FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1919
1919 - 0714.PDF
modified section for the main members. In general it was found that lipping improves the thin section, while, on the other hand, the failing stress in the case of the thicker sections is only a little higher. For instance, the limiting stress for sections for which s/t = 30 is 11.7 tons per sq. in. as against 7.8 for plain angles. Whereas for values of s/t of 7 the limit ing stress of the lipped section is only 17.5, i.e., the same as the plain section. Hence there appears to be no advantage in lipping the sections. In order to obtain the most economical design it is obvious that the dimensions of the main members will require to be varied from place to place. Therefore at the centre of a large machine, in view of the fact that duralumin cannot efficiently be solid drawn in sections thicker than o. 2 in., it may be found that the size of the angle members required become too great to permit them to be solid drawn. It therefore becomes necessary to modify the design. (One such modification is shown in Fig. 29. Plain angles are used to which are riveted longitudinally corrugated plates. To prevent local buckling of the plates the pitch of the rivets should not exceed 15 times the thickness of the plate.) With regard to bracing it has been found that the best type of bracing is quite different from that used in airship girders. In the case of airships, since the shear is small and the main members are of light section with a large pitch for the bracing, it follows that a rigid bracing in the plane of the members is economical. Moreover discontinuous bracing gives a further economy. In the case of aeroplane girders the main members are much heavier and the value of L/K between bracing points so much smaller that the increase of strength to be gained by providing stiffening bracing is not justified. Moreover, since the ,shear is large, it follows that the bracing should be practically continuous to obviate the introduction of local bending moment; further, the Zeppelin type of bracing is inadmissable as this would necessitate double bracing. The problem of the suitable attachment of the fittings presents as great difficulty in the case of large machines as in small ones. It is of such importance that the design should be modified in the first instance to enable this to be done economically. • • In calculating the strength of a spar it is necessary to make allowance for the extra stresses induced by the bending moment introduced by deflection. For wooden spars it is usual to use Perry's formula for a strut subjected to a bending moment. The bending moment due to the lateral load is then (multiplied by VefPe — P to obtain the equivalent bending moment. Where Pe — Euler's crippling load and P = end load in lbs. This makes allowance for the additional 8 - stresses placed ,upon |the |spar |due ,to the bending ^moment obtained by the product of end load times the deflection, and gives fairly satisfactory results for spindled wooden spars. It cannot be used, however, for calculating the strength of metal construction, experiment having proved that the deflection is of the order 10 times that given by vPerry's formula. The deflection of metal spars calculated in tne following way has been found by Mr. Temple to agree with practice. From the ordinary theory of beams the deflectionj in the centre of a girder under side loading is equal to JL L2M 48 EI • Where 8 is the deflection in inches. |L is the length between the supports in inches. M is the maximum bending moment in inches. I is the moment of inertia in inch units about an axis perpendicular to the direction of loading E is Young's modulus for the material of the girder which as a result of bending tests on girders is taken as 5,50*0 tons per sq. in. M 2/ I " D where/is the limiting fibre stress and D is the total depth'of a symmetrical section we have 5. „ J^L L 2± __ 1 A2 48 Since 8 = x D 5,500 /N D 26,400 D' Now in the case of a long strut, if the direct stress is ignored, practically the same law should hold. M Since M ='Pe x 8 . •. Pe = -= but but ~ = % Pe = " M S ^EI L2 • EI La -.8 = ML-* 2EI M I *1 8 = 8 = 2Ly 7T2ED 1 9-87 x 5,500 fJJ • D • D 27,100 The deflections calculated by the above means agree with the results of experiments. Therefore in designing beams to withstand combined stresses the approximate deflection should be calculated by the formula 1 _ / L2 -TT 26,400 D '" (To be continued.) m B ® @ An impression of the Breguet tractor biplane in flight. 714
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
