AIRPLANE PERFORMANCES — HAMLIN AND SPENCELEY 439 



Investigations have revealed clearly the fact that for any given 

 airplane and gross weight, the minimum pounds of fuel expended per 

 1,000 feet of altitude gained in climbing always occurs at the maxi- 

 mum available thrust. This phenomenon is valid irrespective of com- 

 pressibility drag increases. Consequently, an extremely high thrust 

 rocket compared to the other power plants has been chosen because it 

 is practical to do so in this case where the power-plant specific weight 

 is small. The tremendous amount of energy available in an extremely 

 short period of time provides dazzling performance for a decidedly 

 limited duration. Even under maximum range operating conditions 

 at vastly reduced thrust the high S. F. C. still obtaining irrevocably 

 limits range and endurance. The only possible recourse would be 

 some means of launching or commencing rocket flight at high altitudes 

 where exceptionally high air speeds will offset the high rate of fuel 

 consumption. Thus, rocket-powered flight in the present instance is 

 considerably limited in range. 



Next comes the ram-jet, in which case the fuel consumption depends 

 directly upon speed and altitude. Assuming a maximum speed of 700 

 miles per hour at sea level and 550 miles per hour at 30,000 feet, it is 

 apparent that at low altitudes a substantial, though not large improve- 

 ment over the rocket obtains. At altitude, the fuel consumption is 

 not a great deal more than the propeller or turbojet-propelled air- 

 planes. In this case 10,000 pounds of power plant plus fuel results in 

 a practical average duration of 30 minutes. Generally, it may be con- 

 cluded from the figure that the ram- jet is suitable for relatively high 

 maximum speeds at medium to low altitudes for relatively short 

 duration. 



By virtue of a lower power-plant weight the turbojet excels over the 

 propeller up to 15 minutes at sea level and 30 minutes at 30,000 feet, 

 after which the propeller takes over. Considering the decided advan- 

 tage of speed for the former, it may be generalized that for excellent 

 high-speed performance at all altitudes up to 40,000 feet, the turbojet 

 endurance at maximum continuous thrust is a good half-hour. To 

 compare with the rocket and ram- jet a 10,000-pound power plant 

 would show an average duration of some 135 minutes, or 214 hours. 



The propeller and internal-combustion engine is by far the best 

 power plant for long endurance at maximum continuous power but 

 at a considerable sacrifice in speed. Extrapolating an average curve 

 to a 10,000-pound power plant results in an endurance of about 12 

 hours. Note that the ram- jet and turbojet show an increased econ- 

 omy at altitude as does the propeller, but the latter is the only one to 

 show an improvement in high speed at altitude. 



10. Power plant plus fuel volume (fig. 8). — In addition to power- 

 plant weight its size is also a critical factor in the design of the aircraft 



