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:

[Illustration]

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. The line along which the measurement
should be taken is laid down in the aeroplane's specifications.

If a mistake is made and the measurement taken along the wrong line, it
may result in a difference of perhaps 1/4 inch or more to the stagger,
with the certain result that the aeroplane will, in flight, be
nose-heavy or tail-heavy.

After the adjustments of the angles of incidence, dihedral, and stagger
have been secured, it is as well to confirm all of them, as, in making
the last adjustment, the first one may have been spoiled.

OVER-ALL ADJUSTMENTS.--The following over-all check measurements should
now be taken.

[Illustration: The dotted lines on the surface represent the spars
within it.]

The straight lines AC and BC should be equal to within 1/8 inch. The
point C is the centre of the propeller, or, in the case of a "pusher"
aeroplane, the centre of the nacelle. The points A and B are marked on
the main spar, and must in each case be the same distance from the butt
of the spar. The rigger should not attempt to make A and B merely the
sockets of the outer struts, as they may not have been placed quite
accurately by the manufacturer. The lines AC and BC must be taken from
both top and bottom spars--two measurements on each side of the
aeroplane.

The two measurements FD and FE should be equal to within 1/8 inch. F is
the centre of the fuselage or rudder-post. D and E are points marked on
both top and bottom rear spars, and each must be the same fixed distance
from the butt of the spar. Two measurements on each side of the
aeroplane.

If these over-all measurements are not correct, then it is probably due
to some of the drift or anti-drift wires being too tight or too slack.
It may possibly be due to the fuselage being out of truth, but o