The Aeroplane Speaks. Fifth Edition

2. Struts and spars must be symmetrical. By that I mean that the cross-sectional dimensions must be correct, as otherwise there will be bulging places on the outside, with the result that the stress will not be evenly disposed about the centre of strength, and a bending stress may be produced. 3. Struts, spars, etc., must be undamaged. Remember that, from what I have already explained about bending stresses, the outside fibres of the wood are doing by far the most work. If these get bruised or scored, then the strut or spar suffers in strength much more than one might think at first sight; and, if it ever gets a tendency to bend, it is likely to collapse at that point. 4. The wood must have a good, clear grain with no cross-grain, knots, or shakes. Such blemishes produce weak places and, if a tendency to bend appears, then it may collapse at such a point. 5. The struts, spars, etc., must be properly bedded into their sockets or fittings. To begin with, they must be of good pushing or gentle tapping fit. They must never be driven in with a heavy hammer. Then again, a strut must bed well down all over its cross-sectional area as illustrated above; otherwise the stress of compression will not be evenly disposed about the centre of strength, and that may produce a bending stress. The bottom of the strut or spar should be covered with some sort of paint, bedded into the socket or fitting, and then withdrawn to see if the paint has stuck all over the bed.