FIGURE 2-8. STANDARD DOUBLE-LOOP LACING (OPTIONAL)
- Modified Seine Knot Double loop lacing Aft
• Reinforcing Tape (Same width as rib) Capstrip Capsinp S Reinforcing tape (Same width as Rib Cap) Schematic represents lacing around entire rib.
S Starting stitch S = NORMAL STITCH SPACING Wing Leading Edge Fairing Par 2-10 Page 2-16
FIGURE 2-9a. ALTERNATE SEQUENCE TO TIE A MODIFIED SEINE KNOT FOR
RIB LACING Square Knot Half Hitch Step 1. Tie a square knot by passing the short Step 2. Secure the tight square knot with a end of the cord thru the fold-back loop, half hitch at each side.
as illustrated.
Step 3. Route the needle back thru the right hand hole and exit at the next pre-punched lacing location.
Par 2-10 Step 4. Route the needle back thru the exit hole and thru the opposite fabric surface leaving approximately a 3" loop around a finger on the top surface.
Page 2-17
FIGURE 2-9b. ALTERNATE SEQUENCE TO TIE A MODIFIED SEINE KNOT FOR
RIB LACING Step 5. As the needle is returned thru the top surface on the Step 6. The needle tip is routed under cord section "B" to opposite side of the rib cap the loop is rotated to hook and pull cord section "A" aft.
position cord section "A" to the forward side of the needle, then the needle is pulled thru.
Step 7. The needle tip is then routed over the top of cord Step 8. The needle tip is then routed over cord section "D" section "A" and under cord section "B." and passed thru the lacing while holding cord section "D" perpendicular to the surface to avoid cord entanglement.
Page 2-18
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.