ture is identical to the original so that the weight distribution and mass balance are not affected.

(3) 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 the fabric, preparation of plywood surfaces and similar operations, if properly done, will do much toward insuring an attractive and long-lasting job.

(1) Dope-proofing. Treat all parts of the structure which come in contact with doped fabric with a protective coating such as aluminum foil, dope-proof paint or cellulose tape. Clad aluminum and stainless steel parts need not be dope-proofed.

(11) Chafe points. All points of the structure such as sharp edges, bolt heads, etc., which are likely to chafe or wear the covering should be covered with doped-on fabric strips or covered with an adhesive tape. After the cover has been installed, the chafe points of the fabric should be reinforced by doping of fabric patches. Where a stronger reinforcement is required, a cotton duck or leather patch should be sewed to the fabric patch and then doped in place. All portions of the fabric pierced by wires, bolts, or other projections should be reinforced.

(iii) Inter-rib bracing. Conventional wing ribs, which do not have permanent interrib bracing, should be tied in position by means of cotton tape running parallel to the beams. Apply the tape bracing to both the top and bottom capstrips, maintained parallel to the plane of the cover rather than diagonally between the top and bottom capstrips. Apply the tape continuously with one turn around successive capstrips, arranged so that the tape between the ribs is separated from the cover by a distance equal to the depth of the capstrip. Tie the turn of tape around each capstrip by means of a short length of lacing cord.

(iv) Preparation of plywood surfaces for covering. Prior to covering plywood surfaces with fabric, prepare the surface by cleaning and applying sealer and dope.

(a) Cleaning. Sand all surface areas which have been smeared with glue in order 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.

(b) Application of sealer and dope. Apply one brush coat or two dip coats (wiped) of a dope-proof sealer such as that conforming to Specification MILV-6894 thinned to 30 percent nonvolatile content and allow to dry 2 to 4 hours. Finally, before covering, apply two brush coats of clear dope allowing the first coat of dope to dry approximately 45 minutes before applying the second coat.

(4) Seams-(i) Location of seams. Seams parallel to the line of flight are preferable; however, spanwise seams are acceptable.

(ii) Sewed seams—(a) Machinesewed seams (parts D, E, and F of figure 3-1) should be of the folded-fell or French-fell types. Where selvage edges or pinked edges are joined, a plain lap seam is satisfactory.

(b) Hand-sewing or tacking should begin at the point 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 intervals of 6 inches, and the seams should be properly finished with a lock stitch and a knot (fig. 3-5). At the point where the hand sewing or permanent tacking is necessary, the fabric should be so cut that it can be doubled under before sewing or permanent tacking is performed (figure 3-1C). 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.

(c) A sewed spanwise seam on a metal or wood-covered leading edge should be covered with pinked-edge surface tape at least 4 inches wide.

(d) A sewed spanwise seam at the trailing edge should be covered with pinked-edge surface tape at least 3 inches wide. Notches at least 1 inch in depth and 1 inch in width should be cut into both edges of surface tape if it is used to cover spanwise seams on trailing edges especially the trailing edges of control surfaces. The notches should be spaced at intervals not exceeding 6 inches. On tape less than 3 inches wide, the notches should be one-third the tape width. In the event that the surface tape begins to separate because of poor adhesion or other causes, the tape will tear at a notched section thus preventing progressive loosening of the entire length of tape.

(e) A double stitched lap joint should be covered with pinked edge surface tape at least 4 inches wide.

(f) Sewed spanwise seams on the upper or lower surface should be made in a manner that the amount of protuberance is a minimum. The seam should be covered with pinked edge tape at least 3 inches wide.

(g) Sewed seams parallel to the line of flight (chordwise) should not be placed over a rib or be so placed that the lacing will be through or across such a seam.

(iii) Doped seams. (a) A lapped and doped spanwise seam on a metal or woodcovered leading edge should be lapped at least 4 inches and covered with pinked edge surface tape at least 8 inches wide.

(b) A lapped and doped spanwise seam at the trailing edge should be lapped at least 4 inches and covered with pinked edge surface tape at least 3 inches wide. It should be notched at intervals not exceeding 6 inches. (See subdivision (ii) (d) of this subparagraph for notch configuration.)

(5) Covering methods—(1) The envelope method. The envelope method of covering is accomplished by sewing together widths of fabric cut to specified dimensions and machine-sewn to form an envelope which 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.

(ii) The blanket method. The blanket method of covering is accomplished by sewing together widths of fabrics of sufficient lengths 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 placard never-exceed speed of 150 miles per hour or less, the blanket may be lapped at least 1 inch and doped to the frame or the blanket, lapped at least 4 inches at the nose of metal or wood-covered leading edges, doped, and finish with pinkededge surface tape at least 8 inches wide. In fabricating both the envelope and blanket coverings, the fabric should be cut in lengths sufficient to pass completely around the frame, starting at the trailing edge and returning to the trailing edge.

(6) Reinforcing tape. Reinforcing tape of at least the width of the cap

strips should be placed under all lacing In the case of wings with plywood or metal leading-edge covering, the reinforcing tape need be brought only to the front spar on the upper and lower surfaces.

(1) Use of antitear strips. On aircraft with never-exceed speed in excess of 250 miles per hour, antitear strips are recommended under reinforcing tape on the upper surface of wings, and the bottom surface of that part of the wing in the slipstream. Where the antitear strip is used on both the top and bottom surfaces, pass 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. For this purpose the slipstream should be considered as being equal to the propeller diameter plus one extra rib space on each side. Cut antitear strips from the same material as used for covering and wide enough to extend beyond the reinforcing tape on each side so as 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.

(7) Lacing (1) 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. Care should 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 should be joined by the splice knot shown in figure 3-4. The common square knot, which has a very low slippage resistance, should not be used for this purpose. The utmost care should be exercised to assure uniform tension and security of all stitches. The first or starting stitch should be made with a double loop by the method illustrated in figure 3-6. All subsequent stitches should be made with a single loop and tied off with the standard knot for rib lacing (modified seine type), shown in figure 3-5. The spacing between the starting stitch and the next stitch should be one-half the normal stitch spacing. All tie-off knots should be placed on the middle of the reinforcing tape on the bottom surface, or along the edge of the lower capstrip. The seine knot admits

a possibility of improper tightening, resulting in a false (slip) form with greatly reduced efficiency and should not be used for stitch tie-offs. The tie-offs knot for the last stitch should be locked by an additional half-stitch. Where stitching ends, as at the rear beam and at the trailing edge, the last two stitches should be spaced at one-half normal spacing. Under no circumstances should tie-off knots be pulled back through the lacing holes.

(ii) Double-loop lacing. The doubleloop lacing illustrated in figure 3-7 represents a method for obtaining higher strengths than possible with the standard single lacing. When using the double-loop lacing, the tie-off knot should be made by the method shown in figure 3-8.

(iii) Fuselage lacing. Fabric lacing is also necessary in the case of deep fuselages, and on fuselages where former strips and ribs shape the fabric to a curvature. In the latter case the fabric should be laced at intervals to the formers. The attachment of the fabric to fuselages should be so accomplished as to be at least the equivalent in strength and reliability to that used by the manufacturer of the airplane.

(8) Stitch spacing. The stitch spacing should not exceed the spacing approved on the original aircraft. In case the spacing cannot be ascertained due to destruction of the covering, acceptable rib-stitch spacing may be found in figure 3-2. The lacing holes should be placed as near to the capstrip as possible in order to minimize the tendency of the cord to tear the fabric. All lacing cord should be lightly waxed with beeswax for protection. In case waxed braided cord is used, this procedure is unnecessary. (See table 3-2 for accept

able lacing cords.)

(9) Surface tape (finishing tape). All lacing should be covered with tape of at least the quality and width as was used on the original airplane. This tape should not be applied until the first coat of dope has dried. All inspection openings should be reincorporated into the covering, and the fabric around them and along leading edges reinforced with tape. Where wear or friction is induced by moving parts or fittings, a leather patch should be sewed to a fabric patch and doped in place. Pinked surface tape is sometimes applied over the trailing edges of control surfaces and airfoils.

For such application the tape should be at least 3 inches in width and should be notched along both edges at intervals not exceeding 6 inches. (See subparagraph (4) (ii) (d) of this paragraph for notch configuration.) If separation of the tape from the trailing edge begins it will tear at a notched section and thereby prevent loosening of the entire strip.

(10) Special fasteners. (1) When repairs are made to fabric surfaces attached by special mechanical methods, the original type of fastening should be duplicated.

(ii) When selftapping screws are used for the attachment of fabric to the rib structure, the following procedure should be observed:

(a) The holes should be redrilled where found necessary due to wear, distortion, etc., and in such cases a screw one size larger should be used as a replacement.

(b) The length of the screw should be sufficient so that at least two threads of the grip (threaded part) extend beyond the rib capstrip.

(c) A thin washer, preferably celluloid, should be used under the heads of screws and pinked-edge tape should be doped over each screw head.

(c) Doping-(1) Thinning of dopes. Dopes are generally supplied at a consistency ready for brush coats. For spraying operations practically all dopes require thinning. Thinning directions are usually listed on the container label. Where thinning operations are not supplied, thin the dope with a thinner made for the type of dope being used, until suitable brushing and/or spraying properties are obtained. The amount of thinner to be used will depend on the dope, atmospheric conditions, the spraying equipment, the spraying technique of the operator, and the type of thinner employed. The thinning of dopes influences the drying time and tautening properties of the finish and it is necessary that it be done properly. Determine the amount of thinner necessary by using experimental panels in order to ascertain the conditions which prevail locally at the time of application of the dope.

(2) Blushing and use of blush-retarding thinner. (i) Blushing of dopes is very common when doping is accomplished under humid conditions. The

condition is caused by the rapid evaporation of thinners and solvents, which lowers the temperature on the surface, causing condensation of moisture and producing the white appearance known as blush. Blushing tendencies are also increased if strong currents of air flow over the surface when applying dopes or immediately thereafter.

(ii) A blushed finish has very little protective or tautening value. Where the relative humidity is such that only a small amount of blushing is encountered in doping, this condition may be eliminated by thinning the dope with a blushretarding thinner and slightly increasing the room temperature. If it is not possible to correct humidity conditions in the dope room, suspend doping operations until more favorable atmospheric conditions prevail. The use of a large amount of blush-retarding thinner is not advisable because of the undesirable drying properties accompanying the use of this material.

(3) Number of coats. The total number of coats of dope should not be less than that necessary to result in a taut and well-filled finish job. A guide for finishing fabric-covered aircraft follows:

(i) Two coats of clear dope, brushed on and sanded after the second coat.

(ii) One coat of clear dope, either brushed or sprayed, and sanded.

(iii) Two coats of aluminum pigmented dope, sanded after each coat.

(iv) Three coats of pigmented dope (the color desired), sanded and rubbed to give a smooth glossy finish when completed.

(v) Care should be taken not to sand heavily over the center portion of pinked tape and over spars in order not to damage the rib-stitching cords and fabric.

(4) Technique. (1) Apply the first two coats of dope by brush and spread on the surface as uniformly as possible and thoroughly work into the fabric. Exercise care not to work the dope through the fabric so that an excessive film is formed on the reverse side. The first coat should produce a thorough and uniform wetting of the fabric. To do so, work the dope with the warp and the filler threads for three or four brush strokes and stroke away any excess material to avoid piling up or dripping. Apply succeeding brush or spray coats with only sufficient brushing to spread the dope smoothly and evenly.

(ii) When doping fabric over plywood or metal-covered leading edges, care should be taken to insure that an adequate bond is obtained between the fabric and the leading edge. Care should also be taken when using predoped fabric to use a thinned dope in order to obtain a good bond between the fabric and the leading edge.

(5) Applying surface tape and reinforcing patches. Apply surface tape and reinforcing patches with the second coat of dope. Apply surface tape over all rib lacing and over all sewed seams as well as at all other points of the structure where tape reinforcements are indicated.

On

(6) Installation of drainage grommets. With the second coat of dope, install drainage grommets on the underside of airfoils at the trailing edge and as close to the rib as practicable. fuselages, install drainage grommets at the center of the underside in each fuselage bay, located so that the best possible drainage is effected. On seaplanes it is recommended to install special shielded called grommets, sometimes marine grommets, to prevent the entry of spray. Also use this type of grommet on landplanes in that part of the structure which is subject to splash from the landing gear when operating from wet and muddy fields. Plastic-type grommets are doped directly to the covering. Where brass grommets are used, mount them on fabric patches and then dope to the covering. After the doping scheme is completed, open the drain holes by cutting out the fabric with a small-bladed knife. Do not open drainage grommets by punching.

(7) Use of fungicidal dopes. (1) Fungicidal dope is normally used as the first coat for fabrics to prevent rotting. While it may be more advisable to purchase dope in which the fungicide has already been incorporated, it is feasible to mix the fungicide with the dope. Specification MIL-D-7850 specifies that the requirements for cellulose acetate butyrate dope incorporate a fungicide for first coat use on aircraft. The fungicide specified in this specification is zinc dimethyldithiocarbonate which forms a suspension with the dope. This material is a fine powder, and if it is mixed with the dope, it should be made into a paste using dope and then diluted to the proper consistency according to the manufacturer's instructions. It is not practicable to mix the powder with a large quantity of dope.

(ii) Copper napthonate is also used as a fungicide and forms a solution with dope. However, this material has a tendency to bleed out especially on lightcolored fabric. It is considered satisfactory from a fungicidal standpoint.

(iii) The first coat of fungicidal dope should be applied extremely thin so that the dope can thoroughly saturate both sides of the fabric. Once the fabric is thoroughly saturated subsequent coats can be applied at any satisfactory working consistency.

When

(8) Rejuvenation of fabric. using this technique of improving the appearance or condition of the doped surface, care should be exercised to establish initially that the fabric strength itself has not deteriorated beyond safe limits. Experience indicates that rejuvenation may at times cause fabric sag rather than tautening.

(9) Common dope troubles. (1) In cold weather, dopes become quite viscous. Cold dopes pull and rope under the brush, and if thinned sufficiently to spray, lack body when dry. Prior to use, allow dopes to come to a temperature approximately that of the dope room, 24° C. (75° F.).

(ii) Orange peel and pebble effect result from insufficiently thinned dope or when the spray gun is held too far from the surface being sprayed.

(iii) Runs, sags, laps, streaks, high and low spots are caused by improperly adjusted spraying equipment or improper spraying technique.

(iv) Blisters may be caused by water or oil entering the spray gun. Drain air compressors, air regulators, and air lines daily.

(v) Pin holes may be caused by not allowing sufficient time for drying between coats or after water sanding, or they may be due to insufficiently reduced dope.

(vi) Wet areas on a doped surface indicate that oil, grease, soap, etc., had not been properly removed before doping.

(d) Repairs to fabric covering. Repairs to fabric-covered surfaces should be made in a manner that will return che original strength and tautness to the fabric. Sewed repairs and unsewed (doped-on patches or panels) may be made. No fabric or tape should be doped onto a surface which contains aluminum or other color coats. Whenever it is necessary to add fabric reinforcement, the old dope should be removed either by

softening and scraping or by sanding down to the point where the base coat or clear coat is exposed. Clear dope should be used in doping the fabric to the surface. After reinforcement is made, normal finishing procedures may be followed.

(1) Repair of tears in fabric. Tears should be repaired as shown in figure 3-3 by sewing the torn edges together using a baseball stitch and doping a piece of pinked-edge fabric over the tear. If the tear is a straight rip, the sewing is started at one end so that as the seam is made the edges will be drawn tightly together throughout its entire length. If the opening is V-shaped, as is often the case when openings are cut in wings to inspect the internal structure, the sewing should start at the corner or point so that the edges of the cover will be held in place while the seams are being made The sewing is done with a curved needle and well-waxed thread. Clean the surface to be covered by the patch by rubbing the surface with a rag dipped in dope and wiping dry with a clean rag, or by scraping the surface with a putty knife after it has been softened with fresh dope. Dope solvent or acetone may be used for the same purpose but care should be taken that it does not drop through on the inside of the opposite surface causing the dope to blister. A patch of sufficient size should be cut from airplane cloth to cover the tear and extend at least 11⁄2 inches beyond the tear in all directions. The edges of the patch should either be pinked similar to surface tape or frayed out about one-fourth inch on all edges.

(2) Sewed patch repair. When the damage is such that it will not permit sewing the edges together, a sewed-in repair patch may be used if the damage is not longer than 16 inches in any one direction (see fig. 3-3). Cut out the damaged section making a round or oval-shaped opening trimmed to a smooth contour. Clean the area of the old fabric to be doped as indicated in subparagraph (1) of this paragraph. Turn the edges of the patch one-half inch and sew to the edges of the opening. Before sewing, fasten the patch at several points with a few temporary stitches to facilitate sewing the seams. After the sewing is completed, clean the area of the old fabric to be doped as indicated for small repairs and then dope the patch in the regular manner. Apply