1. _All the spars and struts must be perfectly straight._
The above sketch illustrates a section through an interplane strut. If the strut is to be kept straight, _i.e._, prevented from bending, then the stress of compression must be equally disposed about the centre of strength. If it is not straight, then there will be more compression on one side of the centre of strength than on the other side. That is a step towards getting compression on one side and tension on the other side, in which case it may be forced to take a bending stress for which it is not designed. Even if it does not collapse it will, in effect, become shorter, and thus throw out of adjustment the gap and all the wires attached to the top and bottom of the strut, with the result that the flight efficiency of the aeroplane will be spoiled.
[Illustration: Strut straight. Wires and gap correctly adjusted. Strut bent throwing wires and gap out of adjustment.]
The only exception to the above condition is what is known as the Arch. For instance, in the case of the Maurice Farman, the spars of the centre-section plane, which have to take the weight of the nacelle, are arched upwards. If this was not done, it is possible that rough landings might result in the weight causing the spars to become slightly distorted downwards. That would produce a dangerous bending stress, but, as long as the wood is arched, or, at any rate, kept from bending downwards, it will remain in direct compression and no danger can result.
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.