Par 4-58 Page 4-40
f. Patching of Small Holes. Small holes in skin panels which do not involve damage to the stiffening members may be patched by covering the hole with a patch plate in the manner shown in Figure 4-16. Flush patches also may be installed in stressed-skin type construction. An acceptable and easy flush patch may be made by trimming out the damaged area and then installing a conventional patch on the underneath side or back of the sheet being repaired. A plug patch plate of the same size and skin thickness as the opening may then be inserted and riveted to the patch plate. Other types of flush patches similar to those used for patching plywood may be used. The rivet pattern used, however, must follow standard practice to maintain satisfactory strength in the sheet.
g. Splicing of Sheets. The method of copying the seams at the edges of a sheet may not always be satisfactory. For example, when the sheet has cutouts, or doubler plates at an edge seam, or when other members transmit loads into the sheet, the splice must be designed as illustrated in the following examples.
(1) Material: Clad 2024 sheet, 0.032 inch thickness. Width of sheet (i.e., length at splice) = “W” = 10 inches.
(2) Determine rivet size and pattern for a single-lap joint similar to Figure 4-5.
(a) Use rivet diameter of approximately three times the sheet thickness:
3 × 0.032 = 0.096 inch. Use 1/8-inch 2117-T4 (AD) rivets (5/32-inch 2117-T4 (AD) would be satisfactory).
(b) Use the number of rivets required per inch of width “W” from Table 4-10.
(Number per inch: 4.9 × .75 = 3.7 or the total number of rivets required = 10 × 3.7 or 37 rivets.)
See notes in Table 4-10.
Par 4-58 Page 4-41 TABLE 4-9. NUMBER OF RIVETS REQUIRED FOR SPLICES (SINGLE-LAP JOINT) IN BARE 2014-T6, 2024-T3, 2024-T36, AND 7075-T6 SHEET, CLAD 2014-T6, 2024-T3, 2024-T36, AND 7075-T6 SHEET, 2024-T4, AND 7075-T6 PLATE, BAR, ROD, TUBE, AND EXTRUSIONS, 2014-T6 EXTRUSIONS Thickness “t” in Inches No. of 2117-T4 (AD) Protruding Head Rivets Required per Inch of Width “W” No. of Bolts Rivet Size 3/32 1/8 5/32 3/16 1/4 AN-3 .016 6.5 4.9 .020 6.9 4.9 3.9 .025 8.6 4.9 3.9 .032 11.1 6.2 3.9 3.3 .036 12.5 7.0 4.5 3.3 2.4 .040 13.8 7.7 5.0 3.5 2.4 3.3 .051 9.8 6.4 4.5 2.5 3.3 .064 12.3 8.1 .6 3.1 3.3 .081 10.2 7.1 3.9 3.3 .091 11.4 7.9 4.4 3.3 .102 12.8 8.9 4.9 3.4 .128 11.2 6.2 3.2 NOTES:
a. For stringers in the upper surface of a wing, or in a fuselage, 80 percent of the number of rivets shown in the table may be used.
b. For intermediate frames, 60 percent of the number shown may be used.
c. For single lap sheet joints, 75 percent of the number shown may be used.
ENGINEERING NOTES:
a. The load per inch of width of material was calculated by assuming a strip 1 inch wide in tension.
b. Number of rivets required was calculated for 2117-T4 (AD) rivets, based on a rivet allowable shear stress equal to 40 percent of the sheet allowable tensile stress, and a sheet allowable bearing stress equal to 160 percent of the sheet allowable tensile stress, using nominal bolt diameters for rivets.
c. Combinations of sheet thickness and rivet size above the underlined numbers are critical in (i.e., will fail by) bearing on the sheet; those below are critical in shearing of the rivets.
d. The number of AN-3 bolts required below the underlined number was calculated based on a sheet allowable tensile stress of 70,000 pounds per square inch (psi) and a bolt allowable single shear load of 2,126 pounds.
Par 4-58 Page 4-42 TABLE 4-10. NUMBER OF RIVETS REQUIRED FOR SPLICES (SINGLE-LAP JOINT) IN 2017, 1017 ALCLAD, 2024-T3 ALCLAD SHEET, PLATE, BAR, ROD, TUBE,
AND EXTRUSIONS Thickness “t” in Inches No. of 2117-T4 (AD) Protruding Head Rivets Required Per Inch of Width “W” No. of Bolts Rivet Size 3/32 1/8 5/32 3/16 1/4 AN-3 .016 6.5 4.9 .020 6.5 4.9 3.9 .025 6.9 4.9 3.9 .032 8.9 4.9 3.9 3.3 .036 10.0 5.6 3.9 3.3 2.4 .040 11.1 6.2 4.0 3.3 2.4 .051 7.9 5.1 3.6 2.4 3.3 .064 9.9 6.5 4.5 2.5 3.3 .081 12.5 8.1 5.7 3.1 3.3 .091 9.1 6.3 3.5 3.3 .102 10.3 7.1 3.9 3.3 .128 12.9 8.9 4.9 3.3 NOTES:
a. For stringers in the upper surface of a wing, or in a fuselage, 80 percent of the number of rivets shown in the table may be used.
b. For intermediate frames, 60 percent of the number shown may be used.
c. For single lap sheet joints, 75 percent of the number shown may be used.
ENGINEERING NOTES:
a. The load per inch of width of material was calculated by assuming a strip 1 inch wide in tension.
b. Number of rivets required was calculated for 2117-T4 (AD) rivets, based on a rivet allowable shear stress equal to percent of the sheet allowable tensile stress, and a sheet allowable bearing stress equal to 160 percent of the sheet allowable tensile stress, using nominal hole diameters for rivets.
c. Combinations of sheet thickness and rivet size above the underlined numbers are critical in (i.e., will fail by) bearing on the sheet; those below are critical in shearing of the rivets.
d. The number of AN-3 bolts required below the underlined number was calculated based on a sheet allowable tensile stress of 55,000 psi and a bolt allowable single shear load of 2,126 pounds.
Par 4-58 Page 4-43
(c) Lay out rivet pattern with spacing not less than shown in Figure 4-5. Referring to Figure 4-5, item (A), it seems that a double row pattern with the minimum spacing will give a total of 40 rivets. However, as only 37 rivets are required, two rows of 19 rivets each equally spaced over the 10 inches will result in a satisfactory splice.
h. Straightening of Stringers or Intermediate Frames. Members which are slightly bent may be straightened cold and examined with a magnifying glass for cracks or tears to the material. Reinforce the straightened part to its original shape, depending upon the condition of the material and the magnitude of any remaining kinks or buckles. If any strain cracks are apparent, make complete reinforcement in sound metal beyond the damaged portion.
i. Local Heating. Do not apply local heating to facilitate bending, swaging, flattening, or expanding operations of heat-treated aluminum alloy members, as it is difficult to control the temperatures closely enough to prevent possible damage to the metal, and it may impair its corrosion resistance.
j. Splicing of Stringers and Flanges. It is recommended that all splices be made in accordance with the manufacturer’s recommendations. If the manufacturer’s recommendations are not available, the typical splices for various shapes of sections are shown in Figures 4-17 through 4-19. Design splices to carry both tension and compression, and use the splice shown in Figure 4-18 as an example illustrating the following principles.
FIGURE 4-17. TYPICAL STRINGER AND FLANGE SPLICES
TAPERED SPLICE MEMBER NOTE: UNSHADED SECTIONS ARE ORIGINAL AND/OR REPLACEMENT SECTIONS. SHADED SECTIONS ARE CONNECTING OR REINFORCING SECTIONS.
+Wз→ NOTE: FOR MINIMUM NUMBER OF RIVETS REQUIRED, SEE PARAGRAPH 4-58g AND SUBSEQUENT.
Par 4-58 Page 4-44
FIGURE 4-18. EXAMPLE OF STRINGER SPLICE (MATERIAL: 2017 ALLOY)
CUT IN STRINGER , TAPERED SPLICE MEMBER 000 0 01
ON LOWER SURFACE OF WING, MAKE END RIVET SAME SIZE AS SKIN ATTACHING RIVETS.
2d2, MINIMUM A → 3d2, MINIMUM
THE NUMBER OF BOLTS REQUIRED IN THE "BULB" LEG OF THE STRINGER IS DETERMINED FROM TABLE 4-9 FOR 2017-T3 AS FOLLOWS:
tz = 1/16" (Use "064) de= „190" (AN3 bolt) NO. OF BOLTS PER INCH OF WIOTH FROM TABLE = 3.3.
ACTUAL NO. OF BOLTS ON EACH SIDE OF CUT = WX 3.3 = (1.0) × 3.3 = 3.3 BOLTS USE 4 BOLTS
WHEN BOTH RIVETS AND BOLTS
ARE USED, BOLT HOLES SHOULD BE REAMED TO SIZE.
0.0 -KEEP LARGE HOLES BACK FROM END OF SPLICE.
THE NUMBER OF RIVETS REQUIRED IN
THE UPPER LEG OF THE STRINGER IS DETERMINED FROMTABLE 4-9 FOR 2017-T3 AS FOLLOWS:
t, = 1/16" (Use „064") d,= 1/8" (2117-AD rivet) NO. OF RIVETS PER INCH OF WIDTH FROM TABLE = 9.9 ACTUAL NO. OF RIVETS ON EACH SIDE OF CUT = WIx 9.9 = (.5), 9.9 = 4.95 RIVETS. USE 5 RIVETS.
d, (1/8), RIVET DIA. PREFERABLY / 2tTO 3t, BUT NOT GREATER THAN W/4.
(1.0)
- "8" SPLICE BAR. SHADED AREA SPLICE ANGLE "A" GREATER THAN W, Xt,.
GREATER THAN W2xt2. SHADED AREA - STRINGER
AREA REPRESENTING BULB
SECTION A-A Par 4-58 Page 4-45
Par 4-58 Page 4-46 FIGURE 4-19. APPLICATION OF TYPICAL FLANGE SPLICES
AND REINFORCEMENT (
1) To avoid eccentric loading and consequent buckling in compression, place splicing or reinforcing parts as symmetrically as possible about the centerline of the member, and attach to as many elements as necessary to prevent bending in any direction.
(2) To avoid reducing the strength in tension of the original bulb angle, the rivet holes at the ends of the splice are made small (no larger than the original skin attaching rivets), and the second row of holes (those through the bulbed leg) are staggered back from the ends. In general, arrange the rivets in the splice so that the design tensile load for the member and splice plate can be carried into the splice without failing the member at the outermost rivet holes.
Par 4-58 Page 4-47
(3) To avoid concentration of load on the end rivet and consequent tendency toward progressive rivet failure, the splice is tapered at the ends by tapering the backing angle and by making it shorter than the splice bar. (See Figure 4-18.)
(4) The preceding principles are especially important in splicing stringers on the lower surface of stressed skin wings, where high-tension stresses may exist. When several adjacent stringers are spliced, stagger the splices if possible.
k. Size of Splicing Members. When the same material is used for the splicing members as for the original member, the cross-section area (i.e., the shaded areas in Figure 4-17), of the splice material will be greater than the area of the section element which it splices. The area of a section element (e.g., each leg of an angle or channel) is equal to the width multiplied by the thickness. For example, the bar “B” in Figure 4-18 is assumed to splice the upper leg of the stringer, and the angle “A” is assumed to splice the bulbed leg of the stringer. Since the splice bar “B” is not as wide as the adjacent leg, its thickness must be increased such that the area of bar “B” is at least equal to the area of the upper leg of the stringer.
l. The Diameter of Rivets in Stringers. The diameter of rivets in stringers might preferably be between two and three times the thickness “t” of the leg, but must not be more than 1/4 the width “W” of the leg. Thus, 1/8-inch rivets are chosen in the example in Figure 4-18. If the splices were in the lower surface of a wing, the end rivets would be made the same size as the skin-attaching rivets, or 3/32 inch.
m. The Number of Rivets. The number of rivets required on each side of the cut in a stringer or flange may be determined from standard textbooks on aircraft structures, or may be found in Tables 4-9 through 4-11.
Par 4-58 Page 4-48 TABLE 4-11. NUMBER OF RIVETS REQUIRED FOR SPLICES (SINGLE-LAP JOINT) IN 5052 (ALL HARDNESSES) SHEET Thickness “t” in Inches No. of 2117-T4 (AD) Protruding Head Rivets Required per Inch of Width “W” No. of Bolts Rivet Size 3/32 1/8 5/32 3/16 1/4 AN-3 .016 6.3 4.7 .020 6.3 4.7 3.8 .025 6.3 4.7 3.8 .032 6.3 4.7 3.8 3.2 .036 7.1 4.7 3.8 3.2 2.4 .040 7.9 4.7 3.8 3.2 2.4 .051 10.1 5.6 3.8 3.2 2.4 .064 12.7 7.0 4.6 3.2 2.4 .081 8.9 5.8 4.0 2.4 3.2 .091 10.0 6.5 4.5 2.5 3.2 .102 11.2 7.3 5.1 2.8 3.2 .128 9.2 6.4 3.5 3.2 NOTES:
a. For stringers in the upper surface of a wing, or in a fuselage, 80 percent of the number of rivets shown in the table may be used.
b. For intermediate frames, 60 percent of the number shown may be used.
c. For single lap sheet joints, 75 percent of the number shown may be used.
ENGINEERING NOTES:
a. The load per inch of width of material was calculated by assuming a strip 1 inch wide in tension.
b. Number of rivets required was c alculated for 2117-T4 (AD) rivets, based on a rivet allowable shear stress equal to 70 percent of the sheet allowable tensile stress, and a sheet allowable bearing stress equal to 160 percent of the sheet allowable tensile stress, using nominal hole diameters for rivets.
c. Combinations of sheet thickness and rivet size above the underlined numbers are critical in (i.e., will fail by) bearing on the sheet, those below are critical in shearing of the rivets.
(1) In determining the number of rivets required in the example in Figure 4-18, for attaching the splice bar “B” to the upper leg, the thickness “t” of the element of area being spliced is 1/16 inch (use 0.064), the rivet size is 1/8 inch, and Table 4-9 shows that 9.9 rivets are required per inch of width. Since the width “W” is 1/2 inch, the actual number of rivets required to attach the splice bar to the upper leg on each side of the cut is 9.9 (rivets per inch) x 0.5 (inch width) = 4.95 (use 5 rivets).
(2) For the bulbed leg of the stringer “t” = 1/16 inch (use 0.064); AN-3 bolts are chosen, and the number of bolts required per inch of width = 3.3. The width “W” for this leg, however, is 1 inch; and the actual number of bolts required on each side of the cut is 1 x 3.3 = 3.3 (use 4 bolts). When both rivets and bolts are used in the same splice, the bolt holes must be accurately
Par 4-58 Page 4-49 reamed to size. It is preferable to use only one type of attachment, but in the above example, the dimensions of the legs of the bulb angle indicated rivets for the upper leg and bolts for the bulb leg.
n. Splicing of Intermediate Frames. The same principles used for stringer splicing may be applied to intermediate frames when the following point is considered. Conventional frames of channel or Z sections are relatively deep and thin compared to stringers, and usually fail by twisting or by buckling of the free flange. Reinforce the splice joint against this type of failure by using a splice plate heavier than the frame and by splicing the free flange of the frame with a flange of the splice plate. (See Figure 4-20.) Since a frame is likely to be subjected to bending loads, make the length of splice plate “L” more than twice the width “W2,” and the rivets spread out to cover the plate.
4-59. REPAIRING CRACKED MEMBERS. Acceptable methods of repairing various types of cracks in structural elements are shown in Figures 4-21 through 4-24. The following general procedures apply in repairing such defects.
FIGURE 4-20. EXAMPLE OF INTERMEDIATE FRAME STRINGER SPLICE (MATERIAL: 2017-T3 AL ALLOY)
THE NUMBER OF RIVETS REQUIRED IN EACH LEG ON EACH SIDE OF THE CUT IS DETERMINED BY THE WIDTH "W," THE THICKNESS OF THE FRAME "" AND THE RIVET DIAMETER "d" USING TABLE 4- 10 IN A MANNER SIMILAR TO THAT FOR STRINGERS IN FIGURE 4-
20.
NOTE b. IN TABLE 4-10 INDICATES THAT ONLY 60 PERCENT OF
THE NUMBER OF RIVETS SO CALCULATED NEED BE USED IN SPLICES IN INTERMEDIATE.
EXAMPLE: (FOR 2017-T3 aluminum alloy frame)
FLANGE LEG † = .040" d = 1/8" 2117-T4 (AD) W, & W 3 = 6 inch NO. OF RIVETS PER INCH OF WIDTH FROM TABLE 4-10 = 6.2 No. of rivets required = W x 6.2 = . 6 x 6.2 = 3.72 or 4 rivets.
60 percent of 4 rivets = 2.4 rivets.
USE 3 RIVETS ON EACH SIDE OF THE CUT IN EACH FLANGE LEG.
WEB OF ZEE (OR CHANNEL) † = .040" d = 1/8" 2117-T4 (AD) rivet W = 2.0 inches NO. OF RIVETS PER INCH OF WIDTH FROM TABLE 4-10 = 6.2 No. of rivets required = W x 6.2 =
2.0 x 6.2 = 12.4 or 13 rivets.
60 percent of 13 rivets = 7.8 rivets.
USE 8 RIVETS ON EACH SIDE OF CUT IN THE WEB OF ZEE (OR CHANNEL).
"L" SHOULD BE MORE THAN TWICE W2 Thickness of splice plate to be greater than that of the frame to be spliced.
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FIGURE 4-21. TYPICAL METHODS OF REPAIRING CRACKED LEADING AND
TRAILING EDGES AND RIB INTERSECTIONS
CHANNELS
TRAILING EDGE
FORWARD
HORIZONTAL
RIBS SKIN
O VERTICAL SPAR CRACKS CRACKS ANCHOR
NUTS FOR RIB
ATTACH-
MENT OF
FIN TO SKIN
STABILIZER
AND TIP CRACK TO FIN. TRAILING EDGE
REAR SHEAR
FRONT BEAM SPAR CRACKS
LEADING
EDGE AILERON NOTE: ALL REINFORCING PLATES TO BE OF REAR * SAME ALLOY & APPROX. 1.5 THICKNESS OF ORIGINAL Par 4-59 Page 4-51