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Aviation History
1928
1928 - 0443.PDF
MAY 31, 1928 39 THE AIRCRAFT ENGINEER TO FLIGHT In as far as a formula based on the assumption " plane sections remain plane " applies at the moment of failure, it would be true to say that the developed stress was •f = 435 X 1-2 4-66 x 1-2 4-66 2,240 x 0-192 ' 0-192 121 + 291 0-208 + 22-4 = 63-6 tons/sq. in (1) The depth of the spar did not alter appreciably during the test. The second spar, however, failed at a load W of 380 lbs., the greatest deflection measured on the centre line of the web being 1-01 in. The spar contracted in depth at the centre from 3-96 in. to 3-54 in. In developing any new type of spar all the stresses tha may be induced by the external forces and couples should be considered. Usually only the longitudinal stresses are calcu- lated, the shearing stresses being nearly always small by com- parison ; this condition has been found to be true in most of the spar types so far constructed, but it does not neces- sarily follow that the shearing stresses can be neglected for all types. Most stress calculators have at one time or another been incited by the light appearance of a web and the existence of large lateral forces to compute the sheer- ing stresses along the axis of a spar. The writer cannot recall a single instance of this stress having any influence on the spar dimensions. One would, for instance, never trouble to calculate this stress in the case of the box spars described in the last article, but it should not be assumed that this case could never be of any importance. The "developed stress " reckoned on the original moment of inertia was 380 X 13-25 9,120 X 1-01 9,120 2,240 X 0-192 + 2,240 X 0-192 240 X 0-208 =[11-72 + 21-4 + 19-5 = 52-62 tons per sq. in (2) The moment of inertia of the central section of the spar at the instant preceding failure was 0-31 ins.4 approx., the Z being 0-175 ins.3. The developed stress reckoned on this latter figure was therefore 12-85 + 23-5 + 19-5 = 55-85 tons per sq. in (3) An increase of about 6 per cent, over (2). The need for accurate spar measurements being recorded as tests proceed is therefore emphasised. In this particular case the fitting of rib posts resulted in an increase of "developed stress" and load supported of about 14 per cent. Space does not permit of a full investigation into the question of shear stresses in spars, but in the case of single web spars similar in type to Fig. 4 of the last article subjected to heavy shear loads, the maximum horizontal and vertical shear stresses can reach a very high figure if the single web is very thin. For example, in a section 4 ins. deep having a single web 2 ins. by 0-01 in., subjected to a shear force of 1 ton the maximum intensities, horizontal and vertical, of the shear stress would be of the order of 30 tons per square inch. In such a case the shearing stresses could not be neglected. If it is considered expedient we will give this subject closer study in a later article. The following are further items in the normal construc- tion shown in Fig. 1. The ribs in their most elementary form consist of upper and lower booms of channel construction bent to the aerofoil shape, the aforementioned rib posts and three lengths of channel sectioned bracing per rib, one short 404c
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