1. The angle of incidence of the main surface or the tail surface may be wrong. The greater the angle of incidence, the greater the drift. The less the angle, the less the drift.
2. If 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.