ich they are hinged is not a lifting surface, then
they should be rigged to be in a fair true line with it as illustrated
above.

If the controlling surface is, as illustrated, hinged to the back of a
lifting surface, then it should be rigged a little below the position it
would occupy if in a fair true line with the surface in front. This is
because, in such a case, it is set at an angle of incidence. This angle
will, during flight, cause it to lift a little above the position in
which it has been rigged. 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. Nothing can be accomplished in
that way. The only result will be to spoil the control of the aeroplane.

FABRIC-COVERED SURFACES.--First of all make sure that there is no
distortion of spars or ribs, and that they are perfectly sound. Then
adjust the internal bracing wires so that the ribs are parallel to the
direction of flight. The ribs usually cause the fabric to make a ridge
where they occur, and, if such ridge is not parallel to the direction of
flight, it will produce excessive dri