WOOD ON THE WING 



587 



Photograph from Underwood & Underwood, New York. 



WHERE PROPELLERS ARE BUILT IN LARGE QUANTITIES. 



This is a picture taken in the propeller department of one of the great airplane plants now devoted to turning out machines for the United States 

 Government Ash has been preferred for this feature of construction, but the largest single order for propeller-blade material is said to have been 

 for the finest grade of quarter-sawed white oak. Propellers are sometimes made of mahogany or of a combination of mahogany and spruce to al- 

 ternate layers Some propellers are made wholly of black walnut, which does not splinter when hit by a projectile. The sponginess of texture 

 that keeps walnut from splintering is one of the chief reasons for the use of this material in rifle stocks. 



like spruce in general appearance may look as straight 

 and clear, but will occasionally deceive. 



Propellers, like the other parts, are made of built-up 

 pieces. Ash has been preferred; but the largest single 

 order for propeller-blade material is said to have been 

 for the very finest grade of quarter-sawed white oak. 

 Some propellers are made of mahogany, mahogany and 

 spruce in alternate layers, or mahogany and ash. Black 

 walnut has been used in place of mahogany, and some 

 propeller blades are made wholly of black walnut. This 

 is partly because black walnut, hit by projectiles, does 

 not splinter. It has a sponginess of texture which gives 

 it this quality, and furnishes one of the reasons why 

 black walnut is universally in demand for rifle stocks. 



The propellers are subjected to other trials than those 

 of gun. fire, and their normal action makes heavy de- 

 mands on their strength. The very speed of their revo- 

 lutions tends to disrupt them. In a test run with pro- 

 pellers made of wood which had been dried to the lowest 

 possible moisture content, or bone-dry, as they say at the 

 Forest Products Laboratory, the ends of the blades ac- 

 tually exuded sap which was forced out by centrifugal 

 action. In tests, at least, it has been possible to speed 

 the propellers up to such a pitch that the outer end of 

 the blade on an eight-foot propeller travels at the rate 

 of 400 miles an hour. 



Some air-machine engines run at 1700 revolutions a 

 minute, and can be geared up to 2000. An engine of this 

 power would use a nine-foot-six-inch propeller, and the 

 speed of the blade ends would be in the neighborhood of 

 600 miles an hour. A good many thousands of pounds 

 of pressure per square inch are generated by this action 



alone, and propellers have been known to split at the 

 center and fly apart. Even the smallest lack of balance 

 between the two blades is very serious, since the pull of 

 one must counterbalance that of the other. 



In addition there is the gyroscopic force which tends 

 to keep the blades rotating in the same plane. At high 

 soeed this force is hard to overcome, and the cross strains 

 it introduces when there is a change of direction, either 

 up, down, or sidewise, are enormous. 



Yet under conditions of modern warfare, when an avi- 

 ator has to "loop the loop" or plunge, or ascend sharply 

 in maneuvering to bring down, or to escape from, an 

 enemy the machine has to meet and withstand these un- 

 usual tests. 



Ash is used somewhat in propeller blades, but serves 

 its main purpose for engine beds ; maple, birch and 

 cherry have found some place in propeller manufacture ; 

 Douglas fir has been used for struts, and while there is 

 a plentiful supply of this wood it does not have all of the 

 required characteristics. Sugar pine has value, but the 

 commercial output is not large enough to make it wholly 

 dependable. 



Already the demand for woods is forcing a search for 

 substitutes in place of spruce ; of these. Port Orford 

 cedar appears to be the most promising. It is marketed 

 from a comparatively small area in southern Oregon 

 only, and sufficient quantities cannot be gotten out at 

 once. Other substitutes for spruce are eastern white 

 pine and southern white cedar, though it must be ad- 

 mitted that the latter has been suggested because of some 

 of its known advantages and not from actual tests. 



The best of the spruces for airplane manufacture is 



