524 ANNUAL REPOET SMITHSONIAN INSTITUTION, 193 9 



in airplane altitude carries a challenge that can only be met by a 

 combination of adequate engineering design with supreme judgment, 

 skill, and daring on the part of the pilot. 



This question of altitude flying brings us naturally to that of 

 commercial flying at altitudes much less than those just noted, but 

 still at an altitude that will permit of drawing a real advantage 

 from the decreasing resistance to motion with decreasing density of 

 the air and thus, within the conditions of commercial requirements, 

 of realizing higher speeds of transport than would be possible with 

 the same engine power at sea level. Already altitudes of 12,000 to 

 15,000 feet are the rule on long-distance flights. 



We remember that the word "stratosphere" is used for an indefi- 

 nite stratum of the atmosphere lying above an altitude varying 

 somewhere about 35,000 feet, and above which the temperature of 

 the air is nearly constant at about 67° below zero, Fahrenheit. 

 Substratosphere flights may then be considered as those made in that 

 stratum of the atmosphere lying perhaps between altitudes of 20,000 

 and 30,000 feet; though undoubtedly first approaches will be made 

 at altitudes of 15,000 to 20,000 feet. 



Within these limits the difficulties which are met with at altitudes 

 of 40,000 to 50,000 feet and above are present in lesser degree, and 

 it does appear possible to realize, commercially and economically, 

 higher speeds of transport than at lower levels. The interaction of 

 the various factors is very complex, and I shall make no attempt to 

 discuss them in detail, but the general results of flights at altitudes 

 of the order of 20,000 feet are already in the way of being put to 

 commercial test. 



For passenger transport, however, certain special conditions must 

 be met. Most people can tolerate without difficulty altitudes of 

 10,000 to 12,000 feet. It is, of course, quite out of the question to 

 provide passengers individually with oxygen masks and equip- 

 ment. The only practicable solution for both passengers and operat- 

 ing personnel is to place the entire personnel, passengers and crew, 

 within an airtight compartment of the plane in which, above say 

 10,000 feet, the air is kept, by the operation of a suitable compressor, 

 always at a pressure and density corresponding to this altitude. 

 This means a closed metallic structure capable of withstanding the 

 excess pressure within the cabin against the reduced external pres- 

 sure of higher altitudes. Thus with the cabin at the pressure for 

 10,000 feet and with the plane at 20,000 feet, this excess would be 

 about 500 pounds per square foot. With the plane at 30,000 feet, the 

 corresponding figure would be about 800 pounds per square foot. 



Then again the engine must have its own quota of air; but here 

 no compression up to the conditions for 10,000 feet will be accept- 

 able. Full sea-level conditions, or as nearly so as may be practicable, 



