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RAFTSMAN'S CORNER Edited by Chuck Larsen
How To Construct A Scarf Joint Repair To A Wood Wing S p a r . . . In The Wing By Tom Glenn, EAA 127063 1010 13th St.
Bellingham, WA 98225 You own an airplane with wing spars formed of wood. Following an annual inspection your Inspector announces, "I've found some bad news in the right wing — a crack in the rear spar about 3-1/2 feet from the outer end." (I am quoting from an actual experience with my own 1946 Taylorcraft BC12D floatplane last spring.) You climb the stepladder, flashlight in hand, to peer through the 3-1/2 inch inspection hole on the underside of the wing just outboard of the rear strut attach point. At first you see nothing; then as your eyes adjust and focus, you sweep the light beam back and forth and . . . oops! there it is, a crack leading away from a bolt hole. To the unskilled eye, the task of repairing the crack looks (first) impossi-
ble, (second) bewildering and (third) costly. Wrong on all counts. Here's how to proceed. My partner in T-Crafl N39976 is a machinist, skilled in mechanics and metal working with some aircraft experience. I am a long-time amateur woodworker. We combined our skills to repair the cracked spar ourselves, did a firstrate job and saved a good deal of money. Looking back over the job, we observed a number of lessons we learned whose explanation here might save other enthusiasts time, money and anxiety. Here is our step-by-step procedure for performing the job successfully. First, work out an overall program with your FAA Inspector. Level with him. He must supervise your work. Tell him exactly what you propose to do and how you will proceed. Get his agreement before you begin any work. Agree on time intervals at which he will inspect your
progress. Then follow your program. The "Bible" for the performance of all aircraft repair work is FAA Publication AC 43.13-1 A. It contains a wealth of information on all aspects of aircraft rehabilitation and repair. Chapter 1 deals with wood structures. Our description given here tells you how we applied the techniques and methods set forth in Chapter 1. Another reliable guide will be the nearest EAA Chapter. Members of EAA have earned a wide reputation for being skilled, dedicated and generous with their time and counsel. Ask for help. STEP ONE: Remove the wing and set it up flat on padded supports in as roomy and well-lighted workshop space as you have available. In our region that means inside a building and in winter, heated (you will need a modicum of temperature control in STEP TWELVE). STEP TWO: Like a surgeon, you will need to "open up" the patient. If your
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wing covering is serviceable, it will not be necessary to remove and replace it entirely. Instead, slit the covering on both upper and lower surfaces so that it may be laid back to reveal the cracked spar and to give you just enough room to perform the work. STEP THREE: The miracle you are about to perform is called a Scarf Joint built inside the wing without removing the entire spar. (See Sketch A) Scarfing is a method of removing an unsuitable portion of a wood beam or spar and replacing it with a sound piece using a glued joint whose strength is equal to that of the original spar. One key to this process is correct location of the joint. So now, with the wing structure laid bare before you, study the crack and the spar to locate the best spot for the repair. This will be determined by the total length of the scarf joint including reinforcement, or cheek plates, all of which must fall between two adjoining wing ribs (this will avoid having to remodel a wing rib where it attaches to the spar). So select a rib bay inboard of the crack which will produce a scarf cut to remove the least existing spar wood while getting rid of all the unsound material. To calculate the length of the scarf joint, move on to STEP FOUR: Lay out lightly in pencil the line of the scarf joint on the edge of the spar, following the guidelines in Chapter 1 of AC 43.13-1A. In Figure 1.4 we find the critical proportions of the scarf slope to be: optimum (recommended) — 12 to 1 and minimum 10 to 1. Apply these proportions to your specific situation. For example, let's use a spar thickness of 3/4 inch. The optimum scarf length would measure 3/4" x 12 or 9". The minimum allowed is 3/4" x 10 = 7-1/2" long. Your aim is to attain the longest joint up to 12 to 1 which your rib bay will permit. Note that the scarf can slope in either direction, toward you from left to right, or away. Which is determined by how you will operate the hand saw to make the scarf cut in the existing spar. Make it easy on yourself, think ahead: how will I make the cut? Now calculate the dimensions of the two reinforcing or cheek plates, one for each face of the spar. Again, consulting Fig. 1.4 in AC 43.13-1 A, thickness must be 1/4 of the spar thickness: in our example, 3/4" x 1/4 = 3/16" thick. Width is same as spar. Overlap at centerline of spar is 3/4". Overlap at joint end must be at least five times spar thickness, or 5 x 3/4" = 3-3/4" (optimum is 6 x 3/4" = 4-1/2"). So, total
length (acceptable) of each cheek plate is (a) one-half scarf length, from STEP FOUR — 3-3/4"; (b) centerline overlap — 3/4"; (c) end overlap — 3-3/4" = 8-1 / 4" (or 9" optimum). Total length of the optimum completed scarf joint is (a) cheek overlap x 2 (each end) — 9"; (b) length of scarf
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(12:1) — 9". Total is 18" (a length seldom attainable in a rib bay of a light aircraft). The minimum acceptable joint length is (a) cheek overlap x 2 — 7-1/2"; (b) length of scarf (10:1) — 7-1/2" =15". You must find 15" of clear space between ribs but won't need more than 18". Take the longest available space up to 18". Let's say you have 16" available (see Sketch B). Use it all and your scarf will produce a slope of 8-1/2" divided by 3/4" = 11-1/3:1, considerably better than the 10:1 minimum. To complete the layout, draw the line of the scarf on the upper edge of the spar using a sharp-pointed 2 or 3H pencil and a line on each face of the spar where the scarf line intersects it. Your sawcut will follow these three lines. STEP SIX (The Secret): Construct a simple two-piece, glued, wooden jig to accurately guide the sawcut along your scarf line (see Sketch C). The lower piece of the jig clamps to the near face of the spar; the upper piece guides the saw. We made ours of maple and the saw guide part was 2" thick and as long as the rib bay to the left of the sawcut would permit, to give the saw a firm backing to work against. Faces must be true and square. Join the two pieces by using a good shop glue like Elmer's Carpenter's Wood Glue. STEP SEVEN: Clamp the saw jig onto the spar, on the scarf's inboard portion, so that the saw guide aligns perfectly with the scarf line you have drawn. Position the jig so your sawcut will barely save your pencil line on the inboard portion of the scarf. Position the two clamps so as not to interfere with strokes of the saw. We next rubbed powdered graphite on the saw guide face of the jig to make the saw slide easily. STEP EIGHT: Make the sawcut. This will take concentration to hold the saw blade firmly against its guide (with another block of wood held in place by the finger tips of the left hand) and patience because space limitations shorten your saw strokes and your progress will thus be slow. The secret of a proper
cut is to diligently keep the flat of the saw completely and continuously against the guide face of the jig as you stroke. A word on saws: first choice is a Japanese cabinetmaker's saw (Ryoba Noko Giri), a fine-toothed, thin-blade crosscut which cuts on the pull stroke. This saw, in the hands of a careful workman, will give the truest cut. Second choice is a regular back-saw, similar to a hand mitre saw, deep enough to cut through the spar before bottoming on the underside of the "back" or thickened upper edge. The standard hand crosscut or rip saw is not the best tool for this job. A thin, wide, fine-toothed saw is needed; and even though the cut to be made is almost parallel to the grain, a crosscut works better. If you use a back-saw note that, as the sawcut proceeds, the jig can be removed and the depth of the guide block lowered by ripping off successive layers 1/2" to 3/4" thick from the upper surface using a table saw; this will enable the back-saw to cut deeper before bottoming. With no more than about one-fourth of the cut remaining, the jig can be removed altogether; the sawcut in the upper 3/4 of the spar will thereafter provide its own guide to the remainder of the cut. Be careful to follow the two end guidelines on opposite faces of the spar. Any cleanup of the face of the scarf, to remove ridges and otherwise make it flat and true, can be done with a cabinet scraper, a simple and remarkably effective and little used hand tool. Be careful not to injure the feather end of the cut. DO NOT SAND any faces to be mated by gluing. STEP NINE: Remove the scrap end of the spar from the wing. All fittings and their fastenings, including the wingtip frame, attached to this piece of the spar must, of course, be removed first. It can then be eased out of its position in the wing. Save it as a pattern for the replacement piece. STEP TEN: Acquire a suitable piece of spruce or other acceptable wood specie and construct the replacement piece. Follow the guidelines of Fig. 1.2, SPORT AVIATION 61
Ch. 1, AC 43.13-1A for material choice. Also, the September 1984 issue of SPORT AVIATION contains an excellent article evaluating alternative wood species for use in aircraft structural components. Your local aircraft builders supply house will be a reliable source of counsel and materials, or one advertising in SPORT AVIATION. I know of only one manufacturer today still offering aircraft quality sitka spruce in the various sizes and any quantities required by homebuilders, name and address furnished upon request. Suitable aircraft quality plywood for the cheek plates can similarly be obtained. Chamfer the ends of each plate at a 45 degree angle. Remember, when you lay out the new spar piece, the cracked piece you removed is the thickness of the sawcut (kerf) short of the true length needed. The matching scarf can be produced on the new piece by use of any of several woodshop machine tools such as a large-diameter table saw or a bandsaw. It can also be done by hand, as was the scarf in the wing. In any case use an accurate jig and enjoy all the working space you will have while cutting this half of the scarf joint.
Once cut, the new scarf piece can be slipped into position in the wing to verify precise fit to its mating piece. Treat the feather ends gently. STEP ELEVEN: Glue the scarf. Use a glue which conforms to Para. 6 of Ch. 1, or another suitable product (epoxy) developed since that date of publication (1972), one which is recommended by your aircraft building supply house and approved by your FAA Inspector. Carefully follow the manufacturer's directions. Line up the two mating pieces carefully. Use wood pads under all clamp faces and wax paper between pads and work. Wipe away all wet glue dribbles. Maintain temperatures and clamp pressures as specified. Keep glued surfaces meticulously clean. Don't skimp on the details. A tip to hold the scarf accurately in place while setting up the clamps: drive a steel brad into each end of the scarf 1 -112" in from the feather ends and midspar. Drill guide holes slightly undersize first. Length of brad is 1/8" less than thickness of spar; drive brads so that both ends lie below surfaces of spar and thus will not interfere with gluing of cheek plates. Putty the two brad holes not covered by cheek plates in opposite
faces of spar before varnishing. Glue the scarf itself first. After it has set, clean away any hardened glue dribbles and glue the cheek plates (one at a time is easier than two at once). Clean up again and lightly sand in preparation for... STEP TWELVE: Coat all new and reworked old wood with top-quality spar varnish. Again, follow manufacturer's directions. Allow first coat to dry thoroughly before applying second coat (Ch. 1, Section 2, AC 43.13-1A). STEP THIRTEEN: Reassemble and fasten all interior fittings and elements. Have work inspected officially, then proceed with recovering in accordance with Ch. 3 of A/C 43.1 B-1 A. Replace the wing, restore the aircraft to airworthy condition and present for final inspection. This repair, properly done, will serve as well as a complete new spar and save you time, money and a great deal of extra work. Tom recommends the use of the Japanese cabinetmaker's saw. His source of supply is the Japan Woodworker Catalog, 1731 Clement St., Alameda, CA 94501, phone 415/5211810.
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