Learning to fly in the U.S. Army

by E. N. Fales

Instability is a condition whereby an object disturbed has a natural tendency to move as far as possible away from its first position, with no tendency to return. Example: a stick balanced vertically on your finger.

Neutral stability is a condition whereby an object disturbed has no tendency to move farther than displaced by the force of the disturbance, and no tendency to return to its first position.

Now in order that an airplane may be reasonably controllable, it is necessary for it to possess some degree of stability longitudinally, laterally and indirectionally.

Longitudinal stability is its stability about an axis transverse to the direction of normal horizontal flight, and without which it would pitch and toss.

Lateral stability is its stability about its longitudinal axis, and without which it would roll sideways.

Directional stability is its stability about its vertical axis, and without which it would have no tendency to keep its course.

Whenever an airplane does not fly properly, aside from conditions arising from engine or propeller trouble, either its longitudinal, lateral, or directional stability is affected. When its longitudinal stability is affected we call this condition longitudinal instability; likewise, regarding lateral stability and directional stability, referring to these conditions respectively as lateral and as directional instability. The effect of alignment errors will be treated under the foregoing respective heads.

Alignment Errors, Longitudinal.—

1. The Stagger May Be Wrong.—The top surface or wing 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 amount, 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 airplane will then be said to be nose-heavy. A ¼-in. error in the stagger will make a very considerable difference in the longitudinal stability.

2. The Angle at Which the Main Surfaces Are Set Relative to the Fuselage May Be Wrong.—This will have a bad effect especially in the case of an airplane with a lifting tail plane or horizontal stabilizer. If the angle of incidence is too great, the machine will have a tendency to fly “tail-high.” If the angle is too small the airplane may have a tendency to fly “tail-down.”