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Aviation History
1911
1911 - 0994.PDF
SOARING. II. FOLLOWING on our article of last week, it is not without interest to elaborate the discussion of the art of soaring by a few technical considerations as to the conditions that render soaring flight possible. It has been explained that in soaring flight the motorless aeroplane or glider is held aloft by the wind indefinitely in which respect the process differs from gliding flight where the wind only partially contributes to the energy required so that the glider must move always towards the earth. Many years ago, in 1883, to be precise, Lord Rayleigh wrote a letter to Nature in which he laid down the conditions under which soaring could take place. They were two in number and both simple in the extreme. These are :— 1. The wind is not horizontal. 2. The wind is not uniform. Later, F. W. Lanchester offered an extension of these con ditions by stating that " in order that a bird should soar, GLID£G. in STILL MfZ V • VFLOClTy . © / 5ccno/no w/no & • A/*Krt~£. or rtSCf/**T V s Si ASCErrsivE soae/no fa) ~x> v- < V (4) 00. Fig, 1. the wind, being uniform, possesses an upward velocity component." In so far as the conditions of soaring are governed by winds having an upward trend, they are fairly self-evident to the mind, and in any case a glance at the accompanying diagrams, prepared from illustrations in " Flight Manual," will serve to elucidate any difficult or doubtful points. In the first diagram the conditions of a glider in still air are graphically represented by a line sloping towards the earth so as to show the path that the machine will follow. It makes with the horizon an angle which is known as the gliding angle. In the adjacent diagram (2) the line sloping upwards is supposed to represent the direction and velocity of an actual wind having, as is obvious from the position of the arrow, an upward trend, and therefore complying with Lord Rayleigh's condition for gliding as extended by Lanchester. In the third diagram two opposing arrows proceed from a point,which represents the machine hovering in space. It is supported by a wind having an upward trend and velocity precisely equivalent to the natural gliding angle and natural speed of the machine in still air. In other words, it is the compounding of diagrams 1 and 2 when the angle y is equal to the angle 5 and V is equal to v. In diagram 4 the conditions represented are those in which the wind maintains its direction with increased velocity, the result being that the machine travels slowly backwards in the same direction as the wind, thereby gradually gaining in altitude. In diagram 5 the wind is assumed to have a greater upward trend than the gliding angle of the machine, with the result that the machine travels forwards and upwards. In diagram 6 the velocity of the wind is again increased without altering its angle, and the resultant path of the machine becomes more nearly vertical. It is only necessary for the wind to assume a sufficient velocity for directly vertical ascent to actually take place. These diagrams illustrate only some of the phenomena associated with soaring, but they serve the purpose of illustrating the general theory, and those who wish to see other cases similarly illustrated will find a variety of these diagrams in the book above referred to. In practice, winds frequently have an upward trend of sufficient magnitude to account for the soaring of birds, and particularly is this wind with an upward trend in evidence in the vicinity of cliffs, above which soaring birds may generally be seen. Another example of soaring on an upward wind is the sea-gull in the wake of a steamship ; here the hull of the boat forms a sufficient obstruction to deflect a horizontal wind into an obliquely upward direction. In some cases the heat of the sun will warm the earth sufficiently to cause convection to take place in the air so as to give rise to vertical air currents on days when there is no perceptible breeze, and as it does not require a very great vertical component to make soaring possible, such vertical winds as these may be sufficient to account for many of the remarkable instances of soaring in an apparent calm, which have so often been put on record. It is, of course, quite impossible that soaring should take place in a calm or in a steady horizontal wind, and there is no need to merely take Lord Rayleigh's word for it, because a few moments' consideration should serve to convince anyone that either phenomenon would be equivalent to an exhibition of perpetual motion. A popular illustration of the fallacy of a supposition that soaring is possible in a uniform horizontal wind is afforded by the case of a fly in a railway carriage. When the train is in motion, the air in a railway carriage is virtually a uniform horizontal wind ; indeed, it is perhaps the only " natural " example of a uniform horizontal wind that exists. To the fly, however, it appears as still air, for the fly itself is travelling at the speed of the train, and obviously finds the atmosphere no more soarable when the train is in motion than when it happens to be at rest. We have, nevertheless, heard very elaborate arguments put forth in support of the contention that soaring is possible in a uniform horizontal wind, and we admit that the plausibility of some of them has been very nearly as wonderful as the accomplishment of the feat itself would be. Nevertheless, stripped of their deluding drapery, they one and all come down to the condition of the fly in the railway carriage, for whatever may happen initially to the bird that gains altitude by facing the wind and velocity by gliding with the wind, sooner or later the bird and its orbits must both be travelling at the speed of the wind and, therefore, be situated in a relative calm. While it is comparatively easy to appreciate the condition of soaring represented by a wind with an upward trend, and the impossibility of soaring in a steady horizontal wind, it is not quite so easy to see why soaring is possible in a pulsating horizontal wind. The best illustration of this phenomenon that we have ever seen is one that was first used, we believe, by F. W. Lanchester. It consists of a model switchback suitably mounted on wheels and provided with a track for a ball, which, at the beginning of the experiment is situated at the lower end thereof. The ball is perched on a slight eminence from which it can roll down into the first depression under the natural influence of gravity. If left to itself, the ball will only roll a part of the way up the next incline, but if, while the ball is rolling down into the first depression, the switch- 996
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