IP

i From right, the original Sonex Fiberglass wingtip, left and right halves of flow-forming blocks/dies, and the tack-welded wingtip of 5052 0.032-inch aluminum sheet. A CRAFTSMAN HAS TIMES IN HIS

life when he j u s t has to make something out of sheet metal. It may happen after too much of the domestic work of fabric covering, or it might be stimulated by the boredom of s a n d i n g primer, catching up on paperwork, c l e a n i n g the shop, or painting the garden tools. The itch is there. It must be scratched. Wingtips can be frustrating. This is a fact. They

Flow-Forming Wingtips Form Blocks, Part 2

KENT WHITE

do not have to be f a n c y structural tips like those on the Globe Swift, with several stiffening beads on both surfaces, stepped edges, and interior ribs. No, w i n g t i p s can be fairly simple, like those on a North American P-51 Mustang or even a Sonex, and still be plenty interesting. First, they have to fit Pulling the concrete welding jig free makes me glad I nice and tight against the used the wax and the optional embedded steel handle!

end of the wing, and they should match each other fairly closely (even though Steve Wittman was known to have flown lengthy research and testing hours with two completely different tips on Buttercup). Making these wingtips freehand, or "in space," requires some kind of a rigid mock-up or "buck" to fit the formed pieces to. Once the i n d i v i d u a l halves are made to fit the mock-up, they must then be oriented correctly for welding to avoid twisting or warping off the plane of the w i n g . Once the w e l d i n g jig is done and that first pair of tips has been completed, things settle into a smooth flow if more are needed. But getting the first pair done, that can be frustrating. Let's bypass the process of making a pair freehand Sport Aviation

95

Craft & Technique and go on to a method of construction that involves making only one receiver die and a single welding jig for one set of tips. I'll be working from a new Sonex Fiberglas wingtip. No design problems, no development headaches, and this one tip flips to fit either side. I just have to build the three forms from a single existing part. I

begin by making the welding jig. This part is easy because all I need is a rigid form with which to assemble and hold the two halves for welding. I thoroughly wax the inside of the plastic part and then fill it with concrete or "patch all," a fast-setting cement-type material. Adding a welded steel handle while the mix is wet should enable me to

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NOVEMBER 2001

hang onto the thing, as 1 expect it to escape me like a greased pig otherwise. The D-shaped handle is a 3/8inch mild steel round bar welded to a 3/4-by-3/4-inch angle iron. The angle is embedded into the concrete, providing a solid anchor for the exposed handle. As discussed in the September "Craft & Technique," I next make up the receiver into which 1 will pound the sheet to make one-half of the wingtip. As before, 1 hot bend some 1-by-l-inch angle iron to roughly match the outer edge of the part, keeping it flat along the upper flange where the one-half of the tip will mate to the other. Any warping will yield an ill-fitting pair of halves, something even a 1/4-inch "gap" rod might not fill when welding them together. Once the angle is bent and trued to lie flat, I cobble up a crude box to retain the mushy mix of plastic necessary for the molding operation. Don't worry about small leaks or gaps in the box construction because the first smear of mud will close them. I use mild steel anywhere from 18- to 20-gauge in thickness, and I am not particular in which welding method I use to join the parts. Once the box is finished I add a pair of indispensable horizontal legs, one at each end of the box and beneath it to enable easy and essential clamping to the workbench. I finish by sandblasting the box completely, and then by making its mate. By mixing the filler in pint containers I can gradually build up the obvious voids between the part and the cavity. I make a first pass with the plastic, filling the gaps in the box and making preliminary radius edges that are more easily filled to a good density. Checking with the part installed enables void filling early on and sets me up for an accurate final pour. Warm days allow for less added hardener, if my mixing and pouring speed is meant to be done at a re-

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Craft & Technique laxed pace. Additionally, less hardener makes for a stronger mold. I am using good auto body filler,

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All the edges get worked first, with some gently added detail, and then the lower larger areas are worked with the larger tool.

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The finished flow-formed tip halves are smooth ready-to-weld mirror images, and the tooling is ready for more.

up a half-gallon of filler in a gallon can and dump it into the cavity. The welding jig is loaded into the Fiberglas wingtip, the outside of the wingtip is waxed so it will release from the plastic filler, and the nowheavy and slippery tip is smushed down into the bed of wet filler. I wait until the filler is rubbery before trimming the excess; wading in too early just smears it all over the mold, the bench, and the tools. The part lifts easily (thank you wax and handle), and I clean the mold with a knife and sandpaper and then Sport Aviation

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I begin forming with a 2X gun that is throttled down to a nice sedate pace. The small 1.25-inch diam-

eter urethane head is used for edge

radii, and I can easily shape it with a sander to meet any contour requirements. I next form the delicate stepped edge with a steel roundedchisel tool, using very low air pressure, as the mold will erode if I don't. Saving the lowest and largest area for last, I switch to a 3X gun and use the larger forming head with a lowcrown face. Setting the air pressure low enough to move the metal without leaving marks, I gently flow the metal into the mold until it resists me. When this happens 1 re-anneal the sheet, re-clamp it in the correct location, and flow it some more. Some simple planishing may be needed on the first part to smooth out some marks, but the second should be really nice. I do take a little time to work the metal more in places where it w i l l not quite lie down. Sometimes this takes a last an-

neal, and sometimes it takes a little

stretching with a hammer and dolly. Overall, this is a very satisfying way to make parts! Next up—wing ribs.

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 send snail mail to 17167 Salmon Mine Road, Nevada City, CA 95959. , Jd£>

For more information, visit SPORT AVIATION on the Web at www.eaa.org

100

NOVEMBER 2001