July 14, 1923] 



NA TURE 



69 



The International Air Congress, 1923, 



T^HE second International Air Congress since the 

 *^ War was held in London on June 25-30. It 

 was attended by about 600 members representing 

 no less than 20 countries. The Duke of York was 

 president of the Congress, and the Duke of Sutherland, 

 Under-Secretary of State for Air, chairman of the 

 committee. The Congress was opened on June 25 

 with an address from the Prince of Wales. During 

 the week the meetings for papers and discussion were 

 held in the buildings of the Institution of Civil En- 

 gineers. Three days were devoted to these, while two 

 were utilised in visits to works and places of interest 

 to the members. Thus on Tuesday a large party 

 visited the Royal Aircraft establishment at Farn- 

 borough, while on Thursday the National Physical 

 Laboratory attracted many interested members. 



In addition to the official gatherings, receptions 

 were given by the Lord Mayor and the Duchess of 

 Sutherland, while on Friday afternoon the Secretary 

 of State for Air and Lady Maude Hoare entertained 

 the Congress at a garden party at which the Duke 

 and Duchess of York were present. Saturday was 



'i devoted to a final meeting, with the Secretary of 



:_ State for Air in the chair, at which a number of 

 resolutions were passed. The Congress then ad- 

 journed to Hendon to view the Royal Air Force 

 Pageant, and the week closed with a successful 



\ banquet, with the Duke of Sutherland in the chair. 



: Colonel Lockwood Marsh, secretary of the Royal 

 Aeronautical Society, was secretary of the Congress, 

 and received the very cordial thanks of the Congress 



\ for the admirable arrangements by which its success 



: was secured. 



For the papers and discussions the Congress divided 



' into four groups, as follows : — (A) Aerodynamics, con- 

 struction and research ; (B) power plants — fuels, 

 lubrication, airscrews, etc. ; (C) air transport and 

 navigation ; and (D) airships. 



In each of these a number of interesting and 

 important papers were read ; the papers, with the 

 discussions, will be issued shortly in book form, 



I Readers of Nature will probably find most to interest 

 them in Group (A). 



\. Some fifty years ago Lord Rayleigh directed atten- 

 tion to the effect of circulation of air round a cut 

 tennis ball, having spin, as well as forward velocity, 

 in modifying the motion of the ball and causing it to 

 follow a curved path. In his well-known book on 



X aerodynamics, Lan Chester applied the same idea to 

 account for the lift on an aeroplane wing, and de- 

 scribed the manner in which the vortex system set 

 up round the wing was completed by two series of 

 trailing vortices shed off from each wing tip. These 

 carry away part of the energy and thus give rise 

 to a portion of the drag — known now as the induced 

 drag — which resists the motion of the aeroplane. 



Lanchester's work was descriptive and its im- 

 portance was scarcely recognised ; numerical results, 

 figures, and mathematical calculations were needed 



; before its great value was grasped. W^e now see that 

 it contains the solution of the problem ; the intuitive 

 eye of the genius forestalled the slower methods of 

 the mathematician, though laborious calculations and 



I the work of expert draughtsmen and experimenters 



1 were necessary to establish its fundamental truths. 

 Several of the most important papers in Section A 

 were devoted to this subject. 



\ Starting from the known solutions of the flow round 

 an infinite cylinder moving uniformly in a fluid 

 in which there is circulation round the cylinder, 

 Joukowsky and Kutta transformed the motion into 



NO. 2802, VOL. I 12] 



one about a long cylindrical body having a section 

 resembling that of an aeroplane wing, but with an 

 infinitely thin trailing edge. They obtained an ex- 

 pression connecting the lift on such a wing supposed 

 to be of infinite aspect ratio — i.e. infinitely long in 

 comparison with its width in the direction of flow — 

 with the circulation. The motion is thus two-dimen- 

 sional in planes at right angles to the length of the 

 wing. 



One of the stream - lines near the tail leaves the 

 wing at right angles to its upper surface, and unless 

 this point coincides with the trailing edge the motion 

 breaks down and the velocity becomes infinite. By 

 adopting a suitable value for the circulation the stag- 

 nation point can be brought into close coincidence 

 with the trailing edge, the motion becomes steady, 

 and the lift can be determined ; the value so found is, 

 however, some 20 per cent, too great, and the theory 

 does not account for the drag. There would be no re- 

 sistance to the motion of such a wing. 



Major Low, in one of the papers read to the Con- 

 gress, gave an interesting account of a draughtsman's 

 method of applying the Joukowsky theory to a wing 

 of any form. 



This simple two-dimensional theory was modified 

 by Prandtl and his school. He assumes the wing to 

 shed vortices all along its trailing edge from the 

 centre outwards, forming a vortex sheet which at a 

 little distance behind the aeroplane rolls up into a 

 single long vortex trailing away from each wing tip 

 in a direction opposite to that of motion, as in 

 Lanchester's suggestions. Thus the circulation, and 

 hence the lift, falls off as one passes outwards along 

 the wing ; and, assuming a law for its decrease, 

 Prandtl obtains an expression for the lift on a wing of 

 finite aspect ratio, and, by taking into account the 

 effect of the trailing vortices, for the drag considered 

 as due to the action between these and the wing 

 vortex — the induced drag. This accounts for a large 

 percentage of the observed drag. In England, Mr. 

 Glauert has done much in connexion with this theory, 

 which has been applied to the interference of the 

 channel walls on a model under test, to the theory 

 of the propeller leading to Froude's coefficient of 

 0-5 for the induced flow near the propeller, and to 

 other problems. Mr. Glauert's paper gave an im- 

 portant resume of the present position. 



But there is a fundamental difficulty : the fluid is 

 treated as inviscid, and in such a fluid the motion of 

 a body will not set up vortices ; the body will ex- 

 perience no drag. Air is viscous, and the value of 

 the kinematic coefficient of viscosity has an important 

 bearing in aerodynamics, while the shearing forces set 

 up by the viscosity depend on the rate of change of 

 velocity in the direction normal to the flow. Now, 

 since the fluid, if viscous, is at rest relative to the body 

 at all points of its surface, the rate of change of 

 velocity, and therefore the viscous shear, will be greatest 

 close to the surface. The Prandtl theory supposes 

 that such viscous forces are sensible only throughout 

 a very thin film surrounding the surface, which suffices 

 to set up the circulation, and that outside this film 

 the equations of an inviscid fluid may be used. 



Prof. Bairstow in his paper, after a reference to his 

 recent communication read before the Royal Society, 

 suggested that an attempt to relate the circulation 

 theory to the fundamental equations of motion, taking 

 viscosity into account, would lead to a determina- 

 tion of the friction on the surface of the aerofoil, 

 thus giving that part of the drag which is omitted 

 from the Prandtl theory. Promising work on these 



