Page:Popular Science Monthly Volume 92.djvu/422

 Tuning Airplane Wires

Ever notice how a tuning fork vibrates when you strike the correspond- ing note on a piano? Airplane wires are tuned on that principle now

��ONE of the most common troubles on all airplanes has been the difficulty of correctly adjusting the tension of the wires used for bracing the wings. Some of the wires may be tauter than others, after a few hours' fly- ing. There has been no ready means of correcting this fault. In consequence, great stresses have been thrown on some of the \\'ires, while others suf- fered hardly any ten- sion at all.

In tightening wires, mechanics rely en- tirely on their touch and hearing. They twang the wire with their fingers as they would the string of a guitar, listen to the note emitted and judge the tautness accordingly. The error average on the part of a skilled mechanic —after his own work had been tested with scientific instruments — was found to be about 30 -z^,.

It remained for an Italian aviation officer, Carlo Lerici, to hit upon a practical way of helping the mechanic over his difficulty. The accom- panying illustration shows the simplicity and ingenuity of his device, which is really nothing but a multiple tuning fork.

A block of wood holds a series of metallic tongues, resembling those of a mouthorgan. The length of these tongues determines the musical note which they give off

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Airplane wires must all be at the same tension. Here a new multiple tuning fork determines pitch, hence tightness, and fitness for flying

��when struck, and this, of course, depends up- on the number of times they vibrate in a second. Tuning forks vibrate in sym- pathy with strings of corresponding notes. A "G" tuning fork, for example, will vi- brate sympathetically when held close to a piano on which the note "G" is struck. Similarly, when an airplane wire is twanged by a me- chanic, and the Lerici ''\abrometer" is held close to its end, one of the metallic tongues on the instrument will \abrate in sympathj\ The mechanic has only to take note of the vibrating tongue and read the number of vibrations per second, written below it on He then takes the in-

���Thc multiple tuning fork consists of reeds clamped in a block. Each end vibrates at a certain known pitch

��the wooden base, strument successively to the other wires and "tunes" them by tightening or loosen- ing them respectively, until their \abra- tion periods correspond with that of the first wire tested.

According to the inventor, the chance of safety is increased fully lo*^',' by making all wires equally taut. A se- ries of tongues with 20-40 vibrations per second is sutticient for all the mod- ern airplanes used in the war.

The same principle is currently used in de- termining the frequency at which electric genera- tors are operating. Reed- meters are among the simplest existing.

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