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.
Pull to tighten Pull to tighten such information is not available, consider positioning the knot where it will have the least effect on the aerodynamics of the airfoil.
The seine knot admits a possibility of improper tightening, resulting in a false (slip) form with greatly reduced efficiency and must not be used for last stitch tie-offs. Lock the tie-off knot for the last stitch by an additional half-hitch. Under no cir- Knot formed but not tightened. cumstances pull tie-off knots back through the lacing holes.
Load - -→ Load Knot completed, FICURE 3-8. Splice knot.
double loop, using the method illustrated in figure
3-9. All subsequent stitches can be made with a single loop. The spacing between the starting stitch and the next stitch should be one-half the normal stitch spacing. Where stitching ends, such as at the rear spar and the trailing edge, the last two stitches should be spaced at one-half normal spacing.
COVERING FUSELAGES Fuselages are covered by either the sleeve or blanket method, similar to the methods described for covering wings. In the sleeve method several widths of fabric are joined by machine-sewed seams to form a sleeve that will fit snugly when drawn over the end of the fuselage. When the sleeve is in place, all seams should be as nearly parallel as possible to the longitudinal members of the fuselage.
In the blanket method all seams are machine sewed, except one final longitudinal seam along the bottom center of the fuselage. In some cases the blanket is put on it two or three sections and hand sewed on the fuselage. All seams should run fore and aft.
Double-Loop Wing Lacing The double-loop lacing illustrated in figures 3-6 and 3-10 represents a method for obtaining higher strengths than are possible with the standard single lacing. When using the double-loop lacing, make the tie-off knot by the method shown in figure 3-6.
Tie-off Knots All stitches other than the starting stitch must be tied off using the standard knot (modified seine) for rib lacing Figure 3-5. This knot is placed at the edge of the reinforcing tape Figure 3-9. Knots installed on top of the reinforcing tape are subject to increased wear and also have an adverse effect on the aerodynamics of the airfoil.
Tie-off knots usually are placed on the lower surface of low-wing aircraft and on the top surface of high-wing aircraft, to improve the final appearance of the surfaces.
Final location of the knot depends upon the original location selected by the manufacturer. If Fuselage Lacing Fabric lacing is also necessary on deep fuselages and on those where former strips and ribs shape the fabric to a curvature. In the latter case the fabric should be laced to the formers at intervals. The method of attaching the fabric to the fuselage should be at least the equivalent in strength and reliability to that used by the manufacturer of the airplane.
VENTILATION, DRAIN, AND INSPECTION OPEN- INGS The interior of covered sections is ventilated and drained to prevent moisture from accumulating and damaging the structure. Ventilation and drainage holes are provided and the edges reinforced with plastic, aluminum, or brass grommets.
Grommets are doped to the underside of fabric 95
• Standard rib lacing knot Half hitch around stitch loops C with lead A before proceeding to second stitch.
Lead A Lock square knot with half hitch around stitch loops C with end B. Cut off surplus of end B.
Finished first knot First stitch of Capstrip standard lacing 2 - l'ie square knot on capstrip with lead A and end B Lead A End B Stitch loops C Capstrip Start of first knot S/ 2 Refer to 3-5 Details of seine knot S = Normal stitch spacing lacing around entire rib Schematic represents starting stitch First step of Tie square knot on center of capstrip FicURE 3-9. Starting stitch for rib lacing.
Seine Knot Double loop lacing (See Fig. 3-6 for details of knot.)
— Capstrip Capstrip 5/2 lacing around entire rib Starting stitch.
(See Fig. 3-9 for details.)
S = Normal stitch spacing FIGURE 3-10. Standard double-loop lacing.
- NEEDLE
- LACING CORD
CAP STRIP OF RIB
- FABRIC 2 - CROSS OVER UNDER RIB CAP 1 - START NEEDLE THROUGH FABRIC STRIP AND THROUGH FABRIC
CLOSE TO SIDE OF RIB CAP STRIP 3 - PULL EYE END OF NEEDLE
THROUGH THE FIRST HOLE IN FABRIC 4 ~ BRING EYE END OF NEEDLE UP
THROUGH FABRIC OPPOSITE FIRST
HOLE AND FORM CORD ON END OF
NEEDLE, AS SHOWN, TO MAKE HALF HITCH 5 - PULL NEEDLE COMPLETELY OUT 6 - PUT NEEDLE UNDER HALF HITCH AND TIGHTEN HALF HITCH AS SHOWN. AND THROUGH LOOP "K" AS SHOWN -
THEN PULL NEEDLE THROUGH AND
TIGHTEN HALF HITCH - THEN HOLD THUMB AT "I" TO KEEP HALF HITCH TIGHT, AND TIGHTEN LOOP "K", BACK
OF HALF HITCH TO FORM A SEINE KNOT.
FIGURE 3-11. Rib lacing around capstrip.
surfaces wherever moisture may be trapped. It is customary to place one of these grommets on each side of a rib on the underside at the trailing edge.
The grommets are also placed at the lowest drainage points of wings, ailerons, fuselage, and empennage components to provide complete drainage.
Cover Cover Plastic grommets (figure 3-12) are either in the shape of a thin, circular washer or are streamlined.
Plastic grommets are doped to the fabric cover immediately after the surface tape is applied. Streamlined grommets usually are installed with the opening located toward the trailing edge of the surface.
Cover Opening Patch Grommet
- Grommet ~ Grommet Plastic grommet Streamlined Metal grommet plastic grommet FIGURE 3-12. Typical grommets.
Brass and aluminum grommets, also shown in figure 3-12, are mounted on either circular or square fabric patches. The edges of the patch are pinked to provide better adhesion. The patch assembly is doped to the cover after the surface tape is applied.
Inspection doors and access holes are provided in all surfaces, whether fabric or metal covered. One way to provide these openings on fabric-covered surfaces is to dope a zipper-equipped patch in the desired place. Another inspection method for cloth or metal surfaces is to install a framework inside the wing to which a cover plate can be attached by screws. These frameworks are built into the structure wherever access or inspection holes are necessary.
REPAIR OF FABRIC COVERS General Repair fabric-covered surfaces so that the original strength and tautness are returned to the fabric.
Repair all tears or punctures immediately to prevent the entry of moisture or foreign objects into the interior of the structure. Sewn and unsewn repairs are permitted. The type of repair technique to be used depends on the size and location of the damage as well as the never-exceed speed of the aircraft.
When re-covering or repairing control surface fabric, especially on high-performance airplanes, the repairs must not involve the addition of weight aft of the hinge line. The addition of weight disturbs the dynamic and static balance of the surface to a degree that will induce flutter.
Repair of Tears Small cuts or tears are repaired by sewing the edges together and doping a pinked- or frayed-edge patch over the area. The baseball stitch is used in repairing tears. The type illustrated in figure 3-13 enables the damaged edges to be drawn to their original location, thus permitting a tighter repair to be made. The first stitch is started by inserting the needle from the underneath side. All remaining stitches are made by inserting the needle from the top instead of from the bottom so that the points for making the stitch can be more accurately located. The edges are sewn together using the proper thread, The last stitch is anchored with a modified seine knot. Stitches should not be more than 1/4 in.
apart and should extend 1/4 in. into the untorn cover.
Cut two patches of sufficient size to cover the tear and extend at least 1-1/2 in. beyond the tear in all directions (figure 3-14). The fabric used should be at least as good as the original fabric. The edges of the patch should be either pinked or frayed about 1/4 in. on all sides. One patch is saturated with nitrate thinner or acetone and laid over the sewn tear to remove the old finish. The patch is occasionally moistened with a brush until all but the clear 99
1/4" min.
Baseball stitch Start sewing at the point Lock stitching every 8 or 10 stitches, and at ends with SAARA modified seine knot FICURE 3-13. Repair of tears in fabric.
undercoats are soft enough to be removed with a putty knife. Since only the finish under the patch is removed, a neat, smooth repair can be made. A coat of clear tautening dope is applied to the second patch and to the area of the cover from which the finish has been removed. While still wet, this patch is applied to the cover and rubbed so it is smooth and free of air bubbles. Successive coats of clear, pigmented dope are applied until the patched surface has attained the same tension and appearance as the original surrounding surface.
Sewed Patch Repair Damage to covers where the edges oi the tear are tattered beyond joining or where a piece has been completely torn away is repaired by sewing a fabric patch into the damaged area and doping a surface patch over the sewed insert. A sewed-in repair patch may be used if the damage is not longer than 16 in. in any one direction.
The damaged area is trimmed in the form of a circle (figure 3-15) or oval-shaped opening.
fabric insert is cut large enough to extend 1/2 in.
beyond the diameter of the opening. The 1/2-in.
allowance is folded under as reinforcement. Before sewing, fasten the patch at several points with a few temporary stitches to aid in sewing the seams. The edges of the insert are sewed with a baseball stitch.
After the sewing is completed, clean the area of the old fabric to be doped as indicated for repair of tears and then dope the patch in the regular manner. Apply surface tape over any seams that have a second coat of dope. If the opening extends over or closer than 1 in. to a rib or other laced member, the patch should be cut to extend 3 in. beyond the member. After sewing has been completed, the patch should be laced to the rib over a new section of reinforcing tape. The old rib lacing and reinforcing tape should not be removed.
If the fabric covering is damaged at the trailing edge or part of it is torn away as shown in figure 3-16A, it can be repaired as follows. The damaged portion of the panel is removed, and a rectangularor square-shaped opening is made as shown in figure 3-16B. A patch is cut of sufficient size to extend 3/4 in. beyond both sides and the bottom edge of the opening, and 1/2 in. beyond the top.
The edges of the patch are reinforced by being folded under 1/2 in. before being sewed, and each corner is stretched and temporarily held in place with T-pins. Two sides and the trailing edge, as shown in figure 3-16C, are sewed to the old cover with the folded edge extending 1/4 in. beyond both 100
1.5 15
1.5" 1.5" .5". _L -1.5"
•1.5- FIGURE 3-14. Patching over tears. Dash line represents a stitched tear.
ribs. The top of the opening is then sewed. Taping and doping, as shown in figure 3-16D, completes the repair.
Sewed-In Panel Repair When the damaged area exceeds 16 in. in any direction, a new panel should be installed. Remove the surface tape from the ribs adjacent to the damaged area and from the trailing and leading edges of the section being repaired. Leave the old reinforcing tape in place.
Cut the old fabric along a line approximately 1 in. from the center of the ribs on the sides nearest to the damage, and continue the cuts to remove the damaged section completely. The old fabric should not be removed from the leading and trailing edges unless both upper and lower surfaces are being re-covered. Do not remove the reinforcing tape and lacing at the ribs.
Cut a patch to extend from the trailing edge up to and around the leading edge and back approximately to the front beam. The patch should extend (C) (D) approximately 3 in. beyond the ribs adjacent to the damage. FIGURE 3-15. Sewed patch repair.
-Surface tape Rib
•Damaged panel -Trailing edge Opening in panel (A) (B) Side tape Tape end under Hidden stitch ends pinked side tape Baseball stitch - each side and trailing edge before doping New fabric patch • Surface tape All edges of patch folded under 1/2 inch (C) (D) FIGURE 3-16. Repair of panel at trailing edge.
Clean the area of the old fabric to be covered by the patch, put the patch in place, stretch taut and pin. After the patch is pinned in place, fold under the trailing and leading edges of the patch 1/2 in. and sew to the old fabric. Fold the side edges under 1/2 in. and sew to the old cover. After completing the sewing, place reinforcing tape over the ribs under moderate tension and lace down (rib lace or stitch) to the ribs. Remove the temporary pinning.
Give the panel a coat of clear dope and allow to dry. Apply surface tape with the second coat of dope over the reinforcing tape and over the edges of the panel. Finish the doping using regular doping procedures.
This type of repair may be extended to cover both the upper and lower surfaces and to cover several rib bays if necessary. The panel must be laced to all ribs covered.
Unsewed (Doped-On) Fabric Repairs Unsewed (doped-on) repairs may be made on all aircraft fabric-covered surfaces, provided the aircraft never-exceed speed is not greater than 150
m.p.h. A doped-on patch repair may be used if the damage does not exceed 16 in. in any direction. Cut out the damaged section, making a round or ovalshaped opening trimmed to a smooth contour. Use a grease solvent to clean the edges of the opening to be covered by the patch. Sand off the dope from the area around the patch or wash it off with a dope thinner. Support the fabric from underneath while sanding.
For holes up to 8 in. in size, make the fabric patch of sufficient size to provide a lap of at least 2 in. around the hole. For holes over 8 in. in size, make the overlap of the fabric around the hole at least one-fourth the hole diameter with a maximum lap limit of 4 in. If the hole extends over a rib or closer than the required overlap to a rib or other laced member, the patch should be extended at least 3 in. beyond the rib. In this case, after the edges of the patch have been doped in place and the dope has dried, the patch should be laced to the rib over 102