Page:Popular Science Monthly Volume 91.djvu/912

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��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

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