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Wood for Wings: Building with nature's composite
By Mike DiFrisco (originally published in EAA Sport Aviation, May 2000)
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 unfair 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 variety, picture the super-sleek, all-wood Sequoia Falco or the equally pleasing lines of the fast Osprey Aircraft GP-4. These wooden wonders boast performance figures that rival the latest composite speedsters.
So what makes wood so wonderful? Often called nature’s composite, wood may just be the perfect aircraft building material. It’s strong. Pound for pound, wood has twice the tensile strength of aluminum. You can form wood into complex shapes because it becomes pliable after applying copious amounts of hot water. Wood is easy to cut, which means you can build an airplane with common hand tools-saws, chisels, drills, and sandpaper -that most people already have in their shops. Wood is safe, because unlike composite materials, it’s nontoxic. Heck, wood even smells good, is aesthetic in its own right, and is satisfying to work with.
But like everything in this world, wood also has its drawbacks. Because it’s a natural material, stringent manufacturing tolerances do not exist. We have to take it as it comes. Aircraft-grade lumber must meet certain requirements to be categorized as such, but hidden flaws-pitch pockets, knots, and other vagaries of nature-might be waiting to bite the unsuspecting builder. And wood rots. Without proper protection from extreme temperature and humidity, wood will disintegrate. Finally, Sitka spruce, the most common aircraft-grade 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 aircraft-grade Sitka spruce increases (it’s doubled in the past five years).
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 kiln-dried 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 aircraft 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 made from it. This is not hardware-store-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, because wood that gets too dry can become brittle.
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.
Glue is the most common way to fasten wood to wood, and one of the 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. Casein and 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.
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 two-part 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.