Wood is an important component in aircraft construction that dates back to early designs. It is a proven material that is still
used today and is found in many homebuilt aircraft. It is referred to as a natural composite. Wood has many uses
including structural components like wing spars, ribs, fuselage skins and structures. If you are a woodworker and enjoy
working with the material, wood construction may be for you.
So what makes wood so good? The right wood, like Sitka spruce, has a good strength to weight ratio and has twice the tensile
strength of aluminum. Wood is easily worked with common tools that many people already have around their house. It is nontoxic
and can be formed into fairly complex shapes when wet. It is relatively clean and a pleasure to work with.
Being a natural material wood has many variations. Some is suitable for aircraft construction and even more is suitable for firewood. For aircraft, wood must be from the right species of tree and of high quality. Wood must be carefully inspected to determine its usability in different applications, The grain must be tight and straight without flaws such as knots or pitch pockets. And like steel and aluminum, wood must be finished properly to protect it from the elements and other substances like oil and gas.
For use in certified aircraft, wood must go through stringent inspections and be certified. For Experimental aircraft, wood does not need to be certified, although buying certified wood does give confidence. But, in homebuilt applications, you can find perfectly acceptable wood that hasn’t gone through rigorous inspections. But you must know what to look for and inspect it carefully yourself. You would likely be saving money in the process.
Below is a list of the most common and readily available woods homebuilder's use.
The most popular and proven wood for aircraft construction is Sitka spruce. It is not the strongest wood but has a good strength to weight ratio and older trees produce long straight tight grains with few defects which is ideal. Most Sitka spruce comes from British Columbia and Alaska and trees are a half-century old or more. It is not as plentiful as many other conifers, which is reflected in its lower availability and higher price.
Doug fir is a more common conifer that is a substitute for Sitka spruce in many applications. It is stronger than Sitka spruce but is also heavier. It is also more difficult to find long sections of straight tight grains without flaws. You can save some money by using Doug fir for shorter parts, but if you are building a wing spar you will probably be forking out the money for Sitka spruce. If you trust your local wood supplier to label their wood correctly you can find Douglas fir at many locations and it will be less than one tenth the cost of Sitka spruce.
Other woods suitable for aircraft construction include western hemlock, which is 14% stronger than Sitka spruce and white pine which is more readily available in the eastern parts of the country and sugar pine. Balsa wood is a strong and very light wood for nonstructural applications, or to form and cover with composites.
Plywood is another common aircraft building material. But for aircraft construction a special plywood is used, not the hardware store variety. Aircraft-grade plywood is made from imported and domestic mahogany or American birch veneers laminated in a hot press using waterproof glues. The core woods are usually poplar or basswood. Aircraft grade plywood is a very strong and versatile material and can be used in many applications such as fuselages and wing skins. It can be covered with Fiberglass for added strength and protection from the elements.
Suitable wood has minimum specifications for different applications. Initially, look at the wood in general and if you say “that doesn’t look too good” then it isn’t. Things like knots, sap pockets and irregularities in the grain are generally visible with a quick inspection. These can be worked around depending on your part size
The specifications for suitable wood is a minimum of 6 growth rings per inch looking at the end of the wood, but that is a minimum so more is better.
Length wise, there would be no slope in the grain and it would be straight along the long axis of the cut. But if the grain does wander down along the section it should not wander more than 1 inch per 15 inches of length.
If looking at the end again and the grain is not parallel, or 90 degrees to the cut, it would be difficult to bend so should not be used in bending applications.
Knots, sap pockets and other flaws should be avoided. But you still may be able to get good material by cutting flaws out.