Nuts & Bolts

Building Basics WHEN PEOPLE WHO ARE not in the know hear that airplanes have wooden components, their first mental image is of a really old airplane. This is an u n f a i r image. Although wood isn't used much in today's production airplanes, it's still a common building material in many modern—and not-so-modern— homebuilts. You can find wood structures in homebuilts from the ubiquitous Pietenpol Air Camper to the Fisher Flying Products fleet and the geodetic construction of the Loehle replicas. Even some composite airplanes—and many tube and rag designs—use wood for wing ribs and fuselage structures. Lest you think that airplanes made of wood are only of the low and slow

Wood for Wings Building with nature's composite

sandpaper —that most people already MICHAEL DIFRISCO have in their shops. Wood is safe, bevariety, picture the super-sleek, all- cause unlike composite materials, it's wood Sequoia Falco or the equally nontoxic. Heck, wood even smells pleasing lines of the fast Osprey Air- good, is aesthetic in its own right, craft GP-4. These wooden wonders and is satisfying to work with. But like everything in this world, boast performance figures that rival the latest composite speedsters. wood also has its drawbacks. Because So what makes wood so wonder- it's a natural material, stringent manful? Often called nature's composite, ufacturing tolerances do not exist. wood may just be the perfect aircraft We have to take it as it comes. Airbuilding material. It's strong. Pound craft-grade lumber must meet certain for pound, wood has twice the ten- requirements to be categorized as sile strength of aluminum. You can such, but hidden flaws—pitch pockform wood into complex shapes be- ets, knots, and other vagaries of nacause it becomes pliable after apply- ture—might be waiting to bite the ing copious amounts of hot water. unsuspecting builder. And wood rots. Wood is easy to cut, which means Without proper protection from exGl-UE 30INT WITH PRESSURE • SQUEEZE GLUE OUT— OONT STARVE 3OINT

i "NATURE'S COMPOSITE-

GRAIN SLOPE END GRAIN VARNISH HOLES WHERE HARDWARE GOES

POUND TOR

POUND STRONGER THAN ALUMINUM ADHESIVES AND

SCRAP WOOD FOR CLEAN CUT

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you can build an airplane with common hand tools—saws, chisels, drills, and

treme temperature and humidity, wood will disintegrate. Finally, Sitka spruce, the most common aircraftgrade lumber, is becoming harder and more expensive to get. Most Sitka spruce comes from trees in British Columbia and Alaska that are a half-century old or more. As finding and felling these trees becomes more difficult, the price of aircraftgrade Sitka spruce increases (it's doubled in the past five years).

made from it. This is not hardwarestore-variety plywood, mind you, but Mil-Spec aircraft-grade made from imported African mahogany or American birch veneers laminated in a hot press using waterproof glues. The core of aircraft-grade plywood is usually poplar or basswood. Lest you think corrosion is something that only happens to metal,

wood can also develop a sort of organic rust. Known as dry rot, this fungus is caused by too much moisture content and can render wood unusable. So when you store your wood—especially if your building project will span the course of several years—keep it in a well-ventilated area. A place with about 12 to 15 percent relative humidity is best,

Wood Basics

Sitka spruce is the most common wood used in aircraft, and contrary to popular belief, Howard Hughes' Spruce Goose was made of birch—not spruce. Spruce has one of the greatest strength-to-weight ratios and is considered the cream of the crop of natural aircraft building materials. Western hemlock is 14 percent stronger and can be substituted for spruce, according to FAA Advisory Circular 41.13, but it's slightly heavier and is more difficult to find. For Sitka spruce to meet government specification MIL-S-6073, it goes through a series of inspections, starting with experts who supervise the sawing and milling of the lumber at the source. The lumber is kilndried to another government specification, AN-W-2. This spec says the wood must have a specific gravity greater than .36, a minimum density of 24 pounds per cubic foot, grain slope no steeper than one in 15, and a moisture content between 10 and 17 percent. This Type A wood is approved for use in any aircraft

application, including use as supercritical spar material. Type B specs— lumber that is slightly less dense and may have more grain slope—can still be used in some applications, but not in any primary load structures. Plywood is another common air-

craft building material. It covers the wings of the aerobatic One Design and the fuselage of the Falco, and the gussets that strengthen wood wing ribs and fuselage structures are

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Building Basics because wood that gets too dry can become brittle.

Cutting

Wood can be cut and shaped using basic hand and power tools. A table saw is a great timesaver, and some homebuilders wouldn't attempt a wood-based project without a band saw. When cutting wood, grain direction is of paramount importance. Follow the plans or your builder's manual and orient your cuts according to the designer's specifications. Wood is the strongest in the direction of the grain, so if the builder's manual doesn't specify, the long axis of the part you are cutting should be parallel to the grain.

Gluing

Glue is the most common way to fasten wood to wood, and one of the

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most often used aircraft wood glues is T-88, a two-part epoxy structural adhesive. Epoxy-type glues can fill gaps in wood joints up to 1/16 of an inch. Resorcinol glues require a tight joint fit because they do not fill gaps. Follow the designer's recommendations regarding which structural adhesive is best for your project. When gluing two pieces of wood together, make sure the joint is clean and free of sawdust or oils that might interfere with the bonding process. Wood should always be glued under pressure, so have a variety of clamps on hand. You can never have too many clamps, as any builder will attest.

Protecting Wood

To keep the wood's moisture content at the ideal level—dry enough so it doesn't rot, moist enough so it doesn't become brittle—all wood

components must be protected with varnish. Varnish seals in the moisture and prevents excess moisture from attacking your airplane. If you are covering your airplane with fabric, you need to use a twopart varnish because the chemicals used in polyester fabric covering systems will lift and not adhere well to one-part varnishes. You gotta love an all-natural aircraft material that's forgiving of mistakes, is relatively easy to work with, and smells like dad's workshop. And there's nothing like using the same material the Wright brothers used to build their Flyer, which connects you to homebuilding's roots. • For a complete education on working with and inspecting wood, see Ron Alexander's Aircraft Building five-part series in Sport Aviation, beginning in December 1998 and concluding in the April 1999 issue.

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