DURAND— MECHANICAL ASPECTS. 181 



cylinder liners and generally for all parts receiving or carrying the 

 direct load. Further progress here will wait on the skill of the 

 metallurgist in furnishing steels of higher physical properties than 

 those now available. Broadly speaking, the present aeronautic 

 engine, in the most refined designs, exhibits a very near approach 

 to the practicable limit with the materials at present available and 

 further saving in weight must depend chiefly on the work of the 

 metallurgist in developing new and improved materials for use. 



The chief outstanding problems in the design of aeronautic 

 engines are those dealing with the most effective disposition of the 

 available materials of construction, and with the forms, proportions, 

 arrangements and assemblage of the elements in such manner as 

 shall secure the highest practicable degree of reliability of operation. 



In the disposition of the materials with reference to the strength 

 and stiffness required, the fundamental and insistent demand is the 

 saving of weight. This problem is one to be studied partly by the 

 application of mechanics and general engineering principles, and 

 partly by experience. In any given engine there is no question but 

 that there is a certain amount of redundant weight. The problem 

 is to locate it. While, as already noted, the best of modern designs 

 represent apparently a close approach to the ultimate attainable 

 with existing materials, nevertheless the field of design with refer- 

 ence to further refinement is still open and will doubtless well repay 

 further study. This road marks clearly one of the ways whereby 

 future progress and improvement must come. 



The principal problems dealing with improved reliability and 

 with length of operative life may be enumerated as follows: 



1. Oiling system and lubrication generally. 



2. Means for securing all pipes and conduits, whether for oil, water 



or electric wiring, in such manner that jar and vibration 

 cannot cause their rupture or separation at joints. 



3. The reduction of vibration to a minimum by the careful balanc- 



ing of rotating and reciprocating parts so far as practicable. 



4. Adequate bearing surfaces especially for all principal parts, so 



that with a reasonable supply of lubricant there need never 

 be danger of cutting or abrasion. 



5. Adequate crank shaft size and adequate crank shaft bearings, 



