896 



Popular Science Monthly 



it is to a mountaineer. When two fighters 

 seek to kill each other three miles above the 

 earth every bit of extra power counts. All 

 this means that although an engine will 

 weigh only three and one-half pounds and 

 even less for each horsepower developed at 

 sea- level, it weighs very much more for 

 each horsepower the higher it climbs; it 

 is much heavier 

 than it ought 

 to be. 



Painstakingly 

 built as they 

 are, airplane 

 engines must 

 be scrapped 

 or rebuilt at 

 the end of 

 eighty or a 

 hundred hours. 

 By that time 

 the bearings 

 are sure to be 

 scratched; 

 carbonization 

 has set in; 

 other defects 

 appear. It is 

 said that a man 

 is rebuilt every 

 seven years in 

 that ever-re- 

 curring process 

 of discarding 

 old tissues for 

 new. An air- 

 plane engine is 

 rebuilt every 

 few days. New 

 parts are con- 

 stantly substi- 

 tuted for the 

 old, until very 

 little of the 

 original con- 

 struction is left. 

 Even a single 

 steep, long dive 



means overhauling. Flames due to excess 

 of gasoline, and smoke due to the oiling up 

 of the front cylinders, pour out of the ex- 

 haust. The sparkplugs must then be re- 

 newed. Indeed, sparkplugs are the engine- 

 maker's bane. They must be renewed after 

 a few hours in order that they shall not fail 

 in the air. 



The Liberty Motor is not essentially 

 different from other airplane engines. But 

 it will be put together in a new way. 



© Brown and Dawson 



The frame of an airplane wing is made of spruce. It takes 

 only two hundred feet of spruce to make an airplane, but 

 one thousand feet must be examined and eight hundred 

 rejected to obtain two hundred perfect feet. A wing must 

 be as strong as a bridge and yet as light as possible 



Itis standardized. And that means — ? 

 Simply that cylinders can be combined to 

 produce an engine of any desired power ; 

 that the nuts and bolts made in Boston will 

 fit the threads tapped in a part made in 

 Detroit; that the elements of the engine 

 are interchangeable so that a power plant can 

 be improvised on the spot. 



The Sensitive 

 Air -Propeller 

 Propellers 

 are hardly less 

 delicate than 

 motors. They 

 are almost hu- 

 manly sensitive 

 to temperature. 

 In the dry alti- 

 tude of the 

 Mexican bor- 

 der, of northern 

 Africa, of 

 India, and of 

 Bagdad, days 

 are hot and 

 nights cold. 

 Hence propel- 

 lers buckle, 

 warp and fall 

 apart in the air. 

 That disinte- 

 gration is high- 

 ly dangerous ; 

 for a blade may 

 smash thefront 

 of the airplane 

 and kill the 

 pilot. Propel- 

 lers for use un- 

 der such trying 

 circumstances 

 in the tropics 

 are now 'made 

 on the spo*. 

 But wherever 

 they are made 

 the best woods 

 are used, usu- 

 They must be 

 nicety. The 



ally walnut and mahogany. 



balanced with the utmost 



speed at the end of the blades is seven miles 



a minute. Hence the least nick throws the 



propeller out of balance, which may mean a 



wreck. 



No less difficult is the making of the 

 wings. They must be as light as a 

 feather and as strong as a bridge. Flimsiness 

 and strength — could anything be more par- 

 adoxical ? Yet the whole machine itself is a 



