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Aviation History
1911
1911 - 0384.PDF
I/UGHTJ APRIL 29, 11,11. CORRESPONDENCE,. * The name and address of the writer (not necessarily for publication) MUST in all cases accompany letters intended for insertion, or containing queries. Fly Faster for Less Power ? " Also, in a summarised form, in the recent article on the mathematics of the cambered plane, which appeared in FLIGHT, January 21st, page 58. In order to make this application it was necessary to establish an hypothesis that might plausibly represent the actual conditions, and it is assumed, in the rather dim light of present-day knowledge, that the camber of a plane represents the angular deflection of the virtual air stream. Hence the evolution of the term " angle of deflection." Having defined the angle, the span, the sweep and the velocity, we have established, by hypothesis, the change of momentum in a stated mass of air, and the Correspondents communicating with regard to letters which they have read in FLIGHT, would much facilitate ready reference by quoting the number of each such letter. NOTE.—Owing to the great mass of valuable and interesting corre spondence which we receive, immediate publication is impossible, but each letter will appear practically in sequence and at the earliest possible moment. Limits to Construction of Aeroplanes. [1140] The following table of limits will probably interest some •f your readers. The figures are, of course, only approximate :— Quantity. Lower Limit. Velocity Loading Total weight . Useful weight. 20 miles per hour 4 lb. per sq. ft. ... 200 lbs., or say 300 with motor 150 lbs Propeller thrust !i lb. per h.p. Lift per h.p. ...(About 10 lbs. Gliding angle ...|6° Area jioo sq. ft. Travel... ...o ... h.p. hours per lo ... lb. of fuel Determining Causes. Upper Limit Determining Causes. Stability and weight 200 miles per hour of construction Construction Weight of man Weight of man .10 lbs. per sq. ft. ..About 10 tons . ..About 2 tons Axial velocity and About 40 lbs. friction Weight of motor and j About 100 lbs. ... construction Best angle of attack i[About 250 Minimum load ...About 2,000 sq. ft. — ; About 1,500 miles — jAbout 2*5 Strength, weight, and drop of propeller thrust Velocity Size, increase of weight with dimensions Weight of engine and con struction Friction Minimum velocity required Safety] Construction and load Fuel (weight of) Calorific values of fuel Tang Shan, Chili-Li, North China. HERBERT CHATLEY. The Newtonian Method. [1150] As a constant reader of FLIGHT, may I ask you to «xplain the following : In " Artificial and Natural Flight " (Maxim), p. ix of preface and pp. 2, 6 and 62, &c, of the book, the writer derides " Newton's law." What particular law is referred to, and is it the same law as that mentioned in " Flight Manual" N. 8, &c. ? Also at pp. 55 and 56 of Sir Hiram's experiments show that as the angle increases the lift increases. " Flight Manual," No. 20, appears to controvert this. Have I misread either ? A READER. [The laws referred to are the same, but whereas Maxim is discussing the Newtonian method applied to inclined planes, " Flight Manual," N. 8, is dealing with the same theory applied to normal planes. The Newtonian method, as " Flight Manual " explains, is applicable in aerodynamics wherever it is possible to define the change of momentum taking place in the virtual air stream. A normal plane is an obstruction that brings a certain amount of air to rest, but the amount is not exactly defined by the area, because a quantity of the air leaks over the edges. Whereas the Newtonian theory gives full value to the area of the obstruc tion, the average working value for approximations (" Flight Manual," T. 154) is only "6. In dealing with inclined planes the Newtonian method fails because it is impossible, at any rate in the light of present knowledge, to say what change in momentum an inclined flat plane produces in the virtual air stream. The New tonian theory assumed, on purely mathematical logic, that the effect of the angle would be proportional to the square of its sine, hence the formula in " Flight Manual" T. 156. This value, however, practical experiment has shown to be very inaccurate, not because the logic by which it is deduced is at fault, but because the hypothesis stating the conditions does not correctly represent things as they actually are. The nearest approximation to a law defining the effect of an inclined flat plane is given by Duchemin's empirical formula, which can be compared with the Newtonian law in " Flight Manual " T. 156. It will be observed that both laws give an increasing lift with an increasing angle. An attempt to apply the Newtonian method to cambered planes has recently been made editorially in this journal, and may be studied in that series of articles entitled " Can we application of Newton's law gives the resultant lift on first principles. Our theory of the cambered plane does not pretend to be exact, but it does aim at being logical, and it is an attempt to put the mathematics of the cambered plane on a simple straightforward basis that can be understood by everyone and may be readily brought into line with practice as soon as experiments provide the necessary data. In other words, it seeks to establish the cambered plane on the same basis as the normal plane, so that, like the normal plane, it may have its recognised constant, which experimenters will verify from time to time. The constant for the normal plane, for example, has been verified by Hutton, Langley, Dines, Froude and Stanton, with slightly varying results, but the variation is not a source of confusion, because all experi menters have worked with one definite object of establishing the coefficient for this same formula. Consequently, each succeeding experimenter has been able to improve upon the methods of his predecessor and to arrive at greater accuracy in his results. Now in the case of the cambered plane, there is hopeless confusion of thought regarding its fundamental principles, consequently no two experimenters try to find out the same thing. Some regard it merely as a curiously efficient form of fiat plane, for it is not even yet generally recognised that the cambered plane is cambered for fundamental reasons.— ED.] Kite Competition. [1151] In reply to S. Horrock's letter (1064), in which he asks (1) What length of cord; and (2) What was the angle of winning kites. (1) The length of cord or wire was 300 yards. (2) The angles of the winning kites were as follows :—(1) C. W. Hayes, 56°, with a kite 75 sq. ft. area; (2) A. J. Brooke, 530, with a kite 60 sq. ft. area; (3) C. K. Scarf, 410, with a kite 30 sq. ft. area ; (4) W. Jones. 500, with a kite 36 sq. ft. area; (5) H. W. Browse, 55°, with a kite 30 sq. ft. area. But the marks were not only for angle but stability, strength of construction and collapsibility as well. The result will be found on page 822, No. 41, Vol. II, October 8th, 1910. The results of the Major Baden-Powell Challenge Shield Competition may be interesting to this gentleman, and he will find the official account on page 506 (No. 27, Vol. II), July 2nd. Should he require any other information I shall only be too pleased to reply. W. H. AKEHURST, Hon. Sec. K. and M.A.A. Gyroscopic Control. [1152] In the issue of FLIGHT for March 4th a letter appeared from your correspondent Mr. D. Grieg on the subject of " Gyroscopic Control." I read his letter with great interest, but I think that his meaning is somewhat obscure. At the commencement of his letter he states 386
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