1$2 SMITHSONIAN' CONTRIBUTIONS TO KNOWLEDGE VOL. 27 



later, when a new propeller had been substituted for this and the engine was 

 again started up, the starboard propeller also broke. When, upon further trials 

 and replacements of propellers, all had beeu so thoroughly demolished that 

 there was not a complete set remaining, it was seen very clearly that the strains 

 produced on a propeller working in the open air are very much greater than 

 those produced in shop tests, where the air is necessarily quiet. These open- 

 air tests of the propellers had demonstrated that their weakest point was where 

 the steel tubes which received the wooden arms of the blade terminated, and that 

 another, though not so serious, point of weakness was when' the steel arms 

 were brazed to the central huh, the thin metal tending to tear loose even before 

 the brazed joint would give way. It was, therefore, decided to construct im- 

 mediately a new set of propellers in which the steel arms should be made of 

 much heavier tubing, that is, a sixteenth of an inch thick at the end where it 

 was brazed to the central hub, and tapering in thickness to one-thirty-second 

 of an inch at the other end. These arms were further made twelve inches long 

 iu place of being only three inches long as before. This added length carried 

 the steel out beyond the point where the first section brace joined the three 

 arms together, and where they were further strengthened by having the cloth 

 covering tightly stretched around them. In order to utilize such of the hubs of 

 the former propellers as had not been seriously damaged when the propellers 

 broke, it was also decided to try the effect of merely adding an extra length of 

 tube to the short arms by means of a thimble slipped over and brazed to the 

 two parts, which would make these arms twelve inches long. The construction 

 of these propellers was pushed as rapidly as possible; and after their comple- 

 tion no further trouble was at any later time caused by insufficient strength 

 of the propellers. Even in the test of October 7, 1903, when the aerodrome 

 came down in the water at a speed of something like fifty miles an hour, and 

 at an angle of approximately forty-five degrees, no break occurred in either 

 propeller until, when the aerodrome was plunging through the water, a blade 

 of one propeller was broken by the terrific blow which il received when it struck 

 the water under the impulse of the engine driving it at full speed. The severity 

 of this blow is attested by 1h«- fact that the shaft, which was of steel tubing one- 

 eightl inch thick, was twisted about ninety degrees. 



This experience with propellers very strongly emphasizes the fact that on 

 any flying machine the strains which are apt to be met with in the open air must 

 be allowed for in the proportioning of the parts of the machine. Bui since an 

 indiscriminate increase of strength in all the various parts of the machine would 

 entail a prohibitory weight, very careful judgment, based on experience, will 

 have to he exercised in deciding just where added strength must he employed, 

 and also where the " live strains " are not apt to exceed very appreciably the 

 calculation for statical conditions. 



