the alignment of the fuselage, fin in front of the rudder, the
struts or stream-line wires, or, in the case of the Maurice Farman, the
front outriggers, are not absolutely correct--that is to say, if they
are turned a little to the left or to the right instead of being in line
with the direction of flight--then they will act as a rudder and cause
the aeroplane to turn off its course.

3. If any part of the surface is distorted, it will cause the aeroplane
to turn off its course. 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. The
aeroplane would then be said to be "nose-heavy."

A 1/4-inch area in the stagger will make a very considerable difference
to the longitudinal stability.

2. If _the angle of incidence_ of the main surface is not right, it will
have a bad effect, especially in the case of an aeroplane with a lifting
tail-plane.

If the angle is too great, it will produce an excess of lift, and that
may lift up the nose of the aeroplane and result in a tendency to fly
"tail-down." If the ang