592 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 8 



pendent of the air density, and, therefore, of the altitude of flight. 

 If the engine is equally efficient at all air densities, it follows that the 

 work done by the engine must also be the same for any given distance 

 through the air, whatever the altitude. Hence, the minimum ton- 

 miles per gallon must be independent of the altitude. There is no 

 advantage, therefore, under still air conditions, in flying high in order 

 to beat the record for range of flight. 



In practice, of course, one has to allow for the fact that the total 

 weight will gradually diminish as the fuel is used up. And in normal 

 flying one must also allow for the consideration that flight at a speed 



PROGRESS or WORLD'S 

 DISTANCE RECORD. 



Figure 4. 



which corresponds to the most efficient incidence may not always be 

 operationally convenient, and that is what is found convenient 

 cannot but be related to the speed of the prevailing wind, and since 

 winds vary with altitude, the problem is complex. 



Nevertheless, it is a convenient starting point to realize that for 

 air planes having engines of a given efficiency using specific fuels, the 

 maximum possible ton-miles per gallon tends to be independent of 

 altitude. Hence, the natural limit to range of flight depends far 

 less upon the altitude than upon the attainment of high engine econ- 

 omy, of low airplane drag, and of so low a structure weight as to allow 

 really large fuel tanks to be carried. 



