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Aviation History
1913
1913 - 0768.PDF
Photograph of the fractured spars laid out on the grass after the tests. •Flight" Copyright. in actually determining the stresses, because the theorem of three moments no longer applies. In order, therefore, to determine experimentally the strength of various spars, they were erected one after the other with the usual system of hooks and hinges employed in the Wright biplane construction. Loading in the form of bricks carried in boxes hung by straps spaced apart by distances corresponding to the ribs in the wing, was employed to stress the spars. Extra boxes were hung on at the fittings to represent the calculated strut thrusts. The arrange ment is illustrated by the sketch in Fig. 2. The lx>xes represented a load of nearly 4 lbs. per foot run, and one brick per box represented an added load of nearly 3 lbs. per foot run. The bricks were put in the boxes one or two at a time until the spar broke. Measurements of the deflection of the spars in each bay were made after the addition of each four bricks, until the spar was very close to its breaking point. In one or two cases the load was taken off, when the spar was nearly at the breaking point, and measurements were made of the permanent set in the spar. It was found that the spar, after being deflected as much as 2 inches in the centre of a length of 5 ft. 8 ins., would recover perfect straightness within a few iooths of an inch. This clearly snowed that the elastic limit of the wood had not been reached. Moreover, in almost all cases, the broken pieces of the spar were straight, thus indicating that the elastic limit is very close to the ultimate strength. Six different sections were tested, five being constructed of wood, and one being a steel tube. "Flight" Copyright. Fractures of the various spars tested.—Spar No. 1, broke downwards as a beam. Spir No. 2: The upper view shows the fracture, which broke In the inner bay as a beam; the lower views show the fracture that occurred in the same spar at the screw holes of the hook Fitting. The two lower photographs are complementary views of the same fractures It is probable that this lower fracture was the primary failure of the beam. Spar No. 3 showed weakness in the glued joint and also in torsion. Spar No. 4 gave the best results. Spar No. 5 : The upper photograph is a view looking down on the fracture, which failed laterally as a strut; the lower view is a side view of the fracture. This spar is an I'section beam. 794
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