6. The atmosphere is sometimes much damper than at
other times, and this causes wood to expand and
contract appreciably. This would not matter but
for the fact that it does not expand and contract
uniformly, but becomes unsymmetrical, i.e., distorted.
I have already explained the danger of that in
condition 2. This should be minimized by WELL
VARNISHING THE WOOD to keep the moisture out of it.
FUNCTION OF INTERPLANE STRUTS.--These struts have to
keep the lifting surfaces or ``planes'' apart, but this is only
part of their work. They must keep the planes apart, so
that the latter are in their correct attitude. That is only so
when the spars of the bottom plane are parallel with those of
the top plane. Also, the chord of the top plane must be
parallel with the chord of the bottom plane. If that is not
so, then one plane will not have the same angle of incidence
as the other one. At first sight one might think that all
that is necessary is to cut all the struts to be the same length,
but that is not the case.
Sometimes, as illustrated above, the rear spar is not so
thick as the main spar, and it is then necessary to make
up for that difference by making the rear struts correspondingly
longer. If that is not done, then the top and
bottom chords will not be parallel, and the top and bottom
planes will have different angles of incidence. Also, the
sockets or fittings, or even the spars upon which they are
placed, sometimes vary in thickness owing to faulty manufacture.
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