340 AERONAUTICS 



termed " gust-meteorology." This work has already led to valuable results, more 

 especially in recording the sudden deflections of the wind from the horizontal, and the 

 relatiyely small area of gusts. Observations have also confirmed Langley's exposition 

 of the structure of the wind as consisting of a series of rapidly succeeding gusts and lulls 

 of approximately equal intensity; on rare occasions, however, a sudden increase or drop 

 in the velocity of the wind may be sustained for several minutes, a phenomenon which 

 has been held to explain one form at any rate of the aerial disturbances encountered by 

 aviators and known as " holes in the air." 



Recent aerodynamical research has largely substantiated in practice the former 

 theories of air-resistance based on the general laws of fluid motion. In broad terms the 

 resistance of the air has been proved to vary directly as the area of the sur- 

 ^ ace PP se( l to it (with the necessary reservations regarding the shape of the 

 surface), and as the square of the velocity of motion. The coefficient of 

 air-resistance has further been laid down with accuracy; this coefficient, denoted by the 

 symbol K in the case of a flat square plate placed normally to the wind, has a value of 

 0.075 m metric units, and of 0.00143 in British units (Ibs., sq. ft.; ft. per. sec.). The 

 fundamental equation of air-resistance (making due correction for the density of the air) 

 therefore reads R = KSV 2 . The variation due to the shape of the body is, however, 

 very considerable; thus, in the case of a well-designed, curved aeroplane wing, the co- 

 efficient rises from 0.075 to 0.4, and even more a result due to the curve or camber of the 

 wing and its large span. On the other hand, by adopting a shape approximating as 

 nearly as possible to a perfect " stream-line " form, it is possible to reduce resistance to 

 a very low figure, and this is now taken full advantage of in the design of all parts whose 

 resistance to motion through the air impedes progress, as in the case of the hull of a 

 dirigible, or the vertical struts of an aeroplane. Broadly speaking, therefore, recent 

 research has led.to a considerable increase of that portion of air resistance which, in the 

 case of an aeroplane, is converted into the lifting force, and an important reduction of 

 the detrimental portion of resistance, usually termed the "drift." Further experiment 

 must lead to improvement of the ratio of the lift to the drift and involve corresponding 

 modifications of design, which are already clearly foreshadowed. 



The research work which constitutes the present basis of the science of aerodynamics 

 has been carried out at various laboratories founded and supported either by private 

 generosity and enterprise, or by the public funds. The chief of these 

 Provision aerodynamical laboratories are those of M. Eiffel in Paris, the Aero-Techni- 

 for research, ca i Institute founded by M. Deutsch at St. Cyr, the military laboratory at 

 Chalais-Meudon, Dr. Riabouchinsky's institute at Koutchino in Russia, 

 the laboratory of the University of Gottingen directed by Dr. Prandtl, and the new 

 aeronautical branch of the British National Physical Laboratory at Teddington. The 

 work of this latter institution,* which was established in 1910, has given particularly 

 valuable results, due both to its exceptionally fine and complete equipment, and to the 

 fact that its experiments are largely initiated and controlled by the Advisory Committee 

 for Aeronautics, a body appointed by the British government under the presidency of 

 Lord Rayleigh, in 1910, whose reports are annually published in the form of a blue book. 

 The equipment of this laboratory may serve as the general example of the methods 

 whereby aeronautical research work is nowadays carried out. The principal part of the 

 installation consists of two large wind tunnels in which scale models of the objects to 

 be tested are suspended in a current of air drawn through the tunnel by an electric fan. 

 Special care is taken to prevent the formation of eddies, pulsations and other forms of 

 turbulence, and the speed of the air-current can be accurately regulated up to 50 ft. per 

 second. The wind-tunnel is used to determine the air-resistance and pressure distribu- 

 tion on bodies of various shapes, from the hulls of dirigibles to the smallest portions of 

 the structure of an aeroplane. A water channel, on a similar principle, provides the 

 means for studying and photographing the flow of fluids past different forms of bodies. 

 In a separate building is mounted a large whirling arm for testing models of propellers. 

 In addition the equipment comprises machinery for testing the materials employed. 



