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Basic Helicopter FlyingControls

He Wharekura-tin-iKaihautu 0 Aotearoa

THE 0 P E NP0l.YTE(HN|(OF NEWZEALAND

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CONTENTS

Basic Flying Controls 1Cyclic Control 3Collective Control 7Control Mixing 3Tail Rotor Control 10

Control Damping 1nRotor Head Feedback Forces 1nPower Assistance 15Friction Controls 13Artificial Feel 19Trimming Controls 20Magnetic Brake 22

The Collective Lever and Throttle Control ZnThrottle Correlation 26Piston Engine Throttle (Power) Control 26Turbine Engine Power Control 31

Control Systems Maintenance 3HSafety of Personnel 35

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 2

BASIC FLYING CONTROLS

All aircraft have three controllable axes:

Longitudinal »~ PitchLateral - RollNormal - Yaw

A control gives movement about each axis:

Longitudinal - AileronsLateral - ElevatorsNormal —- Rudder

The ailerons and elevators are controlled through the controlcolumn or handwheel, and the rudder is controlled through pedals.

Rotating the handwheel clockwise or counterclockwise orleaning the control column to the right or left, causes theaircraft, through the effect of ailerons, to roll to the right orto the left.

By pulling the control column or handwheel towards him, thepilot raises the nose of the aircraft by raising the elevator'strailing edge; to force the nose down, he pushes the controlcolumn forward, away from him.

Pushing the right rudder pedal forward brings the nose ofthe aircraft round to the right, and pushing the left pedalforward brings the nose to the left.

To sustain ahalancedturn requires a combination of each ofthese controls, the ratio of force or movement of each dependingon the characteristics of the aircraft and the rate of the turn.

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Forces acting on a helicopter in a turn. Lift causes the helicopter to turn when it is banked.

FIG. l An aircraft in a moderate turn

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The pilot starts a turn by banking using the ailerons, andhe then keeps the nose smoothly moving round, without raising ordipping it, by a combination of elevator and rudder movement.Generally, the steeper the turn, the more he moves each controlto maintain balance.

Figure l shows the force distribution needed during the turnto keep the aircraft from returning to its stable straight andlevel original attitude.

A helicopter is controlled in flight by much the sameinteraction of controls and forces. In this assignment, we shallconsider a helicopter's flying and powerplant controls in somedetail.

Cyclic Control

The equivalent of aileron control on a helicopter is sidewaysmovement of the cyclic stick, which, by angular change of the rotorblades‘ pitch, leans the rotor disc in the direction of movementdesired. Figure 2 shows the effect of moving the cyclic stick,which also gives fore and aft control (stick forward, nose down;stick back, nose up).

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The interesting thing about this control is that the bladeangle is changed when the blades are fore and aft, which resultsin rotor disc tilt about 90° later, that is, when the blades areathwartships of the aircraft. The helicopter then moves in thedirection of the low~pitched blade.

Figure 3 shows a relatively simple main rotor controlmechanism with the longitudinal and collective sections of thelinkages hatched for reference.

Sideways movement of the cyclic control column (10) moves thepilot's lateral control rod (1%) lengthwise, pivoting the lateralbellorank (ll) about its rear RH corner. This pushes or pulls

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the lower lateral control rod (15), whose movement is transferredthrough links and bellcranks to the end of the lateral pitch mixerbellcrank (6).

This bellcrank is pivoted so that as it rocks under theinfluence of the control rod, the left and right upper lateralcontrol rods (H and 5) rock the non~rotating section of theswash plate athwartships, appropriately changing rotor blade anglesthrough the pitch control links (1).

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The swash plate allows interconnection between the lowercontrol system, which does not rotate, and the upper controlsystem, rotating with the rotor head. Any tilt of the lowerswash plate is transmitted to the upper swash plate through avery substantial ball or roller bearing, necessary because theupper swash plate changes rotor blade angularity every rotorrevolution.

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We discuss swash plates in some detail in another assignment

_ Thus, lateral control, like aileron control, is by a tiltathwartships. The resulting tilt on the rotor disc causesthe helicopter to bank, like a fixed-wing aircraft.

Figure H shows longitudinal control, which is by fore andaft movement of the cyclic control column. The system shown inFig. H is from the same helicopter as that shown in Fig. 3.

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Longitudinal control

A forward movement of the stick rotates the cyclic controltorque tube (9), the arm on the torque tube brings the controlrod (8) forward, and the linkages and rods thereon result in anupward movement of the upperlongitudinal control rod, tiltingthe lower (or stationary) swash plate forward.

Again, this movement of the cyclic stick has its effect onthe pitch of the main rotor blades, changing each blade throughthe sequence from coarse through fine and back to coarse again

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with each main rotor head revolution, fine pitch in this case beingover the nose of the helicopter.

This movement of the longitudinal control system acts inaddition to and is superimposed upon lateral stick movement-Within the limits of the control stops, full lateral movement canhave imposed on it full longitudinal movement, but such extremeselections are rare during normal flight.

You must remember that the cyclic loads are reciprocal.Thus, loose play in agg control rod will be the source of muchvibration and very rapid wear.

Later in this course we have an assignment on vibrationin helicopters.

The main rotor can be tilted by the two control systems just

described, but superimposed on the cyclic system is the collectivecontrol system.

Collective Control

As Fig. S shows, the pilot has, at his left hand, a leverthat he raises to increase the pitch of all rotor bladestogether and lowers to reduce the pitch of all the rotor bladestogether.

Lifting this collective pitch lezei (ll) rotates the torque

tube (10) and raises the control rod (8). This raises the frontof the collective pitch mixer bellcrank (7), which pivots downat the rear, lowering the upper longitudinal control rod‘sbellcrank pivot. Moreover, tilting the bellcrank (7) back anddown lowers the lateral pitch mixer bellcrank (5), the wholemovement bringing the swash plate assembly down the mast withouttilting it in any direction.

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Thus, through the pitch control rods, an increase of thepitch of all blades equally and together is made while maintainingtheir cyclic relationship. Superimposing collective controlmovements on cyclic control movements and vice versa is calledcontrol mixing.

Control Mixing

Figure 6 shows a demonstration example of a one~leveredmixing unit. It consists of lever A connected at pivot A tothe airframe structure and connected by rod A to the collectivepitch lever so that, in this example, raising the collectivelever raises the free end of the lever up as it turns aboutpivot A.

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Lever B is carried on lever A and is pivoted betweenthe attachment points of rods B and C, that is, lever B see~

saws about its pivot point, B. This lever is connected by rodB to, say, the cyclic fore~and-aft control and by rod C to thestationary swamiplate. In this example, moving the cycliccontrol column forward moves rod B down, thus moving rod C up.

When the collective lever is held fixed in one position andthe cyclic control column is moved fore and aft, lever B seesawsabout pivot B. When the collective lever is raised and thecyclic control column is held fixed in one position, then aslever A is raised, the pivot point of lever B moves to theattachment point of rod B and rod C is raised. Thus, a movementof the collective lever has been added to the fore-and-aft cycliccontrol without the cyclic control column moving.

ln practice, a lateral lever (or levers) is added to lever Aso that collective lever movement is added to both thefore»and-aftand lateral cyclic controls.

The reverse of the movements happens when the collective

lever is lowered. Of course, the cyclic control column can bemoved and its motion transmitted in the normal way while thecollective lever is being moved.

The mixing unit shown in Fig. 7_is taken from an S55 helicopterAll mixing units work on the same principle but their levers andbellcranks are not usually the simple shapes shown in Fig. 7.

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with this simple system in mind, now look again at Fig. 3 land 5.

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Tail Rotor Control

The tail rotor is the equivalent of the rudder of a fixed-wing aircraft

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This small rotor provides:

l. Yaw control in forward flight,

2. Directional control when the helicopter is hovering, and

3. Control of the torque reaction of the main rotor inall stages of flight.

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Figure 8 shows a helicopter tail rotor system, from therudder pedals to the bellcrank that operates the tail rotor pitchchange mechanism.

The rudder pedals are separately mounted on a torque tube,each pair being linked together by bellcranks and push rods.The right~hand bellcrank directly operates a quadrant and cableloop to a bellcrank in the aft of the centre section.

From this aft bellcrank, a push-pull rod runs to the arm onthe tail rotor swash plate bellcrank. On this bellcrank is apin that moves the nonerotating portion of the swash plate in andout in response to the pilot's pedal movement“

A bungee spring is fitted to apply a slight left—pedal

forward preload to the tail rotor controls, this bias balancingthe slight sideways drift caused by the tail rotor.

The cable system, accessible through the right side of thefuselage, is carried round the cargo/passenger compartment.Bellcranks are on bearings, and pulleys are bushed so that thesystem is free-moving but contains the minimum of free play.

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A helicopter has the same axes of control as a fixed—wingaircraft. l~;Collective control is changing the pitch of all rotorblades together and by the same amount.

Cyclic control gives the blades in one place in therotor disc a greater angle of attack than those inanother place.

T\The mixing unit superimposes collective control on thecyclic control movements and vice versa.

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PRACTICE EXERCISE A

State whether each of the following is true or false:

1.

2

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4

5.

The tail rotor counteracts main rotor torque only whenthe helicopter is hovering.

A forward and leftward movement of the cyclic controlcolumn moves the nose of the helicopter down andto the left.

Forward movement of the right rudder pedal moves thenose of the helicopter to the left.

Raising the collective lever causes the helicopter toclimb.

The cyclic, collective, and tail rotor controlmovements are added to each other by the mixing unit.

(Answers on page 37)

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CONTROL DAMPING

Rotor Head Feedback Forces

with a small helicopter that has an articulated rotor head(for example, the Hughes 269 series), when the rotor head and bladesassembly is correctly rigged, adjusted, and balanced, there arefew or no forces fed back from the rotor to the pilot‘scontrols. Thus, no devices are needed to absorb or prevent anyforces being felt by the pilot.

ln the semi rigid and heavier articulated rotor helicopterswith their correspondingly larger rotor»head forces, the effortneeded to move the controls would quickly tire the pilot. Inthese helicopters, hydraulic power assistance is used in both thecyclic and collective controls. Built into the power assistance(servo units) are valves that prevent rotor»head forces beingfed back to the pilot when'Uuahydraulic system is inoperative.Futhermope, the hydraulic lock formed either side of the servopiston when the hydraulic system is in operation also stops feed-back forces. The hydraulic system is usually powered by a pumpdriven by the main rotor gearbox so that, in the event of enginefailure, hydraulic servo assistance is still available duringautorotation. If the hydraulic system should fail, the pilotwould still be able to fly the helicopter in complete safetyalthough, as we said earlier, this would be tiring.

The rotor forces of the bigger helicopters are so large thatmanual control is impossible. These helicopters have two separatehydraulic~powered servo systems. One system is driven by themain rotor gearbox and the other is driven by the engine(s).The two systems are connected electrically and so if the on—linesystem fails, the other system is automatically and 5£_3£§§brought into use.

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Power Assistance

Figure 9 shows a type of servo actuator. We now describethe operation of valves 3 and l0, which together form aninreversiblevalve,andvalve u.

Operation of an irreversible valve: when hydraulic power

is lost or switched off, the lower spring and the poppet valve (10)push up the plunger (12) of the sequence valve (3) while theupper spring holds the valve seat (ll) down. This action stopsthe flow of fluid to the return port (1) and so no hydraulicfluid can now leave the unit unless the valve seat (ll) movesup to relieve the pressure caused by heating of the fluid.

If, with the sequence valve (8) closed, the pilot's controlinput (9) is moved, the hydraulic fluidtrappedin the servo unitis displaced from one side of the actuator piston to the otherby flowing through the slide and sleeve assembly (7). Whencontrol input stops, the slide and sleeve takes up a central positionand hydraulic fluid is again trapped either side of the actuatorpiston, effectively locking the piston in position and passingany feedback forces into the airframe structure through thecylinder barreltrHhni0n‘(5). .

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If, however, the rotor loads become very heavy, thedifferential relief valve (H) lifts and relieves any excessivepressure build-up. when this occurs, a small but damped feedbackis felt in the cockpit controls.

Qperatign of the servo actuapcrz The operation of this typeof actuator - see Fig. 9 - relies upon a small amount of playbetween the input (9) and the output (13). This play permitsthe pilot to move the slide of the slide and sleeve assembly (7)without moving the output (13). Immediately the slide is movedfrom its neutral position, hydraulic fluid is directed to theactuator piston (8) and, because the cylinder is anchored to theairframe at its trunnion (5), the whole assembly moves in thedirection of the displaced sleeve. As the assembly moves, itmoves its rotor head control and progressively returns the sleeveto neutral, thus stopping the movement. This is a simple followthrough action with the actuator piston trying to catch up withthe pilot input movement.

This system is sometimes called a SlOppy link systembecause of the essential clearance at the input control (9),which can be felt by the pilot, when there is no hydraulic pressure,as a slight backlash or slop in the system.

This servo actuator assists the pilot to move the control,allows full manual control should the hydraulic system fail and,through the action of valves 3, H, 6, and 10, stops rotor headforces being fed back to the pilot.

Figure 10 shows a typical installation of cyclic andcollective control actuators and Fig. ll shows a schematic layoutof the hydraulic system for these actuators. Note that thissystem provides assistance for controlling the tail rotor.

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Friction Controls

Lateral and longitudinal adjustable friction controls areprovided to enable the pilot to add some friction to the controlsystems to suit his individual feel for the control response.

The friction controls are also used on the ground to securethe controls while the helicopter is left unattended. Maintenancepersonnel also use the friction controls while maintaining thehelicopter. To a very limited extent, these friction controlsdampen rotor—head feedback forces.

Figure 12 shows typical lateral and longitudinal frictioncontrols for the cyclic control system. They consist of slottedlinks spanned by friction washers held against the faces ofthe link by a spring whose tension is adjusted by a knob on athreaded rod. The range of friction of these controls is usuallyfrom fully free to fully locked.

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Figure 13 shows a typical collective pitch friction control.This uses a slotted guide, friction discs, and a spring. Itis adjusted by an operating handle. As with the cyclic frictioncontrol, you must refer to the particular helicopter maintenancemanual before making any adjustments or replacements.

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Airworthiness Requirements, Volume 1, Leaflet F16.

Artificial Feel

Part of the reason for power assistance is to damp out thereciprocating forces of the rotor heads from the pilot's controlsTo do this totally is unsafe because the pilot needs feedbackfor effective control of the helicopter, which is basically anunstable flying machine.

Stability is achieved by fixing the controls in a positionselected momentarily by the pilot. Movement from that settingshould have a feel proportional to the size of the controlmovement.

555/3/2

_ 29 _

Feel may be provided by bungee or §o£Ee_g£§§§ent cylinders~ see Fig. lH ~ fitted between the control system and thehelicopter structure and, typically, operating in each of the

three control systems, or it may be provided for in the designof the servo actuators and their control valves.

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When a control of the flight system is moved in eitherdirection from neutral, it compresses a spring in the system'sforce gradient cylinder. The further the control is moved, themore the spring is compressed. This compression is transmittedto the pilot's hand control as proportional feedback or feel.

On release, the control tends to move back to neutral,that is, to where the spring in the force gradient cylinder isleast compressed.

Clearly, a pilot cannot be expected to fly for a long timeholding a control column against a spring, or a set of springs,as with the helicopter in an untrimmed condition.

Trimming Controls

Figure l2 shows a load-feel system, similar to that justdescribed, that incorporates a trim function.

Trimming through the force gradient spring is achieved bymoving the spring's point of attachment to the airframe. Ifthe control column is left to follow the trim movement, thewhole spring, housing, and control column moves in the directionof the trim force.

S55/3/2

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The electric motors used to operate the lateral andlongitudinal trim units are controlled by a four~way centreOFF switch on the hand grip of the cyclic control column.

The switch positions are Fwn

LEFT (I) RT

AFT

These switch postions indicate the direction in which therotor tilt takes the helicopter when trim is selected.

Briefly, on selection by the pilot, the trim motor, througha worm gear, drives the rack to which the load-feel spring isattached to extend it from or retract it into the trim housing.This alters the point of attachment of the load~feel spring,taking with it the main control unit.

with a motor driving through a worm and pinion, the positionis maintained, once set, and can be altered only by use of theelectric motor.

The lateral trim assembly shown in Fig. l2 is operatedby a rack and pinion. The longitudinal trim assembly is operatedby an arm. Figure l5 shows an exploded view of the lateral trimmotor and force gradient spring assembly.

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555/3/2

_ 22 _

Magnetic Brake

Another way of changing a force gradient spring setting iswith magnetic brakes or clutches that operate whenever the pilotpresses a trim button on the hand grip of the cyclic controlcolumn.

Until the force trim switch on the instrument control panelis selected ON, the helicopter's flying controls are free to movewithin the constraints of the range stops and with the feel of theservo actuators, if fitted, but without the feel and spring returngiven by the force gradient units.

Once the controls are satisfactorily set and the helicopteris in, say, a cruise attitude, the force trim switch can be used tomagnetically lock the rotary clutches that allow the forcegradient cylinders tethered ends to be adjusted.

rThe magnetic brake consists of a housing containing a solenoid

capped with a rotatable armature. The armature has attached to ita shaft to which the operating lever is clamped. To this operatinglever is attached the tethered end of a force gradient cylinder.

When the force trim switch is selected, all magnetic brakecircuits are energised, the armatures become part of the magneticfields, and the brakes are locked. \

Locking the brakes renders every control movement subject tothe compression of the force gradient spring. If any control move-ment is made, the appropriate force gradient cylinder has its springcompressed. On removal of the force on the control, the forcegradient cylinder returns to its original uncompressed length.

The trim button provides instant control of the trim. Thisbutton, when pressed, breaks the circuits to the magnetic brakes,thus allowing them to free their armatures.

This enables the controls to be reset by the pilot, withoutany restriction, to a new and desired position and there locked,the locks being made as soon as the trim button is released.

555/3/2

-23..

NOTE: The force trim switch, when selected, locks all

A force gradient cylinder and a trim assembly may be fittedin the tail rotor control circuit. This circuit may be servo

magnetic brakes, bringing all force gradientcylinders into action-

The force trim button releases all magnetic brakesmomentarily while control trim changes are made.

assisted. To prevent very fast rotation about the verticalaxes, which will damage the airframe, a snubber of some kind maybe fitted between the tail rotor pedals and the tail rotor servoactuator.

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SUMMARY

The angular changes of pitch forced on the main rotor bythe swash plate result in substantial reciprocal loadsin all parts of the control system-

The effect of the rotor load feedback can be greatlyreduced at the pilot's controls by several methods,notably

l. Power assistance,

2. Hydromechanical damping, and

3. Friction damping (to a lesser extent).

Any helicopter's controls may have any combination offriction damping, power assistance, and force gradientdamping.

All controls may also be trimmable.

The trimming method is to alter the location of the forcegradient springs‘ mountings, electrically or electro-magnetically, to remove any set of the spring from itsneutral or unloaded position.

Trim adjustment switches are invariably on the cycliccontrol column hand grip.

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PRACTICE EXERCISE B

State whether each of the following is true or false:

l. The trim switch is sited on the collective lever.

2. Force gradient devices may be fitted in allcontrol systems to give artificial feel.

3. Tail rotor controls are not servo assisted.

4. The slop in the input linkage of a follow throughactuator is desirable but not essentialfor its operation.

5. An irreversible valve causes a hydraulic lock toform across the servo piston.

(Answers on page 37)

THE COLLECTIVE LEVER AND THROTTLE CONTROL

Positioned for operation by the pilot's left hand, andsometimes known as the power lever or altitude control, the

collective-pitch control lever has two main functions:

1.

2.

It alters the pitch of all main rotor bladessimultaneously and equally in addition to anysettings imposed by the cyclic control.

It carries the engine power control as a twist grip,but it also has automatic connection to the power systemto increase or decrease engine power when thecollective pitch lever is raised or lowered duringpowered flight.

We saw in Fig. 5 how the collective lever is connected tothe main rotor control system, and on page 8 how its operationis superimposed on the cyclic control system through a mixingmechanism.

555/3/2

_ 25 _

in this part of the assignment, we shall see how thecollective and engine controls are interconnected.

A combination of balanced forces acts on a fixed-wingaircraft in straight and level flight at constant speed. If thepilot moves the aircraft out of this balanced state, forcesincrease or decrease, depending on the new attitude of theaircraft- To maintain a set airspeed in a turn, a pilot mustincrease power because any deflection of a control surface fromits faired position increases drag. The same is true of ahelicopter.

NOTE: The collective~pitch control lever is more oftencalled the collective lever or simply as thecollective.

If the helicopter must change direction, climb, or accelerate,or must decelerate rapidly, more power is needed than for straightand level flight at constant speed.

Extra power is needed more often when increasing altitude,but small increases are necessary to reduce sink when changingdirection and for acceleration.

So that power is readily to hand, the throttle lever isoperated by a twist grip on the end of the collective lever.This throttle control is often set to full power in flight, withmovement of the collective lever altering power as necessaryto maintain rotor rev/min.

The engine thus automatically provides the extra power to beabsorbed by the rotor blades when they are operating at a higherangle of attack than previously.

Although not mechanically trimmable, as are the cycliccontrols, the collective lever and the throttle have frictioncontrols that each pilot can adjust to suit his feel for thecontrol.

The helicopter throttle control is almost always a twistgrip, as on a motorcycle, on the forward end of the collective

555/3/2

_.26...

pitch lever. It is sometimes called the collective grip. Thetwist grip friction control's knurled ring is immediately belowthe twist grip. Rolling the twist grip away from the pilot opensthe throttle; rolling it towards the pilot closes the throttle.

Throttle Correlation

The power (throttle) control of both piston— and turbine-engined helicopters is designed so that

l. The pilot can start the engine, ground run it, andbring the engine and rotor rev/min up to theiroperating range without lifting the collectivelever;

2. As the pilot raises the collective lever to increaselift, so the engine power is automatically increasedand vice versa;

3. At full power (full throttle), the pilot may stillraise the collective lever without damaging thecontrol mechanism; and

H. The pilot may greatly reduce power in flight whileat the same time using the collective lever tocontrol the rate of descent of the helicopter.This is necessary for autorotation practice andfor any power—0ff-but-engine—still—running descent.

For these four requirements to be met, the fuel meteringsystem, piston engine or turbine, must have an input from boththe throttle twist grip and the collective lever.

Piston Engine Throttle (Power) Control

The carburettor or fuel injector unit is the only componentthat meters the fuel and air charge into the piston engine. Thismeans that the throttle twist grip control and the collectivelever have to be interconnected and the combined output appliedto the throttle butterfly shaft of the carburettor or fuelcontrol unit.

555/3/2

_ 27 i

This can be done by

1. A push-pull flexible control or rods from thecollective lever assembly to the engine; or

2. A specially designed cam linkage sited inthe control run between the collective leverassembly and the engine.

Figure 33 shows a collective lever and the pilot's throttletwist grip, and Fig. 16 shows an exploded view of the throttlecontrol housing and the installation of the collective leverassembly and push~pull flexible control assembly with its attach-ment to the servo control (fuel injection unit).

Rotating the throttle twist grip shown in Fig. 13 turns thethrottle interconnecting rod shown in Fig. 16 through a pinionand gearshaft assembly. (The drive for the co—pilot's twist gripis shown in A of Fig. 16.) Attached firmly to the throttle inter~connecting rod and gearshaft assembly is the throttle overridebellcrank. Supported and driven by the throttle override bell-crank through the legs of a spring is the throttle bellcrank,which is connected by a push-pull flexible control to the throttlearm of the servo control.

The four design needs set out on page 26 are satisfied by thesystem in Fig. 16 thus:

1. With the collective lever down, the throttletwist grip opens and closes the throttlebutterfly from fully closed to about fullthrottle. When the throttle is on its idlespeed stop, further rotation of the twistgrip compresses the spring sited between theoverride bellcrank and the throttle bellcrank.

2. If, when the throttle is partly opened, thecollective lever is raised, the throttle isopened further. This happens because raisingthe collective lever rotates the completecollective and throttle control assemblyabout its centre line, thus adding collectivelever movement to the existing'throttle setting.

3. At full throttle and with the collective leverraised, further collective lever upward move»ment is possible because the spring betweenthe throttle override bellcrank and the"throttle bellcrank is compressed.

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A. when the throttle twist grip is held closedthe collective lever can be raised and lowered withoutmoving the throttle arm off its idle stop becauseas the collective lever tries to open the throttle,the force applied by the pilot's hand to the throttlegrip keeps it closed, and when the collective islowered, the spring is compressed between the twobellcranks.

The range of movement of the throttle control can be increasedby screwing out the extension of the throttle bellcrank and vice~versa as moving the extension, in or out, alters the distance itsend connection moves for the same angular movement.

This system operates by superimposing collective levermovement on the throttle twist grip movement and by providing aspring gushion at each end of the throttle twist grip range toallow the collective lever movement to overrun it in safety.

Figure l7 shows a throttle and cam control system. Thethrottle linkage to the cam box is moved by a throttle twistgrip turning the extension (ll) through a bevel gear assembly.

The operation of this control system meets the four designneeds in the following ways:

l. with the collective lever down, the throttle twistgrip opens and closes the throttle butterfly fromfully closed to about a three—quarter open positionas the cam follower bearing (9) rides in the slot ofthe moving cam (7). The system is adjusted so thatwhen the throttle lever (3) is on the idle stopscrew (2), the cam follower bearing (9) is justentering the deadpQ$itiOnOf the cam slot towardsits closeg end. Further movement of the throttle twistgrip in a closing direction causes the cam followerbearing to move to the closed end of the cam slotwhile the throttle arm stays firmly on its idle stop.

2. If the throttle is partly opened and the collectivelever is raised, the throttle is opened further.This happens because raising the collective leverrotates the complete collective and throttle controlassembly about its centre line, thus addingcollective lever movement to the existing throttlesetting. The advantage of the cam slot is nowfelt because the cam slot shape can be made sothat it will add power (open the throttle),by the

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correct amount to suit a specific helicopterweight. This means that the throttle opening isautomatically and correctly changed whenever thecollective pitch blade angles are changed, but atonly one helicopter weight. At other weights, smallmovements of the throttle twist grip are needed tokeep engine power and main rotor rev/min properlymatched.

3. At full throttle with the collective lever raised,the throttle lever (3) is fully open and the camfollower bearing has just entered the dead positionof the cam slot at its open end. Further collectivemovement upward is still possible because the camfollower bearing (9) moves to the open end of thecam slot and the throttle lever (3) stays fully open.

H. When the throttle twist grip is held closed, thecollective lever can be raised from its full downposition without opening the throttle lever (3)appreciably at the same time. This is because thecam follower bearing is right against the closedend of the cam slot, and it has to move throughthe dead portion of the slot before it startsto move the throttle open.

In this system, movement of the cam (7) is determined bythe length of the extension (ll), and the co-ordination betweencollective lever and throttle lever movement is decided by theE5535 of the cam slot. The dead areas, or detents, asthey may becalled, at each end of the cam slot allow collective levermovement when the throttle lever is on either of its stops.

Turbine Engine Power Control

A fuel system for a turbo—shaft gas turbine engine consistsof two units.

They are

l. The fuel control, which is mechanically connectedto the throttle twist grip and is driven by thegas producer turbine (N1); and

2. The governor, which is mechanically connected to thecollective lever through a linear actuator and isdriven by the power turbine(Ng).

555/3/2

_ 39 _

In this system, the function of the throttle twist grip isto select on the fuel control,

l. Cut off,

2. Ground idle, or

3. Maximum power.

No allowance is made for the position of the collectivelever. Thus, no mechanical connection is made between thethrottle twist grip and the collective lever.

The function of the collective pitch lever is

l. To select the pitch angle of the main rotor blades,and

2. To instruct the governor to supply enough fuel forthe engine to meet the power demanded.

When set to maximum power, the fuel control computes theamount of fuel needed by the gas producer turbine and the fuelneeded to satisfy the maximum demand of the power turbine. Thisfuel is led to the governor, which computes the fuel to meet thepower demanded of the engine. The fuel needed is thensent to the engine through the £321 control cut-off valve.Excess fuel from the fuel control and the governor is led tothe inlet of the engine-driven fuel pump.

The design meets the four requirements on page 26

l. By starting and running the engine with the fuelcontrol set by the throttle twist grip at ground idle.

2. By selecting maximum power with the throttle twistgrip and raising the collective lever.

3. Because there is no mechanical connection betweenthe throttle and collective controls theycannot interfere one with the other.

H. By selecting ground idle with the throttle twistgrip and then using the collective lever in theusual manner.

Figure 18 shows a gas-turbine control system in schematic form

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The linear actuator' in the control run to the governorprovides a fine adjustment of engine power to engine speed. Itis electrically operated and is controlled by a centre-off two~wayswitch at the end of the collective lever. This i8 the beep SWiT0h-

To prevent accidental selection of cut-off, there is adetent button on the collective lever. To obtain cut—off, thepilot presses the detent button and then rolls the twist gripto the cut—off position.

CONTROL SYSTEMS MAINTENANCE

Control systems in both rotary and fixed-wing aircraft mustbe rigged and maintained to the instructions given in theirmaintenance manuals.

Note the following general points:

l. A complete system should move smoothly and easilyfrom stop to stop. The manufacturer may specifya maximum and/or minimum force to overcome stictionand another force to keep the system moving.

2. There must be no play (wear) anywhere in the systembecause this allows unwanted movement of one part ofthe system relative to another part.

3. Use only the correct type and length of attachmentbolts. Substitution of other bolts can causeweakened attachments and a loss of the freemovement of the system.

Q. Fit attachment bolts the correct way around. Onrotating parts, bolts usually head into wind.

5. when a torque is specified for a nut or bolt, applyit.

6. Do not substitute one form of safety locking for anotherunless this is specified. For example, do not use astiff nut in place of a castle nut and split pin.

7. Install split pins correctly. Single leg bendingis unacceptable.

8. Where lockwire is used, it should be of monelor stainless steel, not soft iron, copper, or brass.

555/3/2

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9.locking must be complete beforewhen you make adjustments to controls, all safety

a test flight ismade and preferably before any ground running is done

Do not secure electrical wiringmoves in a control system. Forsecure a cable of an electronicto the fixed swafiiplatecontrol

You must take especial care not

to any component thatexample, nevervibration analyserrods.

to leave tools, rags,or debris of any kind near any control run because,unlike a fixed—wing aircraft, which has naturalstability and can fly without, say, l00% elevatorcontrol, the helicopter is naturally unstableand each control system is vital for its safeflight. Tools and other obstacles may hinder controlmovement or even jam a system solidly.

. when you have adjusted or replaced a control systemcomponent, consider the effect this may have onanother system with which it is connected. forexample, a throttle adjustment could affect thecollective pitch control.

SAFETY OF PERSONNEL

Remember these points

l. Take care when working on or near a control systemthat it is not operated while your hands are closeto any part. The collective lever and the cycliccontrol column can exert considerable leverage,crushing a finger caught between a moving bellcrankand its support structure. This is even moreimportant to remember when the controls are hydraulicallyassisted.

During the course of maintenance, it is usual to turnthe main rotor. Even a very slowly turning main rotorhas enough power to crush a finger caught betweenthe fixed and rotating halves of the swamiplate.

555/3/2

r35-

? SUMMARYon piston—engined helicopters, the collective pitch Dand throttle twist grip controls are mechanically 1

j interconnected. i

I On turbine—powered helicopters, the pilot uses thei throttle twist grip to select one of three fuel 3~ flows on the gas producer fuel control unit. The ,

collective pitch lever schedules the power turbinegovernor unit as well as selecting the collectivepitch angles. In this installation, the collective §pitch lever and the throttle twist grip are not 1mechanically ihterconnected.

PRACTICE EXERCISE C

Match each of the items in the top list with itscorrect item in thebottom list, writing the numbersof the items in the box below. Use each item only once.

A. Governor

B. Fuel control unit

C. Carburettor

D. Linear actuator

Controlled by

1. The throttle twist grip detent button

2. The throttle twist grip

3. The throttle twist grip beep switch

4. The throttle twist grip and the collective lever

5. The collective lever

6. The cyclic control

Y Y1 ii D 3(Answer on page 37)

555/3/2