436 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1947 



Selecting S.F.C. values, (lb fuel/lb. thrust times hours), maximum 

 continuous thrust operation for each power plant results in the 

 following comparison : 



S. F. C. at maximum continuous thrust 



Air speeds corresponding to the four airplanes are included in the 

 table as a matter of interest. A considerable improvement in eificiency 

 at the higher altitude is noted for the last three power plants compared 

 to the propeller and the price that must be paid for increasing air 

 speeds above 500 m. p. h. is painfully evident. 



8. Variatioti of S. F. C. with reduced thrust (fig. 6) . — Comparison 

 of specific fuel consumption at various level flight speeds is not possible 

 without considering airplane characteristics, because the thrust re- 

 quired depends directlj'' upon the airplane drag. Therefore, the curves 

 drawn in figure 6 necessarily coincide with the particular airplanes to 

 be considered later but the general trends and comparisons are valid 

 in other applications. 



Obviously, the best range performer is the propeller, which, at low 

 thrusts, may be operated on a lean mixture. Here the variables, engine 

 revolutions per minute, manifold pressure, degree of supercharging, 

 fuel mixture setting, and engine operating temperatures must all be 

 carefully controlled in order to obtain maximum economy. Neverthe- 

 less, a reduction in S. F. C. of the order of 50 percent from that at 

 maximum continuous power represents a decided saving. Based on 

 both figures 5 and 6, the propeller has no serious competitor for maxi- 

 mum range or endurance at moderately low air speeds. 



Unlike the propeller, the turbojet evidences a loss in economy when 

 operating at lower than maximum output, representing about a 10 

 percent increase in S. F. C. for cruising conditions. Cruising S. F. C. 

 is better at high altitudes than at low levels in all cases with the 

 exception of the rocket. 



An appreciable improvement in economy of the ram-jet at cruising 

 speeds is shown tending to offset its disadvantage when compared to 

 th turbojet at high speed. Maximum thrust and the corresponding 

 S. F. C. are necessarily established as those produced at the maximum 

 speed of the airplane. 



The rocket, as always, appears as the only power plant whose char- 

 acteristics are independent of its application. In this respect, how- 



