Whirling arm

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Maxim's arm, ready to whirl.
Schematic of Maxim's whirling arm. The operation of the machine was as follows:—the aeroplane g, to be tested was secured to a sort of weighing apparatus which is shown in detail (Fig. 36), and the screw attached to the shaft. Upon starting the engine, a very rapid rotation was given to the screw which caused the radial arm to travel at a high velocity, the whole weight resting on a ball bearing at w. The radial arms and all of their attachments were balanced by a cigar-shaped lead weight s, which was secured to a sliding bar so as to make it easily adjustable. The thrust of the screw caused the screw shaft to travel longitudinally, and this was opposed by a spring connected by a very thin and light wire to the pointer of the index o. As the apparatus rotated rather slowly on account of its great diameter, it was quite possible to observe the lift while the machine was running at its highest speed. The aeroplanes were mounted after the manner of the tray of a grocer's scales (see Fig. 36), and the lift of the aeroplane was determined by what it would lift at r—that is, while the machine was running at a given speed, iron or lead weights were placed in the pail r, until the lift of the aeroplane was exactly balanced [...]."[1]
Fig. 36: the weighing apparatus

A whirling arm is a system, like a wind tunnel, for testing aircraft by moving them through air.

Constructors of whirling arms included George Cayley, Otto Lilienthal, Samuel Pierpont Langley, and Hiram Stevens Maxim.