โ Scribed by A.F. Zahm
No coin nor oath required. For personal study only.
Preface .-When, during the early months of 1918, the new 4' x 4' wind-tunnel, at the Washington Navy Yard, was nearing completion, an aerodynamic balance was needed for it that could measure lift, drag and pitching moment, and could be delivered at an early date . The only balance immediately available in America then was one that had recently been made for sale by Mr. C . B . Kirkham, Chief Motor Engineer of the Curtiss Aeroplane and Motor Corporation, from patterns made by that company for its 4' x 4' tunnel. The balance had been devised for that company, in 1916, by the present writer, then Chief Research Engineer, and is an adaptation of his former balance used early in 1902 . It was reduced to scale drawings by his assistant in the company, Mr . A. P . Thurston, who supervised its construction and made the calibration .
Construction of the Balance .-Fig . i shows the external appearance of the instrument as it was assembled, just after delivery, and mounted on the under side of a plank representing the floor of a wind-tunnel .
The balance consists primarily of a hollow vertical main shaft capable of rotating about its own axis and about two coordinate horizontal axes, one normal, the other parallel to the wind direction . This sturdy main shaft is supported in a bronze hub having four split spokes, accurately milled, into which are bolted four steel-weighing beams provided under their tips with knife-edges resting in V blocks, which themselves rest in the bottoms of four iron lugs jutting down from the base casting screwed to the under side of the tunnel . One pair of the V blocks can be raised to carry the whole vibrating system, or lowered to let it rest on the other pair. Thus the main shaft can vibrate either across or along stream, and thereby measure lift or drag on the wind model .
A ratchet arm fixed to the main shaft turns it through any * Communicated by the Author .
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