of the cyclic pitch control willtilt the swashplate about the universal ball, and movement ofthe collective pitch control will raise or lower the whole assembly,with the universal ball sliding on the rotor drive shaft. Becausethe two control systems are mifed before they arrive at the swash-plate, it can be both raised and tilted at the same time. If itis already tilted, it may be raised or lowered without any changein the angle of tilt, and the tilt may be changed withoutaffecting the height setting.

In Fig. l (b), blade A is positioned immediately above theforeeand-aft control rod attachment. If the cyclic pitch controlis moved forward to give forward flight, the fore—and~aft controlrod will move down and the swashplate will tilt about the axis XX,the fixed scissors will expand, and the rotating scissors willcontract. This position will decrease the angle of attack ofblade A and increase that of blade C. Because of gyroscopiceffect, blade A will be fully flapped down and blade C will befully flapped up when they reach a position 90° later in the planeof rotation. A lateral movement of the cyclic control would tiltthe assembly about the axis YY and, as before, the change in bladeangles is made 90° early. The swashplate assembly is thus offsetabout the centre line of the helicopter to correct for theggroscqpig effect discussed in Assignment 3 of this course.

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FIG. 2 Gimbal~mounted swashplate

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The outer ring (l) turns on the swashplate bearings (2),which are mounted on the stationary swashplate (3). This assemblyis attached to the gimbal ring (Q) by two pivot pins (5), and thegimbal ring is attached to the swashplate support (6) by twopivot pins (7). The collective~pitch sleeve (8) is splined tothe mast and lies in the centre of the swashplate and supportassembly. At its lower end, it is attached to the collective-pitch lever (9) by the yoke and bearing assembly (10). Thecollective-pitch lever (9) pivots about the pivot shaft (ll) andis connected by a system of push~pull rods and bellcranks to thecollective-pitch control column. Operation of this control willraise or lower the rotating collective~pitch sleeve, which isdriven by, and slides on, splines cut on the mast. Mounted atthe top of the collective»pitch sleeve assembly (8) are two scissorlevers (l2), which pivot about centrally positioned bolts (13).At one end of each scissor lever, at position D, is attached acontrol rod going upward to the stabiliser bar assembly, and atthe other end is attached a swivel fork (1%), which is connectedby a swivel (15) to the outer ring (1). The complete swashplateassembly is secured to the top of the transmission by studspassing through holes on the mount flange of the swashplatesupport (6). Fore—and—aft movement of the cyclic—pitch controlcolumn is transmitted to horn A, and the lateral control movementto horn B. In both cases, a system of push-pull rods and bellcranksis used to convey these movements.

The swashplate (3) and outer ring (1) can tilt in anydirection through the action of the gimbal ring (Q). with a swivelfork lying above the lateral horn B and the swashplate tiltedlaterally, one scissor~lever output end moves down while theother scissor output end lifts up. This movement is conductedthrough pushupull rods and levers to the two rotor blades,increasing the pitch on one blade and decreasing the pitch on theother blade. Because the rotor blades are installed with theirspanwise axis at right~angles to the scissor levers, the pitchchange is thus made 90° before the pitch~change effect is to be feltIf collective pitch is increased, the collective»pitch sleeve is

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_ 5 _

lifted up raising the pivot points of both scissor levers bythe same amount, thus lifting the push-pull rods at D by thesame amount. In this manner, any input from the collective controlis superimposed upon, or mixed with, any movement of the cycliccontrol. Alternatively, any cyclic control movement is superimposedupon 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 pitchis transmitted to the rotor head to produce forward or backwardflight. 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 swashplatehas its inner~ring (l) and outer-ring (2) assembly mounted onand attached to the pivot sleeve (3), which incorporates a largespherical surface at its top end. This assembly can slide onthe bearings (Q) up and down on the support assembly (5). Thecollective lever (6) is pivoted and secured to the supportassembly by the idler link (7), and the inner end of the lever isattached 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 clampedto the mast (ll), and the drive linkage is secured to theouter ring by a nut.

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Movement of the cyclic-pitch control column is transmittedby a system of linkages to the intermixing bellcrank, to twohydraulic servo actuators, and then to the control horns C onthe inner ring (l). Prom the intermixing bellcrank onward, thecyclic control movements cannot be considered in terms of separatefore—and-aft or lateral movements, and so the horns at C cannotbe identified separately as the fore-and~aft horn or the lateralhorn.

Movement of the collective-pitch control column is transmittedby a system of linkages through an intermixing bellcrank, wherecyclic movements are superimposed, and a hydraulic servo actuatorto position A on the collective lever (6). As the collective-pitch control column is raised, the inner and outer ring and thepivot sleeve are lifted up, and the push—pull rods connectingcontrol horns B of the outer ring to the rotor head transmitthe movement to the main rotor blades.

Due to the intermixing bellcrank, as the collective pitchis increased, the two cyclic control rods to the horns C areraised by the same amount, the whole assembly being raised bythe collective lever (6). Thus, a cyclic pitch change can besuperimposed upon a collective pitch setting, and a collectivepitch 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. Thisassembly is mounted on a spherical ball bearing and theComplete assembly can slide up and down on the main rotor mast.

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Immediately below the swashplate assembly is the controlmixer assembly shown in Fig. 5, which is connected to theswashplate assembly by the two mixer links and the longitudinallink and is attached to the mast base at the mixer supportbracket. In this installation, the longitudinal link and thelongitudinal—pitch mixer bellcrank also act as the fixedand prevent the stationary star from rotating.

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2. The scissors crank and scissors link, or rotatingscissors WhlCh provide the drive from the rotorhub to the rotating swashplate; and

3. One of the four pitch~control rods that finallytransmit control movement to each main rotor blade.

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TAIL ROTOR CONTROLS

We have seen in Assignment 3 that, for control about thevertical axis, the pitch angle of all tail rotor blades issimultaneously changed by the same amount and in the same directionBecause of this fact, all that is needed to control the tail rotoris a simple mechanical arrangement to transfer control movementsfrom the fixed airframe to the rotating tail rotor.

Two main types of tail rotor pitch—change mechanisms areused. One type uses a simple pitch-control assembly mountedinboard of the tail rotor on the tail—rotor gearbox output shaft.The other type has a control rod passing through the hollowtail rotor gearbox output shaft to a pitch—change head outboardof the tail rotor.

Tail Rotor Pitch-Change Mechanisms

Figure 7 shows a layout of a tail rotor and its pitch—changemechanism. The tail rotor assembly is located on and driven bysplines on the transmission output shaft, being held on the shaftby a retaining nut and centralised by two split, matched conehalves. Immediately inboard of the tail rotor assembly and freeto slide on the output shaft splines is the pitch control assembly.This assembly is moved back and forth on the splines by movementof the station 282 bellcrank, which is itself moved through asystem of push—pull rods by the tail rotor pedals. The swashplateof the pitch—control assembly is connected to the tail rotor bladepitch-change horns by two fixed length pitch-control links.

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The pitch—control assembly of Fig. 7 is shown as an explodedview in Fig. 8. Figure 9 shows a side view of the assemblies,with protective neoprene boots in position.

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Figure l0 shows a tail rotor and tail rotor gearboxassembly with the pitch~change head situated outboard of the tailrotor.

NOTE: Figures 10, ll, and 12 are all different viewsof the same gearbox and tail rotor assembly.Thus, the numbered parts shown are common to allthree figures.

Control movement from the tail rotor pedals is transmittedfrom tube assembly (l) to bellcrank (2) and then to the tail rotorpitch-change mechanism (5) at the back of the tail rotor gearbox (3)

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by a short rod assembly (6). At the pitch—change mechanism inFig. ll, the control movement is fed into the control tube (7),which passes through, and is turned by, a pin and key (8) in thehollow tail rotor shaft. The change from non»rotating to arotating 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 lengthof its keyway.

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The crosshead (13) is located by a pin-(1%) and securedby a nut (15) to the control tube (7). Two pitch links (16)connect the crosshead with the blade-pitch horns (17). Thetrunnion (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 controltube axially inside the tail rotor shaft and will alter the pitchof the tail rotor blades, each one by the same amount, to givethe desired control of yaw.

HORIZONTAL AND VERTICAL STABILISERS

Many helicopters have horizontal and vertical stabilisingsurfaces, which give extra stability in normal cruise flight andso permit the main and tail rotors to be relieved of some oftheir directional control duties. The horizontal stabiliser maybe interconnected with the fore~and—aft cyclic control so thatforward movement of the control moves the trailing edge of thesurface down, and vice versa.

The surfaces usually consist of an airfoil section,often of an unusual shape. Their "neutral" position on thehelicopter can look to be anything but neutral. These surfacesmust be installed, rigged, and maintained to the manufacturers‘instructions if the helicopter is to achieve safety and reachits design performance.

\1, SUMMARY

The swashplate transfers control movements from thenon-rotating cyclic and collective pitch controls tothe rotating rotor head.

Mixing of the collective and cyclic pitch controls maytake place at the swashplate.

To allow for the gyroscopic effect, discussed in Assignment3 of this course,the fixed or stationary swashplate ispositioned so that control changes to the main rotorblades are made 90° of rotation before they are to takeeffect. ‘

,~ The tail rotor pitch~change mechanism transfers controlmovement from the non—rotating tail rotor (rudder) controls tothe rotating tail rotor. A swashplate or a bearing ‘

ip and trunnion assembly is used to make the transfer.

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_ 19 _

PRACTICE EXERCISE A

State whether each of the following statements is trueor false:

1. Another name for a swashplate assembly is anazimuth star_§§§emblg,

2. A swashplate can be tilted in only one direction.

3. A swashplate assembly must be mounted on a sphericalball.

4. The fixed scissor prevents vertical movement ofthe swashplate.

5. The cyclic and collective control movements may be mixedat the swashplate.

6. Mixing of the control movements superimposescyclic control on to collective control, and viceversa.

7. The rotating scissor drives and locates the rotatinghalf of the swashplate.

8. A tail rotor pitch—change head is, in effect,the rotating half of a swashplate.

9. The pitch of a tail rotor blade is controlledcyclically and collectively.

10. The rotating part of the tail rotor pitch—changemechanism can be driven by a key or by splines.

(Answers on page 25)

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STABILISER BAR AND CONTROL ROTOR SYSTEMS

The control rotor system used by the Hiller UH 12 helicopterand the stabiliser bar system used by the older Bell helicopters,help to control the main rotor. Both of these types of aircraftwill 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 rotorhead is underslung and gimbal~mounted. As a result, the rotor canteeter spanwise and rock chordwise. The control rotor, which isan integral part of the rotor head, consists of two small controll-able in pitch aerofoils or paddles mounted at 90° to the main rotorblades. Cyclic control»column movements are fed from the rotatingpart of the wobble Elate (another name for swashplate) to eachpaddle. The forces generated by the rotating paddles tilt themain rotor in the desired direction. In effect, the pilot controlsthe paddles, and the paddles control the main rotor.

Collective control is from the collective pitch~control leverto the yoke assembly at the top of the gearbox, where movement istransferred to a rod inside the rotating main—rotor driveshaft.

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_ 21 _

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On the top of the rotor head is mounted a cross arm, which carriesa push rod to each blade incidence arm and two ballast tubeassemblies, which balance the collective control forces.Movement of the collective pitch lever increases the incidenceof both blades by the same amount and in the same sense.

Stabiliser Bar System

In the stabiliser bar system, the rotor head is gimbal-mounted and free to teeter spanwise and rock chordwise. Eachblade grip is mechanically connected to the other by an equaliserbeam so that the angular positions of the blades on the yokewill always be equal to each other. Mounted, usually below andalways at 90° to the span of the main rotor assembly, and splinedto the mast assembly, is the stabiliser bar and frame. Toquote the manufacturer,

The inertia effect of the stabiliser bar tends to stabilisethe 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 tothe mast (13) and is pivoted at its centre. Its frame isconnected by two short links to the blade dampers (9) of the

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damper frame assembly (5), which is splined and rigidly attachedto the mast. The outputs from the cyclic and collective~pitchcontrol 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) onthe stabiliser bar assembly.

Despite their name, the mixing levers (12) do not mix thefore—and-aft, lateral, and collective controls but mix the inputsto the rotor head from the control rods (7) and the stabiliserbar.

From the levers, the control outputs arecarried to each mainrotor—blade horn by a control link (ll). The hydraulic dampers (9)restrict the pivot rate of the stabiliser bar frame.

Refer to Fig. 1H and consider the events when the cyclicpitch-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) goesdown.

Q. The inertia forces of the rotor head assembly willtry to resist a change in blade~pitch angles.

5. Because of the resistance set up, the control linkswill not move, and their attachments to the mixinglevers (12) become pivot points to allow rods (7)to move.

6. As rods (7) move, the red end of the stabliser baris pushed down, and the white end is lifted up.

7. The stabiliser bar has now had its plane of rotationdisplaced.

8. with the cyclic pitch—control column held to theright, the stabiliser bar will try to return to itsproper plane of rotation.

9. The pivot point of each mixing lever is now theattachment point of rod (7).

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10. The red rod (ll) is now lifted up and thewhite rod (ll) pulled down.

ll. Theitsred

spanwise axis, lifting the leading edge of theblade up and that of the white blade down.

REMEMBER__..........-._--I--: The blades are mechanically connected together with

equaliser beams.

12. The red blade now has a greater angle of attack andwill generate more lift than the white blade.

13. Due to gyroscopic forces or phase lag, the increasein lift takes effect 90° later in the plane ofrotation, and the rotor disc tilts to the right.

1%. As the rotor turns, the white blade comes to wherethe red blade was, and in turn gets the increase inangle of attack.

Q9221: These events all take place together.

The same sequence of events takes place for a fore—and-aft movement of the cyclic pitch control. However, a collectivcontrol movement moves the rods (7) equally and in the samedirection, and the stabiliser bar is not displaced at all. Therate of response to the cyclic pitch-control is governed by thestiffness of the dampers (9). Stiff or hard dampers give avery quick and sensitive response.

The damper rate is decided by the helicopter manufacturerand is checked and adjusted during normal routine servicing.

M

l SUMMARYA control rotor provides a form of power assistanceto the cyclic control.

A stabiliser bar gives a reference horizon base forthe control of the rotor. Its rate of movement isgoverned by two hydraulic dampers.

l

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action in (10) rocks the rotor head assembly about

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PRACTICE EXERCISE B

State whether each of the following statements is trueor false: -

l. A control rotor assists collective pitch changes inthe control rotor system.

2. The control rotor‘s two small aerofoils producethe force needed forcyclic control of the main rotor.

3. A stabiliserbar is rigidly attached to the main rotordrive shaft.

4. In the stabiliser bar system, the cyclic and collectivecontrols are mixed before they reach the stabiliser bar.

5. Two ballasted tubes balance the collective forces inthe controlerotor system.

6. Two hydraulic dampers are fitted in the control~rotorsystem 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 thecollective- and cyclic-control inputs.

9. The timing rate of hydraulic dampers in a flight~controlsystem is important to the handling qualities of thehelicopter.

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 betilted in any direction.

3. False. A swashplate assembly may be ball-mounted, butit may also be gimbal-mounted.

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4. False. The fixed scissor stops the fixed half ofthe swashplate from turning.

9. False. The pitch of the tail rotor blades ischanged 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 cycliccontrol of the main rotor.

8. False. The stabiliser bar is pivoted about acore, which is splined to the mast.

6. False. No hydraulic dampers are needed or fittedin the control-rotor system.

8. False. The collective and cyclic controls aremixed at the swashplate assembly. The stabiliser—bar mixing levers mix the inputs from the swashplatewith the movement of the stabiliser bar.

TEST PAPER 5

l. Describe the purpose and operation of a main rotorswashplate assembly.

2. What are the functions of the fixed and rotating scissors?

3. Why is correct timing necessary for the dampers fitted toa stabiliser—bar assembly?

H. With the aid of a simple sketch, describe a tail rotorpitch—change mechanism, from the non-rotating input atthe tail rotor gearbox to the rotating pitch-changebeam/head of the tail rotor.

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