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Aviation History
1913
1913 - 0116.PDF
(/OGHT FEBRUARY I, 1913. SOME THOUGHTS ON STABILITY AND CONTROL. IV. Longitudinal Stability. The Range of the Front Elevator. The Stability of the Double Curve Wing. 1'ARTof the hypothesis under which the problem of lateral stability has been discussed, assumed the aeroplane to possess sensitive longitudinal stability of the weathercock kind. Weathercock longitudinal stability, as the term itself implies, means that the system tilts and dips in sympathy with the trend of the relative wind in the vertical plane. In fine, the assumption of sensitive weather cock longitudinal stability lays down as an hypothesis that the wings shall preserve a constant angle of incidence. The assumption was primarily necessary in order to satisfy the condi.ions involved in the definition of a gust as a sudden relative spin of the wind. It is self-evident that the change of the wind's direction may take place in the vertical as well as the horizontal plane, and it is essential that the movement in the former direction should not be ignored. By assuming weathercock longitudinal stability in the hypothesis, it became possible to regard all lateral disturbances due to the real wind as originated by movements in the horizontal plane of the system—hence the definition of a gust as a relative spin. In order to consider the problem of stability at all, it is necessary, it seems to me, to lay down some sort of definition of the disturbances that it is proposed to meet and the limitations of the treatment of the subject are necessarily governed by those of the hypothesis itself. Comparatively little is known about those aspects of meteorology that most concern the pilot of an aeroplane, and every day it becomes more apparent that the better understanding of this branch of the subject is of vital consequence to the furtherance of safety in flight. It can, however, do no harm and may indeed be productive of useful results to consider the problem from the stand point of an arbitrary definition as I have endeavoured to do in these articles, for when the complete data is unavailable it is often helpful to spend some time in the study of an incomplete section, provided always that the limitations of the hypothesis be held well in mind. Moreover, it appears to me that the conditions represented by strong vertical currents, such as may be encountered over the sun- heated plains of India and Mexico, and to a lesser extent may be expected to exist over the rivers and lakes of England, are not such as may lightly be included in the study as part of the problem to be solved by a general equation. Their danger lies as much in their power to affect the altitude of flight as in their disturbance of equi librium, and whosoever would avoid being trapped over water or behind barriers that suddenly become impassible, must rely more on his knowledge of meteorology than on the inherent stability of his machine. Thus in so far as it may sometimes appear as if the " stability question" is to be regarded as representing the whole problem of safety in flight, I personally look upon it as one factoi therein only and upon the stable aeroplane or the " safe control," which I have suggested as a possibly preferable compromise, as merely incidental to the general perfection of the art of flying. Adequate engine power, and, above all, its reliability of action are essential to safety apart from every other consideration, and no amount of inherent stability will save the pilot from trouble who wilfully leaves the home field with a weak motor on the off-chance that it may pick up its proper form in the air. The assumption of a constant angle of incidence to the relative wind may seem at first sight to be a singularly limited hypothesis from the standpoint of practical aviation, seeing that the whole basis of speed range in modern aeroplanes lies in the ability to vary the effective angle by the use of the elevator. Even in so far as the assumption draws particular attention to the purpose and use to which the elevator is now put, however, it serves somewhat of an utilitarian purpose—for in this very ability so to control the attitude of the machine in flight the pilot is given a power that is not wholly free from danger. Let us consider for a moment the elementary situation presented by sensitive weathercock longitudinal stability such as is, for example, presented by the flight of the ballasted flat plate, which serves so admirably as a fundamental experiment in aerodynamics. Thus the e.g. of the system is well forward and coincides normally with the centre of pressure. If the angle of incidence should tend to become finer than is proper to the normal conditions, the c. p. advances still further forward and at once introduces a restoring couple that tends to maintain the angle of attack unchanged : the same when the angle increases and the c.p. retreats. A cambered wing of single curvature, that is to say, one in which the surface is wholly concave to the chord, presents precisely the opposite phenomena. When the angle is very fine indeed the c.p. By A. E. BERRIMAN. (Continuedfrom page 87.) may be nearer the trailing edge than the leading edge. As the angle is increased the c.p. moves forward, but when a certain critical angle is reached the c.p. begins to retreat. Finally it coincides with the centre of surface when the wing stands upright against the wind. In flight it is only with the finer angles, those below the critical angle, that we have to deal, and the point of importance that it is necessary to bear in mind is that the natural direction of the travel of the c.p. on a cambered wing of single curvature is the wrong way for inherent stability. A tail set at a lesser angle of incidence confers on the system as a whole a fore-and-aft dihedral angle which is a stabilising principle inasmuch as the variation of the total pressure on the tail plane at different angles is of such a magnitude as to cause the c.p. on the system to have a stable direction of travel. The local travel of the c.p. on the wings is unaffected in itself, but its effect is neutralised. The presence of this tail at so great a distance from the main wings, however, adds weight, and probably decreases the sensitiveness of the system by its damping effect. In the early Wright biplanes the fore and aft dihedral was obtained by holding the front elevator at a greater angle than the planes, and so depended entirely on the pilot. Moreover, the transverse pivot of the system being behind the organ of control produced a combination that was essentially unstable in certain limiting conditions, such as the dive, when an excessive use of the elevator to flatten out might conceivably " take charge " and turn the machine completely over in mid air. A tail in its natural position behind the transverse axis tends to remain there and a fixed tail plane in that place tends to prevent unpleasant consequences ensuing from the incautious use of a front elevator under such circumstances. Taking the system represented by wings of single curvature in conjunction with a fixed tail plane, whether load-carrying or neutral, the result is a tendency towards weathercock longitudinal stability. As that is, therefore, what modern designers try to acquire, it seems not inappropriate to adopt this quality as an hypothesis in preceding arguments. Furthermore, if the machine possessed no elevator, the angle of incidence would tend to be invariable, and the horizontal flight speed a fixed quantity. An elevator is necessary, however, for the reason that weather cock stability represents a state of oscillation rather than steadiness, and it is essential to give the pilot means for steadying his machine at his discretion. The elevator Aeld at an angle, however, virtually increases or decreases the effective angle of the tail, and by so doing it temporarily alters the neutral axis about which the machine is the stable equilibrium. As a means of intelligently opposing the oscillations of recovery, so as to increase the steadiness of flight, the elevator performs its proper and legitimate purpose. Its secondary use, which has become of even greater consequence in practical flying, however, is that indicated above, in which the resetting of the neutral axis of equilibrium causes a temporary change in the angle of incidence of the wings and so alters the speed at which the machine is sup ported in horizontal flight. In this way variable speed is acquired with modern aeroplanes, and the practice being only safe within limits is thus open to abuse either through lack of appreciation of the consequences or through over-confidence in personal skill. There is, as has been shown, a normal attitude in which the system is stable, with the elevator neutral, and consequently the pilot has at his command means for altering the horizontal speed either in such a way that it is reduced or increased in value. In the former case the limiting condition is reached when the aeroplane gets " stalled " through, becoming excessively cabre. This attitude is dangerous in so far as it is a pasition from which it is impossible safely to extricate the machine without diving and this sort of dive seems often to involve more head room than most people have seen reason to suppose is necessary. Whether the need for horizontal speed range as at present obtained is of fundamental importance is a question that involves other matters than those of stability, but it seems important to realise the possibilities that attend this method. Neither the cabre" nor the vol pique" attitudes should be possible with weathercock longitudinal stability that is not tampered with by the holding of the elevator at abnormal angles for considerable periods of time. The presence of a tail, it has been explained, is due to the inherent instability of the cambered wing of single curvature, but in modern machines the tail organs include an equally important member in the rudder. As, however, the preceding discussion on directional and lateral stability has shown the apparent uselessness of Il6
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