292 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1962 



because improved fuels have allowed higher compression ratios (from 

 about 4.0 in 1903 to present values up to 8.0) . 



Best weight per horsepower in 1903 was the Manly engine, at 2.6 

 pounds. This value came down to 1 pound in 1935 and has gone 

 slightly below that since (see table 1) . 



One of the most remarkable improvements has been in reliability 

 and reduced maintenance. The very early aircraft engines were over- 

 hauled after every flight. The approved overhaul period for the best 

 modern transport engines is now as high as 2,600 hours. 



Further improvements in piston engines would have been made 

 had it not been for the introduction of jets and turbines, which have 

 virtually put an end to intensive development of the large piston en- 

 gine. Jet engines are also under development for certain categories 

 of small aircraft. 



PISTON-ENGINE FAMILY TREE 



Figure 21 shows the piston-engine family as it has developed, finally 

 culminating in the Vee-12 liquid-cooled engines as represented by the 

 Rolls-Eoyce and Packard "Merlin," and the 18-cylinder air-cooled 

 radials; and the Pratt and Whitney E-2800 and the Wright 3350, 

 both of which remain in air-transport service, as do also the Pratt and 

 Wliitney R-1830 and Wright E-1820 (in the DC-3 airplane). There 

 are also some Pratt Whitney 9-cylinder "Wasps" in service in me- 

 diimi-powered airjDlanes, especially in Canada, and a few remain in 

 service in the old Ford trimotors dating from about 1930. 



TURBINE, JET, AND ROCKET ENGINES 



Figure 21 includes a family tree for rocket, turbo-jet, and turbo- 

 propeller engines. The history of these developments is so recent and 

 so well covered in the literature (particularly by Schlaifer and Gibbs- 

 Smith) that no attempt will be made to cover it here. There could 

 well be a paper of similar length, or even a whole volume, devoted to 

 this important and revolutionary development in aircraft propulsion. 



NATIONAL CREDITS 



It is interesting to review the contributions of the various nations 

 in the field of aircraft propulsion. Table 2 summarizes this subject. 

 It is evident that the U.S.A. and France have been the principal con- 

 tributors to early engine development, while England has made sig- 

 nificant contributions in late piston and early turbine engines, and 

 Germany was the first to fly rocket and jet engines. 



CONCLUSION 



If the art and science of aircraft propulsion develop as fast in the 

 next 50 years as they have since the Wright brothers' initial flight. 



