It is able to lift owing to a certain amount of slack
in the control wire holding it--and one cannot adjust the
control wire to have no slack, because that would cause it
to bind against the pulleys and make the operation of it too
hard for the pilot. It is therefore necessary to rig it a little
below the position it would occupy if it was rigged in a fair
true line with the surface in front. Remember that this
only applies when it is hinged to a lifting surface. The
greater the angle of incidence (and therefore the lift) of the
surface in front, then the more the controlling surface will
have to be rigged down.
As a general rule it is safe to rig it down so that its trailing
edge is 1/2 to 3/4 inch below the position it would occupy if in
a fair line with the surface in front; or about 1/2 inch down for
every 18 inches of chord of the controlling surface.
When making these adjustments the pilot's control levers
should be in their neutral positions. It is not sufficient
to lash them. They should be rigidly blocked into position
with wood packing.
The surfaces must not be distorted in any way. If
they are held true by bracing wires, then such wires must be
carefully adjusted. If they are distorted and there are no
bracing wires with which to true them, then some of the
internal framework will probably have to be replaced.
The controlling surfaces should never be adjusted with
a view to altering the stability of the aeroplane.
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