LIMITS TO HUMAN FLIGHT — WIMPERIS 591 



By these expedients it is possible, theoretically at any rate, to 

 postpone indefinitely the failure of the human mechanism, and the 

 limit to the altitude performance becomes solely that of the engine. 

 Does nature here impose a definite limit? At 50,000 feet the air 

 density is but a ninth of that at ground level, and since for practical 

 reasons there is no question of supplying the engine with oxygen, the 

 task of supercharging the cylinders adequately is a severe problem, 

 beyond the capacity in fact of any single supercharger. Efforts to 

 increase the altitude still higher than the present limit must impose 

 more and more severe duties on the supercharging mechanism, which, 

 therefore, must needs increase in bulk and weight. In the limit, this 

 very increase in weight makes the attainment of higher altitudes more 

 difficult. So far as present day possibilities in design are concerned, 

 I may cite the estimate given by Mr. Barnwell in his lecture at Bristol 

 in February last that if one went "all out" for an altitude record, a 

 height of 61,000 feet should be attainable. 



There must at any future time be a design limit to what is possible 

 in the light of the materials then available, but it would be rash to 

 forecast finality since the one thing which does seem limitless is the 

 fertility of the brain of a designer once new materials become avail- 

 able. At present we have no means of predicting what new materials 

 may arrive in the future. We are but beginning to grope our way 

 through knowledge of crystal lattice structure towards a coming 

 technique of alloy design. 



THE RANGE OF FLIGHT 



It is far from easy to find any limits imposed by the laws of nature 

 on the greatest distance that an airplane can fly without refuelling. 

 How, for instance, does range of flight depend on the size of the air- 

 craft, the power of the engine, and the height chosen? It is sometimes 

 thought strange that airplanes of what I may call the same vintage, 

 using the same fuel and with equally efficient engines, show when 

 flying at their most economical speed precisely the same figure of 

 ton-miles per gallon (the figure is the same as that commonly obtained 

 in the ordinary motor car). This is true whatever the size of the air- 

 plane may be, and whatever the altitude of flight. I suggest that 

 the simplest way to understand this seemingly strange result is this. 

 At all air densities there is one particular incidence for a given air- 

 plane which gives the maximum value of the ratio of lift to drag, and, 

 therefore, the lowest value of the drag for that particular airplane. 

 Hence, when flying level at this incidence, the lift being equal to the 

 weight, the drag must have a constant value. (I ignore for the 

 moment the effect of any change of weight due to fuel consumption.) 

 Now, with a constant drag force the work done to cover a specific 

 distance through the air must be a fixed and definite amount, inde- 



