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Aviation History
1928
1928 - 1013.PDF
OCTOBER 25, 1928 81 THE AIRCRAFT ENGINEER SUPPLEMENT TOFLIGHT crto a so kja 1/5I" z c/5 U Of a 8» 10 jiii • i ' tT w 1 1 I 1 1 1 I [ I BEAR y 111111111 NG S" yy rRESS «/ 7 i/ i 11111111 (STEE // y yf / FIG.l. y -. - : niiiinr 2 o a Z nO n 20 30 40 50 60 70 80 90 100 I BEARING STRESS IN TONS PER SQUARE INCH I '- / 1 1 ( 1 1 1 1 1 1 FIG 2. i I i I i i I i i : i i i i \t i i i : ; 3 4 ! ITl (POISSONS RATIO). Cn-N Ck-fc Kit;. 1 shows this graphically for steels taking fc as equal to ft, and fs as equal to half ft plus 5 all in tons per sq. in. Young's Modulus (E).—This can be determined by any of the standard test methods. Modulus of Rigidity (N or G).—Ditto. Poiasons ratio.—This ratio is very seldom given and little used, but greater use could and should be made of it. E.g.. the mathematical relationship between the four elastic- constants " E," " X," " K," and " m," are to be found in many text-books. 1 ' 2 •Let C,, = 2 ( 1 -r ) and C K = 3 (1 )ml \ m; EThen - = ml E B and — = CK Fit'. 2 shows Cw and Vk plotted against " m." It will be noticed that " m " cannot be less than 2, or else " CK "" has a negative value which is an impossibility. The use of the above curve has and would prevent the use of incongruous values for E and N. Moreover. Poissons ratio is usually constant for any material, irrespective of its condition. The Effect of Workshop processes on the Normal Design Stresses of the Materials. If any material is likely to be used in a condition other than normal, e.g., softened, annealed, etc., a definite set of values should be given for that particular state, along with the values for the normal state. A process that is being greatly used (and its use is on the increase) is welding. In general, the effect of welding is to reduce the strength of the material, whilst the use of a subsequent heat treatment may replace part, if not all. the lost strength. Strength values should be given for normal- welded not heat treated, and welded heat treated. The second class needs qualifying in the case of long struts welded at the ends only, e.g.. long struts of constant section usually fail at the centre by tending, while the ends that are only subjected to pure compression are much over strength and could, therefore, be of weaker material. Thus, a long strut of constant section with welded ends and not heat treated would probably suffer no diminution of strength due to the welding. The Effect on the Design Stress Value due to different Types of Loading All the design stresses given are for a normally steady load. It is well known that materials fail at a much lower stress when they are subjected to a rapidly varying load. All structural members of an aircraft have fluctuating loads, but fortunately the frequency is so low, and does not affect the strength, with the one possible exception: the engine structure. Here, the frequency of vibration is a function of the engine revolutions. The members themselves do not appear to be affected except in the case of engines having a very uneven torque. The parts that appear to suffer are the end attachments by shear and bearing. A good empirical rule for dealing with fluctuating loads has been suggested by Dr. Arnold, i.e., the equivalent constant load is equal to the range of load plus the maximum load. Conclusion.—The three main advantages which would accrue if a clear-cut design stress policy, as outlined above, were agreed upon. (1) Consistent design stresses by all the aircraft construc- tors. (2) True design figures of merit could be obtained by using the formula given by A. J. Sutton-Pippard in R. & M. 729 in conjunction with the design stresses instead of the proof stresses. (3) Manufacturers of raw materials wishing to introduce a new material to the aircraft industry would know exactly what information is required by the aircraft designer. TECHNICAL LITERATURE SUMMARIES OF AERONAUTICAL RESEARCH COMMITTEE REPORTS These Reports are published by His Majesty's Stationery- Office, London, and may be purchased directly from H.M. Stationery Office at the following addresses : Adastral House, Kingsway, W.C. 2; 28, Abingdon Street, London, S.W.I ; York Street, Manchester ; 1, St. Andrew's Crescent, Cardiff -T or 120, George Street, Edinburgh ; or through any book- seller. REPORT ON THE DEVELOPMENT OF A HOT-WIRE RATE or DESCENT METER. By G. W. H. Gardner, B.Sc, and F. W. Meredith. B.A. Presented by the Director of Scientific Research, Air Ministry. R. &' M. No. 1144 (Ae. 312). (16 pages and 10 diagrams).) August, 1927. Price Is. net. The Kcucral purpose of the investigation was to provide an instrument(or aerodynamic research whieh will indicate varying rates of descent of an aeroplane.An instrument involving the principle of the hot-wire anemometer has been developed and tested in flight and its characteristics have been treatedtheoretically. The instrument developed will indicate the rate of descent of an aeroplanewithin a range of 0-60 ft. per second with an accuracy of ± 1 ft. per second and has an indication time-lap to 1 /e of its amplitude of less than one second.The instrument is bulky and is not recommended for other purposes than aero- dynamic research.At the request of the Aeronautical Research Committee a rate of descent recorder involving the principle of the existine hot-wire rate of descent indi-cator is now being designed.
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