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Aviation History
1911
1911 - 0684.PDF
AEROPLANES AND GUSTS. IN discussing the disturbing effect of gusts upon aeroplanes it is usual to first describe a particular machine. This is not quite convincing when the object is to determine how machines in general may be made steady in gusts, and a method dealing with the aeroplane as an abstraction, possessed of the minimum essential properties, has certain advantages. Every glider capable of steady flight may be assumed to possess a self-righting tendency, or a tendency to fly in a plane substantially at right angles to gravity. Now, in its final analysis, the direction of gravity is nothing more than the direction in which the glider, regarded for the moment as a wingless mass, tends to accelerate relative to the air, so that anything which changes the direction of this acceleration tendency changes, in effect, the direction of gravity regulating the flight path relative to the air, and the plane at right angles in which the glider tends to fly. In the diagram the central point, A, represents the position of a self-righting glider suddenly attacked by an acceleration of the air from the left at 40 ft. per sec. per second (i.e., a gust of strength 40) represented by the line, DA, drawn 40 units in length to a convenient scale. Special attention is drawn to the fact of the strength of the gust being measured by the acceleration at the Q TB TI»UC wevti. c *°* K moment, so that continuation of the gust means continuation of the acceleration, not continuation of a mere velocity of the air. Since DA represents the gust acceleration, AD, or the same line measured in the opposite direction, represents the acceleration tendency of the glider relative to the air in consequence of the gust, and AB drawn vertically downwards, 32'2 units in length, represents the familiar acceleration tendency due to common gravity. The resultant of AD and AB is AE, which, on account of its representing the whole acceleration tendency relative to the air during the gust, and therefore entirely superseding common gravity in determining the subsequent flight path, is called the " resultant relative gravity." In considering the flight path during, the gust it is now only necessary to view the diagram along EA, so that AE may be thought of as gravity, and to regard FAF1, at right angles to AE, as a virtually level plane with respect to which the glider rights itself in the usual manner, according to the way it is flying at A. Needless to say all flight paths are relative to the air, not to the ground, but being relative paths they fulfil our object in correctly representing changes in pose. It is essential to understand the acceleration is not itself a velocity, and, therefore, will not immediately make any change in the magnitude and direction of the velocity of the glider relative to the air. From this it follows that if the glider were proceeding along AC at its natural speed, it will find itself, the instant the gust commences, running down steeply with respect to AF and AE, and will, therefore, pursue the familiar kind of oscillating righting path, AGH, according to the strength of the righting tendency. If the righting tendency is made less strong, and especially if the glider is as well damped as it should be, the more satisfactory path, AI, will be pursued instead of AGH. It is the turning up into the path, AG or AI, which constitutes the well known disturbance of the head gust. If the glider flies in the direction, AC1, at its natural speed the moment before the rear gust, DA, attacks, viewing the diagram as befiire, along EA shows the path during the gust to be of the form of AG1!!1 or AI1, according to the strength of the righting tendency and the damping. The running at the ground in this case is the well known and very objectionable disturbance of the rear gust. From these considerations it appears the righting tendency, or " natural stability," as it is more usually called, is only suitable for giving a stable pose in calm air, or an underlying average pose in gusty air, and is fundamentally incapable of aiding the momentary stability of pose in gusty air because of its being the very means whereby gusts disturb the pose. Taking it for granted an underlying righting tendency cannot be dispensed with, the diagram suggests improved stability in gusts may be sought in the discovery of additional dampers of the oscillations, and beyond that, in some means of actually resisting the acceleration tendency, AD, which is the root of the disturbance. A large, light, freely-revolving, constant-pitch propeller, with a considerable amount of fly-wheelage, gives forces of the required nature, and might be employed till better means are known. Such means cannot prevent the pose changing ultimately by the angle, CAF or C'AF1, if the gust continues, since such change is inevitable with the least degree of righting tendency, but they may slow down the turning into paths like AI and AI1, and reduce the curvatures so much that a gust of average duration will not have time to greatly disturb the pose. Inspection of the diagram also suggests means may be discovered of causing the righting tendency itself to temporarily disappear, or even reverse, at the onset of a gust, so that for a sufficient number of seconds the glider may not deviate appre ciably from AC or AC. If the glider at A had been flying away through the paper, DA would have been a side gust, and the lateral righting tendency would have deter mined a lateral righting path resembling AG or AI according to the strength and damping of the righting tendency, but with the usual superimposed left-hand circling movement. The righting tendency is again the source of the disturbance of pose, and is accordingly only in order when just strong enough to determine the underlying average lateral pose, while keel surface, in cutting down the acceleration tendency, AD, relative to the air, appears to be a positive advantage. This theory does not profess to deal with rotating gusts which act on one wing more than the other, but only with linear gusts, and the former are probably the worst disturbers of lateral pose. The diagram may be drawn with equal facility for a gust in any direction and of any strength. For all gusts ot strength 40, the locus of D is obviously the circle round A of radius 40, and the locus of E the similar circle round B, shown dotted in the diagram. The relative gravity, AE, due to a gust, 40, may therefore range from AK, two and a quarter times as great as common gravity, but in the usual direction, to AL, only one quarter of common gravity, but acting upside down. The former represents the condition of maximum stress in the glider (two and a quarter times normal), while the latter represents the curious " hole in the air " condition in which stresses are reversed, and the glider commences to turn upside down. The turning being somewhat slow and undecided the gust does not usually last long enough to make it serious, provided the aviator, and other movable parts, keep in place under the reversed gravity. In the diagram certain dotted constructions illustrate the modifications when the gust grows less suddenly. In the atmosphere a gust acceleration like DA cannot be instantaneously established owing to the air being compressible, and then AE does not become AE at once, but moves more or less slowly from AB through the positions marked 1, 2, 3, 4, while the virtual level, AF, moves through its corresponding positions, I, 2, 3, 4, instead of becoming AF at once. The righting paths then necessarily become less abrupt in curvature, like AJ and AJ1, with less tendency to be succeeded by oscillations, since the glider has more time to adjust itself to the changing conditions while they are changing. In tracing the effects of certain severe rearward and downward gusts, it will be found the slow growing of the gust makes all the difference between the glider turning over backwards through about three right angles, or dipping more slowly towards the ground. June 12th, 1911. S. L. WALKDKN. 1 -- X 686
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