Aeroplane Construction and Operation

by John B. Rathbun

Longitudinal Stability. In Figs. 9-10-11-12 the machine was assumed to be flying in still air, the attitudes of the machine being simply due to changes in the loading or to a change in altitude. The actual case is more complicated than this, for the reason that the machine is never operating in still air but encounters sudden gusts, whorls, and other erratic variations in the density and velocity of the air. Each variation in the surrounding air causes a change in the lift of the wings, or in the effect of the tail surfaces, and hence tends to upset the machine. If such wind gusts would always strike the wings, body, and tail simultaneously, there would be no trouble, but, unfortunately, the air gust strikes one portion of the machine and an appreciable length of time elapses before it travels far enough to strike another. Though this may seem to be a small fraction of time, it is in reality of sufficient duration to have a material effect on the poise of the aeroplane. Vertical gusts due to the wind passing over buildings, hills, cliffs, etc., not only tend to upset the machine, but also tend to change the altitude since the machine rises with an up gust and sinks with a down trend in the Stream.

Assume a machine as in Fig. 9 to be traveling steadily along a horizontal path in still air. A sudden horizontal gust now strikes the machine from the front, thus causing a sudden lift in the main wings. As this gust strikes the wings before the tail, the tail will stand at the old altitude while the wings are lifted, thus giving the position shown by Fig. 10. After passing over the wings it lifts the tail, this effect probably not being sufficient to restore the wing and the tail to their old relative attitude since the gust generally loses velocity after passing the wings. A head gust of this type often strikes the front wings diagonally so that it never reaches the tail at all. To remedy this upsetting action of the gust, the pilot must move his rear elevator so that the elevator is in the position shown by Fig. 12, that is, the flap must be turned down so as to raise the tail.

A gust striking from behind may, or may not affect the elevator flaps, this depending on their position at the time that the gust strikes. If the flaps are turned up, the rear end will be raised by the gust and the machine will head dive: if turned down, the gust will depress the tail, raise the head and tend to "stall" the machine. If the tail is of the lifting type, the rear entering gust will reduce the relative velocity, and the lift, and cause the tail to drop. On passing over the tail and striking the wings, the rear gust will reduce the velocity and cause a loss in lift. This will either cause the machine to head dive or drop vertically through a certain distance until it again assumes its normal velocity.

All of these variations cause a continually fore and aft upsetting movement that must be continually corrected by raising and lowering the elevator flaps, and in very gusty weather this is a very tedious and wearing job. It requires the continual attention of the pilot unless the action is performed automatically by some mechanical device, such as the Sperry Gyroscopic, or else by some arrangement of the surfaces that give "inherent" stability. Control by means of the elevator flaps (which raise and lower the body in a fore and aft direction, as shown) is known as "longitudinal control," and when the machine is so built that correction for the longitudinal attitude is obtained "inherently," the machine is said to be "longitudinally stable." Modern machines can be made very nearly longitudinally stable, and are comparatively unaffected by any than the heaviest gusts.

Lateral Stability. The gusts also affect the side to side, or "lateral" balance by causing a difference in lift on either end of the wings. Should the gust strike one tip before the other, or should it strike one tip harder than the other, the tendency will be to turn the machine over sidewise. This is a more difficult problem to solve than the longitudinal moment, although perfect inherent stability has been attained in one or two machines without the use of additional automatic control mechanism. Inherent lateral stability has always been attended by a considerable loss in the efficiency of the aeroplane and speed due to the peculiar arrangements in the main lifting surfaces. At present we must make a decision between efficiency and stability, for one feature must be attained at a sacrifice in the other. Contrary to the general opinion, perfect stability is not desirable, for almost invariably it affects the control of a machine and makes it difficult to maneuver. Should the stability appliances be arranged so that they can be cut out of action at will, as in the case of the Sperry Gyroscopic Stabilizer, they will fulfill the needs of the aviator much more fully than those of the fixed inherent type. The first thoroughly stable machine, both longitudinally and laterally, was that designed by Lieutenant Dunne, and this obtained its distinctive feature by a very peculiar arrangement of the wing surfaces. It was excessively stable, and as with all very stable machines, was difficult to steer in a straight line in windy weather, and was correspondingly difficult to land.

Fig. 12.A. Diagram of the Tractor Biplane

The first machine of the ordinary biplane type that proved inherently stable was the R. E.-1 designed in England by Edward Busk. This machine was flown from Farnborough to Salisbury Plain, and during this flight the only control touched was the vertical rudder used in steering. Since then, all English machines have been made at least partially stable, the degree depending upon the service for which it was intended. It has been found that in fighting, a very controllable machine is necessary, hence stability must be sacrificed, or the control surfaces must be made sufficiently powerful to overcome the stable tendency of the machine. War machines are made to be just comfortably stable over the range of ordinary flight speeds, and with controls powerful enough so that the inherent stability can be overcome when maneuvering in battle. The present war machine always contains an element of danger for the unskilled pilot.

Dihedral Angle. This was the first lateral stability arrangement to be applied to an aeroplane, but is only effective in still air. In rough weather its general tendency is to destroy stability by allowing dangerous oscillations to take place. Fig. 13 is a front view of a monoplane in which the wings (w) and (w') are set at an angle (d), this angle being known as the "dihedral angle." The dotted line (m-m) shows the line of a pair of perfectly horizontal wings and aids in illustrating the dihedral. Assuming the center of lift at CL on the wings, it will be seen that an increase in the dihedral raises the center of lift above the center of gravity line C. G. by the amount (h). With the center of gravity below the center of lift it is evident that the weight of the machine would tend to keep it on a level keel, although the same effect could, of course, be attained in another way. The principal righting effect of the dihedral is shown by Fig. 14 in which the wings (w) and (w') are set as before. The machine is tipped or "listed" toward the left (seen from aviator's seat) so that wing (w') is down. By drawing vertical lines down until they intersect the horizontal line X-X (the line of equilibrium), it will be seen that wing (w') presents more horizontal lift surface than (w) since the projected or effective wing length (C) is greater than (b). Since (w') presents the greater surface, the lift (L) tends to restore the machine to its original level position. If the wings were both set on the same straight line, the projected lengths (b) and (c) would be the same and there would be no restoring effect.