178 SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE VOL. 27 



space in which they had to be applied, they were never really large enough 

 for the work they had to do when transmitting the full power of the large en- 

 gine. They gave continual trouble, and were the source of delay which, while 

 it cannot be accurately measured, since there were often other causes, yet might 

 be conservatively estimated at not less than three or four months. Such a de- 

 lay, when reckoned in retrospect, can easily be seen to have caused an expense 

 which would have sufficed for almost any change in the bearings, bed plates, 

 etc., had the change been made immediately after the bearings were found to 

 give trouble. With the better steel which it is now possible to obtain for the 

 races of the bearings, and with the high-grade balls now obtainable, the bear- 

 ings could be readily replaced without changing any other parts and still be 

 amply strong for the work. 



Propellers 



Both the tests on the whirling-table and the actual residts with the models 

 had shown that propellers which were true helices formed out of wood were 

 rather more efficient than those constructed by the use of a hub in which were 

 inserted wooden arms, forming a framing over which cloth was tightly drawn. 

 But the very great difference in the cost of construction and the facility with 

 which the latter type could be repaired in case of damage — the wooden ones 

 were practically of no use if once they were much injured — made it seem ad- 

 visable to construcl all the propellers for t he large aerodrome in the manner 

 just explained. Several pair of small propellers had been built on this plan, 

 some as early as 1895, and one very important advantage had been found to 

 he possessed by this type besides cheapness and facility of repair. Wooden 

 propellers of even so small a diameter as one metre had been found to suffer 

 a quite appreciable bending of the blades, due to the thrust produced by them, 

 even though the blades had been made of considerable thickness. In planning 

 a propeller 2.5 metres in diameter for the large aerodrome it was seen that in 

 order to make the blade sufficiently strong to withstand its own thrust it would 

 be necessary to make it inordinately thick, which, of course, would mean a con- 

 siderable increase in weight. Tn fact, it was seen that the weight of the larger 

 propellers would increase practically as the cube of the diameter; which, for 

 the 2.5-metre propeller, would involve a weight of something over fifteen times 

 the weight of those one metre in diameter. The other type, which for conve- 

 nience we will call " canvas covered," permitted the bending moment produced 

 on the blade by the thrust to he taken up by sruy-wires running from the cor- 

 ners of the blades to a central post projecting from the hub of the propeller, 

 and it was found that in this way a considerable saving in weight could be 

 effected. 



