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.
22/46 The same effect is obtained in a control surface by the addition of weight. Since most of the repairs (A) to control surfaces are aft of the hinge center line, resulting in a trailing-edge-heavy condition, the weight is added forward of the hinge center line.
The correct re-balance weight must be calculated and installed in the proper position.
Chord line Re-balancing of Movable Surfaces The material in this section is presented for fami- Tail down underbalance liarization purposes only, and should not be used when re-balancing a control surface. Explicit in- Plus (+ ) condition structions for the balancing of control surtaces are given in the service and overhaul manuals for the specific aircraft and must be followed closely.
Any time repairs on a control surface add weight fore or aft of the hinge center line, the control Chord line surface must be re-balanced. Any control surface that is out of balance will be unstable and will not remain in a streamlined position during normal flight. For example, an aileron that is trailing-edge heavy will move down when the wing deflects up- (B) ward, and up when the wing deflects downward.
Such a condition can cause unexpected and violent maneuvers of the aircraft. In extreme cases, futtering and buffeting may develop to a degree that could cause the complete loss of the aircraft.
Re-balancing a control surface concerns both Nose down overbalance static and dynamic balance. A control surface that is statically balanced will also be dynamically balanced. Minus (-) condition Static Balance Static balance is the tendency of an object to remain stationary when supported from its own Chord line center of gravity. There are two ways in which a control surface may be out of static balance. They are called underbalance and overbalance.
When a control surface is mounted on a balance stand, a downward travel of the trailing edge below the horizontal position indicates underbalance, Some manufacturers indicate this condition with a plus (+) sign. Figure 2-784 illustrates the underbalance condition of a control surface.
An upward movement of the trailing edge, above the horizontal position (figure 2-78B), indicates overbalance. This is designated by a minus (-) sign. These signs show the need for more or less weight in the correct area to achieve a balanced control surface as shown in figure 2-78C.
A tail-heavy condition (static underbalance) causes undesirable fight performance and is not (C) Level-horizontal position Balanced condition FIGURE 2-78. Control surface static balance.
usually allowed. Better flight operations are gained by nose heaviness static overbalance. Most manu- 82
facturers advocate the existence of nose-heavy con• trol surfaces. Inboard hinge fitting Dynamic Balance Dynamic balance is that condition in a rotating body wherein all rotating forces are balanced within themselves so that no vibration is produced while the body is in motion. Dynamic balance as related to control surfaces is an effort to maintain balance when the control surface is submitted to movement on the aircraft in flight, It involves the placing of weights in the correct location along the span of the surfaces. The location of the weights will, in most cases, be forward of the hinge center line.
RE-BALANCING PROCEDURES Requirements Repairs to a control surface or its tabs generally increase the weight aft of the hinge center line, requiring static re-balancing of the control surface Outboard hinge fitting system as well as the tabs. Control surfaces to be rebalanced should be removed from the aircraft and supported, from their own points, on a suitable stand, jig, or fixture (figure 2-79).
Trim tabs on the surface should be secured in the neutral position when the control surface is mounted on the stand. The stand must be level and be located in an area free of air currents. The control surface must be permitted to rotate freely about the hinge points without binding. Balance condition is determined by the behavior of the trailing edge when the surface is suspended from its hinge points. Any excessive friction would result in a false reaction as to the overbalance or underbalance of the surface. Section A - A or jig, a neutral position should be established with When installing the control surface in the stand FIGURE 2-79. Field type halancing jigs.
the chord line of the surface in a horizontal position (figure 2-80). Use a bubble protractor to de- Hinge center line Bubble protractor termine the neutral position before continuing balancing procedures. Sometimes a visual check is all Chord line that is needed to determine whether the surface is balanced or unbalanced.
Any trim tabs or other assemblies that are to remain on the surface during balancing procedures should be in place. If any assemblies or parts must be removed before balancing, they should be removed.
METHODS At the present time, four methods of balancing Support stand FIGURE 2-80. Establishing a neutral position.
(re-balancing) control surfaces are in use by the various manufacturers of aircraft. The four methods are commonly called the calculation method, 83
scale method, trial weight (trial and error) method, and component method.
The calculation method of balancing a control surface is directly related to the principles of balancing discussed previously. It has one advantage over the other methods in that it can be performed without removing the surface from the aircraft.
In using the calculation method, the weight of the material from the repair area and the weight of the materials used to accomplish the repair must be known. Subtracting the weight removed from the weight added will give the resulting net gain in the mount added to the surface.
The distance from the hinge center line to the center of the repair area is then measured in inches.
This distance must be determined to the nearest one-hundredth of an inch (figure 2-81).
The next step is to multiply the distance times the net weight of the repair. This will result in an in.-lbs. (inch-pounds) answer. If the in.-lbs. result of the calculations is within specified tolerances, the control surface will be considered balanced. If it is Bubble protractor Trim tab Rudder Measurement in inches Hinge center line repair area Center of Chord line FIcURE 2-81. Calculation method measurements.
not within specified limits, consult the manufacturer's service manuals for the needed weights, material to use for weights, design for manufacture, and installation locations for addition of the weights.
The scale method of balancing a control surface requires the use of a scale that is graduated in hundredths of a pound. A support stand and balancing jigs for the surface are also required. Figure 2-82 illustrates a control surface mounted for rebalancing purposes. Use of the scale method requires the removal of the control surface from the aircraft.
Hinge center line
- Adjustable support Support stand Mounting bracket Weight scale FICURE 2-82. Balancing setup.
CHAPTER 3
AIRCRAFT FABRIC COVERING AIRCRAFT FABRICS marking of specification numbers along the selvage General edges to permit identification of the fabric in the Most aircraft in production today are of all-metal field.
construction. However, many aircraft in service still The following definitions are presented to simuse fabric for covering wings, fuselages, and con- plify the discussion of fabrics. Some of these terms trol surfaces. In the United States cotton fabrics are shown graphically in figure 3-3.
have long been the standard material for covering (1) Warp-The direction along the length of aircraft. Cotton fabrics are still used, but other fabrics, such as linen, Dacron, and fiber glass, are (2) Warp ends-The woven threads that run gaining in popularity. the length of the fabric.
Organic and synthetic fibers are used in the man- (3) Filling, woof, or weft-The direction ufacture of fabrics or cloth for covering aircraft. across the width of the fabric.
The organic fibers include cotton and linen; the (4) Count-The number of threads per inch synthetic fibers include fiber glass and heat-shrinka- in warp or filling.
ble synthetic fibers. (5) Ply-The number of yards making up a Three of the most common heat-shrinkable syn- thread.
thetic fibers available are a polyamide, manufac- (6) Bias-A cut, fold, or seam made diagotured and marketed under the trade name Nylon; nally to the warp or fill threads.
an acrylic fiber called Orlon; and a polyester fiber (7) Calendering-The process of ironing known as Dacron. fabric by threading it wet between a se- Fabric Quality and Strength Requirements ries of hot and cold rollers to produce a smooth finish.
In the original manufacture of a fabric-covered (8) Mercerization-The process of dipping aircraft, the quality and strength of the fabric, sur- cotton yarn or fabric in a hot solution of face tape, lacing cord, thread, etc., are determined takes diluted caustic soda. This treatment by the aircraft's never-exceed speed and the pounds causes the material to shrink and acquire per square foot of wing loading. The never-exceed greater strength and luster.
speed for a particular aircraft is that safe speed (9) Sizing-Material, such as starch, used to beyond which it should never be operated. The stiffen the yarns for ease in weaving the aircraft wing loading is determined by dividing its total wing planform area (in square feet) into the (10) Pinked edge-An edge which has been maximum allowable gross weight. cut by machine or shears in a continuous All fabric, surface tape, reinforcing tape, machine series of V's to prevent raveling.
grade aircraft textile material. The materials must thread, lacing cord, etc., used for re-covering or repairing an aircraft's cover should be of high- (11) Selvage edge-An edge of cloth, tape, or webbing woven to prevent raveling.
also be at least as good a quality and of equivalent Cotton Fabrics strength as those originally used by the aircraft Grade A airplane cloth is a 4-oz. mercerized manufacturer. fabric made of high-grade, long-staple cotton. It is Acceptable fabrics for covering wings, control calendered to reduce the thickness and lay the nap surfaces, and fuselages are listed in figures 3-1 and so that the surface will be smooth. There are from
3-2. Fabrics conforming to AMS (Aeronautical 80 to 84 threads per in., warp and fill. The mini- Material Specifications), incorporate a continuous mum tensile strength is 80 Ibs. per in. of width, 85
Minimum Minimum tentearing sile strength Thread count Materials Specification strength new deteriorated per inch Use and remarks (undoped) (undoped) Airplane cloth Society Auto- 5 pounds 56 pounds 80 mini- For use on all airmercerized motive En- warp and per inch. mum, 84 craft. Required on cotton gineers AMS fill. maximum aircraft with wing (Grade "A"). 3806 (TSO- warp and loadings greater C15 refer- fill. than 9 p.s.f. Reences this quired on aircraft spec.). with placarded never-exceed speed greater than 180 m.p.h.
MIL-C-5646 Alternate to AMS
• 3806.
Airplane cloth MIL-C-5643 Alternate to MIL-C- cellulose 5648 or AMS 3806 nitrate pre- (undoped), Finish doped. with cellulose nitrate dope.
Airplane cloth MIL-C-5642 Alternate to MIL-C.
cellulose 5646 or AMS 3806 acetate (undoped). Finish butyrate, with cellulose acepredoped. tate butyrate dope.
Airplane cloth Society Auto- 4 pounds 48 pounds 80 minimum, For use on aircraft mercerized motive En- with wing loadings cotton. gineers AMS of 9 p.s.f. or less.
3804 (TSO- C14 references this spec.).
Airplane cloth 2F777 mercerized cotton.
Aircraft linen.. British 7F1 This material meets the minimum strength requirements of TSO- Ç15.
Figure 3-1. Textile fabrics used in covering aircraft.
Materiais Specification Yarn Size Minimum ten- Yards pe Use and remarks sile strength pounc Reinforcing tape, MIL-T-5661 150 pounds Used as reinforcing tape on fabric cotton. per one. and under rib lacing cord.
half-inch Strength of other widths approx.
width in proportion.
Lacing waxed braidedpre- MIL-C-5040 ... cord, 80 pounds 310 mini- Lacing fabric to structures.
double. mum. less already waxed, must be cotton. lightly waxed before using.
Lacing cord, special U.S. Army No. 20/3/3/3........ 85 pounds
6-27. double.
Lacing cord, braided MIL-C-5848 80 pounds 170 minicotton single mum.
Lacing cord thread; MIL-T-6779 9 ply 59 pounds 620 minilinen and linen single. mum.
hemp. 11 ply .. 70 pounds 510 minisingle. mum.
Lacing cord thread; MIL-T-5880 ... Ticket No. 62 pounds 480 minihigh-tenacity cot- single.
Machine thread cor- Federa. V.T- 20/4 ply. .. 5 pounds 5,000 nor- Use for all machine sewing.
Hand sewing thread V-T-276b. 8/4 ply. .... 14 pounds 1,650 nor- Use for all hand sewing. Use fully single. waxed thread.
Surtace tape cotton MIL-1-5083 ... 80 Ibs/in. Use over seams, leading edges, trai (made from AN- ing edges, outer edges and ribs, C-121). pinked, scalloped or straight edges.
Surface tape cotton. Same as fabric ... Same as Alternate to MIL-T-5083.
used. fabric used.
PECURE 3-2 Miscellaneous textile materials.
warp and fill. The term "4 ounce" indicates that the normal weight of the finished cloth is 4 oz./sq. yd.
for 36. and 42-in. widths. Fabric of this grade and weight is acceptable for covering any aircraft fabric surface.
Linen Fabrics Unbleached linen fabric is used extensively in England and to a limited degree in the United States. This fabric is practically identical to Grade 87
Pinked edge Warp 1 Woof pigs Selvage edge Selvage edge FIGURE 3-3. Fabric terms.
A cotton fabric insofar as weight, strength, and threads per inch are concerned.
Dacron Fabrics Dacron is a very smooth monofilament, polyester fiber manufactured by the condensation of dimethyl terephthalate and ethylene glycol. A generally standard style and weight of Dacron cloth has evolved for use as aircraft covering. It is a plain weave with a weight of about 3.7 oz./sq. yd. This heavy-duty fabric has a tensile strength of approximately 148 Ibs./in. and can be used to replace Grade A cotton and linen fabrics.
A fine weave, medium weight Dacron fabric is used when a minimum covering weight and a very smooth finish are desired. The medium weight fabric has a tensile strength of approximately 96 lbs./in., weighs about 2.7 oz./sq. yd., and can also be used as a replacement for Grade A cotton fabric.
Glass Cloth Glass cloth, or fiber glass cloth, is made from fine-spun glass filaments which are woven into a strong, tough fabric. Glass cloth used for aircraft covering has a plain weave and weighs about 4.5 oz./sq. yd.
Glass cloth is not affected by moisture, mildew, chemicals, or acids. It is also fire resistant.
Glass cloth applications generally fall into the following classes:
(1) Class A is a complete or partial reinforcement of a serviceable fabric covering. No direct structural attachment of the glass cloth is provided. This composite covering is considered airworthy until the underly.
ing conventional fabric deteriorates below the values listed in figure 3-1.
(2) Class B is a reinforcement of a fabric covering wherein the glass cloth is provided with the same direct structural attachment as that used with the original covering. This composite covering is considered airworthy until the underlying conventional fabric has deteriorated to a strength less than 50% of the minimum tensile strength values for new fabric listed in figure 3-1.
(3) Class C is replacement coverings applied either independently or over a conventional covering. The glass covering should possess all the necessary characteristics for airworthiness and is in no way dependent upon the underlying covering if one is present.
MISCELLANEOUS TEXTILE MATERIALS Surface Tape Surface tape is the finishing tape that is doped over each rib or seam to cover the stitching and provides a neat, smooth, finished appearance. It can be obtained with serrated or pinked edges, or with a straightedge impregnated with a sealing compound. The compound-impregnated edges or pinked edges provide better adhesion to the fabric covering- Surface tape is made from Grade A fabric in various widths from 1¼ to 5" and from glider fabric in 1½ and 6" widths. Cotton surface tape may be used with Grade A cotton, linen, or Dacron fabric. Surface tape is also available in Dacron fabric, which should be the first choice when covering an aircraft with Dacron fabric. Linen surface tape frequently is used with fiber glass covering, especially for covering screwheads. If fiber glass tape is used, it is difficult to remove the irregularities caused by the screwbeads.
Using linen tape to cover screwheads gives a smooth, finished appearance.
Surface tape or finishing tape should be placed over all lacing, seams (both machine- and handsewn), corners, edges, and places where wear is likely to occur. Two-inch tape generally is used for this purpose. Pinked surface tape is sometimes applied over the trailing edges of control surfaces and airfoils. For such application the tape must be at least 3 inches in width and if the aircraft "neverexceed speed" is greater than 200 mph, notch the tape at equal intervals not to exceed 18" between notches. Notching of trailing edge is unnecessary if the never exceed speed is under 200 mph. If the tape begins to separate from the trailing edge, it will tear at a notched section and thereby prevent loosening of the entire strip.
Tape is applied over a second wet coat of dope which is applied after the first coat has dried. Another coat of dope is applied immediately over the tape. The tape adheres firmly to the covering because both surfaces of the tape are impregnated with dope.
Reinforcing Tape Reinforcing tape is used over ribs between the fabric covering and the rib stitching to prevent the stitching cord from cutting through the fabric. It is also used for cross-bracing ribs and for binding.
Reinforcing tape is fabricated from cotton, Dacron, fiber glass, or linen materials. A tape made from fiber glass on acetate with a pressure-sensitive adhesive is also available.
Reinforcing tape is available in a variety of widths conforming to the different widths of ribs or rib capstrips. The tape should be slightly wider than the member it covers. A double width is sometimes necessary for very wide members.
Reinforcing tape is used under all lacing to protect the fabric from cuts. This tape should be under a slight tension and secured at both ends. For wings with plywood or metal leading edge covering, the reinforcing tape is extended only to the front spar on the upper and lower surfaces.
Sewing Thread Thread is made with a right or left twist that is identified by various terms. Machine, machine twist, left twist, or Z-twist indicates a left-twist thread;
S-twist indicates a right-twist thread.
An unbleached silk-finish, left-twist cotton thread is used to machine sew cotton fabrics. Silk-finish refers to a thread which has been sized to produce a hard, glazed surface. This finish prevents the thread from fraying and becoming weak. A thread having a tensile strength of at least 5 lbs. per single strand should be used. An unbleached white cotton, silk-finish thread is used in hand sewing cotton fabrics. This thread must have a strength of at least 14 Ibs. per single strand.
Dacron fabrics are sewn with Dacron sewing thread. Glass fabrics, when sewn, are sewn with special synthetic threads.
Thread for hand sewing and lacing cord should be waxed lightly before using. The wax should not exceed 20% of the weight of the finished cord. A beeswax free from paraffin should be used for waxing.
Rib Lacing Cord Rib lacing cord is used to sew the fabric to the ribs. The cord must be strong to transmit the suction on the upper surface of the wing from the fabric to the ribs, which, in turn, carry the load into the main wing structure. The cord must also resist fraying caused by the flexing action of the fabric and wing ribs. Dacron, linen, glass, or cotton cords are used for rib lacing cord.
Special Fasteners When repairs are made to fabric surfaces attached by special mechanical methods, the original type of fastening should be duplicated. Screws and washers are used on several models of aircraft, and wire clips are used on other models. Screws or clips may not be used unless they were used by the manufacturer of the aircraft. When self-tapping screws are used to attach fabric to metal rib structure, the following procedure should be observed.
Worn or distorted holes should be re-drilled, and a screw one size larger than the original should be used as a replacement. The length of the screw should be sufficient so that at least two threads of the grip (threaded part) extend through and beyond the rib capstrip. A thin washer, preferably celluloid, should be used under the heads of screws, and pinked-edge tape should be doped over each screwhead.
SEAMS A seam consists of a series of stitches joining two or more pieces of material. Properly formed seam stitches possess the following characteristics:
(1) Strength. A seam must have sufficient strength to withstand the strain to which it will be subjected. The strength of a seam is affected by the type of stitch and thread used, number of stitches per inch of seam, tightness of the seam, construction of the seam, and the size and type of needie used.