AERONAUTICS: Fighting Noise

To pilots, passengers, increasingly to groundlings, is airplane noise a plague.

To military airmen it is a serious limitation on operations, an absolute barrier to surprise maneuvers. Uncounted attempts have been made to silence the roaring engines—the latest last week at Roosevelt Field, L. I. by a Miss El Dorado Jones, of Manhattan, formerly of Moline, Ill.

Puffing upon one cigaret after another, Miss Jones directed mechanics in attaching to the Cirrus engine of a Moth biplane a muffler of her own invention. As the plane sped along the runway and over the hangars there were noises—of thrumming propeller, snapping pistons, vibrating metal—but there was no bark of exhaust.

Miss Jones’s device, about 18 in. long, outwardly resembles an ordinary Ford automobile muffler. Inside is a series of small “pinwheels” which retard the speed of the exhaust gases—”chewing up” the sound waves without creating excessive back pressure upon the engine. (The latter factor, involving loss of power, has been the principal drawback to most attempts at muffling.) The pilot who tested the Jones muffler in flight said the engine lost none of its normal speed—1,900 r.p.m.

Miss Jones, head of El Dorado Inventions, has been manufacturing automobile mufflers since 1913 when her ears were first assailed by an unmuzzled Ford in a Moline garage. Her idea for the airplane muffler she gleaned from contemplation of a grease-vapor exhaler set into the wall above a restaurant stove. Another of her products is a 2-lb. electric flatiron. Fiftyish, Miss Jones believes in the ability of women over 40, substitutes them for men at workbenches in her machine shops whenever possible.

New in detail, the Jones muffler is not entirely new in principle. Among many other efforts, one like hers was created by one William Deal and exhibited in the 1929 Aeronautical Exposition of the American Legion in Manhattan by Curran Machine Works of Long Island City. Other methods under experiment are:1) to reduce the speed of the exhaust gases by rapid cooling and 2) employment of sound-absorbing material in the muffler —with the corresponding danger of the material catching fire.

Propellers. Effective silencing of air craft engines would be, roughly speaking, only half the battle. Much of the noise of planes — some say 50%, some say “most” is caused by the propeller itself.*The Aeronautics Research Division of the Department of Commerce has been studying the problem for two years, is still uncertain as to the exact process by which propeller noise is created. Of the solution, however, it is certain : “The only positive method known of reducing propeller noise is to reduce the tip speed by using a geared propeller.” Geared propellers are in use, will be more generally adopted as difficulties of weight and construction are overcome.

Other Noises. With slowspeed propeller and muffled engine, airplane noises will be reduced below the point of oppression. But wire-bracing, gears, cams, valves, engine cowling, various parts of unbraced sheet metal in airplane construction will, by their vibration, for some time keep planes from becoming as silent as mod ern automobiles.

*In tests conducted by the Army Air Corps at McCook Field, Dayton, O., propellers were whirled by highspeed electric motors. Observers passing outside the laboratory reported the din was “like a battery of Liberty engines.”