Nuts & Bolts

Craft & Technique

Some primer and some paint make it look like what it ain’t (plastic). But we left the weld seams showing through the paint, just for fun.

THE PROTOTYPE PROCESS CALLS FOR Patterning the scoop’s front the fabrication of either parts view is next, using the same or a single aircraft to prove the cut-and-snip-and-tape airworthiness of a new design method as on the profile. It, or to improve some aspect of too, is taped in place to fill out it. Once the prototype is the 3-D aspect. Once finished Hand-working a custom modification proven, production fabricawith all the little trimming tion methods are employed, and taping adjustments, we KENT WHITE usually with much of the determine the width of the dimensional and assembly base flange that will connect information taken from the protoWorking from a 3-D pattern is the scoop to the cowling, taking type. essential, so I begin by cutting white into account how much it will take This hand-working process also poster board into a rough profile. to step over any rivet lines, stiffenkeeps old birds in the air or modifies After a few versions that are too fat, ers, or brackets already in place. I them. Some old aircraft use an too short, or too long we arrive at now tape out this “safe area” for fasexternal scoop to feed fresh cold air one that satisfies both the carb inlet tening the scoop onto the cowling, into the carburetor, increasing area and complements the look of and then I make the base flange fit horsepower by providing air of the existing custom cowling. To within those confines. greater density. The owner of a cus- simulate the finished product I careTransferring the cardboard pattom Stearman wanted such a scoop, fully tape this cardboard piece into terns to 1/2-inch plywood, which is and rather than borrowing one place, and sometimes spray paint it cut to match, gives us the start of a from another design that sorta to match the cowling, making the hard 3-D mockup or “station buck.” looked okay, we designed and built appearance as real as possible. I fasten these first two pieces to a a custom scoop. plywood base using drywall screws. The frontal area was designed first The finished scoop is hand-formed The “master” panel is gently formed in cardboard and then checked for aluminum, ready for primer, paint, to the “buck,” before stretching size and shape by mocking it up on and installation. and shrinking it to final fit. the airplane.

Getting the Scoop

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Craft & Technique Seeing the wood framework for the first time, I realize that it needs a few more pieces for a more “solid feel” and give my metal parts more support and confirmation when fitting them. Interpolating the curves as I go, I make a few “paper dolls,” transfer them to plywood, and then screw them in place. A flexible ruler checks the contours and a last bit of sanding gives the edges a smooth series of curves. To cover this buck with metal I start by making some panels out of butcher paper. I flow the paper over the contours, imagining where I’ll put the welds and what corners I can get myself into. Breaking a part into bite-sized pieces is a stumbling block to many craftsmen, but I look for pieces that are a handy size to work with. If at all possible, I try to have breaks, joints, or seams on high crowns. Centering the seam on the high crown limits the amount of weld-related distortion

and confines it to the immediate vicinity of the weld, rather than letting it infect a large area of carefully worked metal. Maybe it’s Murphy’s Law, but making a new part is usually a learning experience because you always find a better way to make the second one. After tinkering with the paper, I decide on a four-piece scoop. The main section will be on the top, from front to back, then two sides, and with a little chin section under the intake opening. After cutting out the paper patterns, I tape them on the sheet metal, allowing for about a 1/2 inch of excess, total. Using a variety of snips and shears to cut out the material, make one long continuous cut by keeping the cutter fully engaged in the material. Otherwise, the cut becomes jagged with “Dutch daggers” created by reengaging the cutter in a series of new directions, segmenting the

curves. Deburring the edges (or wearing lightweight Kevlar gloves) will save blood and leave a nicer looking pair of hands at the dinner table. Typical layout tools include the black marker, soft lead pencil, ballpoint pen, and layout dye. With the fluids or dyes, remember that heavy scribing will cause stress risers. These risers can lead to premature part failure when operating stresses and vibration work on that nick, gouge, or cut and grow them into a crack. Take great care to sharpen the scribe or dividers and use only very light pressure when scratching the surface. Use a scribed line only when shearing or cutting; use marking pens for hems, bends, and folds. Mark every detail possible, like “bend up,” “fold down,” “inside,” “right,” and “up.” Even though you may feel foolish being so anal, think how you’ll feel after making two LEFTS.

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When deciding which piece to form first I always start with the easy ones because I never know if I’ll quit or get fired. Seriously, some parts may have a master piece, one that must precede the others if the others are to have the proper confirmation throughout the assembly and fitting process. The scoop’s large top center section will work well to locate all the other pieces, and because its shape is autonomous, it’s the “master.” Shaping the .050-inch 3003 halfhard aluminum into the center panel’s proper contours requires a little stretching across the main bend, simple radius bends along the sides, and a little shrinking on the sides at the bend. I put the stretch in first, using a shot bag and a mallet. The sides are radiused over a Tstake having a 1-inch diameter using the plastic dead-blow hammer, and the little edge shrinks are done with an auto body dolly and the dead-blow hammer. Trying the panel for fit several times, I mark inconsistencies as I go. I make sure to stretch it a bit more where it hits the buck and shrink it a bit where it lifts too high. With the top finished, I lay out the sides, blanking them large enough to flange their bottom edges and to overlap their top edges. The sides need to curve from top to bottom, with a little edge shrinking at the upper corners and a nice sharp radius at the lower rear to transition onto the back of the scoop. After the sides meet the top and fit the buck, look at the proposed weld seam. Plan to keep it centered on the scoop’s corner radius. To create a straight trim line, connect points along the center with a flexible ruler and create a line of marker pen dots. Whether the line masters from the top or the side makes no difference, as the goal is simply to have the seam straight and centered on the radius. After trimming the panel, reinstall it on the buck and scribe off it onto the related part. Cutting the Sport Aviation

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Craft & Technique

The sides are next, with the “chin” being last.

scribe line right on the money makes a butt weld and not a lap weld. Make witness marks at a few points along the way, to ensure correct alignment when you weld. Only when all three parts are fit and trimmed to each other do you think about tack welding them together. Don’t mind cooking the buck when tacking the parts in proper alignment; the aroma of pinewood smoke is a small sacrifice for accurate parts. In some cases, I fit against plastic, fiberglass, or plaster and thus avoid tack welding on the buck, but in this case—it’s time for the barbecue! After welding the sides and gently working the seams true again, refit the assembly and begin to work out the front, or air inlet. Bend a simple U over the ever-handy Tstake that’s set in against the front of the scoop. Work the lower corners into each area to make the tight little transition we saw earlier at the back. Little details like these make the part look nicer. Tangling around the opening with every small contrivance I can bang the metal with, I finally get the thing formed, fit, snipped, and tacked. I finish working it as I weld, a nifty little process I call working in. (You will see this later, too.) I prefer gas welding over TIG for this panel working because TIG beads are thick and heavy and the penetration is mediocre without expensive back purge. The oxy98

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All four pieces are formed and fit, with witness marks keeping them aligned during tack welding.

Checking left and right contours with a contour gauge.

acetylene weld beads are faster to make, cheaper, and thinner and flatter, enabling one to easily planish the work smooth. The appearance side is then bumped up and down gently with a dolly to level it up. When all contours are fairly close to confirmation, planishing with a spoon or slapper may commence. Metal finishing is defined as “smoothing by removing metal” and includes grinding, filing, sanding, and polishing. FAA Advisory Circular 43.13-1B, Acceptable Methods and Practices, Chapter 4, Section 5, Paragraph e7 says: “Do not file or grind welds in an effort to create smooth appearance, as such treatment causes a loss of strength.” A light kiss with the file on the high spots of the weld-bead only should allow for good planishing, which will suffice for any paint-grade part, leaving some good primer-surfacer coats to finish up.

After welding and planishing the entire scoop, the intake is formed to receive a bit of detail.

An edge return is the last part made, a little detail that finishes the intake. I make it by measuring the opening’s circumference, cutting a 3/4-inch wide strip of aluminum stock, forming it to fit the opening minus the edge radius, and then welding the ends to close the loop. Then the band is radiused at the edge to fit the opening. When installed for the first time, the fit is a little…surprising. Time to gather up your courage and tap around the face of the opening, closing up the gap, and truing it up in all planes for welding. When it is somewhat close, tacking commences, and at the same time tapping away with the spoon to get the contour just right. When tacking is done, I weld the whole thing from A to Z. A little planishing, a touch with the file, and it is ready for paint! EAA Technical Counselor Kent White achieved master technician’s status in 1976 at Harrah’s Auto Collection, where he restored metal components for aircraft and autos. He started his own metal restoration company in 1977 and now teaches, writes, and develops tools for metalworking while he still pounds out parts. He encourages any welder or metalworker, man or woman, to contact him in regard to preserving the traditions of aircraft metalworking. To contact him, call 530/292-3506, e-mail [email protected], or snail mail 17167 Salmon Mine Road, Nevada City, CA 95959.