Notice how the ends of each piece are ferruled, usually with copper or tin. This is to prevent the bolt pulling out with the grain of the wood, and also prevents splitting and end checking and gives a uniform base on which the pressure comes.
It is generally advised not to paint wood as it tends to conceal defects from inspection. So varnish only.
Wrapping wooden members with linen or cord tightly and doping this, both to make waterproof and to still further tighten, increases the resistance to splitting. The absence of warping tendencies determine often what wood to choose.
The selection of lumber and detection of flaws is a matter of experience and should be cultivated. It is, however, nothing more than the extension of the knowledge that leads a man to pick out a good baseball bat.
Woods.—1. Spruce.—Should be clear, straight-grained, smooth and free from knot holes and sap pockets, and carefully kiln-dried or seasoned. It is about the lightest and for its weight the strongest wood used. It is ordinarily used as a material for spars, struts, landing gear, etc., as it has a proper combination of flexibility, lightness and strength.
2. White Pine.—A very light wood used for wing ribs, and small struts.
3. Ash.—Springy, strong in tension, hard and tough, but is considerably heavier than spruce. Used for longerons, rudder post, etc.
4. Maple.—Used for small wood details, as for blocks connecting rib pieces across a spar or for spacers in a built-up rib.
5. Hard Pine.—Tough and uniform and recommended for long pieces, such as the wooden braces in the wings.
6. Walnut, Mahogany, Quarter-sawed Oak.—The strength, uniformity, hardness and finishing qualities make these woods favorites for propeller construction.