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.
(2) Elasticity. The elasticity of the material to be sewed determines the degree of elasticity desirable in a seam. Elasticity is affected by the quality of thread used, tension of the thread, length of stitch, and type of seam.
(3) Durability. The durability of a seam is determined by the durability of the material. Tightly woven fabrics are more durable than loosely woven fabrics which tend to work or slide upon each other. For this reason, the stitches must be tight and the thread well set into the material to minimize abrasion and wear caused by contact with external objects.
(2) Good appearance. The appearance of a seam is largely controlled by its construction. However, appearance should not be the principal factor when constructing covers. Due consideration must be given to strength, elasticity, and durability.
Sewed Seams Machine-sewed seams (figure 3-4) should be of the folded-fell or French-fell types. A plain lapped seam is satisfactory where selvage edges or pinked edges are joined.
All machine sewing should have two rows of stitches with 8 to 10 stitches per inch. A lockstitch is preferred. All seams should be as smooth as possible and provide adequate strength. Stitches should be approximately 1/16 in. from the edge of the seam, and from 1/4 to 3/8 in. from the adjacent row of stitches.
Hand sewing is necessary to close the final openings in the covering. Final openings in wooden wing coverings are sometimes closed by tacking, but sewing is preferable. A 1/2-in. hem should be turned under on all seams to be hand sewn. Prepar-
- 1/4" or 3/8" atory to hand sewing, the fabric on wooden wings can be held under tension by tacks; fabric on metal wings can be pinned to adhesive tape pasted to the trailing edge of the wings.
Hand sewing or tacking should begin where machine sewing stops and should continue to the point where machine sewing or uncut fabric is again reached. Hand sewing should be locked at 6-in.
intervals and the seams should be properly finished with a lockstitch and a knot (figure 3-5). Where hand sewing or permanent tacking is necessary, the fabric should be so cut that it can be doubled under before it is sewed or permanently tacked. After hand sewing has been completed, the temporary tacks should be removed. In hand sewing there should be a minimum of four stitches per inch.
A double-stitched lap joint should be covered with pinked-edge surface tape at least 4 in. wide.
Spanwise seams on the upper or lower surface should be sewed with a minimum protuberance. The seam should be covered with pinked-edge tape at least 3 in. wide.
A spanwise seam sewed at the trailing edge should be covered with pinked-edge surface tape at least 3 in. wide. Notches (V-shaped) at least 1 in.
deep and 1 in. wide should be cut into both edges of the surface tape if it is used to cover spanwise seams on trailing edges, especially the trailing edges of control surfaces. For application on aircraft with never-exceed speed of over 200 mph the tape should be notched at equal intervals not to exceed 18" between notches. If the tape begins to separate because of poor adhesion or other causes, it will tear at a notched section, thus preventing progressive loosening of the entire length of tape.
Sewed seams parallel to the line of flight (chordwise) may be placed over a rib, but the seams should be placed so that the lacing will not be through them.
- 1/16" 1/16"- • 1/2" Stitching 1/16" - 1/16"
- 1/16" →/1/16" French fell seam Folded fell seam Plain overlap seam FIGURE 3-4. Machine-sewed seams.
strength are to be obtained from the material se- Half hitch lected. A good covering job is important not only from a strength and appearance standpoint, but also because it affects the performance of the airplane.
All covering must be taut and smooth for best performance.
All fabric materials to be used in covering should (A) First step be stored in a dry place and protected from direct sunlight until needed. The room in which the sewing and application of the covering is done should be clean and well ventilated.
Preparation of the Structure for Covering One of the most important items in covering aircraft is proper preparation of the structure. Dope proofing, covering edges which are likely to wear (B) Second step operations, if properly done, will do much toward the fabric, preparing plywood surfaces, and similar After pull no. 1 ensuring an attractive and long-lasting job.
hold stitch tension by pressing here Dope Proofing during pull no. 2 Treat all parts of the structure that come in contact with doped fabric with a protective coating, Pull no. 1-10 Ib. guch as aluminum foil, dope-proof paint, or celluto tighten stitch lose tape. Clad aluminum and stainless steel parts Pull no. 2-10 lb. need not be dope proofed.
to tighten knot Chafe Points (C) Third step All points of the structure, such as sharp edges or boltheads, that are likely to chafe or wear the covering should be covered with doped-on fabric strips or taped with cellophane or other nonhygroscopic adhesive tape. After the cover has been installed, the chafe points of the fabric should be reinforced by the doping-on of fabric patches. Where a (D) Completed knot stronger reinforcement is required, a cotton duck or FIGURE 3-5. Standard knot for rib lacing leather patch should be sewed to the fabric patch (modifed seine knot). and then doped in place. All portions of the fabric Doped Seams
(1) For a lapped and doped spanwise seam on a metal-or-wood-covered leading edge, lap the fabric at least 4 inches and cover with pinked-edge surface tape at least 4 inches wide.
(2) For a lapped and doped spanwise seam at the trailing edge, lap the fabric at least 4 inches and cover with pinked-edge surface tape at least 3 inches wide.
APPLYING COVERING General The proper application of cloth on the surfaces is essential if a good appearance and the greatest pierced by wires, bolts, or other projections should be reinforced. Patches should fit the protruding part as closely as possible to prevent the entrance of moisture or dirt.
Inter-Rib Bracing A continuous line of reinforcing tape may be used to successively tie the rib sections between the spars together at equally spaced intervals to hold the ribs in correct alignment and prevent their warping. Wing ribs that do not have permanent inter-rib bracing should be tied in position with reinforcing tape. Approximately half way between the front and rear spar, apply the tape diagonally between the top and bottom capstrip of each suc- 91
Pull No. 2-5 lb. Stitch loops Sand all surface areas that have been smeared to tighten stitch around capstrip with glue to expose a clean wood surface. Remove loose deposits such as wood chips and sawdust.
Remove oil or grease spots by carefully washing with naphtha.
From preceeding After cleaning the surfaces, apply one brush coat Pull No. 1-10 Ib . stitch or two dip coats (wiped) of a dope-proof sealer, to tighten loops such as that conforming to Military Specification Operation No. 1 MIL-V-6894 thinned to 30% nonvolatile content, and allow to dry 2 to 4 hrs. Finally, before coverto complete knot. Pull No. 4-10 1b. Edge of capstrip ing, apply two brush coats of clear dope, allowing the first coat of dope to dry approximately 45 min.
before applying the second coat.
Edge of capstrip Covering Practices The method of fabric attachment should be identical, as far as strength and reliability are concerned, to the method used by the manufacturer of Pull No. 3 the airplane to be re-covered or repaired. Fabric lacing Direction of / pull snug threads are parallel to the line of flight. However, it may be applied so that either the warp or fill is usually preferable for the warp threads to be parallel to the line of flight. Either the envelope method or blanket method of covering is acceptable.
Operation No. 2 The envelope method of covering consists of sew- Pull No. 4 ing together widths of fabric cut to specified dimensions and machine sewn to form an envelope that can be drawn over the frame. The trailing and outer edges of the covering should be machine sewn unless the component is not favorably shaped for sewing, in which case the fabric should be joined by hand sewing.
Completed knot FIGURE 3-6. Standard knot for double loop lacing.
cessive rib from the wing butt rib to the tip rib.
Tape is to be continuous and will be anchored with one turn around each individual rib cap strip.
Preparing Plywood Surfaces for Covering Before covering plywood surfaces with fabric, prepare the surface by cleaning and applying sealer and dope.
In the blanket method of covering, widths of fabrics of sufficient lengths are sewn together to form a blanket over the surfaces of the frame. The trailing and outer edges of the covering should be joined by a plain overthrow or baseball stitch. For airplanes with a placarded never-exceed speed of 150 m.p.h. or less, the blanket may be lapped at least 1 in. and doped to the frame or the blanket; it may be lapped at least 4 in. at the nose of metal- or wood-covered leading edges, doped, and finished with pinked-edge surface tape at least 4 in. wide. In both the envelope and blanket coverings, the fahric should be cut in lengths sufficient to pass completely around the frame, starting at the trailing edge and returning to the trailing edge. Seams parallel to the line of flight are preferable; however, spanwise seams are acceptable.
Before applying cotton or linen fabrics, brush on several coats of clear, full-bodied nitrate dope on all 92
points to which the fabric edges will be cemented.
If the structure is not doped, the dope used to cement the fabric edges will be absorbed by the surface as well as by the fabric. This will result in a poor bond to the structure after the dope has dried.
Dacron fabric can be attached to the structure by using either nitrate dope or specially formulated cements.
After securing the cover, cotton and linen fabrica may be water-shrunk to remove wrinkles and excess slack. The fabric must be dried thoroughly before doping begins. Dacron may be heat-shrunk by using an electric iron set at 225° F. or by using a reflector heater. Do not apply excessive heat, because the Dacron, as well as the understructures of wood, may be damaged.
Shrinking should be done in several stages on opposite sides to shrink the entire area uniformly.
Remove the excess slack with the initial application of heat. The second pass will then shrink the fabric to the desired tautness und remove most of the remaining wrinkles. Nonshrinking nitrate and butyrate dopes are available and produce no further shrinking or tightening. Regular dopes will pull the fibers and strands together and can damage light structures. A nonshrinking dope must be used when Dacron is heat-shrunk to its final tautness.
Taping Sewed seams, lapped edges, and rib stitching or screws must be covered with pinked-edge surface tape. Use surface tape having the same properties as the fabric used for covering.
Apply the tape by first laying down a wet coat of dope, followed immediately by the tape. Press the tape into the dope. Work out any trapped air and apply a coat of dope over the surface of the tape.
COVERING WINGS Wings may be covered with fabric by the envelope, blanket, or combination method. The envelope method is preferable and should be used whenever possible.
The envelope method of covering wings consists of sewing together several widths of fabric of definite dimensions and then running a transverse (spanwise) seam to make an envelope or sleeve.
The advantage of the envelope method is that practically all sewing is by machine, and there is an enormous saving of labor in fitting the covering.
The envelope is pulled over the wing through its open end, which is then closed over the butt by hand sewing. When the envelope is used in repairing a portion of a surface the open end is fitted to extend 3 inches beyond the adjacent rib. If the envelope is of the proper dimensions, it will fit the wing snugly. When possible, the spanwise seam should be placed along the trailing edge.
In the blanket method several widths of fabric are machine sewed together and placed over the wing with a hand-sewed, spanwise seam along the trailing edge. Care must be taken to apply equal tension over the whole surface.
The combination method uses the envelope method as much as possible, and the blanket method on the remainder of the covering. This method is applicable to wings with obstructions or recesses that prevent full application of an envelope.
After the cover is sewn in place and shrunk, reinforcing tape of at least the width of the capstrip is placed over each rib and the fabric is laced to each rib. Except on very thick wings, the rib lacing passes completely around the rib. On thick wings the rib top and bottom cap strips are individually laced. In lacing any covering to a wing, the lacing is held as near as possible to the capstrip, by inserting the needle immediately adjacent to the capstrip.
The rib should not have any rough or sharp edges in contact with the lacing, or it will fray and break.
Each time the lacing cord goes around the rib it is tied, and the next stitch is made at the specified distance.
In order not to overstress the lacing, it is necessary to space the stitches a definite distance apart, depending on the speed of the airplane. Because of the additional buffeting caused by the propeller slipstream, the stitching must be spaced closer on all ribs included within the propeller slipstream. It is customary to use this closer spacing on the rib just outboard of the propeller diameter as well.
The stitch spacing should not exceed the approved spacing on the original covering of the aircraft. If the spacing cannot be ascertained because of destruction of the covering, acceptable rib stitch spacing may be found in figure 3-7. The lacing holes should be placed as close as possible to the capstrip to minimize the tendency of the cord to tear the fabric. All lacing cord should be lightly waxed with beeswax for protection.
Anti-tear Strips In very high speed airplanes difficulty is often experienced with rib lacing breaking or with fabric tearing in the slipstream.
On aircraft with never-exceed speeds in excess of 250 m.p.h., anti-tear strips are recommended under reinforcing tape on the upper surface of wings and on the bottom surface of that part of the wing in the slipstream. Where the anti-tear strip is used on both the top and the bottom surfaces, extend it continuously up to and around the leading edges and back to the trailing edge. Where the strip is used only on the top surface, carry it up to and around the leading edge and back on the lower surface as far aft as the front beam or spar. For this purpose the slipstream should be considered as heing equal to the propeller diameter, plus one extra rib space on each side. Cut anti-tear strips from the same material as that used for covering, and cut them wide enough to extend beyond the reinforcing tape on each side to engage the lacing cord. Attach the strips by applying dope to that part of the fabric to be covered by the strip and applying dope freely over the strip.
Single-Loop Wing Lacing Both surfaces of fabric covering on wings and control surfaces should be securely fastened to the ribs by lacing cord or any other method originally approved for the aircraft.
All sharp edges against which the lacing cord may bear must be protected by tape to prevent abrasion of the cord. Separate lengths of lacing cord should be joined by the splice knot shown in figure 3-8. The common square knot, which has a very low slippage resistance, should not be used to splice lengths of cord. The utmost care should be used to assure uniform tension and security of all stitches.
Rib stitching usually is started at the leading edge of the rib and continued to the trailing edge.
If the leading edge is covered with plywood or metal, start the lacing immediately aft of these coverings. The first or starting stitch is made with a NOTES
1. If original rib stitch spacing cannot be determined, use spacing indicated in these curves.
2. Lacing to be carried to leading edge when velocity exceeds 275 m.p.h.
Maximum spacing of rib lacing - inches •Spacing other than in slipstream 3 2 Spacing in slipstream 100 150 200 250 300 Placard never exceeds speeds - m.p.h. (indicated).
(Curves presume leading edge support reinforcement such as plywood, metal,) FIcURE 3-7. Rib stitch spacing chart.