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Aviation History
1913
1913 - 1156.PDF
1/j.lGHT " (5) That the pectoral cord certainly assists the smooth working of the two-winged type. " (6) That the amplitude need not be excessive, which means unsymmetrical working, an arc of 60 being quite sufficient for practical purposes. (7) That the span be made as small as possible, any increase in the same retarding the smooth working of the wings. " (8) That the experimentalist must be prepared to exercise the greatest patience, as model after model is sure to be broken when testing. In fact, the science of ornithopter model flying is a life study, and the £$ prize offered for such stringent conditions is ridiculously inadequate. " A model ornithopter is in essence a scale working model of a full-sized machine, and very few aerodynamical scientists would care to give the fruits of their labour away for this amount. The question arises, did the promoters of this contest realise what they were asking for ? I doubt it. My own original model, in reality a scale model, flies nothing like 100 ft., but I hope to shortly exhibit its capabilities, and explain the difficulties to be overcome at one of the meetings on Wimbledon Common." [A series of articles on this subject commences below.—V. E. J.] Mr. N. F. H. Clarke's Experiment. Mr. R. G. Hale (H. M.S. " Cyclops "), writing re the above, says : " The following experience which I had with one of my early models may, perhaps, be of interest. The machine was of the Canard type V frame, the wing tips of the main plane being swept back and given a pronounced negative camber. The model was a hand-launched one, weight 7 ozs. I was tuning up at the time of the occurrence, which took place at Shotly. On being launched the model began to climb, but, swerving to the right, fouled some telephone wires, with the result that it capsized. The machine then vol planed upside down for about 4 or 5 yards, and then suddenly dived. I naturally expected a bad smash, but about 10 ft. from the ground the model flattened out, and again commenced to climb, finally gliding to earth. Subsequently the left wing was found to lie cracked. In conclusion, please accept my thanks for the valuable assistance I have derived from the model pages of FLIGHT." The Problem of the Ornithopter. In considering the question of flight through the air, we have three main factors to take into consideration—(1) the necessify for support ; (2) that for propulsion ; (3) that of stability. The first can obviously be obtained by mere flotation, as in the case of the ordinary balloon. Or it can be derived from the mechanical re actions of tbe air by virtue of its inertia, as in the case of flapping birds, and the mechanically propelled aeroplane. In the case of an aeroplane, numbers 1 and 2 are distinct; but, in the case of a bird both- functions are performed by the same mean?. Calculations have been made to demonstrate theoretically that in the latter case, or in the case of the properly constructed wing flapping models, that such apparatus requires less work for propulsion than in the case of screw-propelled aeroplanes, i.e., machines with fixed wings, because the air is attacked at a better angle and because the weight of the body ip itself assists in producing the all-necessary powerful flaps of the wings. The wing'is sustained by the vertical component due to the speed and the angle of incidence, whilst in its flapping action the wing in the down stroke acts as an aeroplane gliding downwards obliquely and gains in rapidity over the relative wind owing to its OCTOBER 25, 1913. negative angle of incidence. Whilst on the up stroke it also acts as an aeroplane lifted by the air pressure, resultant from the forward velocity produced by the down strokes, its angle of incidence now being positive, both actions in the case of the bird being rendered effective by the weight of its' body. It must, however, not be forgotten that this flapping motion is not performed at any point fixed in space, but in a yielding medium. We must also allow for considerable losses due to friction, body resistance and for possible loss of altitude or height on the up stroke from the sinking of the body in the yielding air. It would appear, however, that this sinking or falling need not take place if the wing on the up stroke has an adequate positive angle of attack, whereby by means of the increased lift produced the lost height is either regained or it may even be increased. In the calculations referred to above, it has been usual to assume that on the down stroke there is no "drift," because the wing presents only its edge to the relative wind by reason of its negative angle ; and also that the skin coefficient is so small that it may be neglected. Assuming (for the sake of argument) such to be the case, then—in order to fly with "flapping wings"—the "drift" must be encountered but half the time (this, of course, supposes the up and down stroke to be equal in duration, which, as a matter of fact, they are not). If D be the drift, R the body resistance, V the velocity of the machine, W the work required, we have W (?•»)' Model float designed and constructed by Mr. C. Ian Burrell. 1182 It is, however, obvious that a constant small angle of incidence cannot be maintained; even soaring birds cannot doit, since they must encounter various eddies and wind gusts, varying wind veloci ties causing them to change their course, rise, &C, &c, many ot which necessitate an increase in the angle of incidence, and there fore of " drift " resistance. Due allowance must also be made for framework resistance. Again, in order to sustain the weight during the down stroke, to propel the machine, and also to store up part of the energy as has been more than once suggested in a spring or com pressed air cylinder during the up stroke—to assist in the succeeding down stroke—the down stroke must produce an increased pressure and the power to do this must be taken into account: in other words, the wing cannot descend effectively at a negative angle and so evade the "drift" during the down stroke. Pettigrew (" Aerial Locomo tion," p. Hi) says : " In the aerial machines so far as yet devised, there is no sympathy between the weight to be elevated and the lifting power, whilst in natural flight the wings and the weight of the flying creature act in concert and reciprocate, the wings elevating the body one instant and the body by its fall elevating the wings the next." Further on, he says : " Weight assisted by elastic ligaments or springs, which recover all wings in flexion, is to be regarded as the mechanical expedient resorted to by nature in supplementing the efforts of all flying things." It would thus appear, provided the above conclusions are true, that the muscles in case of birds can be imitated more or less accurately by using some form of "spring" in their place. If we hang a weight on a spiral spring the latter is stretched, if we pull at the weight and let it go—the spring with its weight will oscillate up and down so many times a minute. Assuming that this is a mechanical reproduction of what, really happens in the case of birds—remembering, however, that in the latter case the load or pressure is added at each stroke for a continuous action—the ques tion which arises is this : Is it an essential feature of any successful form of ornithopter model that something in the nature Of springs must be employed ; bearing in mind that the wing (natural or artificial) is, in the first case, and should be, in the second, both resilient and elastic ; and that the medium air is itself highly elastic ? Springs necessitate extra weight, and are not set in motion without the expenditure of energy. Action and reaction are always equal and opposite. Nevertheless, basing our statement on the result of practical experiments, it would appear that such are an essential feature of successful ornithopter flight. (To be continued.') Answers to Correspondents. D. MCKAY.—The silk is usually glued. If the varnish has no slackening effect on the fabric, it would be advan tageous to varnish it after it was glued on—so much depends on the varnish. Try both plans on a small plane and compare resu'ts. Whatever dope or proofing is used, it should have a tightening effect on the fabric, and not the reverse; save for floats we have not found varnish very satisfactory—why not use a light soft-proofed silk, such as the Bragg-Smith ? Your other query does not fall within the scope of this section. '.
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