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Aviation History
1926
1926 - 0210.PDF
SUPPLEMENT TO FLIGHT 36 THE AIRCRAFT ENGINEER MARCH 25, 1926 Above a certain limit the length of the bubble is without appreciable effect on the rate of rise. For bubbles rising " slowly " in a given tube, i.e., at such a rate that not more than one thickened annulus of liquid appears between the bubble and the walls towards the rear end of the bubble, it has been found that the rate of rise is inversely proportional to the kinematic viscosity provided that the surface tension is constant. An approximate determination has been made of the form of the function of rja upon which, in addition to the viscosity, the rate of rise depends. In certain conditions, e.g., when light oils are examined in a tube of such a diameter as to give a convenient rate of rise, the rate may vary about 14 times as fast as the diameter and six times as fast as the surface tension, but these effects become very much less pronounced when viscous liquids are used in wide tubes. The air bubble viscometer may be used with confidence for the approximate comparison of viscosities of materials of the same class. A simple modification is suggested which enables a check to be made on the assumed constancy of surface tension. AMERICAN NATIONAL ADVISORY COMMITTEE REPORTS. The National Advisory Committee for Aeronautics in the United States of America corresponds to our own Aero- nautical Research Committee, and perhaps it may be said that the publications issued by the American body score over those published in this country by being issued much more rapidly than is usually the case with the British Reports and Memoranda. Two distinct classes of reports are issued, the first being known as Technical Reports. These are printed and illustrated by photographs and or drawings and diagrams. The second class are known as Technical Notes, and are issued in mimeographed form so as to enable them to be rapidly distributed to a somewhat smaller, but directly interested circle of readers. Copies of the Reports and Notes can be obtained from the Superintendent of Documents, Government Printing Office. Washington, D.C.. U.S.A., but the American N.A.C.A. have a Technical Assistant in Europe, whose office is at 18, Rue Tilsitt, Paris. France, from whom, we understand, copies can be obtained. Readers who are in a hurry to obtain copies of the reports are advised to write to him, as usually some considerable time will be saved by so doing. In so far as space permits, we hope to publish regularly in THE AIRCRAFT ENGINEER summaries of these American Reports and Notes as they are issued. In the meantime, the following summaries of the Reports published in 1925, and list of Technical Notes issued last year, may be of interest. Summaries of Technical Reports Published in 1925. Report No. 210, entitled " Inertia Factors of Ellipsoids for Use in Airship Design.1' by L. B. Tuckerman, Bureau of Standards.—This report is based on a study made by the writer as a member of the special committee on design of Army semi-rigid airship " R.S.I " appointed by the National Advisory Committee for Aeronautics. The increasing interest in airships has made the problem of the potential flow of a fluid about an ellipsoid of considerable practical importance. In 1833, George Green, in discussing the effect of the surrounding medium upon the period of a pendulum, derived three elliptic integrals, in terms of which practically all the characteristics of this type of motion can be expressed. The theory of this type of motion is very fully given by Horace Lamb in his " Hydrodynamics," and appli- cations to the theory of airships by many other writers. Tables of the inertia coefficients derived from these integrals are available for the most important special cases. These tables are adequate for most purposes, but occasionally it is desirable to know the values of these integrals in other cases where tabulated values are not available. For this reason it seemed worth while to assemble a collection of formulae which would enable them to be computed directly from standard tables of elliptic integrals, circular and hyperbolic functions and logarithms without the need of intermediate transformations. Some of the formulae for special cases (elliptic cylinder, prolate spheroid, oblate spheroid, etc.) have been published before, but the general forms and some special cases have not been found in previous publications. Keport No. 211, entitled "Water Model Tests for Semi- rigid Airships," by L. B. Tuckerman, Bureau of Standards.— The design of complicated structures often presents mathe- matical problems of extreme difficulty which are frequently insoluble. In many cases, however, the solution can be obtained by tests on suitable models. These model tests are becoming so important a part of the design of new engineering structures that their theory has become a neces- sary part of an engineer's knowledge. For balloons and airships water models are used. These are models about 1/30 the size of the airship hung upside down and filled with water under pressure. The theory shows that the stresses in such a model are the same as in the actual airship. In the design of the Army semi-rigid airship R.S.I no satisfactory way was found to calculate the stresses in the keel due to the changing shape of the bag. For this purpose a water model with a flexible keel was built and tested. This paper gives the theory of the design, construction, and testing of such a water model. Beport No. 212. entitled " Stability Equations for Airship Hulls." by A. F. Zahm.—In the text are derived simple formulae for determining, directly from the data of wind- tunnel tests of a model of an airship hull, what shall be the approximate character of oscillation, in pitch or yaw. of the full-scale airship when slightly disturbed from steady forward motion. Beport No. 213, entitled " A Resume of the Advances in Theoretical Aeronautics made by Max M. Munk," by Joseph S. Ames.—In order to apply profitably the mathematical methods of hydrodynamics to aeronautical problems, it is necessary to make certain simplifications in the physical conditions of the latter. To begin with, it is allowable in many problems, as Prandtl has so successfully shown, to treat the air as having constant density and as free of vis- cosity. But this is not sufficient. It is also necessary to specify certain shapes for the solid bodies whose motion through the air is discussed, shapes suggested by the actual solids— airships or airfoils—it is true, but so chosen that they lead to solvable problems. In a valuable paper presented by Dr. Max M. Munk, of the National Advisory Committee for Aeronautics, Washing- ton, to the Delft Conference in April, 1924, these necessary simplifying assumptions are discussed in detail. It is the purpose of the present paper to present in as simple a manner as possible some of the interesting results obtained by Doctor Munk's methods. (To be continued.) CORRESPONDENCE In order to leave as much space as possible for new articles in THE AIRCRAFT ENGINEER each month, letters dealing with the various points raised will be published in the Correspon- dence columns of Flight week by week. Letters referring to the article by Mr. Frank Courtney on " Stalled Flight and Control," which appeared in the February 25 issue, will be found in Flight of March 4 (Mr. R. Reynolds and Capt. B. Thomson). March 11 (Capt. G. de Havilland and Mr. Courtney), and March 18 (Mr. R. Reynolds). ERRATA Although a correction was published in Flight of March 11 of two errors which crept into Mr. North's article on " Aircraft Performance," published in THE AIRCRAFT ENGINEER of February 25, it is thought that some of our readers may have missed these corrections, and consequently they are repeated here. On p. 13 the last words of the first paragraph should have read " gun fire " and not " gun power " as printed. On p. 15 the last word of the first line of column 2 should have been " mechanistic " and not " metaphysic." Mr. R. Reynolds writes to point out that a small error crept into his letter published in the Correspondence column of Flight of March 18. In the seventh line from the bottom the words " stalled control " should read " slotted control.' 178/
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