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.
Strobe light.
Reflectors FicuRE 2-74. Blade tracking with strobe light.
The angle of the flag to the chord of the blade is important. If the angle is too great, the marks will be long and the flag will flutter excessively. If the angle is too straight, the blade may cut the flag.
The most satisfactory angle is about 80° to the chord line of the blade. The marks on the flag will then be approximately 3/16 in. to 1/4 in. long. The flag method of tracking can be used not only to ascertain the relative positions of the blades but also the flight characteristics of the blades at different r.p.m. and power settings.
In order to plot the flight characteristics of a set of blades, it is necessary to take a trace at different
r.p.m. settings and record the results. A minimum of three traces is necessary to produce a satisfactory plot. Four traces are desirable to produce a plot on heads having three or more rotor blades. When the tracking plot is completed, one blade is chosen as a reference blade. Usually, the reference blade is the center blade of a plot on a multi-blade rotor system and the lower blade on a two-blade rotor system. If the center or lower blade of a plot shows unusual flight characteristics, another blade may be chosen as the reference blade. A blade track that rises with an increase in r.p.m. is a climbing blade; one that lowers with increase of r.p.m. or power is a diving blade. When a climbing blade and a diving blade cross, it is termed a crossover. Because of the climbing and diving tendencies of imporperly rigged blades, it is possible to have all blades at a common point at certain r.p.m. and power settings, but out of track at other r.p.m. or power settings.
The most common error in blade tracking is to bring the blades into track with trim tabs at cruise
r.p.m. only. The blades may then be at the meeting point of a crossover and will spread at different
r.p.m. power settings, or forward speed; an out-oftrack condition will result. The correct tracking procedure is to maintain a constant blade spread at all r.p.m. power settings and flight speeds. A constant spread can be held only by proper adjustment of trim tabs. After a constant spread has been established with trim tabs, it is necessary to bring the blade tips into a single path of rotation with the pitch links. Bending the trim tabs up will raise the blade and bending them down will lower the blade.
Bending of trim tabs should be kept to a minimum because tab angle produces excessive drag on the blades. The setting of the tabs on main rotor blades (if provided) should be checked to eliminate outof-balance moments which will apply torque to the rotor blades. The tab setting is checked for correctness by running the rotor at the prescribed speed and ensuring that the cyclic-pitch control column remains stationary. Out-of-balance moments impart a stirring motion to the column.
Leading edge- Curtain 70° to 80° Blade Blade Curtain
- Pole Position of chalk mark (approximately 2 inches long) Line parallel to longitudinal axis of helicopter Pole Handle Curtain 1/2" max spread (typical) Approximate position of chalk marks Pole • FIGURE 2-75. Tracking.
PRINCIPLES OF BALANCING OR RE-BALANCING The principles that are essential in the balancing or re-balancing of the control surfaces are not too difficult to understand if some simple comparison is used. For example, a seesaw that is out of balance may be compared to a control surface that does not have balance weights installed, as in figure 2-76.
From this illustration, it is easy to understand how a control surface is naturally tail (trailing edge) heavy.
It is this out-of-balance condition that can cause a damaging flutter or buffeting of an aircraft and therefore must be eliminated. This is best accomplished by adding weights either inside or on the leading edge of the tabs, ailerons, or in the proper location on the balance panels. When this is done properly, a balanced condition exists and can be compared to the seesaw with a child sitting on the short end of the plank.
The effects of moments on control surfaces can be easily understood by a closer observation and study of a seesaw and two children of different weights 80
Hinge center line
UNBALANCED CONDITION Seesaw Chord line Balance weight Control surface BALANCED CONDITION Hinge center line FIGURE 2-76. (A) Unbalanced, (B) Balanced conditions.
seated in different positions thereon. Figure 2-77 illustrates a seesaw with an 80-lb. child seated at a distance of 6 ft. from the fulcrum point of the seesaw. The weight of the child tends to rotate the seesaw in a clockwise direction until it touches the ground. To bring the seesaw into a level or balanced condition, a child is placed on the opposite end of the seesaw. The child must be placed at a point equal to the moment of the child on the left side of the seesaw.
• 6 ft.
80 lb. child Weight x distance = Moment 80 lb. × 6 feet = 480 ft. Ibs. = Moment FIGURE 2-77, Moment.
Assume that the child is placed a distance of 8 ft.
to the right of the fulcrum point. A simple formula can be used to determine the exact weight that the child must have to balance or bring the seesaw into a leveled condition.
To produce a balanced condition of the seesaw (or control surface), the counterclockwise moment must equal the clockwise moment. Moment is found by multiplying weight times distance. Therefore, the formula to balance the seesaw is:
W= X D = W1 X D1.
W2 would be the unknown weight of the second child. Dy would be the distance (in feet) from the fulcrum that the second child is seated (8). Wi would be the weight of the first child (80 lbs.). D1 would be the distance (in feet) from the fulcrum that the first child is seated (6).
Finding the weight of the second child is now a simple matter of substitution and solving the formula as follows:
W½ X D½ = W1 X D1 We X 8 = 80 Ibs. X 6 480 Ibs.
W2 = Wz = 60 Ibs.
So the weight of the second child would have to be 60 lbs. To prove the formula:
60 Ibs. X 8 ft. = 80 Ibs. X 6 ft.
480 ft. Ibs. = 480 ft. Ibs.
This would result in a balanced condition of the seesaw since the counterclockwise moments around the fulcrum are equal to the clockwise moments around the fulcrum.