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Aviation History
1909
1909 - 0314.PDF
MAY 29, 1909. FLAPPING WING MACHINES. JUST AS OUR ARTICLE IN "FLIGHT" OF FEBRUARY 6TH EXPLAINED THE ELEMENTARYPRINCIPLE OF AN AEROPLANE, SO DOES THE PRESENT ARTICLE DEAL IN SIMPLE TERMS WITH A CERTAIN ASPECT OF FLAPPING FLIGHT. IN our first article on '•' How Men Fly " we explained how the air could be made to support a flying machine by pushing thin surfaces through it at a high speed. Many thinkers have wondered, however, why it is that man does not first try to fly, like the bird, by flapping wings. It would seem, according to their arguments, the proper thing to do, and in the very remote days it was, according to the records which have been left of those times, unquestion- ably a governing idea among the majority of practical workers. That those very early pioneers of flight failed with their crude appliances is no cause for wonder when modern inventors are still unsuccessful; and there are, even now, a few undaunted believers in the field who profess to see the ultimate solution of practical flight in the flapping-wing device. Progress, however, has gone, as usual, along the line of least resistance in evolving the aeroplane instead of the "flapper," and consequently it was only proper to commence such a series of articles as this by dealing first with the funda- mental principles of what is, after all, the only practical type of machine at the present day. ,. Flat-winged " Flappers," The most elementary idea of the beating wings seems to be that of using plain flat surfaces striking downwards upon the air, as is done, for instance, in the Collomb orthopter. This point of view presumably supposes that the most important result to be achieved by wing movement is that of ascending vertically to a higher level in the air. It ignores propulsion as a possible direct result of flapping, and apparently views artificial flight as being primarily composed of two separate and distinct evolutions, follow- ing one another in continual sequence. The machine first rises vertically, and then travels obliquely by gliding, until another rise is necessary—the process may be likened to sliding down the balusters after ascending in the lift. Any machine having been lifted to a position above the earth's surface, has a latent or potential energy due to its weight and height (foot-pounds potential = weight x height) which, assuming it be suitably built and mani- pulated, it can utilise for propulsion by gliding at a very gradual angle, so that in the course of its descent it travels a long way from its starting-point. Once in motion, such a machine would also, owing to its momen- tum, travel obliquely upwards during the subsequent lifting strokes of its wings, and thus the resultant motion would probably be more switchback-like than zigzag, as was implied by the analogy suggested above. It is most unlikely, however, that such a machine would be efficient either as a glider or as an orthopter. Instead of dismissing the simple flat wing type of machine, however, we purpose in this article to refer to one or two other fundamental principles associated with its action, as the problem is one in which we have found many readers to be interested. If a simple orthopter is to lift itself straight up into the air by the downward beats of its wings, provision must obviously be made whereby the upward wing flaps are rendered null and void. A plain flat suiface flapping up and down would react equally in both directions on the air, so that at the best the machine could but perform a series of hops ; to maintain the motion of ascent it is necessary to " feather " on the upward stroke, as is done in the idle stroke when sculling. Some form of valve would, for convenience, be employed to give this feathering effect by allowing the air to pass freely through the wings on the up stroke while the full wing surface remains effective on the down stroke. Wings with Valves. The fundamental question as to whether it is theoreti- cally possible to make a machine with valvular wings, which could continue to raise itself vertically in the air by beating them, is one which, so far as the mere mathe- matical solution is concerned, may be answered in the affirmative, but man himself has not yet shown that his constructive arts are equal to the practical realisation of this fact. It is open to anyone to convince themselves, by any such simple experiment as flapping a sheet of cardboard, that beating the air is a means of creating a force, and the laws relating to this " wind resist- ance"—dealt with in the first article of this series—• are known to a certain extent already, by virtue of data collected in other branches of scientific research. It is merely a question of surface, speed, and weight: the greater the weight to be carried, the greater the surface necessary to support it; and the greater the surface in proportion to the weight, the slower need the wings flap downwards in order to get the necessary " grip " on the air. Speed plays a more important part than surface in obtaining a lifting effect out of the air, as anyone may demonstrate for themselves by experimenting with a piece of cardboard as already mentioned. If the rate of flapping—which for the moment may be assumed to represent the speed—is doubled, the lift would be about four times as great, whereas, if it is the surface which is doubled, the lift would be only twice as great. Although the lifting effect which is produced is so essentially associated with movement it is very important to properly grasp the principle upon which the air is really utilised as a means of enabling the machine to lift itself bodily in space. The air itself does not exert an upward thrust of an active character such as would be comparable with the lifting effect derived from muscular effort or mechanical appliances. The air is a purely passive agent, for at the most it merely operates as a member against which the wings abut whilst they exert a leverage to lift the body of the machine upwards. Normally the still air is unable to support anything heavier than itself, but it resists being set in motion quickly, and when this reaction is equal to the load, the air may for convenience be considered as approximating to the solid state. When a Wing Lifts. This purely imaginary solid state of the air is reached with any given machine of the flapping-wing type when that machine actually lifts itself from the ground by its own efforts, for from the moment that the body of the machine commences to move upwards the air is to all intents and purposes forming a solid fulcrum at some point imme- diately under the wings. Suppose, for example, that a flapping-wing machine of the valvular-wing type is resting on the ground, with its wings stretched more Or less straight up into the air (Fig. 1). As they begin to flap, 316
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