The surface is cambered,
i.e., curved, to pass through the air with the least possible
drift. If, owing perhaps to the leading edge, spars, or trailing
edge becoming bent, the curvature is spoiled, that will
result in changing the amount of drift on one side of the aeroplane,
which will then have a tendency to turn off its course.
LATERAL INSTABILITY (FLYING ONE WING DOWN).--The only possible
reason for such a condition is a difference in the lifts
of right and left wings. That may be caused as follows:
1. The angle of incidence may be wrong. If it is too
great, it will produce more lift than on the other side of the
aeroplane; and if too small, it will produce less lift than on
the other side--the result being that, in either case, the aeroplane
will try to fly one wing down.
2. Distorted Surfaces.--If some part of the surface is
distorted, then its camber is spoiled, and the lift will not be
the same on both sides of the aeroplane, and that, of course,
will cause it to fly one wing down.
LONGITUDINAL INSTABILITY may be due to the following reasons:
1. The stagger may be wrong. The top surface may have
drifted back a little owing to some of the wires, probably
the incidence wires, having elongated their loops or having
pulled the fittings into the wood. If the top surface is not
staggered forward to the correct degree, then consequently
the whole of its lift is too far back, and it will then have a
tendency to lift up the tail of the machine too much.
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