278 



ANNTJAL REPORT SMITHSONIAN INSTITUTION, 1962 



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Figure IS. — Aviation fuel performance chart. 



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cient supercharging at sea level to take advantage of the increased 

 knock limit. 



VIBRATION CONTROL 



Powerplant vibration presents two kinds of problems in aircraft. 

 One is "external" vibration, or vibration of the powerplant with rela- 

 tion to the airplane itself. The other is "internal" vibration, that 

 is, vibration of parts within the powerplant. Considerable external 

 vibration from engine and propeller was accepted as normal in the 

 early days of aviation. In my experience it became of concern first 

 in 1920, with the Hispano-Suiza V-8 300-hp. engine, a larger version 

 of the original model. This engine, like all V-8's up to that time, had 

 cranks at 180 degrees, which gave a strong second-order horizontal 

 vibration. It also had an unusually large torque variation, due to 

 its large cylinders and high mean effective pressure. Pilots com- 

 plained of discomfort with this engine. 



About 1921 the Wright Aeronautical Corporation, which built the 

 300-hp. Hispano engine, built one with the cranks at 90 degrees 

 which eliminated the horizontal shake. Vibration-measurement at 

 that time was in a crude state, and the improvement obtained was 

 demonstrated on the test stand by the fact that, with the 90-degree 

 shaft, a penny would remain on the crankcase, while with the 180- 

 degree shaft the penny would quickly bounce off. 



The next test was to mount two engines in similar Thomas-Morse 

 fighters, one with the 180-degree shaft and one with the 90-degree 

 shaft. A number of engineers ran these engines on the ground, and 

 a number of pilots flew them. The consensus was that there was 



