450 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1947 



Next in high speed comes the turbojet, which excels the propeller 

 at all altitudes, being 37 percent faster at sea level and 8 percent at 

 25,000 feet, the top speed of 550 miles per hour representing a de- 

 cided advantage. Maximum speed for this airplane occurs at sea level 

 only because of the influence of Mach Number on drag. Otherwise, 

 a considerable increase in speed with altitude up to 20,000 to 30,000 

 feet would be found. Although at altitudes between 25,000 and 40,000 

 feet the speeds are not greatly in excess of those for the propeller, this 

 margin will improve with further development of turbojet engines. 



A Mach Number = 1.0 line is shown in the figure, representing the 

 magic barrier to speed performance, being the condition of maximum 

 airplane drag coefficient. In order to achieve sonic velocity, the thrust 



FiatmE 18. — Maxinmm and best climbing speeds. 



required at sea level would be 1,000 percent of that for the speed 

 actually attained or 650 percent in the case of a 30,000-foot altitude. 

 Sonic velocities with this type of airplane, therefore, appear impos- 

 sible at the present time. 



Regarding speed for best climb, which occurs at 65 percent of 

 maximum speed at sea level and 75 percent at 20,000 feet, a consider- 

 able tactical advantage over the propeller airplane is realized. A 

 greater distance covered during climb, since rates of climb for both 

 are comparable, also offers a considerable advantage in being able to 

 protect a given target from a base relatively far distant. An appre- 

 ciable performance margin also obtains above 40,000 feet because of 

 the higher ceiling of the turbojet airplane. 



