As a rule such diagonal measurements are
taken from the bottom socket of one strut to the top socket
of another strut, but this is bad practice, because of possible
inaccuracies due to faulty manufacture.
The points between which the diagonal measurements
are taken should be at fixed distances from the butts of the
spars, such distances being the same on each side of the
aeroplane, thus:
It would be better to use the centre line of the aeroplane
rather than the butts of the spars. It is not practicable
to do so, however, as the centre line probably runs through
the petrol tanks, etc.
THE DIHEDRAL BOARD.--Another method of securing
the dihedral angle, and also the angle of incidence, is by
means of the dihedral board. It is a light handy thing to
use, but leads to many errors, and should not be used unless
necessary. The reasons are as follows:
The dihedral board is probably not true. If it must be
used, then it should be very carefully tested for truth before-
hand. Another reason against its use is that it has to be
placed on the spars in a position between the struts, and
that is just where the spars may have a little permanent
set up or down, or some inaccuracy of surface which will,
of course, throw out the accuracy of the adjustment. The
method of using it is as follows:
The board is cut to the same angle as that specified for
the upward inclination of the surface towards its wing-
tips. It is placed on the spar as indicated above, and it
is provided with two short legs to raise it above the flanges
of the ribs (which cross over the spars), as they may vary
in depth.
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