LIMITS TO HUMAN" FLIGHT — WIMPERIS 585 



be overcome by a change of wing section, but since the wing section 

 cannot well be changed during flight, no compromised wing section 

 can be the best both at subsonic and supersonic speeds. 



The forms of aerofoil employed for the air speeds customary today 

 have been studied unremittingly — even exhaustively. Much less is 

 known of those suited to supersonic speeds though work on airscrews 

 gives us some lead. But it seems that sharper leading edges will be 

 needed and wings less deep than at present. Even so, the best lift 

 to drag ratios may be nearer 5 than 20, and this, if it continues, will 

 have striking consequences. Let us consider what could be the 

 physical requirements of an airplane flying at a speed equal to the 

 velocity of sound. An airplane weighing a ton will have a drag of 

 one-fifth of a ton on wing drag only, and perhaps as much as one- 

 third of a ton over all. This for a speed equal to the velocity of sound 

 will require 1,500 horse-power per ton — and, as this is the net figure, 

 the gross may be roughly put at 2,000. But a present-day engine of 

 this size would require the whole of this weight allowance and there 

 would be none left for the airframe and its contents! If speeds such 

 as these are to be attained it cannot therefore be by the engine as we 

 know it today. 



In point of fact the limit to speed is reached even more rapidly than 

 these considerations indicate, since the air speed just above the top 

 of the wing is appreciably higher than the air speed of the machine 

 itself. Hence the critical boundary will be reached while the air 

 speed of the airplane is still substantially below the velocity of sound. 

 Taking everything into consideration, there is much to be said for 

 assessing the maximum possible speed of level flight with the present 

 type of engine as over 500 but less than 600 miles per hour. It is 

 true, however, that if we could suppress everything but laminar drag, 

 or if there were some entirely new prime mover invented giving vastly 

 greater thrust than an internal combustion engine of the same cubic 

 capacity (even one using the very latest fuels of the highest octane 

 numbers), it might be still possible to force the airplane through the 

 air at even higher speeds. But there is not the lease present indication 

 of either of these being practically possible. So 600 miles per hour 

 is likely to remain the limit to the speed of human flight. 



The highest speeds today are all sea-level speeds, but the greatest 

 speeds in the near future are likely to be attained at high altitudes, 

 though as they will be the achievement of military types we shall not 

 hear much about them; a few years later they will again be attained 

 at sea level — since at sea level the velocity of sound barrier is not so 

 soon encountered. At sea level, therefore, it is that human flight 

 will reach the limit imposed by nature's unclimbable fence. 



