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Aviation History
1915
1915 - 0266.PDF
their Morane monoplane, they encountered three Alba- tros biplanes, which are said to have speedily turned tail on seeing the Morane. Gilbert and his mechanic singled out one, and started in pursuit. They soon over hauled the slower German machine, and at times were flying only fifteen metres apart. Although the hostile aviators were armed with a machine gun, and Bayle only with a cavalry rifle, the Germans seemed distressed, and were constantly dodging about in an endeavour to escape. On one occasion the Albatros dived under the Morane at a distance of a couple of metres only. Bayle says that he thought the Germans intended to ram them, but realising that this was not the case he got in a couple of shots with his rifle. The chase had, unfortunately, to be given up at Montdidier owing to lack of ammunition. On their return to the French lines Gilbert and Bayle, upon examining their machine, found that one bullet had pierced the wing above their heads, while another had first gone through the reserve petrol tank and then through a tool chest in which, amongst other things, were some picture post-cards belonging to Gilbert. The mechanic, who during the pursuit had taken off his gloves in order to be able to handle his rifle better, bad his hands so frostbitten, the temperature being 16 degrees below zero, THE SCREW By F. W. LANCH Introduction. i. It can hardly be said that in the past the theory of the screw- propeller has l>een established on an altogether satisfactory footing. The more general theory of propulsion as laid down by Rankine and the late Mr. W. Froude, although admittedly incomplete, is in effect one of the most successful of the applications of the Newtonian theory,t and may be regarded as practically sufficient; when, how ever, we pass from the general theory to the implement of propulsion, the screw propeller, the outcome of the early attempts cannot be regarded as conclusive in any respect. Mr. W. Froude introduced into his treatment of the subject the notion of regarding itbe blade as made up of a number of annular elements. So long as this idea is not pushed too far it is of the greatest utility, and may be said to have taken a permanent place in propeller theory ; certain restrictions, however, are necessary. The author's method of treatment, as set forth in his " Aerial Flight," (Vol. I., Chap. IX), is founded on the theory of sustenta- tion and of the aerofoil developed earlier in the same work. In it the Froude conception of the blade as the sum of its annular elements is adopted, but the component elements are given, as to their pressure reaction values, &c, the l>enefit of the aspect ratio of the blade as a whole. The theory is developed in respect only of the condition of highest efficiency, or optimum condition, and is carried to the extent of being made the basis of an actual design. The investigation in question and the rules and procedure laid down by the author have been followed anyd worked to at the Royal Air craft Factory with a very fair measure of success, the utility of the method being fully admitted. In the present paper the author go<s far beyond anything attempted in the original investigation and deals with the propeller more generally, under other than the optimum condition ; also as a special case (in Part I), a theoretical solution is given to the problem of the stationary screw or " helicopter. To some extent the results of a recent paper by the author, "A Contribution to the Theory of Propulsion and the Screw Propeller " (Inst. Naval Architects), are utilised ; though mainly this is trie case in respect to the " helicopter " problem. In dealing broadly with the screw propeller, the author has adopted a result given in an earlier paper by Dr. R. E. Froude,J in which it is shown that, in the case of a propelling instrument of a purely hypothetical kind, one-half the acceleration must take place in front of the propeller and the other half behind it. This result undoubtedly applies in some degree in the case of a screw propeller, but how much before the medial plane of the propeller is reached and how much after we do not exactly know. It being necessary in the investigation (Part III) to assume something, Dr. Froude's result has been taken * A Paper read before the Institution of Automobile Engineers, April 14th, * The theory of the hypothetical medium of Newton. : " On the Part played in Propulsion by Differences of Fluid Pressure' Free. Inst. Naval Architects, 1889. APRIL 16, I9I5. that he had to go to hospital. He is now in flying trim once again, and none the worse for his adventure, whilst amongst his dearest souvenirs is one of the picture post cards pierced by a German bullet and bearing the fol lowing inscription by Gilbert: "A men vieilami Bayle, d mon devout- mechanicien, en souvenir de notre chasse a I'Albatros du 18 Novembre, 1914, a Amiens." ".'BOLUS." PROPELLER.* ESTER, M.Inst.C.E. as it stands. It will be noted, however, that the author's treatment admits of any other or better ascertained result being adopted ; a few forms of algebraic expression only will need modification, and the arithmetical results will need correcting accordingly—the method is quite elastic. 2. The author takes the present opportunity of pointing out a difference between his present and former treatment which is of some moment. In his "Aerial Flight " (Vol. I.) the theory of sustentation and of the aerofoil in which the propeller theory is founded is based to a certain (and essential) degree on the experi mental results at that time available, mainly the work of Dines and Langley ; in the present investigation the whole of this is discarded, and in its place (in spite of all the wind channel work that has since been done), the author has elected to use a purely theoretical con ception of the nature of the fluid motion. This is as set forth in the paper on "The Aerofoil" read last month before this Institution ; it is a definite fact that the results given by experiment are, so far as relevant to the present subject, considerably less reliable than those which may be deduced from the most elementary theoretical considerations. The form of treatment adopted is, however, such that when this state of affairs is reversed, as in due course it may be, it will be merely necessary to substitute a curve plotted from experiment for one calculated from theory, in the graphic portion of the work to bring things up to date. It may be briefly stated here that the author's recent paper gives an account of a theory of dynamic support founded directly on vortex motion. It is shown that the aerofoil gives rise to a vortex pair or system in its wake, and that the supporting reaction, the resistance, and (approximately) the sectional form may all be correctly deduced as related to the said vortex system. Beyond this it is shown that the vortex system commonly generated by an aerofoil of approximately uniform camber is very closely that which would be set up in a two-dimensional region by the movement of a plane (or line, if strictly in two dimensions) through a very short distance by an impulsive or nearly impulsive force ; this happens luckily to be a form which has been studied and solved by the mathematician, and the streamline system is that given in Fig. 1. The two-dimensional region is presumed to lie "athwart stream," and the width of the plane in Fig. 1 corresponds to the effective span of the aerofoil; it has to be assumed that the plane be with drawn immediately the impulsive force has set the fluid in motion. The two-dimensional treatment would not be legitimate were it not for the fact that the vortex is shown to comprise a cyclic component around the aerofoil itself. The subject at the best is not in theory the easiest to understand, and in any case some knowledge of the hydrodynamics of mathematical theory must be presumed, other wise the facts mast be taken on trust.Sj g Reference should be made to the paper cited for a fuller exposition ; also to the author's " Aerial Flight." Vol. I. For the study of the mathematical treatment of hydrodynamics and vortex motion, either Lamb or Basset may be recommended. 266
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