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In connexion with the series of articles on rotary‐wing aircraft by Dr. Bennett which concluded last month, our attention was called to this paper by Professor von Kármán. Although it is now to some extent of historical interest only, it did not at the time of its appearance receive the attention it deserved and we believe that this translation will be of interest to those concerned with the subject

This monograph is the second of the series on the general subject of Aerodynamic Theory which is being prepared by recognised authorities under a grant from the Guggenheim Fund for the Promotion of Aeronautics, the first volume of which was reviewed in AIRCRAFT ENGINEERING, Vol. VI, September, 1934, p. 249. The preparation of the present volume is the joint work of Dr. Th. von Kármán, Director of the Guggenheim Aeronautics Laboratory, California Institute of Technology, Pasadena, and Dr. J. M. Burgers, Professor of Aero‐ and Hydrodynamics at the Technische Hoogeschool, Delft.

DURING the past eight or ten years the speeds of most types of aeroplanes have been practically doubled. Part of this impressive advance has resulted from the use of increased power, but most of it has come from the reduction of aerodynamic drag. The largest and most obvious “built‐in head winds” such as exposed engine cylinders, landing gear struts and wires were first eliminated and attention was then directed to successively smaller factors. The stage has now been reached where it is necessary to consider the effects on drag of such items as rivets, sheet‐metal joints and other irregularities on the surfaces exposed to air flow.

THE extensive series of investigations to be discussed in the present paper has its origin in an attempt to clarify the reason for certain discrepancies, which have long plagued aerodynamicists, between the results of tests on similar aerofoils carried out in different wind tunnels. It has been well known for many years that the Reynolds number has an important influence on aerofoil characteristics. It is therefore highly desirable that aerofoil tests, to be useful for full‐scale predictions, be made at as large a value of the Reynolds number as possible. For several years the variable density wind tunnel of the National Advisory Committee for Aeronautics has been able to employ Reynolds numbers considerably higher than were attainable in any other wind tunnel. Its results have, therefore, very properly been generally accepted as furnishing the standard aerofoil data for aeroplane designers in the United States, and to some extent also in Europe. It appears from the present investigation that another factor, turbulence, may be of the same order of importance as the Reynolds number in determining certain aerofoil characteristics. In discussing the possible effects of this factor, it is desirable that as wide variations in its magnitude as can be obtained should be considered. The turbulence characteristics of the N.A.C.A. variable density tunnel and of the wind tunnel at the Guggenheim Aeronautics Laboratory at the California Institute of Technology (referred to in the figures as Galcit) are as different as those of any two large contemporary wind tunnels. It is for this reason that in the following discussion results obtained in these two tunnels are compared.

TODAY aviation is a most influential factor in our lives and Brooklyn a most influential factor in aviation. This was clear to all who attended the very successful conference on High Speed Aeronautics organized as a feature of the Centennial year by the Department of Aeronautical Engineering and Applied Mechanics of the Institute. Over 600 research workers and technicians assembled at the Engineering Societies Building, New York, to hear and to discuss papers by scientists and engineers from America, England, France, Germany, Italy and Sweden.

A CONSIDERABLE literature about skin friction or boundary‐layer theory exists both in English and in German and there is general agreement as to the best practicable method of dealing with a change from model to full scale. In the resume now to be given no new ideas or facts are introduced.

TURN INDICATORS TURN indicators are primarily used to indicate deviations from a straight course. When flying in fog, it is practically impossible to keep to the course by the compass alone, and the turn indicator is of service in enabling the pilot to preserve a straight course without reference to landmarks on the ground. The instrument also gives important information as to whether the aircraft is correctly banked on a turn, as described in the section headed “Cross Level” above.

There can be few scientific and engineering subjects which have progressed as rapidly as the theory and practice of high‐speed aerodynamics and jet propulsion during recent years. The number and scope of papers that have poured out from research establishments, universities and the industry are such that it has been impossible for all but the very gifted to keep ahead of developments in more than a few limited aspects of the subjects.

IN the first of these articles it was pointed out that normal supersonic flow can be described theoretically, to a first approximation, by the linearized equation of motion. This has the form of the wave equation and governs first order disturbances to fields of uniform flow; for example, flow past thin wings or slender bodies at small angles of incidence, and flow through ducts of varying cross‐section. In the same way small disturbances in a purely subsonic stream can be described by a linearized equation of motion, which can be reduced to Laplace's equation by contracting the co‐ordinate normal to the direction of flow. Transonic flow, in which regions of both supersonic and subsonic flow occur, is not so easily represented.

Under this heading are published regularly abstracts of all Reports and Memoranda of the Aeronautical Research Committee, Reports and Technical Notes of the U.S. National Advisory Committee for Aeronautics, and publications of other similar research bodies as issued