AIRPLANE PERFORMANCES — HAMLIN AND SPENCELEY 433 



100 800 30O 400 900 600 700 BOO 900 



TRUE AinSlfCD - U.P.M 



Figure 3. — Thrust vs. air speed at 40,000 feet. 



but maximum speed performance compared to the propeller-driven 

 airplane affords a sharp contrast. 



From the standpoint of self-sufficiency, the ram- jet is the least 

 desirable since an auxiliary means of propulsion is required for take- 

 off. Although thrust varies roughly as the velocity to the 2.0 to 2.5 

 power, it falls off with atmospheric density. In other words, thrust 

 is a direct function of differential pressure, which, in turn, is deter- 

 mined by the airplane drag characteristics. This results in a relatively 

 low airplane ceiling but an excellent maximum speed at sea level where 

 the air forces become prohibitive for sonic air speeds. Loss of energy 

 due to shock waves in the duct entrance has been taken into account. 



6. Specific engine weight (fig. 4) . — Important in the airplane design 

 configuration is the power-plant installation weight. For comparison, 

 the complete power-plant weight, exclusive of fuel-supply system, per 

 unit of maximum available thrust, is presented in figure 4. (See also 

 sec. 9.) When the reciprocating engine is used for high-altitude op- 

 eration it becomes an extremely complicated and heavy power plant; 

 in fact, beyond 40,000 feet it becomes prohibitively so. Also, the 

 improvement in specific engine weight with decreasing air speed and 

 altitude again classifies propeller propulsion as best suited for rela- 

 tively low air speeds and altitudes. Even when compared at low alti- 

 tudes, the reciprocating power-plant dry weight is some 20 times as 

 heavy as the rocket motor. 



The turbojet oft'ers a considerable improvement in engine weight 

 which will probably continue to be improved. Again, with increas- 



