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.
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