tings._ 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withdrawn to see if the paint has stuck all over the bed.

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.

[Illustration]

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. This must be offset by
altering the length of the struts. The best way to proceed is to measure
the distance between the top and bottom spars by the side of each strut,
and if that distance, or "gap" as it is called, is not as stated in the
aeroplane's specifications, then make it correct by changing the length
of the strut. This applies to both front and rear interplane struts.
When measuring the gap, always be caref