0 lb. That is a very great difference, and, since weight is of the
greatest importance, the design of an aeroplane is always such as to,
as far as possible, keep the various wooden parts of its construction in
direct compression. Weight being of such vital importance, and designers
all trying to outdo each other in saving weight, it follows that the
factor of safety is rather low in an aeroplane. The parts in direct
compression will, however, take the stresses safely provided the
following conditions are carefully observed.

CONDITIONS TO BE OBSERVED:

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.

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,
th