FIGURE 2-9c. ALTERNATE SEQUENCE TO TIE A MODIFIED SEINE KNOT FOR
RIB LACING Step 9. Pull cord section "D" perpendicular to the fabric surface to remove all slack in the cord back to the last rib lacing knot while working the loose knot to the right side. Do not pull cord section "E."
Half Hitch Par 2-10 Step 10. After all slack is removed by pulling cord section "D," switch hands and place a thumbnail on the loose knot formed on the right-hand side, then secure the knot by pulling firmly perpendicular to the fabric surface on cord section "E."
Step 11. After completing all lacing in the same sequence the end is secured with a half hitch after the modified seine knot. The knot is pulled to the inside by routing the needle thru the wing before cutting the cord to leave the end inside.
Page 2-19
b. During the installation of lacing cord through a wing or any other component, special attention should be given to avoid interference with the routing of any control cable, bellcrank, or any other movable item. To prevent chafing and cutting of the lacing cord, control cables or any other movable items should be tensioned or positioned to their normal alignment before rib lacing and checked afterwards to ensure adequate clearance. When a lace cord will be chafed by a moving component, a blindstitch may be made around the top and bottom rib caps as illustrated in Figure 2-11.
FIGURE 2-10. SPLICE KNOT
PULL TO TIGHTEN PULL TO TIGHTEN
KNOT FORMED BUT NOT TIGHTENED LOAD — - LOAD
KNOT COMPLETED The splice knot is made by crossing the ends of the cord, and making four complete wraps with the small end of the free piece around the end of the standing piece. The end is then doubled back through the formed loop.
other free end is wrapped and doubled back.
The long ends of the cords are then pulled until the knot is tight. The short ends are cut close to the knot. This finishes the splice knot.
Par 2-10 Page 2-20
FIGURE 2-11. BLINDSTITCH LACING SQUARE KNOT SECURED WITH HALF
HITCHES Route the needle adjacent to the rib cap at opposite sides.
Return the needle thru the exit hole and thru a third hole opposite the first entry hole.
a half hitch at each side. Complete the square knot with passing the short end of Tie a square knot by the cord thru the foldback loop, as illustrated.
Secure the tight square knot Route the needle back through the with a half hitch at each side, fabric and pull the knot inside before cutting the cord.
c. Stationary structures interfering with needle routing may be circumvented by aligning the needle forward or aft adjacent to the rib cap. Pull the needle through the wing and then return through the same hole and exit at the desired adjacent location.
Par 2-10 Page 2-21
Par 2-10 Page 2-22 NOTE: The first lace on a wing rib should be spaced from the leading edge fairing no more than half the required lace spacing for the balance of the rib.
d. Both surfaces of fabric covering on wings and control surfaces must be securely fastened to the ribs by lacing cord or any other method originally approved for the aircraft. Care must be taken to insure that all sharp edges against which the lacing cord may bear are protected by tape in order to prevent abrasion of the cord. Separate lengths of lacing cord may be joined by the splice knot shown in Figure 2-10 or tied off. The first loop is tied with a square knot as illustrated in Figures 2-5 and 2-9a, and the knot secured with a half hitch on each side after the lacing is pulled tight around the rib. The needle is then routed through the wing and around the rib cap at the next rib lace location with the cord and knot remaining on top of the fabric surface as illustrated in Figures 2-5, 2-6, and 2-8. An alternate method is to route the needle under the fabric and out through the next lace location, then back down through the wing as illustrated in Figures 2-9a through 2-9c. A modified seine knot is then tied as illustrated in Figures 2-4 through 2-9c.
(1) Rotate each lace loop to place the knot at the side of the rib cap to reduce the protrusion and aerodynamic interference before moving to the next lace location, or the cord routed under the fabric to the next lace location as illustrated in Figures 2-9a through 2-9c. The end cord is then cut off leaving a minimum of ¼ inch stub. Lacing tension should be uniform.
(2) Repeated pulling of long lengths of lacing cord may remove wax coating from the cord and cause fraying. Convenient lengths of rib lacing cord may be used to lace long or thick ribs. The end of each length is tied off with a half hitch as illustrated in Figure 2-9c, or if needed, separate lengths of lacing cord may be joined by using the splice knot illustrated in Figure 2-10.
(3) Lacing is installed through other components, where applicable, in the same manner as a wing. Single, wide space lace attachments, usually used on empennage surfaces, are tied with a square knot and half hitch on each side, the same as a starting wing rib lace illustrated in Figure 2-9a, steps 1 and 2. The lace may be rotated to place the knot under the fabric before cutting the cord.
e. Blind lacing on a fuselage, wing rib caps above and below a fuel tank, and any other component, when used by the original aircraft manufacturer, should be reinstalled in the same location and spacing as installed by the original aircraft manufacturer. The lace cord is routed around the stringer, rib cap, or other structure using an appropriate length, single or double pointed, curved needle as illustrated in Figure 2-11. Blind laces are tied with a square knot, then pulled tight and secured with a half hitch at each side. The lace may be rotated to place the knot under the fabric surface before cutting the cord.
2-11. STITCH SPACING.
a. Rib lace spacing on wings, formerly referred to as stitch spacing, should be no greater than the spacing used by the original aircraft manufacturer. When the original spacing cannot be determined the maximum spacing illustrated in Figure 2-12 should be used on the wings and wing control surfaces.
Par 2-11 Page 2-23 FIGURE 2-12. FABRIC ATTACHMENT SPACING
b. W hen the original lace spacing on the empennage surfaces and fuselage, is not known, a maximum spacing of two times the spacing shown in Figure 2-12 for the slipstream area (prop wash) on the wings may be used.
c. The installations of fabric attachments such as screws, rivets, wire clips, and rib lacing should be delayed until the fabric is stabilized and pulled taut with dope. This action is delayed to avoid pulling wing ribs and other structures out of alignment or tearing the fabric at attachment points as the fabric becomes taut. All lacing should be installed adjacent to the structure to which the fabric is being laced, to avoid tearing the fabric and/or creating slack in the cord loop when a load is applied. Where plastic washers were used by the aircraft manufacturer to provide increased pull-through resistance, under the heads of rivets or screws, the same diameter aluminum washer may be used as replacement. Aluminum washers are used because they are not affected by solvents found in adhesives or dopes, nor do they become brittle because of age or cold weather.
2-12. FASTENERS. Several light aircraft designs employ screws, rivets, or single-wire metal clips to secure the fabric to the wing.
a. Screws holding the old fabric can be removed after spinning a small sharpened tube around each screw or using a razor blade to cut and peel away the finishing tape. Care must be
Par 2-12 Page 2-24 taken not to mark or scribe the underlying metal or wood structure. Blind rivets through ribs can be removed by drilling in the center to undercut the head.
b. Single-wire clips may be removed without damage to the rib by inserting a wide, thin screwdriver blade under the clip and carefully twisting. Apply a lifting force at the clip end to pull it up through the hole.
NOTE: It is important that any damage found to ribs, such as oversize rivet or screw holes, and cracks or breaks in the rib cap, should be tagged immediately for easy location and repair later.
c. When repairs are made to fabric surfaces attached by special mechanical methods, duplicate the original type of fastener. When self-tapping screws are used for the attachment of fabric to the rib structure, observe the following procedure:
(1) Redrill the holes where necessary due to wear, distortion, etc., and in such cases, use a screw one size larger as a replacement.
(2) Extend the length of the screw beyond the rib capstrip at least two threads.
(3) Install a thin washer, preferably aluminum, under the heads of screws and dope pinked-edge tape over each screw head.
2-13. FINISHING TAPE.
a. Finishing tape (surface tape) is installed after the fabric has been pulled taut with the initial dope application. This procedure is performed to prevent ripples from forming in fabric panels adjacent to newly applied tapes. Ripple formation is caused by the inability of the combined tape and fabric to tighten uniformly with adjacent fabric when additional dope is applied.
b. In addition to the tape widths required to be installed over fabric seams specified in paragraph 2-7, finishing tape should be installed as weather protection over all rib lacing, screws, rivets, wire clips, or other devices used to secure fabric. This includes wings, control surface ribs, empennage surface ribs, and fuselage stringers, where so installed by the original aircraft manufacturer. Tape width should be sufficient to bond the fabric a minimum of 3/8 inch on each side of all fabric attachments. Two inch width tape is normally used. Tapes over wing rib lacing should extend a minimum of 1/2 inch past each end of any reinforcing tapes. Random or widely spaced attachments may be covered by individual sections of fabric or finishing tape.
c. Installation of finishing tapes for additional wear resistance is recommended over the edges of all fabric-forming structures. This includes fuselage stringers, longerons, leading and trailing edges, false or nose ribs, control surfaces, and empennage ribs not already covered and protected by a finishing tape that is required to be on a fabric seam or fabric attached to the structure. Compound surfaces, such as wingtip bow and empennage surfaces, are more conveniently taped using bias cut finishing tape, which easily conforms to the compound contour, rather than notching linear cut tape to fit the surface. Bias cut tape will be reduced to
Par 2-13 Page 2-25 approximately two thirds the original cut width when pulled tight around a wingtip bow and should be considered when selecting the width of tape for the various locations.
d. Finishing tapes are applied by coating the fabric surface over which the tape will be applied with dope, applying the tape over the wet dope film, then brushing the tape firmly onto the fabric surface. This action will assure a good bond by thoroughly saturating and wetting the finishing tape.
2-14. INSPECTION RINGS AND DRAIN GROMMETS.
a. Inspection Rings. Inspection access is provided adjacent to or over every control bellcrank, drag-wire junction, cable guide, pulley, wing fitting, or any other component throughout the aircraft which will be inspected or serviced annually. They are installed only on the bottom side of the wings except where installed on the top surface by the original manufacturer.
(1) Cutting the holes may be delayed until needed; however, all covers should be finished in matching colors with any trim lines and stored until needed. Spraying matching colors a year later is expensive and time consuming.
(2) The 3-9/16 inch inside diameter cellulose acetate butyrate (CAB) plastic inspection access rings have become popular and bond satisfactorily with Nitrate Dope or Fabric Cement.
Any metal inspection hole reinforcements of a particular shape or special design or size, installed by the original manufacturer, should be reinstalled after cleaning.
(3) Tapes or patches over aluminum reinforcements are optional, but recommended in the prop-wash areas on the wings and forward fuselage bottom.
(4) Fabric patches over plastic rings are strongly recommended because plastic is not a stable material, becomes brittle at low temperatures, and fatigues and cracks from prop blast vibration. Plastic rings are often cracked during removal and installation of spring, clip-held covers. Patches with a minimum 1-inch overlap, should be installed with dope.
b. Drain Grommets. Atmospheric temperature changes cause the humidity in the air to condense on the inside of aircraft surfaces and pool in all low areas. Rainwater enters through openings in the sides and top, and when flying, everywhere throughout the structure. Taxiing on wet runways also splashes water up through any bottom holes. Therefore, provisions must be made to drain water from the lowest point in each fabric panel or plywood component throughout the airframe while in a stored attitude. Drain holes also provide needed ventilation.
(1) Install drain grommets on the underside of all components, at the lowest point in each fabric panel, when the aircraft is in stored attitude. Seaplane grommets, which feature a protruding lip to prevent water splashes through the drain hole, are recommended over drain holes subject to water splashing on land planes as well as seaplanes. The appropriate size holes must be cut through the fabric before installing seaplane grommets. Plastic drain grommets may be doped directly to the fabric surface or mounted on fabric patches then doped to the covering.
Installing a small fabric patch over flat grommets to ensure security is optional. Alternate brass grommets are mounted on fabric patches, then doped to the fabric.
Par 2-14 Page 2-26
(2) After all coating applications and sanding are completed, open all holes through flat drain grommets by cutting through the fabric with a small-blade knife. Do not attempt to open drain holes by punching with a sharp object because the drain hole will not remain open.
2-15. –2-19. [RESERVED.]
Par 2-20 Page 2-27 SECTION 2. APPLICATION OF DOPE
2-20. GENERAL. Nitrate dope and butyrate dope are manufactured by treating cellulose, derived from wood pulp or cotton linter with select acids, then dissolving in a blend of solvents and adding plasticizers for flexibility. After a brush or spray application on fabric, the film develops tension and strength as the solvents evaporate. The tension and strength will increase in proportion to the total film thickness. The fabric functions as a film former and carries no load until a crack develops in the dope film. An excessively thick dope film will develop too much tension and may warp or damage a light airframe.
a. Viscosity adjustments for brush or spray applications may also vary between brands.
When the viscosity adjustment ratio is not provided or is unclear, the product manufacturer should be contacted for detailed instructions. If instructions cannot be obtained or the source of the dope is unknown, the dope should be considered suspect and its use is not advised.
b. Dope, which has been stored for an extended period of time or under adverse conditions, should be suspected of becoming acidic and should be tested before being used on cotton or linen fabric. In some cases, fresh production dope has also been found to have a high acid content and will begin to deteriorate cotton or linen in a period of a few months. The acid content of nitrate or butyrate dope should not exceed 0.06 percent, calculated as acidic acid. An acidity test can be performed by most testing laboratories if high acid content is suspected.
c. Butyrate dope is superior to nitrate dope in weather exposure tests. However, nitrate dope provides better adhesion to natural fiber than butyrate dope. The adhesion of butyrate dope to natural fibers is adequate; it is not necessary to use nitrate dope for the first application and butyrate dope for all other applications. The presence of naphtha in nitrate dope, manufactured in accordance with canceled Mil Specs formulas, causes nitrate dope to be incompatible with butyrate dope; therefore, nitrate dope should not be applied over butyrate dope for repairs or refinishing; however, butyrate dope may be applied over nitrate dope.
NOTE: Nitrate or butyrate dope thinners and retarders should not be substituted for each other, nor should automotive coating-type thinners be used.
d. Clear dope produces the most tension and strength. Aluminum-pigmented dope will weigh slightly more than clear dope and develop less tension and strength for the same film thickness. Pigmented color finishes will produce the least tension and strength due to the higher ratio of plasticizers.
e. During the coating-buildup procedure, solvents released from each succeeding coat will penetrate and be absorbed into the previous dope film, temporarily releasing the tension and increasing the drying time between coats as the dope film becomes thicker. If elapsed time between coats exceeds several weeks at temperatures above 70 °F, it is recommended that several spray coats of an appropriate dope thinner or dope with retarder and/or rejuvenator added be applied to the lightly sanded, dried dope film to open the surface and provide cohesion for the next coat. This will reduce the possibility of surface cracks caused by dissimilar tension between the old and new dope film.
Par 2-20 Page 2-28
f. All dope coats through the final finish may be applied with a brush; however, brush marks will be noticeable in the finish. With increasing environmental concerns, high pressure airless and high-volume low-pressure (HVLP) paint spray equipment is recommended over conventional siphon and pressure pot spraying equipment. High pressure airless and HVLP paint spraying equipment will greatly reduce paint over-spray and fogging. A spray gun, single coat is applied by overlapping each consecutive pass 50 percent of the fan width. A double coat is applied by repeating the coating application in the same direction, or at a 90° angle to the first coat (cross coat) before the first coat has flashed off or dried dust free. For safety and helpful tips for doping, see Tables 2-3 and 2-4.
TABLE 2-3. SAFETY TIPS FOR DOPE/PAINT SAFETY TIPS.
Always ground the aircraft structure while sanding and painting.
Do not use an electric drill as a dope/paint mixer.
Wear leather-soled shoes in the dope/painting area.
Have an adequate, approved ventilation system.
Wear cotton clothes when doping or painting.
Wear an approved face mask or respirator when spraying.
Follow all the manufacturer’s instructions.
TABLE 2-4. TIPS FOR DOPING HELPFUL TIPS FOR DOPING.
Limits for optimum application of dope: relative humidity 20 to 60%; temperature range 65 to 75 °F.
Drying time will vary with temperature, humidity, amount of thinner used, and whether or not retarder was added to the mixture.
Do not recoat until the surface is completely dry and all active solvents have left the dope film.
Spray all coats except the first three or four clear coats, to avoid brush marks.
Over thinning is preferred to under thinning.
Addition of retarder will produce a smoother coat, but drying time between coats will be extended.
To get a clean line for the trim colors, apply a light coat of clear dope directly on the masking tape prior to painting. This will help eliminate the trim color from running under the masking tape.
Remember to always bring the dope to room temperature before using.
Rubbing compound and wax polish may be applied after all solvents have escaped (usually 2 weeks, depending upon the weather).
Par 2-21 Page 2-29
2-21. DOPE APPLICATION PROCEDURE (Natural Fabrics).
a. Step 1. After the cotton or linen fabric is installed in accordance with the procedures specified in paragraphs 2-7 and 2-8, the fabric is wetted with distilled water to remove wrinkles and fold creases, which will show in a gloss finish. Water may be applied by rubbing with a clean sponge or rag, or by using a paint spray gun. Do not use tap water. It may contain minerals which will contaminate the fabric.
(1) As water is absorbed by the fibers, the threads swell, resulting in temporary tauting of the fabric panel. The fabric should be allowed to dry before dope application, otherwise the water in the fibers will interfere with the dope penetration and adhesion.
(2) After the fabric has dried, the first coat of dope is applied, brushing in one direction to set the nap with a clean, non-shedding, 2- to 6-inch wide, semi-soft, long bristle paint brush.
(3) To offset the deteriorating effect of mildew or other fungus on natural fibers, especially in damp climates, it is recommended that a fungicide be added to the first coat of dope. The preferred fungicide is zinc dimethyldithiocarbamate powder, which should be prepared per the manufacturer’s instructions. If no manufacturer’s instructions are available the zinc powder may be stirred in at a ratio of 4 ounces, to one gallon of un-thinned nitrate or butyrate dope, after the powder is wetted to a paste with a 50/50 ratio of dope and thinner.
(4) Pre-mixed fungicidal dope, manufactured in accordance with the formula specified in MIL-D-7850, will have a transparent purple tint to indicate the fungicide additive. Dope manufactured with other colors to identify the manufacturers’ products sold under proprietary trade names may or may not have a fungicidal additive.
(5) The viscosity of the dope should be adjusted to uniformly wet the fabric, indicated by the fabric becoming translucent so that it penetrates through the fabric but does not drip or run down the opposite side. Any dope-runs or pooling on the opposite side will shrink and distort the fabric, and may be visible on the finished surface.
(6) The ideal temperature for application of dope or other coatings is 65 to 75 °F and the humidity should be less than 65 percent. As a general rule, each 10 °F increase or decrease in ambient temperature will increase or decrease drying time by 100 percent. Dope should be allowed to warm to room temperature prior to attempting to adjust the viscosity.
b. Step 2. Depending upon the quality of the dope and the ratio of thinning, the fabric should start to become taut after the first brushed coat of dope has dried approximately 1 hour at 70 °F. A second, heavier coat is applied by cross brushing at 90 °F to the first coat. Viscosity should be adjusted only as necessary to brush out a heavy uniform coat. If the fabric is not taut, with all sag removed, after the second coat has dried approximately 2 hours, a third coat may be applied.
c. Step 3. After the fabric has become semi-taut and stabilized with the initial dope application, and the rib lacing and other fabric attachments are completed as detailed in paragraphs 2-9 through 2-12, it is ready for “dressing out” as described in paragraphs 2-13 and 2-14.
Par 2-21 Page 2-30 NOTE: “Dressing out” means applying all the finishing tapes, reinforcing patches, inspection access ports, and drain grommets, etc.
d. Step 4. After the covering is a dressed out, one or more coats of clear dope are brushed over all finishing tapes and fabric reinforcing patches. This will balance the thickness of the dope film with the previously coated areas of the fabric. It is very important that the porosity of the fabric be filled while brushing to avoid pinholes showing in the finish.
e. Step 5. After drying at least 2 hours at 70 °F, a third heavy coat of clear dope is applied over the entire surface, preferably with a paint spray gun if brush marks are to be avoided. After the third coat of dope has dried at least 2 hours at 70 °F, the fabric should be taut and the dope film should show a gloss, depending upon the dope quality and the ratio of thinner added. If not, a fourth coat of clear dope may be applied, in the same manner as the third coat.
NOTE: Three to four clear coats of dope film showing a uniform gloss combined with the aluminum-pigmented coats and finish coats is considered satisfactory for light aircraft up to 9 lb. per square foot (psf) wing loading.
Five to eight clear coats, depending upon the quality of the dope and resulting film thickness, are recommended for higher wing loading aircraft to assure the covering does not stretch and lose tension.
f. Step 6. After the clear coats are found to be satisfactory, two heavy cross-coats of aluminum-pigmented dope are applied with a spray gun to provide protection from UV rays.
Tests have shown that UV radiation will deteriorate cotton, linen, and polyester fabric; however, polyester fabric deteriorates at a rate half that of cotton or linen under identical exposure conditions. UV radiation does not deteriorate glass fabric. Aluminum-pigmented dope blocks UV radiation and provides a sanding base. A gauge of UV protection in the field is to block all visible light from penetrating through the fabric. Drying time between the two coats should be at least 1 hour at 70 ºF.
(1) An option to premixed aluminum dope is to use aluminum-pigment paste.
Aluminum paste should be prepared per the manufacturer’s instructions. If no manufacturer’s instructions are available, mix 3 ounces (by weight), of 325-mesh aluminum-pigment paste, to 1 gallon of unthinned, clear dope. The aluminum paste should first be mixed to a cream consistency with a 50/50 ratio of dope and thinner before mixing into the unthinned clear dope.
A higher ratio of aluminum-pigment added to the dope may cause a loss of primer-coat and finish-coat adhesion, and peeling may occur especially when high tack tape is used to mask for the trim colors and registration numbers.
(2) The viscosity of the mixed aluminum-pigmented dope should be adjusted for satisfactory spray gun application.
g. Step 7. After two coats of aluminum-pigmented dope have dried at least 4 hours at 70 ºF, the surface may be wet sanded with # 280 grit (or finer) waterproof sandpaper. The aluminum-pigmented dope should be sanded only to develop a smooth surface, not sanded completely off to the clear dope undercoats. Do not sand over screwheads, rib lacing, or any structural sharp edges that will quickly cut through fabric and require patching. Additional coats