Sir yoseph Petavel 191 



It is impossible to study a model aeroplane when it 

 is actually flying. At Teddington, therefore, they have 

 adopted the expedient of fixing the model and forcing 

 the air past it. Specially designed apparatus records how 

 the wind-pressure affects the model, and the velocity and 

 direction of the air-flow past any particular part of it. 



This is important, for if anything in the design of a 

 'plane produces air-eddies in any given wind-direction 

 the lifting-power of the machine will be reduced. How 

 then can it be proved that any given design will produce 

 an even current as it travels through the air? Or, if it 

 produces eddies that cause it to ' drag/ how can those 

 eddies be measured and the cause of them exposed ? 



These seem to be impossible questions, but the National 

 Physical Laboratory has answered them by creating a 

 simple apparatus. They fix a fine platinum wire, sus- 

 pended between two prongs, to that part of the model 

 which they wish to study, and an electrical current is 

 passed through the wire, heating it. Then the wind is 

 turned on. If there is an even current of air, with no 

 obstacles to produce an eddy at the point under test, the 

 wind will cool the whole length of wire evenly, and the 

 instrument which carefully records every variation of 

 temperature of that little wire will show that result. 

 But if eddies are present in the wind-force at that given 

 point, the wire will one minute be in a fast current of air 

 — and therefore cool — and then for a fraction of a second 

 in a slower current or a calm produced by the eddy. In 

 that fraction of time the temperature of the wire will 

 increase, and that increase will be recorded, thus proving 

 conclusively the presence of unequal air-currents due to 

 the design of the machine. 



That is one small example of how these wind tunnels 

 lay bare secrets which might otherwise not be revealed 

 except by years of research. Investigations have also 

 been made with airship models, and these have been 



