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Aviation History
1934
1934 - 0290.PDF
FLIGHT, MARCH 22, 1934 Fig. 7 : Sheet-web spar. Correctly spaced stiffeners.Booms not deformed. Tandum 604. Flat sheet metal girders with very thin metal webs. Part I. H. Wagner. 9.—N.A.C.A. Technical Memorandum 605. (604) Part II. H. Wagner. 10.—N.A.C.A. Technical Memorandum 606. Part III. H. Wagner. 11.—N.A.C.A. Technical Memorandum 592. Metal cover- ing of airplanes. G. Mathar. 12.—N.A.C.A. Technical Note 469. A summary of design formulae for beams having thin webs in diagonal tension. Paul Kuhn. 14.—N.A.C.A. Technical Note 366. Torsion in box wings. John B. Wheatley. 15.—N.A.C.A. Technical Note 461. Effect of rivet heads •' ;• on the characteristics of a 6-ft. by 36-ft. Clark Y metal aerofoil. C. H. Dearborn. (Throughout; the reference and figure numbers, as used in the original paper, are adhered to. In order to assist designers, Mr. Pollard co-related the mathematical part of his paper, and the formula referred to in a series of appen- dices, together with the bibliographical references, will be dealt with-at a subsequent date in THE AIRCRAFT ENGI- :NEER.—ED.J The Discussion 7' Mr. H. E. Wimperis (Diretcor of Scientific Research) •presided at the meeting and, in opening the discussion, he remarked that the work carried out at Bristol was con- sidered to be very important, as would be recognised by the fact that the Air Ministry had helped to advance it. It was always easy to get money, he said, but it was not always easy to get men able to carry out research woik of this character. Generally speaking, there was not rusually much trouble in finding funds for really good pieces of research work, but it was very rare to find the kind of man who could do the work and pay a dividend on the expenditure. In Mr. Pollard, however, working with Capt. Barnwell, they had found a combination which : amply satisfied the Air Ministry, and he was glad of the •opportunity of saying so on the present occasion. The work that had been done bore testimony to the ability of Mr. Pollard, who frankly confessed that there were enough variables in the construction to keep the problem new for some years to come. One of the features of this method of construction was the great stiffness which it offered. Research work recently in regard to wing torsion, flutter of every kind and tail buffeting had indicated how easy it would be to build an aeroplane which would be amply strong enough for everything, but utterly failing in stiffness for flying so that the kind of construction which would give the strength that is needed and at the same time the stiffness, which required to be increased, he suspected, with increasing speeds, was very necessary. Mr. S. C. Redshaw Imperial College, Civil Engineering Staff), referring to the statement in the paper that in the box-type of construction the stresses in spar webs can be estimated by Wagnerian or other methods, whereas with the skin the case was different, since that part of the structure is subjected to both shear and longitudinal forces, suggested that that was not altogether justified. The spar webs would be called upon to take direct stresses caused by bending, and, in addition, shear stresses and the web would tend to wrinkle close to the compression flange. Thus the strength of the spar .would be lowered accord- ingly. If the strength of the web was calculated from sheer stresses only, a wrong estimate of the strength would probably be obtained. With regard to the author's remarks concerning atomic-hydrogen welding, he believed the average welder was not able to weld thinner than 18-gauge. The author had mentioned 0.02 in. for the thickness of the covering. Did this render atomic-hydrogen welding too expensive or impossible to be generally used? Mr. W. D. Douglas (Mechanical Testing Department, R.A.E., Farnborough) remarked that stressed skin struc- tures become extremely complicated from the structural point of view as soon as the skin is fitted with a stiffening system. It was very frequently convenient to consider a structure like a stiffened-stressed skin structure as divided up into components, but whilst it was convenient it was sometimes dangerous to draw conclusions founded on the assumption that the parts could be regarded as separate structural units. In general, a stiffened skin might be regarded as a tubular form of construction fitted with stiffening members, some of which ran longitudinally—and might be called stringers—and others ran transversely— and might be called rings or hoops. The author had referred to the investigations at present being carried out by the Air Ministry, in which considerable attention was being paid to various new forms of spacings of the stringers. He drew attention to a quotation by the author, on page 9, from Mr.-Redshaw, in which he stated that the theoretical critical stresses in a curved-skin construction in compres- sion—he believed—were practically independent of the length of the specimens. It was known that with short specimens there were certain end effects, due to the impos- sibility of introducing shear, and end stresses as they existed in the complete fuselage. Therefore, whether or not we could justifiably use short specimens could not be determined merely by calculations, but must be checked by experiment. Further on, the author referred to the effect of end load on the shear buckles in the webs, and suggested that probably these would be affected by the end load. In this connection it was interesting to look at Fig. 7. In Fig. 8 : Spar booms, each stabilised by two sheets at right-angles (wing covering). 290
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