He Wharekura-tin-i Kaihautu 0 Aotearoa
THE 0 P E N P0l.YTE(HN|( OF NEWZEALAND
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Basic Helicopter Flying Controls 555—3—2
CONTENTS
Basic Flying Controls Cyclic Control Collective Control Control Mixing Tail Rotor Control
1 3 7 3 10
Control Damping Rotor Head Feedback Forces Power Assistance Friction Controls Artificial Feel Trimming Controls Magnetic Brake
1n 1n 15 13 19 20 22
The Collective Lever and Throttle Control Throttle Correlation
Zn 26
Piston Engine Throttle (Power) Control Turbine Engine Power Control Control Systems Maintenance Safety of Personnel
26 31 3H 35
COPYRIGHT
This material is for the sole use of enrolled students and may not be reproduced without the written authority of the Principal, TOPNZ
555/3/2
_ AIRCRAFT ENGINEERING
HELICOPTERS
ASSIGNMENT 2 BASIC FLYING CONTROLS
All aircraft have three controllable axes: Longitudinal »~ Pitch Lateral - Roll Normal - Yaw A control gives movement about each axis: Longitudinal - Ailerons Lateral - Elevators Normal —- Rudder The ailerons and elevators are controlled through the control column or handwheel, and the rudder is controlled through pedals. Rotating the handwheel clockwise or counterclockwise or leaning the control column to the right or left, causes the aircraft, through the effect of ailerons, to roll to the right or to the left. By pulling the control column or handwheel towards him, the pilot raises the nose of the aircraft by raising the elevator's trailing edge; to force the nose down, he pushes the control column forward, away from him. Pushing the right rudder pedal forward brings the nose of the aircraft round to the right, and pushing the left pedal forward brings the nose to the left. To sustain ahalancedturn requires a combination of each of these controls, the ratio of force or movement of each depending on the characteristics of the aircraft and the rate of the turn.
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Forces acting on a helicopter in a turn.
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The pilot starts a turn by banking using the ailerons, and he then keeps the nose smoothly moving round, without raising or dipping it, by a combination of elevator and rudder movement. Generally, the steeper the turn, the more he moves each control to maintain balance. Figure l shows the force distribution needed during the turn to keep the aircraft from returning to its stable straight and level original attitude.
A helicopter is controlled in flight by much the same interaction of controls and forces. In this assignment, we shall consider a helicopter's flying and powerplant controls in some detail.
Cyclic Control The equivalent of aileron control on a helicopter is sideways movement of the cyclic stick, which, by angular change of the rotor blades‘ pitch, leans the rotor disc in the direction of movement desired.
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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Cyclic control
The interesting thing about this control is that the blade angle is changed when the blades are fore and aft, which results in rotor disc tilt about 90° later, that is, when the blades are athwartships of the aircraft. The helicopter then moves in the direction of the low~pitched blade. Figure 3 shows a relatively simple main rotor control mechanism with the longitudinal and collective sections of the linkages hatched for reference. Sideways movement of the cyclic control column (10) moves the pilot's lateral control rod (1%) lengthwise, pivoting the lateral bellorank (ll) about its rear RH corner. This pushes or pulls
555/3/2
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the lower lateral control rod (15), whose movement is transferred through links and bellcranks to the end of the lateral pitch mixer bellcrank (6).
This bellcrank is pivoted so that as it rocks under the influence of the control rod, the left and right upper lateral control rods (H and 5) rock the non~rotating section of the swash plate athwartships, appropriately changing rotor blade angles through the pitch control links (1). .---\
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The swash plate allows interconnection between the lower control system, which does not rotate, and the upper control system, rotating with the rotor head. Any tilt of the lower swash plate is transmitted to the upper swash plate through a very substantial ball or roller bearing, necessary because the upper swash plate changes rotor blade angularity every rotor revolution.
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We discuss swash plates in some detail in another assignment _ Thus, lateral control, like aileron control, is by a tilt athwartships. The resulting tilt on the rotor disc causes the helicopter to bank, like a fixed-wing aircraft. Figure H shows longitudinal control, which is by fore and aft movement of the cyclic control column. The system shown in Fig. H is from the same helicopter as that shown in Fig. 3. r;-\ 4\\\
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A forward movement of the stick rotates the cyclic control torque tube (9), the arm on the torque tube brings the control rod (8) forward, and the linkages and rods thereon result in an upward movement of the upperlongitudinal control rod, tilting the lower (or stationary) swash plate forward. Again, this movement of the cyclic stick has its effect on the pitch of the main rotor blades, changing each blade through the sequence from coarse through fine and back to coarse again
555/3/2
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with each main rotor head revolution, fine pitch in this case being
over the nose of the helicopter. This movement of the longitudinal control system acts in addition to and is superimposed upon lateral stick movement-
Within the limits of the control stops, full lateral movement can have imposed on it full longitudinal movement, but such extreme selections 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 much vibration and very rapid wear.
Later in this course we have an assignment on vibration in helicopters. The main rotor can be tilted by the two control systems just
described, but superimposed on the cyclic system is the collective control system.
Collective Control As Fig. S shows, the pilot has, at his left hand, a lever that he raises to increase the pitch of all rotor blades together and lowers to reduce the pitch of all the rotor blades together. Lifting this collective pitch lezei
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Thus, through the pitch control rods, an increase of the pitch of all blades equally and together is made while maintaining their cyclic relationship. Superimposing collective control movements on cyclic control movements and vice versa is called control mixing.
Control Mixing Figure 6 shows a demonstration example of a one~levered mixing unit. It consists of lever A connected at pivot A to the airframe structure and connected by rod A to the collective pitch lever so that, in this example, raising the collective lever raises the free end of the lever up as it turns about pivot A.
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Lever B is carried on lever A and is pivoted between the attachment points of rods B and C, that is, lever B see~ saws about its pivot point, B.
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B to, say, the cyclic fore~and-aft control and by rod C to the stationary swamiplate. In this example, moving the cyclic control column forward moves rod B down, thus moving rod C up. When the collective lever is held fixed in one position and
the cyclic control column is moved fore and aft, lever B seesaws about pivot B. When the collective lever is raised and the cyclic control column is held fixed in one position, then as lever A is raised, the pivot point of lever B moves to the attachment point of rod B and rod C is raised. Thus, a movement of the collective lever has been added to the fore-and-aft cyclic control without the cyclic control column moving. ln practice, a lateral lever (or levers) is added to lever A so that collective lever movement is added to both thefore»and-aft and lateral cyclic controls. The reverse of the movements happens when the collective lever is lowered.
Of course, the cyclic control column can be
moved and its motion transmitted in the normal way while the
collective lever is being moved. The mixing unit shown in Fig. 7_is taken from an S55 helicopter All mixing units work on the same principle but their levers and bellcranks are not usually the simple shapes shown in Fig. 7.
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Tail Rotor Control The tail rotor is the equivalent of the rudder of a fixedwing aircraft This small rotor provides: l.
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Directional control when the helicopter is hovering, and
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Control of the torque reaction of the main rotor in all stages of flight.
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Figure 8 shows a helicopter tail rotor system, from the rudder pedals to the bellcrank that operates the tail rotor pitch change 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 cable loop to a bellcrank in the aft of the centre section. From this aft bellcrank, a push-pull rod runs to the arm on the tail rotor swash plate bellcrank. On this bellcrank is a pin that moves the nonerotating portion of the swash plate in and out 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 balancing the slight sideways drift caused by the tail rotor.
The cable system, accessible through the right side of the fuselage, is carried round the cargo/passenger compartment. Bellcranks are on bearings, and pulleys are bushed so that the system 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—wing aircraft.
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The mixing unit superimposes collective control on the cyclic control movements and vice versa. -
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PRACTICE EXERCISE A
State whether each of the following is true or false: 1.
The tail rotor counteracts main rotor torque only when the helicopter is hovering.
2
A forward and leftward movement of the cyclic control column moves the nose of the helicopter down and to the left.
3.
Forward movement of the right rudder pedal moves the nose of the helicopter to the left.
4
Raising the collective lever causes the helicopter to climb.
5.
The cyclic, collective, and tail rotor control movements 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 blades assembly is correctly rigged, adjusted, and balanced, there are few or no forces fed back from the rotor to the pilot‘s controls. Thus, no devices are needed to absorb or prevent any forces being felt by the pilot. ln the semi rigid and heavier articulated rotor helicopters with their correspondingly larger rotor»head forces, the effort needed to move the controls would quickly tire the pilot. In these helicopters, hydraulic power assistance is used in both the cyclic and collective controls. Built into the power assistance (servo units) are valves that prevent rotor»head forces being fed back to the pilot when'Uuahydraulic system is inoperative. Futhermope, the hydraulic lock formed either side of the servo piston when the hydraulic system is in operation also stops feedback forces. The hydraulic system is usually powered by a pump driven by the main rotor gearbox so that, in the event of engine failure, hydraulic servo assistance is still available during autorotation. If the hydraulic system should fail, the pilot would still be able to fly the helicopter in complete safety although, as we said earlier, this would be tiring. The rotor forces of the bigger helicopters are so large that manual control is impossible. These helicopters have two separate hydraulic~powered servo systems. One system is driven by the main rotor gearbox and the other is driven by the engine(s). The two systems are connected electrically and so if the on—line system 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 describe
the operation of valves 3 and l0, which together form aninreversible valve,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 the upper spring holds the valve seat (ll) down. This action stops the flow of fluid to the return port (1) and so no hydraulic fluid can now leave the unit unless the valve seat (ll) moves up to relieve the pressure caused by heating of the fluid. If, with the sequence valve
(8) closed, the pilot's control
input (9) is moved, the hydraulic fluidtrappedin the servo unit is displaced from one side of the actuator piston to the other by flowing through the slide and sleeve assembly (7). When control input stops, the slide and sleeve takes up a central position and hydraulic fluid is again trapped either side of the actuator piston, effectively locking the piston in position and passing any feedback forces into the airframe structure through the cylinder barreltrHhni0n‘(5). ' ’ ' ”‘ ’“ "
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555/3/2
_ 15 _
If, however, the rotor loads become very heavy, the differential relief valve (H) lifts and relieves any excessive pressure build-up. when this occurs, a small but damped feedback is felt in the cockpit controls. Qperatign of the servo actuapcrz The operation of this type of actuator - see Fig. 9 - relies upon a small amount of play between the input (9) and the output (13).
This play permits
the pilot to move the slide of the slide and sleeve assembly (7) without moving the output (13).
Immediately the slide is moved
from its neutral position, hydraulic fluid is directed to the actuator piston (8) and, because the cylinder is anchored to the airframe at its trunnion (5), the whole assembly moves in the direction of the displaced sleeve. As the assembly moves, it moves its rotor head control and progressively returns the sleeve to neutral, thus stopping the movement. This is a simple follow through action with the actuator piston trying to catch up with the pilot input movement. This system is sometimes called a SlOppy link system because 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 head forces being fed back to the pilot. Figure 10 shows a typical installation of cyclic and collective control actuators and Fig. ll shows a schematic layout of the hydraulic system for these actuators. Note that this system provides assistance for controlling the tail rotor.
555/3/2
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Friction Controls Lateral and longitudinal adjustable friction controls are
provided to enable the pilot to add some friction to the control systems to suit his individual feel for the control response. The friction controls are also used on the ground to secure the controls while the helicopter is left unattended. Maintenance personnel also use the friction controls while maintaining the helicopter.
To a very limited extent, these friction controls
dampen rotor—head feedback forces. Figure 12 shows typical lateral and longitudinal friction controls for the cyclic control system. They consist of slotted links spanned by friction washers held against the faces of
the link by a spring whose tension is adjusted by a knob on a threaded rod. The range of friction of these controls is usually from fully free to fully locked.
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Cyclic system friction controls:
Figure 13 shows a typical collective pitch friction control. This uses a slotted guide, friction discs, and a spring.
is adjusted by an operating handle.
It
As with the cyclic friction
control, you must refer to the particular helicopter maintenance manual before making any adjustments or replacements. 555/3/2
_ 19 _
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Whenever the friction controls have been worked on, a
_du piicate inspection nest be made to satisfy New Zealand Civil Airworthiness Requirements, Volume 1, Leaflet F16. Artificial
Feel
Part of the reason for power assistance is to damp out the
reciprocating forces of the rotor heads from the pilot's controls To do this totally is unsafe because the pilot needs feedback for effective control of the helicopter, which is basically an unstable flying machine.
Stability is achieved by fixing the controls in a position selected momentarily by the pilot. Movement from that setting should have a feel proportional to the size of the control movement.
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 the helicopter structure and, typically, operating in each of the three control systems, or it may be provided for in the design
of the servo actuators and their control valves.
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When a control of the flight system is moved in either direction from neutral, it compresses a spring in the system's force gradient cylinder. The further the control is moved, the more the spring is compressed. This compression is transmitted to 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 is least compressed. Clearly, a pilot cannot be expected to fly for a long time holding 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 just described, that incorporates a trim function. Trimming through the force gradient spring is achieved by moving the spring's point of attachment to the airframe. If the control column is left to follow the trim movement, the whole spring, housing, and control column moves in the direction of the trim force.
S55/3/2
W Q1 _ The electric motors used to operate the lateral and longitudinal trim units are controlled by a four~way centre OFF 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 the rotor tilt takes the helicopter when trim is selected. Briefly, on selection by the pilot, the trim motor, through a worm gear, drives the rack to which the load-feel spring is attached 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 position is maintained, once set, and can be altered only by use of the electric motor. The lateral trim assembly by a rack and pinion. by an arm.
shown in Fig. l2 is operated
The longitudinal trim assembly is operated
Figure l5 shows an exploded view of the lateral trim
motor and force gradient spring assembly.
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555/3/2
_ 22 _
Magnetic Brake Another way of changing a force gradient spring setting is with magnetic brakes or clutches that operate whenever the pilot presses a trim button on the hand grip of the cyclic control column. Until the force trim switch on the instrument control panel is selected ON, the helicopter's flying controls are free to move within the constraints of the range stops and with the feel of the servo actuators, if fitted, but without the feel and spring return given by the force gradient units. Once the controls are satisfactorily set and the helicopter is in, say, a cruise attitude, the force trim switch can be used to magnetically lock the rotary clutches that allow the force gradient cylinders tethered ends to be adjusted. r The magnetic brake consists of a housing containing a solenoid capped with a rotatable armature.
The armature has attached to it
a shaft to which the operating lever is clamped. To this operating lever is attached the tethered end of a force gradient cylinder. When the force trim switch is selected, all magnetic brake circuits are energised, the armatures become part of the magnetic
fields, and the brakes are locked.
\
Locking the brakes renders every control movement subject to the compression of the force gradient spring. If any control movement is made, the appropriate force gradient cylinder has its spring compressed. On removal of the force on the control, the force gradient cylinder returns to its original uncompressed length. The trim button provides instant control of the trim.
This
button, 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, without any 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 magnetic brakes, bringing all force gradient cylinders into actionThe force trim button releases all magnetic brakes momentarily while control trim changes are made.
A force gradient cylinder and a trim assembly may be fitted in the tail rotor control circuit.
This circuit may be servo
assisted. To prevent very fast rotation about the vertical axes, which will damage the airframe, a snubber of some kind may be fitted between the tail rotor pedals and the tail rotor servo actuator.
SUMMARY
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1
The angular changes of pitch forced on the main rotor by the swash plate result in substantial reciprocal loads in all parts of the control systemThe effect of the rotor load feedback can be greatly
reduced at the pilot's controls by several methods, notably l.
Power assistance,
2.
Hydromechanical damping, and
3.
Friction damping (to a lesser extent).
I |
Any helicopter's controls may have any combination of friction damping, power assistance, and force gradient
damping.
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All controls may also be trimmable.
The trimming method is to alter the location of the force gradient springs‘ mountings, electrically or electromagnetically, to remove any set of the spring from its neutral or unloaded position. Trim adjustment switches are invariably on the cyclic control column hand grip.
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555/3/2
._2L|.....
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 all
control systems to give artificial feel. 3.
Tail rotor controls are not servo assisted.
4.
The slop in the input linkage of a follow through actuator is desirable but not essential for its operation.
5.
An irreversible valve causes a hydraulic lock to
form across the servo piston.
(Answers on page 37)
THE COLLECTIVE LEVER AND THROTTLE CONTROL
Positioned for operation by the pilot's left hand, and sometimes known as the power lever or altitude control, the collective-pitch control lever has two main functions: 1.
It alters the pitch of all main rotor blades simultaneously and equally in addition to any settings imposed by the cyclic control.
2.
It carries the engine power control as a twist grip, but it also has automatic connection to the power system to increase or decrease engine power when the collective pitch lever is raised or lowered during powered flight.
We saw in Fig. 5 how the collective lever is connected to the main rotor control system, and on page 8 how its operation is superimposed on the cyclic control system through a mixing mechanism.
555/3/2
_ 25 _
in this part of the assignment, we shall see how the collective and engine controls are interconnected.
A combination of balanced forces acts on a fixed-wing aircraft in straight and level flight at constant speed. If the pilot moves the aircraft out of this balanced state, forces increase or decrease, depending on the new attitude of the aircraft- To maintain a set airspeed in a turn, a pilot must increase power because any deflection of a control surface from its faired position increases drag.
The same is true of a
helicopter. NOTE:
The collective~pitch control lever is more often called the collective lever or simply as the collective.
If the helicopter must change direction, climb, or accelerate, or must decelerate rapidly, more power is needed than for straight and level flight at constant speed. Extra power is needed more often when increasing altitude, but small increases are necessary to reduce sink when changing direction and for acceleration. So that power is readily to hand, the throttle lever is operated by a twist grip on the end of the collective lever.
This throttle control is often set to full power in flight, with movement of the collective lever altering power as necessary to maintain rotor rev/min. The engine thus automatically provides the extra power to be absorbed by the rotor blades when they are operating at a higher angle of attack than previously. Although not mechanically trimmable, as are the cyclic controls, the collective lever and the throttle have friction controls that each pilot can adjust to suit his feel for the
control. The helicopter throttle control is almost always a twist grip, as on a motorcycle, on the forward end of the collective 555/3/2
_.26...
pitch lever.
It is sometimes called the collective grip.
The
twist grip friction control's knurled ring is immediately below the twist grip. Rolling the twist grip away from the pilot opens the throttle; rolling it towards the pilot closes the throttle.
Throttle Correlation The power (throttle) control of both piston— and turbineengined helicopters is designed so that l.
The pilot can start the engine, ground run it, and bring the engine and rotor rev/min up to their operating range without lifting the collective lever;
2.
As the pilot raises the collective lever to increase lift, so the engine power is automatically increased and vice versa;
3.
At full power (full throttle), the pilot may still raise the collective lever without damaging the control mechanism; and
H.
The pilot may greatly reduce power in flight while at the same time using the collective lever to control the rate of descent of the helicopter. This is necessary for autorotation practice and for any power—0ff-but-engine—still—running descent.
For these four requirements to be met, the fuel metering system, piston engine or turbine, must have an input from both the throttle twist grip and the collective lever.
Piston Engine Throttle (Power) Control The carburettor or fuel injector unit is the only component that meters the fuel and air charge into the piston engine.
This
means that the throttle twist grip control and the collective lever have to be interconnected and the combined output applied to the throttle butterfly shaft of the carburettor or fuel control unit.
555/3/2
_ 27 i
This can be done by 1.
A push-pull flexible control or rods from the collective lever assembly to the engine; or
2.
A specially designed cam linkage sited in the control run between the collective lever assembly and the engine.
Figure 33 shows a collective lever and the pilot's throttle twist grip, and Fig.
16 shows an exploded view of the throttle
control housing and the installation of the collective lever assembly and push~pull flexible control assembly with its attachment to the servo control (fuel injection unit). Rotating the throttle twist grip shown in Fig. 13 turns the throttle interconnecting rod shown in Fig. 16 through a pinion and gearshaft assembly. (The drive for the co—pilot's twist grip is shown in A of Fig. 16.)
Attached firmly to the throttle inter~
connecting rod and gearshaft assembly is the throttle override bellcrank. Supported and driven by the throttle override bellcrank through the legs of a spring is the throttle bellcrank, which is connected by a push-pull flexible control to the throttle arm of the servo control. The four design needs set out on page 26 are satisfied by the system in Fig. 16 thus: 1.
With the collective lever down, the throttle twist grip opens and closes the throttle butterfly from fully closed to about full throttle.
When the throttle is on its idle
speed stop, further rotation of the twist grip compresses the spring sited between the override bellcrank and the throttle bellcrank. 2.
If, when the throttle is partly opened, the collective lever is raised, the throttle is
opened further.
This happens because raising
the collective lever rotates the complete
collective and throttle control assembly about its centre line, thus adding collective lever movement to the existing'throttle setting. 3.
At full throttle and with the collective lever raised, further collective lever upward move» ment is possible because the spring between the throttle override bellcrank and the" throttle bellcrank is compressed.
555/3/2
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Collective and throttle control assembly
555/3/2
comm.
_ 29 i
A.
when the throttle twist grip is held closed the collective lever can be raised and lowered without moving the throttle arm off its idle stop because as the collective lever tries to open the throttle,
the force applied by the pilot's hand to the throttle grip keeps it closed, and when the collective is lowered, the spring is compressed between the two bellcranks. The range of movement of the throttle control can be increased by screwing out the extension of the throttle bellcrank and vice~ versa as moving the extension, in or out, alters the distance its end connection moves for the same angular movement. This system operates by superimposing collective lever movement on the throttle twist grip movement and by providing a
spring gushion at each end of the throttle twist grip range to allow the collective lever movement to overrun it in safety. Figure l7 shows a throttle and cam control system.
The
throttle linkage to the cam box is moved by a throttle twist grip turning the extension (ll) through a bevel gear assembly. The operation of this control system meets the four design needs in the following ways: l.
with the collective lever down, the throttle twist grip opens and closes the throttle butterfly from fully closed to about a three—quarter open position as the cam follower bearing (9) rides in the slot of the moving cam (7). The system is adjusted so that when the throttle lever (3) is on the idle stop screw (2), the cam follower bearing (9) is just entering the deadpQ$itiOnOf the cam slot towards its closeg end.
Further movement of the throttle twist
grip in a closing direction causes the cam follower bearing to move to the closed end of the cam slot
while the throttle arm stays firmly on its idle stop. 2.
If the throttle is partly opened and the collective lever is raised, the throttle is opened further. This happens because raising the collective lever rotates the complete collective and throttle control assembly about its centre line, thus adding collective lever movement to the existing throttle setting. The advantage of the cam slot is now felt because the cam slot shape can be made so that it will add power (open the throttle),by the
555/3/2
- 30
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0 1. CONTROL CABLE. 2. IDLE STOP SCREW. . 3.TH HOTTLE LEVER. 4.CAM SHAFT AND LEVEFI. 5.PUSH-PULL ROD. 6_CAM FOLLOWEH LEVER 7. CAM. 8, CAM FOLLOWEH. 9. CAM FOLLOWER BEARING. 10. CAM BOX. 11, EXTENS!ON. 12.THROTTLE CONTROL SHAFT. 13. COLLECTIVE TORQUE TUBE. 14 COLLECTIVE PITCH LEVER AND THROTTLE CONTROL
Collective and throttle (cam) control assembly
555/3/2
—
31 —
correct amount to suit a specific helicopter weight. This means that the throttle opening is automatically and correctly changed whenever the collective pitch blade angles are changed, but at
only one helicopter weight. At other weights, small movements of the throttle twist grip are needed to keep engine power and main rotor rev/min properly matched.
3.
At full throttle with the collective lever raised, the throttle lever (3) is fully open and the cam
follower bearing has just entered the dead position of the cam slot at its open end. Further collective movement upward is still possible because the cam follower bearing (9) moves to the open end of the cam slot and the throttle lever (3) stays fully open. H.
When the throttle twist grip is held closed, the collective lever can be raised from its full down position without opening the throttle lever (3) appreciably at the same time. This is because the cam follower bearing is right against the closed end of the cam slot, and it has to move through the dead portion of the slot before it starts to move the throttle open.
In this system, movement of the cam (7) is determined by the length of the extension (ll), and the co-ordination between collective lever and throttle lever movement is decided by the E5535 of the cam slot.
The dead areas, or detents, asthey may be
called, at each end of the cam slot allow collective lever movement when the throttle lever is on either of its stops.
Turbine Engine Power Control A fuel system for a turbo—shaft gas turbine engine consists of two units. They are l.
The fuel control, which is mechanically connected to the throttle twist grip and is driven by the gas producer turbine (N1); and
2.
The governor, which is mechanically connected to the collective lever through a linear actuator and is driven by the power turbine(Ng).
555/3/2
_ 39 _
In this system, the function of the throttle twist grip is to select on the fuel control, l.
Cut off,
2.
Ground idle, or
3.
Maximum power.
No allowance is made for the position of the collective lever. Thus, no mechanical connection is made between the throttle 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 for
the engine to meet the power demanded. When set to maximum power, the fuel control computes the amount of fuel needed by the gas producer turbine and the fuel needed to satisfy the maximum demand of the power turbine. This fuel is led to the governor, which computes the fuel to meet the power demanded of the engine. The fuel needed is then sent to the engine through the £321 control cut-off valve. Excess fuel from the fuel control and the governor is led to the 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 fuel control set by the throttle twist grip at ground idle.
2.
By selecting maximum power with the throttle twist grip and raising the collective lever.
3.
Because there is no mechanical connection between the throttle and collective controls they cannot interfere one with the other.
H.
By selecting ground idle with the throttle twist grip and then using the collective lever in the usual manner.
Figure 18 shows a gas-turbine control system in schematic form 555/3/2
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The linear
actuator'
in the control run to the governor
provides a fine adjustment of engine power to engine speed. It is electrically operated and is controlled by a centre-off two~way switch at the end of the collective lever. This i8 the beep SWiT0hTo prevent accidental selection of cut-off, there is a detent button on the collective lever.
To obtain cut—off, the
pilot presses the detent button and then rolls the twist grip to the cut—off position.
CONTROL SYSTEMS MAINTENANCE
Control systems in both rotary and fixed-wing aircraft must be rigged and maintained to the instructions given in their maintenance manuals. Note the following general points: l.
A complete system should move smoothly and easily from stop to stop. The manufacturer may specify a maximum and/or minimum force to overcome stiction and another force to keep the system moving.
2.
There must be no play (wear) anywhere in the system because this allows unwanted movement of one part of the system relative to another part.
3.
Use only the correct type and length of attachment bolts. Substitution of other bolts can cause weakened attachments and a loss of the free
movement of the system. Q.
Fit attachment bolts the correct way around. On rotating parts, bolts usually head into wind.
5.
when a torque is specified for a nut or bolt, apply it.
6.
Do not substitute one form of safety locking for another unless this is specified. For example, do not use a stiff nut in place of a castle nut and split pin.
7.
Install split pins correctly.
Single leg bending
is unacceptable.
8.
Where lockwire is used, it should be of monel or stainless steel, not soft iron, copper, or brass.
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when you make adjustments to controls, all safety locking must be complete before a test flight is made and preferably before any ground running is done
9.
lU.
ll.
12
.
Do not secure electrical wiring moves in a control system. For secure a cable of an electronic to the fixed swafiiplatecontrol
to any component that example, never vibration analyser rods.
You must take especial care not to leave tools, rags, or debris of any kind near any control run because, unlike a fixed—wing aircraft, which has natural stability and can fly without, say, l00% elevator control, the helicopter is naturally unstable and each control system is vital for its safe flight. Tools and other obstacles may hinder control movement or even jam a system solidly. when you have adjusted or replaced a control system component, consider the effect this may have on another system with which it is connected. for example, a throttle adjustment could affect the collective pitch control.
SAFETY OF PERSONNEL
Remember these points l.
Take care when working on or near a control system that it is not operated while your hands are close to any part. The collective lever and the cyclic control column can exert considerable leverage, crushing a finger caught between a moving bellcrank and its support structure. This is even more important to remember when the controls are hydraulically assisted.
2.
During the course of maintenance, it is usual to turn the main rotor. Even a very slowly turning main rotor has enough power to crush a finger caught between the fixed and rotating halves of the swamiplate.
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SUMMARY on piston—engined helicopters, the collective pitch and throttle twist grip controls are mechanically interconnected.
j I
On turbine—powered helicopters, the pilot uses the throttle twist grip to select one of three fuel flows on the gas producer fuel control unit. The collective pitch lever schedules the power turbine governor unit as well as selecting the collective pitch angles. In this installation, the collective pitch lever and the throttle twist grip are not mechanically ihterconnected.
i ~
PRACTICE EXERCISE C Match each of the items in the top list with its
correct item in thebottom list, writing the numbers of 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
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