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Aviation History
1915
1915 - 0882.PDF
[/j]CHT THE By ALGERNON NOVEMBER 12, 1915. ARRIVAL" OF THE AEROPLANE. E. BERRIMAN, M.I.A.E., A.F.Ae.S., Chief Engineer of the Daimler Co., Coventry, England. (Continuedfrom fage 861). The above particular citations, incomplete as they are, suffic; to tt Stream Line Forms. THEORETICALLY, it can be shown that obstructions of fish-like or torpedo shape should experience a resistance that is wholly frictional by nature ; that is to say, their bulk when thus enclosed by a surface of suitable contour should not give rise to any of the resist ance that would be occasioned by the exposure of its flat cross section.* Such shapes are commonly called stream-line.t Actually, the result of tests on many different models shows that while the character of their resistances approximates to the friction law, their coefficients always exceed Zahm's constant for a truly flat surface. /*« <,*•*•<. IO KLOV- — nuw Fig. 1.—Diagram illustrating the nature of the fluid flow round a streamline form in motion. The region of dead water behind the tall end of the streamline form is indicated. It appears from the photographic observation of fluid flow around model stream-line forms that the stream-line flow discontinues its adherence to the contour of the figure as it approaches the tail end of the model.± In the immediate vicinity of the tail there is thus a surface of discontinuity§ enclosing a region of dead water and turbulence, which condition necessarily augments the resistance. See Fig. I. For this reason, it has been suggested that the nature of the resistance of slream-line forms should properly be regarded as dual ; that is to say, partly conforming to the law of surface friction, and partly conforming to the v2 law appropriate to normal surfaces. Apparently, however, the assumption of the frictional law for stream-line forms approximates to the truth sufficiently for practical purposes, provided that the coefficient appropriate to any particular form is determined by experiment. No single value of the coefficient obtains for any variety of shapes, but the ratio of the flat-plate coefficient, determined by Zahm, to the stream-line coefficient, determined in respect to a number of different models that have been tested, is in the order of 1 : 2. Assuming this to hold good for some imaginary cafe, and approxi mating the exposed surface of a stream-line body in terms of a cylinder rr DL (where D — diameter and Z = length) we get a ratio of surface/section = 4i/Z>, from which we may obtain a rough idea of the kind of fundamental relationship that might be expected to exist between the resistance of a stream-line form and the face resistance of its sectional area. Thus, in the example previously cited, a ratio of resistances of 300 : 1 was established in respect to single-face and double flat- surface resistance. In a stream-line body, only the outside surface is in contact with the air, but if, as has been suggested, its coefficient of resistance is twice the flat surface, then the above ratio of 300 : 1 may be allowed to stand. We thus obtain : _ . r Surface Ratio 01 areas = section _ . . unit surface Ratio of resistances —r-— unit section _, . . . , whole surface Ratio of resistances -r r— zr~ whole section == 4 D 300 300 D Or, for a particular case in which the ratio of length to diameter is, for example, 7'5, the resistance of a stream-line body might be expected to be about one tenth the resistance of its cross sectional area exposed as a flat plate. Actually, the resistances of stream-line bodies are of this order of magnitude, although varying widely among themselves. The best results thus far recorded at the National Physical Laboratory show a particular form with a ratio of length to diameter of 4 : 1, which offered 0*07 times the resistance of its cross sectional area regarded as a flat plate. * For proof, tee Lanchester's " Aerodynamics," 3rd edition, lection 9. t Lanchester's definition is as follows : " A stream-line body is one tl motion through a fluid does not give rise to a surface of discontinuity." dynamics," 3rd edition, section 23 11 11 at in its "Aero- : For an illustrated account of these experiments see the " Technical Report of the Advisory Committee for Aeronautics," 191 i-ta, page 95. 5 For a general discussion on discontinuous motion, see Lanchester's " Aero dynamics," 3rd edition, section 33. indicate the enormous importance of the subject—an importance that has already become a predominating influence where modern aeroplane design is based on scientific research. In the aeroplane of to-day, eveiy effort is made to profit by these lessons. The engine, the pilot, and the passenger are placed in tandem, and, as far as possible, the body of the machine is streamlined round them. Most of the struts in biplane construction are made of timber and are rough-turned to stream-line section on a copying lathe, being afterwards finished to gauge size on a sand belt. With steel tubes,. a few oval sections are already on the market and are suitable for some purposes, but in many cases circular-section tubesl! are fitted with light aluminium or wooden fairings, in order lo convert their exterior surface into streamline form. Leaving aside the question of stability, which must necessarily be discussed separately, it is in the above-mentioned direction that aeroplane design has shown the most notable improvement of late years. Success in this department really resolves itself into a capacity for taking infinite pains, and certainly the Royal Aircraft Factory, where this class of design has predominated, has shown great consistency of purpose in its efforts to reduce the resistance of its machines. National Research and Private Enterprise. At this point, it may perhaps be best if I digress for a moment in order to explain the position of the Royal Aircraft Factory and its importance to the country at the present time. For the past six years, the official development of scientific aeronautics in Great Britain has been directed by an Advisory Committee appointed by the Prime Minister. Its President is Lord Rayleigh, its Chairman is Dr. Glazt brook. The formation of this Committee was an accomplishment for which the Government deserves much credit from the world at large, for the Technical Reports of this Committee constitute an unrivalled source of reliable information that might otherwise have taken half a century or more to have seen the light in its present lucid form.. The research itself, which the Advisory Committee directs, takes place at the National Physical Laboratory and at the Royal Aircraft Factory. In the Aeronautics Branch of the National Physical Laboratory the experiments are all conducted on small scale models. At theR.A.F.—an institution evolved from the old Military Balloon Factory, and now under civilian control, with Mr. Mervyn O'Gorman, C.B., as the Superintendent —the experiments are life size. Besides these two Experimental Departments, which are given over to aeronautical research, there is the invaluable work in meteorology, which is conducted partly under the direct control of the Committee and partly under the direction of the Meteorological Office. The reports of this section are also embodied in the annual Blue Books. It must not be supposed for a moment that the development of private enterprise in aeroplane construction, nor the practice of the art of flying has waited upon the work of the Advisoiy Committee, the N.P.L., or the R.A.F. On the contrary, it is to such sportsmen as the Wrights in America ; Farman, Delagrange and Bleriot in France, A. V. Roe and S. F. Cody in England that the flying machine first learned to walk, so to speak, in its proper element.If They had few text-books**—and they wrote none. The Work of ihe Wings. I will now revert to the subject of aeroplane resistance, and discuss as briefly as may be the part of it that is due to the wirgs. Hitherto,. I have referred only to the resistances of struts, wires, body, etc., in short, to the many and various constructional features inseparable from the design of a useful machine. These, nevertheless, play no part in the wing structure proper. In the aggregate, these various resistances are commonly referred to as the " body resistance." No matter what their precise nature, they all increase as the square of the speed,tt and the significance of reducing that due to even one II The circular section itself offers a very high resistance, owing to the surface of discontinuity commencing practically at the full diameter. _«[ For the history of the development of the aeroplane and the work of the pioneers, see "Aviation," p. 172, et seq. For a fuller account of the work of Cayley, Wenham, Walker, Lana, Pilcher, Stringfellow, and Horelli, see " The Aeronautical Classics," published by the Aeronautical Society. For original articles on their own work by Lilicntbal, Cbanute, Maxim, Pilcher and Langley, see Means' " Epitome of the Aeronautical Annual." ** The earliest publication that can claim to be regarded as a really scientific text-book was Lanchester's " Aerial Flight," the first volume of which, called "Aerodynamics," appeared in 1907, and is only beginning to-day to be appre ciated in the way it deserves. tt Approximately. See previous notes on the " z/2 " law, and the laws of dynamical similarity. 882
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