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Aviation History
1919
1919 - 1077.PDF
AUGUST 14, 1919 5 •• •,„*,•• . V *V ,4/wV"* ' EXPERIMENTAL DESIGN AND TESTING OF AEROPLANE RIBS BY GEORGE B. FULLER AND LESSITER MILBURN. THE function of the rib in an aeroplane is to carry the load with the least possible distortion and with the proper factor of safety. Inasmuch as no mathematical solution is readily applied to the average aeroplane rib, a method of experimental design was adopted to produce a rib which should meet the requirements with the least possible weight, and at the same time utilise the least possible time, labour and material expense in production. The aeroplane rib is, at best, a very light and non-rigid member. The rib shown in Fig. 1 has a 94-in. chord, is £ in. used after the first five tests, to give the most severe condition. Bearing in mind the above conditions of loading and methods of fastening, there were some five methods of apply ing the load to be considered, as follows :— 1. A system of equalising levers, linked together, which would distribute the load over the rib, the lengths of the levers being proportioned to give proper loads at the points required. 2. A system of loading the rib. through rubber bands of varying strength, to agree with the load-grading curve. Fig. 1.—The final rib selected from the results of the tests has a chord of 94 ins., weighs 111, ozs., and carries an ultimate load of not less than 555 lbs. wide, weighs 11J oz. and carries an ultimate load of not less than 555 lbs. This rib was the final selection from the results of a series of tests made upon ribs of various types which were designed by approximate mathematical methods. The conditions demanded of a rib-testing machine follow :— 1. A rib-testing machine should hold the rib and apply the load in the same manner as the air load when the rib is in the wing. 2. The distribution of the load along the chord must not vary from the predetermined load grading when the rib deflects as the load is applied. 3. The machine should be so arranged that the load may be varied from zero to the breaking load, and an accurate means of measuring the load at any point must be provided. •0 re 5C a s I i I 35 _ ^4 7q_ /70_ T fc, \ *.TO - N 4 70 s s 1 \ ' Z A3 t S * STATIONS •\ LGA ^AC D ^ CfiAl LC *V£ff • L i INO CM AC H& / "P: A6L \ DcilA TJ 1 I JT^ re. • M C A 2 V TtST F 7X c . . 1 L_L ! ns! ! ' |i| i r\ r e t 10 11 « & '* is it 17 *e ff ic -* \ 3 ! 'ctttmts ft IB AtC IS LOAD 660 IBS Fig 2.—Curve showing the distribution of load along the rib 4. An accurate means of measuring the distortion of the rib under load should be provided. As the load on the rib varies continually with the varying angle of incidence of the wing, the most severe case was chosen. The supports of the rib, also, are not all the same. At or near the drift struts the ribs may be said to have a <t FRONT 5PAK 3. A system of springs used in the same manner as the rubber bands. 4. A system of hydraulic cylinders, the area of each being proportional to the load, as taken from the load-grading curve at the particular point, all having the same pressure. 5. A system of levers resting on knife edges, each lever being independent of any other and having its own load applied in a pail, or the like. After a number of sketches had been made and the points of advantage and disadvantage of each of the above methods fully considered, the fifth or last method was adopted and used in all the following tests. Description of the Machine Used The machine was constructed of wood, with a platform at about the height of a man's arm, on which the rib was to be placed. Guides of J-in. pipe were secured to the platform to maintain the rib in a vertical position. At the rear of the platform knife edges were mounted on a horizontal beam. Over these knife edges 20 ash levers were set, each having a metal plate to take the knife edge. Both ends of these levers were cut in the form of an arc, having the centre at the knife edge, so that the lever ratio was always the same. At the rear end of each lever a pail was attached and the front ends were connected to the stirrups by wires. The lower end of the wire was slipped through the stirrup, bent up and secured with a small washer so that the stirrups could be readily fastened to the wires. The distance from one stirrup to the next was 4.7 ins. Each lever was counterbalanced with a weight, so that when the pails were empty the lever rested evenly on the knife edge. To reproduce the condition of non-rigid support of the ribs mentioned above, the test ribs were made up on dummy spars as shown in Fig. 1. Two U-bolts were passed over each spar and connected to the floor by steel cables, leaving the spars free to move without respect to each other. Method of Loading The load may be applied by putting either water or sand in the pails. When the machine was designed it was intended that a water load be used. A tank was to be placed over the loading pails with 20 lines of hose, one leading to each pail. In the end of each hose a nozzle was placed having an exact diameter and calibrated for various heads. The diameters were such that the water would flow into the pails according to the ratio established from the load grading curve. The flow of water was to be controlled by a gate valve operating on all the lines of hose at the same time. '<t REAK 5?A« 1 ? Fig. 3.—Rib section to scale obtained by tracing photograph showing total deflection under distributed load fixed or rigid support, while near the centre of the bay, the rib supports may be considered as not being fixed, due to the spring of the beam. In these tests a non-rigid support was • Courtesy Automotive Industries, N.Y. On account of delay in securing apparatus for water load ing, sand loading was substituted. A small measure was made for each of the load pails, and the capacity of each cup was so proportioned that when one round of cups of sand had IO79
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