462 SECTIONAL TRANSACTIONS.— G. 



Tuesday, August 23. 



Symposium on Vibration (lo.o). 



Prof. C. E. Inglis, O.B.E., F.R.S. — Resonance in relation to mechanical 

 vibrations {Demonstration). 



Dr. F. H. Todd. — Vibration in ships. 



A ship, being a steel structure, is particularly liable to vibration, and a 

 little may be felt in practically every vessel. The exciting forces arise in 

 the propellers or from lack of balance in the main or auxiliary machinery. 

 The vibration which results may be either of a local nature, affecting only 

 a small part of the structure, or may be a general vibration of the w^hole 

 hull girder. When synchronism arises between the frequency of the 

 disturbing force and a natural frequency of the whole hull, serious resonant 

 vibration occurs, even though the magnitude of the disturbing force may 

 be quite small. 



Vibration which is found to exist on a completed ship may be reduced in 

 amount by improving the balance of the machinery, ensuring accuracy of 

 manufacture in the propellers, perhaps by a change in design of the latter 

 with a consequent change in propeller revolutions per minute, by the use 

 of a vibration damper and, if it is of a local nature, by local stiffening of the 

 structure. 



Resonant vibration of the whole hull could be avoided in the finished ship 

 if the natural hull frequencies could be calculated in the design stage, since 

 the frequencies of the disturbing forces would then still be capable of 

 adjustment. Methods for calculating these natural frequencies have been 

 developed, considering the hull girder as a simply supported beam immersed 

 in water, and the calculated values have been compared with those obtained 

 by observation on board ship. The first natural frequency for vertical 

 vibration, with two nodes in the length of the vessel, can now be predicted 

 with reasonable accuracy, but further data are required for different types of 

 ships, and, particularly, reliable figures for the first horizontal frequency. 



When it is remembered that a ship is the home of a crew for months at a 

 time, and of passengers for perhaps several weeks, the importance of the 

 effects of vibration upon comfort cannot be overestimated. 



Major B. C. Carter. — Vibration in aircraft. 



The first part of the paper comprises descriptions of types of vibration 

 that occur in aircraft, and the second part deals with means of investigating 

 such vibration. 



After brief reference to aircraft vibration due to aerodynamic forces, the 

 vibratory disturbances that may be produced by engines and airscrews are 

 considered. The manner in which torsional vibration of crankshaft- 

 airscrew systems comes about is then explained and tables of natural 

 frequencies are given in respect of representative direct-drive and geared 

 engines : methods of reducing torsional vibration are commented upon 

 and brief reference is made to lateral vibration of crankshafts. There 

 follows an examination of airscrew-blade vibration. 



Mechanical and optical apparatus and instruments for investigating ' 

 vibration are next described and some typical records are given. Included 

 in this are the Cambridge and the R.A.E. vibrographs, the D.V.L. and the 



