Craftsman's Corner

Aircraft wood is best cut with sharp tools, and sandpaper ... EAA member John ... wood. The technician told him that those are fine as long as the dust is removed ...
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Ben Owen

Part 2

FLAT- NO

45'-MAX I

Aircraft wood is best cut with sharp tools, and sandpaper is not necessarily the proper way to finish a surface to be glued. Epoxy works best on reasonably rough, unsanded finishes, and a sharp knife or the sharp edge of a piece of glass makes a better finishing tool than some sandpapers. EAA member John Zark of Streamwood, IL says he spoke to a technician at Chem Tech, Inc. (T-88 Epoxy) since he was using their adhesive, a garnet sandpaper and an aluminum oxide sanding disc on his wood. The technician told him that those are fine as long as the dust is removed from the wood by wiping with the edge of a piece of glass. He also said to clean up any wood that is very rough and hairy (cut with a dull blade) as the epoxy tends to stick to the hairs and does not give a good bond. At least not as good as it could be. The technician also advised against using regular sandpaper and wet or dry paper. It seems wet or dry leaves a residue, which can cause poor bonding. Aircraft wood is very expensive and should be stored and preserved properly. It should not be set on cement floors or stuffed into the roof rafters without proper flat support. Wood should be supported at least every four feet to keep it from warping, and the ends should be shellacked, varnished or coated with paint to keep it from checking. Fungicides should be applied when storing in a damp climate, and before varnishing. Polyurethane varnish is an excellent finish for wood, inside and out. Wings covered with plywood panels should have the panels carefully placed on the wings and a pencil outline drawn of ribs and spars where the glue goes. These lines should be taped over, and the entire inside surface should be varnished, then the tape removed and glue applied where the tape was before gluing it down. Some thought should be given to preserving the interior of wood structures on aircraft. For instance, since the wing has dihedral, and most

42 MAY 1989

NOT OK.

of these aircraft are taildraggers, a place where water can collect is against the outside edge of the rib at the front face of the spars, front and back. This is the point at which holes should be drilled and seaplane grommets inserted to help with drainage. It should not be possible for water that works its way inside structures to sit in pools against aircraft wood. Aircraft wood is usually light and strong, but doesn't have much resistance to rot. By the way, there is no such thing as "dry rot", a term used of old as the water had dried out by the time the rot was noticed. Water is required to be present against the wood to give the rot fungus a place to gain a foothold., You wouldn't expect in metal aircaft construction to be able to leave a 1/8" gap between materials, nor would you expect to be able to do this with composite structures. Aircraft woodworking does require craftsmanship, and even though modern epoxies can fill gaps, you can see from the prior discussion of the older glues, properly fitted joints are required. It is typical in aircraft construction to discard a lot of parts that simply aren't of high enough quality. That is where the education comes in . . . the recreation comes later! Another part of education is in learning that glues and, particularly, fungicides should be handled with care. They can be dangerous to your health, so please read the labels first. It might also interest you to know that the military specifications for wood and plywood are not enforced by the government. We have to trust the manufacturer who puts the stamp on aircraft

wood indicating it is to Mil. Spec, and that it is indeed of that quality. When the government was checking and approving wood, the specification called for checking at least one in every 20 pieces of wood, and it was still necessary for the user to know enough about wood to be able to select the quality needed. I recently had the occasion to see some rough cut aircraft spruce that had been improperly stored and supported on about 10 foot centers, it had warped, sagged and its ends had checked because the wood was not varnished or painted on the ends. About $400 worth of aircraft spruce was no longer suitable for aircraft use because of improper storage and care. Some of the spruce lumber we see comes from trees that are hundreds of years old. The supply is extremely limited in aircraft spruce. Quite frankly, it deserves all the respect and care we can give it. Aircraft plywood is another matter. Quality has to be carefully selected. "Candling" with a strong light underneath thin plywood can help you to detect voids. A major manufacturer of wooden rowing shells (rowboats) fabricates his own plywood from veneer and slow-drying epoxy. One of the beauties of epoxy is that it almost totally eliminates the need for pressure in bonding. Pressure is an absolute requirement for the old glues in bonding. An excellent free two-page article on how to determine the proper pressure for gluing aircraft wood is available from EAA's Information Services office (send stamped self-addressed envelope) if you decide on using other than epoxy.