AIRCRAFT PROPULSION — TAYLOR 279 



no noticeable difference in vibration of the airplane. Probably, the 

 engine torque variation was so large in both cases as to obscure the 

 improvement in sidewise shake. In any case, the 90-degree shaft was 

 not approved, although it soon became standard on V-8 engines for 

 nonaircraft use. Such was the state of vibration analysis in 1922 ! 



Reduction of engine vibration became essential in the early days 

 of commercial aviation when passenger comfort became important. 

 In this case, radial engines were used. Draper and Bentley made a 

 serious study of radial-engine shaking forces and moments in 1937-38. 

 One solution lay in flexible engine mounts to reduce the transmis- 

 sion of vibration to the airplane structure. This involved a problem 

 of "droop" due to gravity when the engine was mounted at its rear, 

 as in the case of radials. There was also the problem of decoupling 

 the several modes of vibration in order to avoid numerous critical 

 speeds. This problem was solved by the moimt patented by E. S. 

 Taylor and K. Browne, which has been widely used since. The prin- 

 ciple employed is an arrangement of links which have the effect of 

 supporting the engine at its center of gravity, although the actual 

 flexible mounts are at the rear. Koppen has used very flexible de- 

 coupled engine mounts in light airplanes with good effect since about 

 1939. Another contribution to reduction of engine vibration was 

 the adoption by Wright and Pratt and Whitney, in the late 1930's, of 

 second-order rotating weights to balance the second-order shaking 

 component characteristic of the master-rod system in radial engines. 



Internal vibration of reciprocating engines has been most serious 

 in the propeller-crankshaft system. This type of vibration originates 

 chiefly from the torque variation inherent in piston engines and may 

 be destructive when resonance is involved. 



The Liberty engine of 1917 had a torsional resonant speed of 1900 

 r.p.m. with the usual propeller. Its rating at 1700 r.p.m. was close 

 enough to cause accessory-gear breakage as previously mentioned. 



Serious trouble with torsional vibration was experienced in the 

 1920's in dirigible airships using long shafts between engine and 

 propeller. This type of vibration also held back the development of 

 metal propellers, discussed later in this article. 



A very critical case of crankshaft-propeller vibration appeared 

 with the introduction of the geared version of the Wright 9-cylinder 

 1820-cubic inch radial engine in 1935. This problem was quickly 

 and brilliantly solved by E. S. Taylor and R. Chilton, who developed 

 the pendulous counter-weight, which effectively counteracted the 

 principal torque components of the engine and prevented breakages 

 in the drive system. The basic concept was by E. S. Taylor, for 

 which he received the Reed Award in 1936. Chilton contributed the 



