in a straight line through or between structural members of the aircraft. Fairleads should never de- straight line. flect the alignment of a cable more than 3° from a Checknut threaded rod end Tube Adjustable Rivets antifriction rod end Adjustable rod end clevis Control or push-pull rod Quadrant Rear spar Torque tube Cable Push-pull rod Horn Rudder aileron interconnect cables B Torque tube Centerline of airplane Rig pin hole Sector Bellcrank D Sector Shaft Control wheel Bearing Drum E Cable drum FIGURE 2-60. Flight control system mechanical linkages.
Pressure seals are installed where cables (or rods) move through pressure bulkheads. The seal grips tightly enough to prevent excess air pressure loss but not enough to hinder movement of the cable. Pressure seals should be inspected at regular intervals to determine that the retaining rings are in place. If a retaining ring comes off, it may slide along the cable and cause jamming of a pulley.
Pulleys are used to guide cables and also to so that the flange of the rod end is interposed between the ball race and the anchored end of the attaching pin or bolt as shown in figure 2-61.
Flange.
change the direction of cable movement. Pulley Anchored bearings are sealed, and need no lubrication other end than the lubrication done at the factory. Brackets fastened to the structure of the aircraft support the pulleys. Cables passing over pulleys are kept in place by guards. The guards are close-fitting to prevent jamming or to prevent the cables from slipping off when they slacken due to temperature vari- Peening ations.
MECHANICAL LINKAGE Various mechanical linkages connect the cockpit controls to control cables and surface controls.
These devices either transmit motion or change the direction of motion of the control system. The linkage consists primarily of control (push-pull) rods, torque tubes, quadrants, sectors, bellcranks, and cable drums.
Control rods are used as links in flight control systems to give a push-pull motion. They may be adjusted at one or both ends. View A of figure 2-60 shows the parts of a control rod, Notice that it consists of a tube having threaded rod ends. An adjustable antifriction rod end, or rod end clevis, attaches at each end of the tube. The rod end, or clevis, permits attachment of the tube to flight control system parts. The checknut, when tightened, prevents the rod end or clevis from loosening.
Control rods should be perfectly straight, unless designed to be otherwise, when they are installed.
The bellcrank to which they are attached should be checked for freedom of movement before and after attaching the control rods. The assembly as a whole should be checked for correct alignment, When the rod is fitted with self-aligning bearings, free rotational movement of the rods must be obtained in all It is possible for control rods fitted with bearings to become disconnected because of failure of the peening that retains the ball races in the rod end.
This can be avoided by installing the control rods FIGURE 2-61. Rod end flange interposed between the bearing race and the end of the attaching bolt.
Another alternative is to place a washer, having a larger diameter than the hole in the fange, under the retaining nut on the end of the attaching pin or bolt.
TORQUE TUBES Where an angular or twisting motion is needed in a control system, a torque tube is installed. View B of figure 2-60 shows how a torque tube is used to transmit motion in opposite directions.
Quadrants, bellcranks, sectors, and drums change direction of motion and transmit motion to parts such as control rods, cables, and torque tubes. The quadrant shown in figure 2-60B is typical of flight control system linkages used by various manufacturers. Figures 2-60C and 2-60D illustrate a bell.
crank and a sector. View E illustrates a cable drum. Cable drums are used primarily in trim tab systems. As the trim tab control wheel is moved clockwise or counterclockwise, the cable drum winds or unwinds to actuate the trim tab cables.
STOPS Adjustable and nonadjustable stops (whichever the case requires) are used to limit the throw-range or travel movement of the ailerons, elevator, and rudder. Usually there are two sets of stops for each 68
of the three main control surfaces, one set being located at the control surface, either in the snubber cylinders or as structural stops (figure 2-62), and the other at the cockpit control. Either of these may serve as the actual limit stop. However, those situated at the control surface usually perform this function. The other stops do not normally contact each other, but are adjusted to a definite clearance when the control surface is at the full extent of its travel. These work as over-ride stops to prevent stretching of cables and damage to the control system during violent maneuvers. When rigging control systems, refer to the applicable maintenance manual for the sequence of steps for adjusting these stops to limit the control surface travel.
through a cable system by a spring-loaded plunger (pin) that engages a hole in the control surface mechanical linkage to lock the surface. A spring connected to the pin forces it back to the unlock position when the cockpit control lever is placed in the "unlock" position. An over-center toggle linkage is used on some other type aircraft to lock the control surfaces.
Control surface locking systems are usually so designed that the throttles cannot be advanced until the control surfaces are unlocked. This prevents taking off with the control surfaces in the locked position.
A typical control lock for small aircraft consists of a metal tube that is installed to lock the control wheel and rudder pedals to an attachment in the cockpit. Such a system is illustrated in figure 2-63.
Stop Rudder horn FIcURE 2-62. Adjustable rudder stops. Lock assembly
CONTROL SURFACE SNUBBERS AND LOCKING
DEVICES Various types of devices are in use to lock the control surfaces when the aircraft is parked or moored. Locking devices prevent damage to the control surfaces and their linkages from gusts and high-velocity winds. Common devices that are in use are the internal locking brake (sector brake) spring-loaded plunger, and external control surface locks.
Internal Locking Devices The internal locking device is used to secure the ailerons, rudder, and elevator in their neutral positions. The locking device is usually operated FIGURE 2-63. Typical control lock assembl or small aircraft.
Control Surface Snubbers Hydraulic booster units are used on some aircraft to move the control surfaces. The surfaces are usually protected from wind gusts by snubbers incorporated into the booster unit. On some aircraft an auxiliary snubber cylinder is connected directly to the surface to provide protection. The snubbers hydraulically check or cushion control surface movement when the aircraft is parked. This prevents wind gusts from slamming the control surfaces into their stops and possibly causing damage.
External Control Surface Locks External control surface locks are in the form of channeled wood blocks. The channeled wood blocks slide into the openings between the ends of the movable surfaces and the aircraft structure. This locke the surfaces in neutral. When not in use, these locks are stowed within the aircraft.
Tension Regulators Cable tension regulators are used in some flight control systems because there is considerable difference in temperature expansion of the aluminum aircraft structure and the steel control cables. Some large aircraft incorporate tension regulators in the control cable systems to automatically maintain a given cable tension. The unit consists of a compression spring and a locking mechanism which allows the spring to make correction in the system only when the cable system is in neutral.
type designed for different kinds of cable, cable sizes, and cable tensions.
One type of tensiometer is illustrated in figure
2-64. With the trigger lowered, place the cable to be tested under the two anvils. Then close the trigger (move it up). Movement of the trigger pushes up the riser, which pushes the cable at right angles to the two clamping points under the anvils. The force that is required to do this is indicated by the dial pointer. As the sample chart beneath the illustration shows, different numbered risers are used with different size cables. Each riser has an identifying number and is easily inserted into the tensiometer.
Pointer lock Riser
AIRCRAFT RIGGING Control surfaces should move a certain distance Anvils in either direction from the neutral position. These movements must be synchronized with the movement of the cockpit controls. The flight control system must be adjusted (rigged) to obtain these requirements.
Generally speaking, the rigging consists of the following: (1) Positioning the flight control system in neutral and temporarily locking it there with rig pins or blocks, and (2) adjusting surface travel, Trigger system cable tension, linkages, and adjustable stops to the aircraft manufacturer's specifications. Sample only Example When rigging flight control systems, certain items of rigging equipment are needed. Primarily, this No. 1 Riser No. 2 No. 3 equipment consists of tensiometers, cable rigging Dia. 1/16 3/32 11/8 Tension 5/32 3/16 7/32 1/4 tension charts, protractors, rigging fixtures, contour Lb.
templates, and rulers. 12 16 21 30 12 20 19 23 29 40 17 26 22 32 Measuring Cable Tension 26 37 30 To determine the amount of tension on a cable, a tensiometer is used. When properly maintained, a tensiometer is 98% accurate. Cable tension is determined by measuring the amount of force needed to 120 46 50 make an offeet in the cable between two hardened steel blocks, called anvils. A riser or plunger is pressed against the cable to form the offset. Several manufacturers make a variety of tensiometers, each FICURE 2-64. Tensiometer.
In addition, each tensiometer has a calibration table (figure 2-64) which is used to convert the dial reading to pounds. (The calibration table is very similar to the sample chart shown below the illustration.) The dial reading is converted to pounds of tension as follows. Using a No. 2 riser (figure 2-64) to measure the tension of a 5/32-in.
diameter cable a reading of "30" is obtained. The actual tension (see calibration table) of the cable is 70 Ibs. Observing the chart, also notice that a No. 1 riser is used with 1/16, 3/32-, and 1/8-in. cable.
Since the tensiometer is not designed for use in measuring 7/32- or 1/4-in. cable, no values are shown in the No. 3 riser column of the chart.
When taking a reading, it may be difficult to see the dial. Therefore, a pointer lock is present on the tensiometer. Push it in to lock the pointer. Then remove the tensiometer from the cable and observe the reading. After observing the reading, pull the lock out and the pointer will return to zero.
Cable rigging tension charts (figure 2-65) are graphic tools used to compensate for temperature variations. They are used when establishing cable tensions in flight control systems, landing gear systems, or any other cable-operated systems.
To use the chart, determine the size of the cable that is to be adjusted and the ambient air temperature. For example, assume that the cable size is 1/8-in. in diameter, that it is a 7 x 19 cable, and the ambient air temperature is 85° F. Follow the 85° F. line upward to where it intersects the curve for 1/8-in. cable. Extend a horizontal line from the point of intersection to the right edge of the chart.
The value at this point indicates the tension (rigging load in pounds) to establish on the cable. The tension for this example is 70 lbs.
Surface Travel Measurement The tools for measuring surface travel primarily include protractors, rigging fixtures, contour templates, and rulers. These tools are used when rig.
ging flight control systems to assure that the desired travel has been obtained.
Values include 10 percent structural deflection Design limit rig load 340
• 320
• 300 280 Rigging 260 load in
• 240 pounds Cable sizes - 220 1/4 7 x 19 • 200 3/16 7 × 19 • 180
- 160 5/32 1 × 19 140 1/8 7 × 19-
• 120 3/32 7 × 7. - 100 1/16 7×7 - 80
- 60
• 40
• 20
- 0 -65-60-50-40-30-20-10 0 10 20 30 40 50 60 70 80 90 100 110 120130140150 160 Temperature in degrees fahrenheit FIGURE 2-65, Typical cable rigging chart.
Protractors are tools for measuring angles in degrees. Various types of protractors are used to determine the travel of flight control surfaces. One protractor that can be used to measure aileron, elevator, or wing flap travel is the universal propeller protractor. Notice that this protractor (figure 2-66) is made up of a frame, a disk, a ring, and two spirit levels. The disk and ring turn independ.
ently of each other and of the frame. (The corner spirit level is used to position the frame vertically when measuring propeller blade angle.) The center spirit level is used to position the disk when meas uring control surface travel. A disk-to-ring lock is provided to secure the disk and ring together when the zero on the ring vernier scale and the zero on the disk degree scale align. The ring-to-frame lock prevents the ring from moving when the disk is moved. Note that they start at the same point and advance in opposite directions. A double 10-part vernier is marked on the ring.
The procedure to use for operating the protractor to measure control surface travel is shown at the bottom of figure 2-66.
Rigging Fixtures and Contour Templates Rigging fixtures and templates are special tools (gages) designed by the manufacturer to measure control surface travel. Markings on the fixture or template indicate desired control surface travel.
Rulers In many instances the aircraft manufacturer gives the travel of a particular control surtace in degrees and inches. If the travel in inches is provided, a ruler can be used to measure surface travel in inches.
RIGGING CHECKS The purpose of this section is to explain the methods of checking the relative alignment and adjustment of an aircraft's main structural compo.
nents. It is not intended to imply that the procedures are exactly as they may be in a particular aircraft. When rigging an aircraft, always follow the procedures and methods specified by the aircraft manufacturer.
Structural Alignment The position or angle of the main structural components is related to a longitudinal datum line parallel to the aircraft center line and a lateral datum line parallel to a line joining the wing tips. Before checking the position or angle of the main components, the aircraft should be leveled.
Small aircraft usually have fixed pegs or blocks attached to the fuselage parallel to or coincident with the datum lines. A spirit level and a straight edge are rested across the pegs or blocks to check the level of the aircraft. This method of checking aircraft level also applies to many of the larger types of aircraft. However, the grid method is sometimes used on large aircraft. The grid plate (figure 2-67) is a permanent fixture installed on the aircraft floor or supporting structure. When the aircraft is to be leveled, a plumb bob is suspended from a predetermined position in the ceiling of the aircraft over the grid plate. The adjustments to the jacks necessary to level the aircraft are indicated on the grid scale. The aircraft is level when the plumb bob is suspended over the center point of the grid.
Certain precautions must be observed in all instances. Normally, rigging and alignment checks should not be undertaken in the open. If this cannot be avoided, the aircraft should be positioned with the nose into the wind.
The weight and loading of the aircraft should be exactly as described in the manufacturer's manual.
In all cases, the aircraft should not be jacked until it is ensured that the maximum jacking weight (if any) specified by the manufacturer is not exceeded.
With a few exceptions, the dihedral and incidence angles of conventional modern aircraft cannot be adjusted. Some manufacturers permit adjusting the wing angle of incidence to correct for a wing-heavy condition. The dihedral and incidence angles should be checked after hard landings or after experiencing abnormal flight loads to ensure that the components are not distorted and that the angles are within the specified limits.
There are several methods for checking structural alignment and rigging angles. Special rigging boards which incorporate, or on which can be placed, a special instrument (spirit level or clinome.
ter) for determining the angle are used on some aircraft. On a number of aircraft the alignment is checked using a transit and plumb bobs or a theo- 72
Ring vernier scale Disk degree scale 30 20 10 0 20 30 Ring adjuster Center spirit level
- Disk-to-ring lock on ring engages only when zeros on scales are aligned.
Disk - Ring-to-frame lock Corner spirit level on frame folded in 1 With disk-to-ring lock in the deep slot, turn disk adjuster to lock disk to ring.
2 Move control surface to neutral. Place protractor on control surface and turn ring adjuster to center bubble in center spirit level (ring must be unlocked from frame).
3 Lock ring to frame with ring-to-frame lock.
4 Move control surface to extreme limit of movement 5 Unlock disk from ring with disk-to-ring lock.
6 Turn disk adjuster to center bubble in center spirit level.
7 Read surface travel in degrees on disk and tenths of a degree on vernier scale.
FIcuRE 2-66. Using the universal propeller protractor to measure control surface travel.
dolite and sighting rods. The particular equipment (1) Wing dihedral angle.
manuals. to use is usually specified in the manufacturer's (2) Wing incidence angle.
(3) Engine alignment.
When checking alignment, a suitable sequence (4) Horizontal stabilizer incidence.
should be developed and followed to be certain that (5) Horizontal stabilizer dihedral.
the checks are made at all the positions specified. (6) Verticality of the fin.
The alignment checks specified usually include: (7) A symmetry check.
Aft Lower nose jack Raise right jack Raise left jack Lower left jack Lower right jack Raise nose jack Scale reading one unit per inch of jack movement FIGURE 2-67. Typical grid plate.
Checking Dihedral The dihedral angle should be checked in the specifed positions using the special boards provided by the aircraft manufacturer. If no such boards are available, a straight edge and a clinometer can be used. The methods for checking dihedral are shown in figure 2-68.
It is important that the dihedral be checked at the positions specified by the manufacturer. Certain portions of the wings or horizontal stabilizer may sometimes be horizontal or, on rare occasions, anhedral angles may be present.
Checking Incidence Incidence is usually checked in at least two specified positions on the surface of the wing to ensure Straight edge and Special dihedral board with adjustable level spirit level incorporated.
FIGURE 2-68. Checking dihedral.
that the wing is free from twist. A variety of incidence boards are used to check the incidence angle.
Some have stops at the forward edge which must be placed in contact with the leading edge of the wing.
Others are equipped with location pegs which fit into some specified part of the structure. The purpose in either case is to ensure that the board is fitted in exactly the position intended. In most instances, the boards are kept clear of the wing contour by short extensions attached to the board. A typical incidence board is shown in figure 2-69.
Bubble level..
Straight edge and adjustable level Stop - Incidence board Chord line FIGURE 2-69. A typical incidence board.
When used, the board is placed at the specified locations on the surface being checked. If the incidence angle is correct, a clinometer on top of the board will read zero, or within a specified tolerance of zero. Modifications to the areas where incidence boards are located can affect the reading. For example, if leading-edge deicer boots have been installed, this will affect the position taken by a board having a leading edge stop.
Checking Fin Verticality After the rigging of the horizontal stabilizer has been checked, the verticality of the vertical stabilizer relative to the lateral datum can be checked.
The measurements are taken from a given point on either side of the top of the fin to a given point on the left and right horizontal stabilizers (figure 2-70). The measurements should be similar within prescribed limits. When it is necessary to check the String or tape measure lateral datum FIGURE 2-70. Checking fin verticality.
alignment of the rudder hinges, remove the rudder and pass a plumb bob line through the rudder hinge attachment holes. The line should pass centrally through all the holes. It should be noted that some aircraft have the leading edge of the vertical fin offset to the longitudinal center line to counteract engine torque.
Checking Engine Alignment Engines are usually mounted with the thrust line parallel to the horizontal longitudinal plane of symmetry. However, this is not always true when the engines are mounted on the wings. Checking to ensure that the position of the engines, including any degree of offset, is correct depends largely on the type of mounting. Generally, the check entails a measurement from the center line of the mounting to the longitudinal center line of the fuselage (figure 2-71) at the point specified in the applicable manual.
Symmetry Check The principle of a typical symmetry check is illustrated in figure 2-71. The precise figures, tolerances and checkpoints for a particular aircraft will be found in the applicable service or maintenance manual.
On small aircraft the measurements between points are usually taken using a steel tape. When measuring long distances, it is suggested that a spring scale be used with the tape to obtain equal tension. A 5-lb. pull is usually sufficient.
Where large aircraft are concerned, the positions where the dimensions are to be taken are usually chalked on the floor. This is done by suspending a plumb bob from the checkpoints, and marking the floor immediately under the point of each plumb bob. The measurements are then taken between the center of each marking.
ADJUSTMENT OF CONTROL SURFACES In order for a control system to function properly, it must be correctly adjusted. Correctly rigged control surfaces will move through a prescribed are (surface-throw) and be synchronized with the movement of the cockpit controls.
Rigging any system requires that the step-by-step procedures be followed as outlined in the aircraft maintenance manual. Although the complete rigging procedure for most aircraft is of a detailed nature that requires several adjustments, the basic method follows three steps:
FIGURE 2-71. A typical method of checking aircraft symmetry.
(1) Lock the cockpit control, bellcranks, and the neutral (no trim) position, and the surface tab the control surfaces in the neutral posi- is usually adjusted to streamline with the control tion. surface. However, on some aircraft the trim tabs
(2) Adjust the cable tension, maintaining the rudder, elevators, or ailerons in the neutral position.
(3) Adjust the control stops to limit the control surface travel to the dimensions given for the aircraft being rigged.
The range of movement of the controls and control surfaces should be checked in both directions from neutral.
The rigging of the trim tab systems is performed in a similar manner. The trim tab control is set to may be offset a degree or two from streamline when in the "neutral" position. After the tab and tab contro. are in the neutral position, adjust the control cable tension.
Pins, usually called rig pins, are sometimes used to simplify the setting of pulleys, levers, bellcranks, etc., in their neutral positions. A rig pin is a small metallic pin or clip. When rig pins are not provided, the neutral positions can be established by means of alignment marks, by special templates, or by taking linear measurements.
If the final alignment and adjustment of a system are correct, it should be possible to withdraw the rigging pins easily. Any undue tightness of the pins in the rigging holes indicates incorrect tensioning or misalignment of the system.
After a system has been adjusted, the full and synchronized movement of the controls should be checked. When checking the range of movement of the control surface, the controls must be operated from the cockpit and not by moving the control surfaces. During the checking of control surface travel, ensure that chains, cables, etc., have not reached the limit of their travel when the controls are against their respective stops. Where dual controls are installed, they must be synchronized and function satisfactorily when operated from both positions.
Trim tabs and other tabs should be checked in a manner similar to the main control surfaces. The tab position indicator must be checked to see that it functions correctly. If jackscrews are used to actuate the trim tab, check to see that they are not extended beyond the specified limits when the tab is in its extreme positions.
After determining that the control system functions properly and is correctly rigged, it should be thoroughly inspected to determine that the system is correctly assembled, and will operate freely over the specified range of movement. Make certain that all turnbuckles, rod ends, and attaching nuts and bolts are correctly safetied.
The flight control units located in the cockpit (figure 2-72) of all helicopters are very nearly the same. All helicopters have either one or two of each of the following: (1) Collective pitch control, (2) cyclic pitch control, and (3) directional control pedals. Basically, these units do the same things, regardless of the type of helicopter on which they are installed. But this is where most of the similarity ends. The operation of the systems in which these units are installed varies greatly according to the helicopter model.
Cyclic control stick - Controls attitude and direction of flight Throttle (Motorcycle-Grip type). - Controls RPM Collective pitch stick - Controls altitude Pedals - Maintain heading FICURE 2-72. Controls of the helicopter and the principal function of each.
Rigging the helicopter coordinates the movements of the flight controls and establishes the relationship between the main rotor and its controls and between the tail rotor and its controls. Rigging is not a difficult job, but it requires great precision and attention to detail. Strict adherence to rigging procedures is a must. Adjustments, clearances, and tolerances must be exact.
Rigging of the various flight control systems can be broken down into three major steps.
(1) Step one consists of placing the control system in a particular position; holding it 77 in position with pins, clamps, or jigs; and adjusting the various linkages to fit the immobilized control component.
(2) Step two consists of placing the control surfaces in a specific reference position;
using a rigging jig (figure 2-73), a precision bubble protractor, or a spirit level to check the angular difference between the control surface and some fixed surface on the aircraft.
(3) Step three consists of setting the maximum range of travel of the various components.
Main rotor rigging protractor FIcURE 2-73. A typical rigging protractor.
This adjustment limits the physical movement of the control system.
After completion of the static rigging, a functional check of the flight control system must be accomplished. The nature of the functional check will vary with the type of rotorcraft and system concerned, but usually includes determining that:
(1) The direction of movement of the main and tail rotor blades is correct in relation to movement of the pilot's controls.
(2) The operation of interconnected control systems (engine throttle and collective pitch) are properly coordinated.
(3) The range of movement and neutral position of the pilot's controls are correct.
(4) The maximum and minimum pitch angles of the main rotor blades are within specified limits. This includes checking the fore-and-aft and lateral cyclic pitch and collective pitch blade angles.
(5) The tracking of the main rotor blades is correct.
(6) In the case of multi-rotor aircraft, the rigging and movement of the rotor blades are synchronized.
(7) When tabs are provided on main rotor blades, they are correctly set.
(8) The neutral, maximum and minimum pitch angles and coning angles of the tail rotor blades are correct.
(9) When dual controls are provided, they function correctly and in synchronization.
Upon completion of rigging, a thorough check should be made of all attaching, securing, and pivot points. All bolts, nuts, and rod ends should be properly secured and safetied.
Blade Tracking When the main rotor blades do not "cone" by the same amount during rotation, it is referred to as "out of track." This may result in excessive vibratjon at the control column. Blade tracking is the process of determining the positions of the tips of the rotor blade relative to each other while the rotor head is turning, and of determining the corrections necessary to hold these positions within certain tolerances. Tracking shows only the relative position of the blades, not their path of flight. The blades should all track one another as closely as possible. The purpose of blade tracking is to bring the tips of all blades into the same tip path throughout their entire cycle of rotation.
In order to track rotor blades with minimum time and maximum accuracy, the correct equipment must be available. The equipment generally used to track blades includes:
(1) Tracking flag with flag material.
(2) Grease pencils or colored chalk.
(3) Suitable marking material.
(4) Reflectors and tracking lights (figure 2-74).
(5). Tracking stick.
(6) Trim-tab bending tool.
(7) Trim-tab angle indicator.
Before starting a blade tracking operation, new or recently overhauled blades should be checked for proper incidence. Trim tabs should be set at zero on new or overhauled blades. Trim tabs of blades in service should not be altered until blade track has been determined.
One means of tracking is the flag tracking method (figure 2-75). The blade tips are marked with chalk or grease pencil. Each blade tip should be marked with a different color so that it will be easy to determine the relationship of the tips of the rotor blades to each other. This method can be used on all types of helicopters that do not have jet propulsion at the blade tips. The man holding the flag faces in the direction of blade rotation, watching the retreating blades. Facing away from the oncoming blades permits the flag holder to observe the blades as they come in contact with the flag.