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Aviation History
1922
1922 - 0633.PDF
OCTOBER 26, 1922 of most modern commercial aeroplanes, a reduction of struc ture weight from 34 per cent, to 26 per cent, increases the revenue load from 34 per cent, to 42 per cent., an increase of nearly 25 per cent, in the utility of the aeroplane. In the case of the high performance aeroplane, the increase will be seen to be vastly greater, and in many cases makes possible a type of aeroplane which will be placed out of court with the heavier structure weight. I have every reason to believe that the structure weight of aircraft can be reduced from an average of 33 per cent, to an average of 25 per cent, to 27 per cent, by the use of metal construction, an advantage which, while extremely beneficial for commercial aeroplanes, is absolutely imperative in the case of military machines. I do not suggest that the whole of this gain can be obtained at once, but theoretical considerations and practical experience both indicate that there is a reasonable probability of arriving at such a figure at no very distant date, provided experiment on broad lines is continued unhampered by the necessity of obtaining immediate results from experimental expenditure. Let us consider how this retrenchment of weight is to be accomplished. It can be shown from first principles that it is possible to make structural members of metal considerably lighter than of timber, and this has been borne out by prac tical experience. There are two physical properties of struc tural materials which are of first order of importance in con sidering the weight of the structure manufactured from them. Firstly, the ratio of the modulus of elasticity to the specific gravity, and secondly, the manner in which this ratio falls off with increase of stress intensity. This latter property is generally considered by focussing the attention upon the yield point of the material at which, in the ordinary normalised carbon steels, a critical change in the value of E occurs. In the case of timber, which is a vegetable growth itself of complex structure, it is only possible to discuss the change of elasticity with stress in reference to a similar critical case, and indeed since the cause of failure in light compression members of wood and metal is somewhat dissimilar, the difficulty of an exact comparison is enhanced. The foll^ing table sum marises the elastic moduli of a number of*materials, either most commonly used in aircraft construction or by reason of their properties most suitable. In the last column of the table these values have been made specific by dividing them by the specific gravity of the material, and from this it will Material. Spruce (Spec. 2 V.i) Steel Duralumin alum, alloy (Spec. 2 L. 3) Magnesium alloys . . ' . . " E" (tons/sq. in. 670 13,000 Specific Gravity. °-45 7.8 4.790 2,400 85 '75 E/Sp. Gr. 1,490 1,667 1,680 1.37° be seeu that in the case of these materials there is very little to choose between them, though metal is at some advantage. If we were to imagine that the structure of an aeroplane were composed entirely of " Euler " struts, in which the modulus of elasticity would be the only property of the material of importance, it would be apparent that that form of con struction, which would give us members with the greatest radii of gyration, would give us the lightest structure. The second table furnishes the critical stress which will be obtained in the outer compression fibres of a strut or a beam, at failure. This stress is again made specific by dividing by the specific gravity of the material, and in this case it is found that there are marked differences between the various types of material showing an advantage for certain of the light alloys over timber, and of certain classes of steels over both. If an aeroplane were constructed entirely of short struts failing in pure compression and of ties failing in pure tension, it is obvious that, supposing the other physical properties of the material to be suitable, the greatest advantage could be obtained by using the material having the highest critical stress, so far as compression failures were concerned, and the highest ultimate stress so far as the tension members were concerned. These maxima are usually to be found in the same material. In making these statements, it is postulated that the design of the member is such as to enable the critical stress to be realised. In other words, for the purpose of our argument, this stress is the critical stress by definition, and the circumstances under which it can be realised will be discussed later. We can now consider the properties of the materials in a more general way. Fig. 2 shows the ratio of the intensity of load to the free- length radius of gyration of struts manufactured from materials of different critical stress. This figure will be familiar to most as representative of the characteristic curves of Mr. Southwell and Prof. Robertson. The first inspection of this figure will show that in order to realise the advantage of materials having a high critical stress, it is necessary that the ratio of l\k should be kept as low as possible. It is at this point that the principal structural characteristic of the aeroplane exercises an important influence. The aero plane is a very large structure for its weight, which means that struts with a low Ijk must have very thin walls where they are made of dense material, such as steel, or, in a lesser degree, aluminium and its alloys. It is a well-known experimental fact that, where the thickness of the wall is small compared with the radius of curvature and the length of the arc, the intensity of load indicated in Fig. 2 is not realised owing to the crinkling or buckling of the shell at a stress less than that which has been assumed as the critical stress. The lightest strut, therefore, will be that one in which the compromise between low Ijk and high critical stress is best effected. (To be concluded.) THE ROYAL AIR. FORCE London Gazette, October ij, 1922 General Ditties Branch Flight Lieut. H. H, James is granted a permanent comma., retaining present substantive rank and seny. ; Sept. 16, 1919 (Gazette Sept, 16, 1919, appointing him to short service commn,, is cancelled). Medical Branch The follg. are granted short service commits, as Flight Lieuts., with effect from, and with seny. of, the dates indicated :- F. E. Johnson ; Sept. 30. R. G. J. McCullagh ; Oct. 4. Chaplains' Branch The Rev. J. G. Stephens is granted styort service commn., with relative rank of Squadron Leader ; Sept. 12. Memorandum Lieut. S. V. Towers relinquishes temp, commn. on ceasing to be empld., and is permitted to retain rank ; Oct. 8. ROYAL AIR FORCE INTELLIGENCE The following appointments in the Royal Air Force i Appointments. notified: — Air Vice-Marshal: Sir J. M. Salmond, K.C.B., C.M.G., C.V.O., D.S.O., from Headquarters, Inland Area, to command Royal Air Force, Iraq. 1.10.22. Wing Commander : D. L. Allen, A.F.C., from R.A.F. Depot (Inland Area), to command Headquarters, Constantinople Wing. 28.9.22. Squadron Leaders : H. J. Down, from Headquarters, Coastal Area, to Headquarters, Constantinople Wing (Supernumerary) 5.10.22. J. Rylands, from No. 1 Stores Depot, to Headquarters, Constantinople Wing. 5.10.22. A, J. O. Wigmore, M.B., from No. 14 Squadron (Middle East) to Palestine) Wing Headquarters (Middle East), n.8.22. G. Blatherwick, from School of Photography (Inland Area), to No. 1 School of Technical Training (Boys) (Halton). (Supernumerary.) 16.10.22. Flight Lieutenants: F. J. Cooke from Headquarters, R.A.F., Iraq, to R.A.F. Depot (Inland Area) (Supernumerary). 20.8.22. J. C. Smyth, from Aircraft Depot (India) to R.A.F. Depot (Inland Area) (Supernumerary). 21.6.22. G, H, H. Maxwell, M.B., from R.A.F. Depot (Inland Area) to Headquarters, R.A.F. (Middle East) (Supernumerary). 16.9.22. A. E. Jenkins, from No. 1 Flying Training School (Inland Area) to R.A.F. Depot (Inland Area). 26.9.22. J. R. Crolius, M.B., from No, 4 Squadron (Inland Area) to R.A.F. Depot (Inland Area), 21.9.22. T. McClurkin, M,B., from Research Laboratory and Medical Officers School of Instruction (Coastal Area) to R.A.F, Depot (Inland Area). 25.9.22. G. S. Ware, M.B., from Research Laboratory and Medical Officers' School of Instruction (Coastal Area) to R.A.F. Depot (Inland Area). 25.9.22. E. B. Mason. The notification which appeared in R.A.F. Intelligence, No. 77, dated 5.9.22, wherein this Officer was posted from No. 25 Squadron to School of Photography, with effect from 25.9.22, is hereby cancelled. J. S. T. Fall, D.S.C., A.F.C., from Marine and Armament Experimental Establishment (Coastal Area), to No. 56 Squadron (Inland Area). 1.11.22. J. McGowan Glen, M.C., from No. 4 Squadron (Inland Area) to R.A.F. Depot. (Supernumerary.) 26.9.22; and from R.A.F. Depot (Inland Area)lto No. 5 Flying Training School (Inland Area). 16.10.22, for duty as Adjutant, T. Duminv, from R.A.F. Depot (Inland Area) to Headquarters (Coastal Area), 16.10.22." C. P. Barber, from R.A.F. Depot (Inland Area) to Armament and Gunnery School (Inland Area). 16.10.22. A. Durston, A.F.C., from Headquarters, R.A.F. (Mediterranean), to H.M.S. " Argus " (Coastal Area). (Supernumerary.) 23.9.22. J. A. Sadler, from No. 267 Squadron (Mediterra nean) to H.M.S. " Argus " (Coastal Area). (Supernumerary.) 23.9.22. J. H. Hagon, from School of Photography (Inland Area) to R.A.F. Base (Leuchars) (Coastal Area). 16.10.22. W. R. Curtis from No. 267 Squadron (Mediter ranean), to H.M.S. "Argus" (Coastal Area). (Supernumerary.) 23.9.22. J. K, Ritchie Landells, M.B., from No. 2 Squadron (No. 12 Wing, Ireland) to Headquarters (No, 12 Wing, Ireland). 4.10.22. T. McClurkin, M.B., from No, 2 Squadron (Inland Area) to Research Laboratory and Medical Officers' School of Instruction (Coastal Area), 11.jo.22. P. Huskinson, M.C., from Boys' Wing (Cranwell) to R.A.F. Cadet College (Flving Wing) (Cranwell). 9.10.22. A. W. Mylne, from R.A.F. Cadet College (Flying Wing) (Cranwell) to R.A.F. Cadet College (Ground Wing) (Cranwell). 9.10.22. R. B. Waite, M.B.E., from Headquarters, Inland Area, to Air Ministry (Director of Per sonnel). 9.10.22. A. W. Symington, M.C., from R.A.F. Depot (Inland Area) to Half-Pay List. 1.10.22. 633
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