AIRPLANE PERFORMANCES — HAMLIN AND SPENCELEY 455 



Maximum range for the turbojet, althougli considerably less than 

 for the propeller, occurs at 150 miles per hour faster air speed, or 

 a 45-percent reduction in time for equivalent range. Speed for best 

 range increases with altitude in both cases. An outstanding character- 

 istic of the turbojet is its relative insensitivity to air speed insofar 

 as range is concerned, the only power-plant variable being engine 

 revolutions per minute. In the case of the propeller, as previously 

 mentioned, a number of variables must be carefully controlled within 

 close limits to attain optimum range. 



The ram-jet suffers still greater in range, but maximum range occurs 

 at extremely high air speeds, above 500 miles per hour. In this case 

 only, speed for economical operation decreases with altitude. 



Eange characteristics for the rocket indicate a wide speed range, 

 350 to 500 miles per hour, with an exceedingly limited range, about 

 100 miles. 



Figure 21 shows the variation in maximum range with altitude, 

 including the distance traveled during climb. The relative maximum 

 ranges and the altitudes at which they occur, up to 40,000 feet, are 

 compared : 



Maximum range 



Altitude 



Piopellor 

 Turbojet. 

 Ram -jet. 

 Rocket... 



Sea level 

 40,000 

 30, 000 

 30,000 



Whereas the propeller has the greatest range, it occurs at sea level. 

 At altitudes above 35,000 feet, range for the turbojet is superior. Both 

 air-stream engines and the rocket show maximum range increases with 

 altitude of from 18 percent to 250 percent, contrary to the propeller. 



CONCLUSIONS 



1. Propeller airplane. — (a) A practical maximum speed limitation 

 not much in excess of 500 miles per hour is apparent. Maximum 

 speeds obtain in the 20,000- to 30,000-foot range. 



(h) Because of superior range, the propeller type of propulsion 

 cannot be supplanted at the present time. Maximum range is rela- 

 tively insensitive to altitude up to engine critical altitude. 



(c) Greater pay loads possible in addition to range make com- 

 mercial application and long-range bombers most attractive. 



(d) Cruising and climbing speeds are slow. 



(e) Operation above 40,000 feet appears impractical except possibly 

 in the case of power plants incorporating the gas turbine. 



