Form and Function: Aircraft Wood

Newcomers to aviation may be surprised to learn that the primary structural components in some airplanes are made from wood. The use of wood in new aircraft is increasingly rare, but you still see it. The Pitts aerobatic biplanes, which have educated countless aerobatic pilots and won so many aerobatic titles, have wood wing spars and ribs.

Many of the vintage and classic airplanes still flying have wood structures. Not all wood airplanes are small, either. The de Havilland Mosquito, a twin-engine British bomber from World War II, and the huge HK-1, which is often called the ?Spruce Goose,? are both all-wood airplanes.

If you were around aviation before World War II, you?d be familiar with wood as a primary aircraft structural material. The Wright Flyer was virtually all wood, except for fittings, fasteners, and the cotton-cloth wing and canard skins. Wood continued to be an extremely significant aircraft structural material until World War II, when production capacities, supplies of quality wood, and rapidly increasing structural stresses made wood structures obsolete.

Generally, you?ll see two types of wood in aircraft ? solid wood and plywood. Solid wood is used for such parts as wing spars, rib-cap strips, and fuselage stringers. Plywood is used for such things as floor boards, wing skins, and gusset plates.

Aircraft-grade plywood bears little resemblance ? in quality or price ? to what you find in the lumber yard. It has no defects, knots, holes, or gaps. The grain in each ply (or layer) is at either 45- or 90-degree angles to adjacent plies, and the plies are bonded with high pressure and temperature using waterproof glue. A thin sheet of aircraft plywood can easily cost more than $100.

Wood, when applied to aircraft structures, has significant positive ? and negative ? characteristics. On the positive side, wood has a high strength-to-weight ratio, meaning that it?s strong for its weight. Wood also has good flexibility characteristics, in that it will flex an indefinite number of times without fatiguing and eventually failing as metal does.

Wood?s flexibility enables it to absorb and dampen vibration. Anyone who?s ever flown behind a wood prop will tell you how smooth it is compared to the same aircraft/engine combination driving a metal prop. Finally, wood is easy to shape and it doesn?t require expensive, heavy tooling.

On the negative side, it?s becoming difficult to get aircraft-quality wood in the sizes required for parts such as wing spars, and with its rarity comes a high price. In an aircraft such as a Taylorcraft BC12-D, the front wing spar is .75 inches thick, 6 inches wide, and more than 16 feet long. Federal regulations require that the wood?s edge grain runout be less than 1:15 (when parallel to a level surface, the grain doesn?t deviate from parallel more than one inch over a 15-inch distance). The wood should have no knots, grain imperfections, or discoloration that indicates any kind of fungal activity. The blank piece of spruce for a spar such as this will cost well over $5 a foot.

Because finding aircraft-grade wood in the necessary size is difficult, you?ll see splices on many spars at some point along their length. Splicing a spar isn?t difficult, but the splice must be in the proper location, and the quality of workmanship and materials must be excellent. A properly executed spar splice will actually be stronger than a single piece of wood.

At 125?F wood loses approximately 25 percent of its structural strength. In direct summer sunlight, the internal temperature of a wood wing tied down on a paved ramp can easily top 180?F if the wing is painted a dark color and isn?t properly ventilated. Wood structures are made strong enough to offset this loss, but the extra beef means a weight penalty.

Wood is also subject to attack by fungus, minute plants that grow and feed on wood cells when the wood?s moisture content rises above 20 percent. Ideally, wood used in aircraft structures should have a moisture content between 10 and 12 percent. A coat of varnish will protect wood parts; but if you leave it unprotected, the wood naturally absorbs moisture from the air. Ever wonder why wood doors in your house stick or won?t close as the seasons change? If wood absorbs moisture it swells, fosters the growth of fungus, eventually gets soft (decay or rot), and will fail under stress.

Wood used as a structural member must withstand tension and compression stresses, which means it must be designed accordingly. As a spar flexes under a load, the bottom of the spar is under tension, and the top side is compressed. Wood is only half as strong in compression as it is in tension so the spar must handle the compression forces at work on the top side. When the time comes to re-cover the wood wings of an aerobatic aircraft, the spars often reveal compression failures.

Fastening wood components to other wood components isn?t as easy as finding a hammer and nails. Glue is the connection of choice, and to get a good bond the wood parts must fit together perfectly. A bad glue joint can be the structure?s weak link. Even if the joint is good, the glue itself might weaken its grip with time.

In decades past casein glue was the aviation classic. But this glue, which is made from milk by-products and water, has been obsolete for aircraft use for years because of its poor long-term adhesion, and its tendency to dry and crack at the glue line. Modern glues have all but eliminated the problems associated with the obsolete glues. Today, either resorcinol or epoxy-type glues, which are vastly superior to anything else available, are what bond wood parts together.

Resorcinol and epoxy glues are two-part products consisting of a resin and a hardener, and they are virtually impervious to any type of moisture or fungus. When properly mixed, applied to a properly prepared surface, and cured, the glue joint is much stronger than the wood on each side of it. The wood will fail long before the glue joint does.

The finishes applied to wood today also are superior to those used in the past. If applied and maintained properly, today?s wood finishes are virtually impervious to moisture and fungus. Many antique and vintage aircraft have succumbed long before their time because yesterday?s inferior finishes didn?t protect their structures from moisture and fungus.

If you fly or own an aircraft with wood components, you should be on the lookout for several things. Wood in good condition has a very sharp, solid sound when you tap it (do not pound!) with a light plastic mallet or screwdriver handle. Wood under attack by moisture or fungus will sound hollow and soft, and it also will have a musty or moldy smell (if you can differentiate that odor from the surrounding aircraft smells of fuel, oil, paint, etc).

Some aircraft, such as the Bellanca Viking series, have wings using wood structural components and wood skins. Generally, wood aircraft skins are covered with aircraft fabric and finished with either aircraft dope or one of the newer, and far superior polyurethane finishes.

You should inspect the surface closely for cracking paint or fabric and for minor surface damage that can allow moisture and fungus to reach the wood structure. Bulges or loose fabric indicates that moisture has gotten under the surface and is attacking the wood. Also check for nail or screw heads that protrude above the smooth surface.

You should store your aircraft in a covered and well-ventilated hangar where moisture and contaminates such as birds and dirt cannot reach the wooden surfaces. With proper care wooden structures offer nearly unlimited service.