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:

[Illustration: Points A, B, and C, must be the same fixed distances
from the butt as are Points D, E, and F. Distances 1 and 2 must equal
distances 3 and 4.]

The above applies to both front and rear bays.

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 beforehand. 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:

[Illustration]

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. A spirit-level is then placed on the board, and the
wires must be adjusted to give the surface such an inclination as to
result in the bubble being in the centre of the level. This operation
must be performed in respect of each bay both front and rear. The bays
must then be diagonally measured as already explained.

YET ANOTHER METHOD of finding the dihedral angle, and at the same time
the angle of incidence, is as follows:

A horizontal line is taken from underneath the butt of each spar,
and the set measurement is either the angle it makes with the spar,
or a fixed measurement from the line to the spar taken at a specified
distance from the butt. This operation must be performed in respect of
both main and rear spars, and all the bays must be measured diagonally
afterwards.

[Illustration]