LIMITS TO HUMAN - FLIGHT — WIMPERIS 589 



is taking place. The relation of this to the spinning airplane is of 



interest. 



THE EXTREME ALTITUDE OF FLIGHT 



I come now to the problem of high flying. What are the conditions 

 which limit the altitude of flight? We know that the higher one flies 

 the lower the air density and the smaller, therefore, the quantity of 

 oxygen that passes into the human lungs and into the engine cylinders. 

 The engine itself is really almost human. It breathes in air, it takes 

 in fuel, it changes it chemically, gets rid of waste products, maintains 

 its own warmth and does not like being either too hot or too cold. 

 Like the human body it is liable to suffocation and needs super- 

 charging or the equivalent thereof. There are two effects to consider, 

 the effect of oxygen want in the human body and of a similar lack in 

 the airplane engine. It so happens that the effect on the former 

 operates much more rapidly than it does on the latter. Both are 

 accustomed to working under normal atmospheric conditions at sea 

 level where the pressure is about 15 pounds per square inch, and the 

 proportion of oxygen in the atmosphere about one-fifth of the whole. 



We are fortunate in having a careful study of this question in a 

 lecture delivered by Prof. G. T. R. Hill before the Royal Society of 

 Arts in 1935. I think his conclusions are entirely correct — certainly 

 on this point — and it is worth summarizing them. Since the air 

 pressure at 35,000 feet is just one-fifth of what it is at sea level, it 

 follows that human beings will be as well off with oxygen at that alti- 

 tude as by breathing ordinary air at sea level. But experience has 

 shown that even without any artificial supply of oxygen, flight at 15,000 

 feet is possible; the lungs can adapt themselves to that condition of 

 slight oxygen starvation — provided that no particular bodily exertion 

 is called for. Supplying pure oxygen at 42,000 feet would correspond 

 to this same degree of starvation. But when 45,000 feet is reached 

 oxygen starvation reaches the fainting point — a condition similar to 

 that which arises in the absence of oxygen at some 20,000 feet. The 

 supply of pure oxygen to the pilot, making the oxygen richness approxi- 

 mately five times as large as it would otherwise be, has, therefore, the 

 effect of postponing the normal fainting point for the average pilot to 

 some 45,000 feet, which corresponds very closely to the altitude 

 records obtained in recent years when the pilot was fed in this way. 

 Thus Uwins reached some 44,000 feet in 1932, and Donati 47,000 in 

 1934. The opinion expressed by Wing Commander G. S. Marshall 7 

 some 4 years ago was that while the safe limit of height for a person 

 breathing pure oxygen was 44,000 feet on a flight straight up and down 

 again, he would not state this to be the limit of possible human endur- 

 ance. How high birds can fly I do not know, but Hugh Ruttledge 



' Royal Aeronautical Society, January 1933. 



