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.
Whichever method is used, be sure that after the job is
done the spars are perfectly straight.
STAGGER.--The stagger is the distance the top surface
is in advance of the bottom surface when the aeroplane
is in flying position. The set measurement is obtained as
follows:
Plumb-lines must be dropped over the leading edge of
the top surface wherever struts occur, and also near the
fuselage. The set measurement is taken from the front of the
lower leading edge to the plumb-lines. It makes a difference
whether the measurement is taken along a horizontal line
(which can be found by using a straight-edge and a spirit-
level) or along a projection of the chord.
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