Page:Popular Science Monthly Volume 90.djvu/687



HE screw propeller of an aeroplane or airship is made like a pair of blades on an ordinary electric fan. They are, however, about a dozen times larger, and capable of throwing back a column of air several thousand times more powerful. When such a propeller is revolved at high speed, it will push with tremendous force against the air; and with equal force will the aeroplane be pushed ahead. Its operation is just like a propeller pushing a ship, only that the aeroplane propeller works against a very much lighter substance.

The first aeroplane propellers were copies of windmills. Their blades were of canvas stretched on a wooden framework. They were, however, very inefficient. The next step in advance was made when aluminum blades were riveted to steel arms which projected from the driving-shaft from the motor. But the great defect in both of them was their lack of clean-cut outline. Unless a propeller blade is made rigid and in one unbroken piece, the slightest projection or un-evenness serves only to churn the air. At the high speed at which the propeller revolves, the violent churning will hinder rather than help the aeroplane in its progress. The solid one-piece wooden propeller, invented by Hiram Maxim over twenty-three years ago, practically eliminated this defect. Its outline is unbroken and its surface is polished as smooth as glass. Nothing in the world is given so high a finish, and its clean-cut blades churn

the air very little. It has one great drawback, nevertheless, in that it is affected by the weather. Changes in degrees of moisture, heat and cold warp the propeller in time. As a matter of fact, the wooden propeller of one of our military aeroplanes which ran the risks of the Mexican climate was warped suddenly while the aeroplane was high in the air, and came to grief. the next step, has only recently been developed. It consists of a thin one-piece steel framework upon which a glossy coating of lacquer is baked. The steel is pressed into just the right form by a huge steel die. The surface of the steel is treated with chemicals. This makes the steel spotlessly clean so that it can "take" the lacquer. A heavy coating of the unbaked baked lacquer is then evenly over the entire surface of the steel. The two substances are forced together under great pressure and are then brought to a high temperature. The steel and the lacquer amalgamate and one rigid piece is formed. The light lacquer strengthens the inner framework, and the propeller itself is considerably lighter on account of it than if it were of solid steel. When the propeller is cooled and polished, it forms a surface as smooth as a lacquered Japanese box. This spells efficiency. The weather-proof quality of the lacquer also enables the propeller to withstand all climates indefinitely.