586 



AMERICAN FORESTRY 



Copyright 1917 by International Film Service, New York. 



FASHIONING A PROPELLER FOR A WAR SHIP OF THE AIR. 

 The speed of a propeller's revolutions make heavy demands on their strength. Some idea of this strain is afforded by the statement that some 

 engines run at 1700 revolutions a minute and can be geared up to 2000. An engine of this power would use a nine and one-half foot propeller and 

 the speed of the blade ends would be approximately 600 miles an hour. Such speed subjects the blade to pressure of a good many thousand pounds 

 to the square inch and propellers are apt to split at the center and fly apart unless made of perfect material and with great care. 



used the more likely are 

 they to be free from de- 

 fects. Further, the lamina- 

 tion divides the stresses 

 and prevents them from 

 coming in full force on any 

 one grain. It is, on the 

 whole, another exemplifica- 

 tion of the adage that 

 in union there is strength. 

 Still another advantage 

 comes from the fact that 

 the laminated stock can be 

 built to form curves, or 

 can be bent to a curve 

 without splitting or weak- 

 ening the piece. The 

 planes are curved from 

 front to back, and the ribs 

 upon which they are 

 stretched form the basis of 

 this curve. 



The main reason for the 

 use of spruce is its unifor- 

 mity of structure and free- 

 dom from defects. Other 

 woods have desirable me- 

 chanical properties, but 



Copyright 1917 by Underwood & Underwood, New York. 



FEW NAILS ARE PERMITTED IN AIRPLANES 

 Nails are not a great deal used in the building of airplanes. Each post. 



beam or rib is made up of thin layers glued together. -- - 



One part of the construction in which nails or tacks are used 



Nails weaken the 

 structure. One part of' the construction in whicTl nails _ 

 is in covering the wing frames, as pictured herewith. This must be done 

 with the utmost care and requires the employment of men of skill and 

 intelligence. 



lack what might be termed 

 the reliability of spruce. 

 Upon the wood's reliabil- 

 ity the safety of the 

 aviator depends, and in 

 turn this may mean the 

 safety of a whole brigade 

 of men on the ground, 

 whose movements the avi- 

 ator is directing. In other 

 objects made of wood there 

 can be a slight margin of 

 material which is not quite 

 perfect, and this is recog- 

 nized in the lumber grad- 

 ing rules ; but not so with 

 airplanes. 



Spruce has the quality 

 of being what it appears 

 to be on the surface. It 

 does not have hidden de- 

 fects, and this material 

 frankness makes it a fa- 

 vorite. If one picks out a 

 stick of spruce that looks 

 good is clear and straight 

 he may be sure that it is 

 good. Other woods very 



