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Aviation History
1928
1928 - 0903.PDF
73 SEPTEMBER 27, 1928 THE AIRCRAFT ENGINEER SUPPLEMENT TO FLIGHT by these two groups of manoeuvres, then from the foregoing the ratio between the design and the yield (or proof) stress must be the same. It has been the practice for a number of years to use the ultimate stress as the design stress when determining the sizes of bracing wires, and under normal manoeuvres the wires do not want tightening and it is safe to assume that the stresses produced have not exceeded the yield stresses of the material. As the yield point is 80 per cent, of the ultimate stress for the steel the bracing wires are made of, it would be on the safe side to take the ratio of abnormal to normal manoeuvres as 1 to -8; therefore the ratio of design stress to yield point stress should be the same. (To be concluded.) 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 ; or 120, George Street, Edinburgh ; or through any book- seller. THE EXPERIMENTAL DETERMINATION OF THE TRAJECTORY OF AIRCRAFT BOMBS.—By H. E. AYimperis. M.A. K. & M. No. 1121 (Ae. 294). (25 pages and 13 diagrams.) June, 1928. Price Is. 6d. net. Early in the late war it became apparent that a scientific study wouldneed to be made of the ballastic properties of aircraft bombs. This was necessary in order that knowledge might be (rained of those factors relatingto their flight whiiii enter into the design of an aiming device. The first question was to determine whether the relatively simple trajectoryof a particle falling through a medium in which the resistance was proportional to the square of the velocity would be followed, even approximately, bybombs designed chiefly in relation to what they had to contain, rather than in reference to their probable ballastic coefficients. Full-scale experimentswere made at various Air Stations, and use was made of a coal mine shaft n Yorkshire. It was found that the particle trajectory was sufficientlyclosely followed for the practical purposes of the moment, and a technique of bomb' sighting was built thereon. This led to the introduction into theAir Forces of Great Britain of the system of sighting here described. A full-scale study was also made of the under-water trajectory of a bombdropped above the sea. This was required lor the purposes of the anti- submarine aircraft patrol. FURTHER DEVELOPMENT OF AUTOGYRO THEORY. PARTS I AND II.—By 0. X. H. Lock, M.A. R. & M. No. 1127 (Ae. 299). (43 pages and 3 diagrams.) March. 1927. Price Is. 9d. net, The general theory of the autogyro given by (ilauert in R. * M. 1111,*is based on certain simplifying approximations and assumptions. Tiie object, of the present paper is to develop the theory still further by removingsome of the approximations. The approximations of R. & M. 1111 may be classified as follows : —(1) The coefficient of axial velocity through the disc is constant over the disc, and is a small quantity.(2) The lift coefficient of a blade element is proportional to the incidence. and the profile drag coefficient is constant.(3) The flapping motion is expanded as a Fourier's series, and co- efficients of cos 2*jj. sin 2tft, etc.. are neglected.(4) Squares and higher powers of the ratio of the forward speed to the tip speed (/j. -" V cos i 1?!J) arc neglected throughout.Pan I.—Assumption 4 is dispensed with, all powers of n beng retained. Tt had been remarked that it is theoretically possible to eliminate theflapping motion of an autogyro by substituting a suitable mechanical varia- tion of the blade angle round the circle. The present investigation verifiesthat the two machines are. in fact, identical, and explains the fictitious dis- crepancy of the values of maximum lift drag for the two machines. After working out the force components, thrust and longitudinal force forboth cases and verifying their identity, an alternative method of determining the drag is developed, based on considerations of energy loss : it has beenverified that the resulting formulas' give results identical with those already obtained, and being more simple, take the place of the rather complicatedformula- for the longitudinal force. The ratio of the value of maximum lift drag of the present investigations(the true value for heavy blades subject to assumptions l and 2) to the value given in R. and M. 1111 is as follows :— Blade angle 0° 2° 4°Ratio 1025 1-44 2 1 On account of the errors introduced by assumption!! 1. 2 and :i (especially 2)the actual value of lift drag is probably lower than that obtained here, and may be in fact closer to the value obtained in li. and M. 1111.Part II takes account of the general term in the Happing motion, so as to remove the restriction to infinitely heavy blades. TESTS OF A METAL AIRSCREW IN A CLOSED TUNNEL FOR COMPARISON WITH AMERICAN TESTS IN AN OPEN JET TUNNEL. —By H. C. H. Townend, B.Sc., and J. H. Warsap. R. & M. Xo. 1137 (Ae. 307). December. 1927. (4 pages and 5 dia- grams.) Price (id. net. In this country airscrews have hitherto been tested in wind tunnels of theclosed type, and the results obtained in such tunnels are subject to a correc- tion for the effect of the walls, which becomes appreciable when the screwis not small in comparison with the tunnel. In wind tunnels of the open-jet type the interference of the wall is theoretically negligible. In America open-jet tunnels are normally employed for such tests, and relatively larger air- screws are habitually tested than is considered desirable with closed tunnels. By testing a series of similar screws of different diameters in an open-jettunnel it, has been verified* that the interference is negligible for screws up to 4 ft. diameter in a jet whose diameter is "4 ft. : and in order to provide acheck on the accuracy of the th'eoretica! correction commonly adoptedt in this country it was decided to request the loan of a metal airscrew previouslytested in an open-jet tunnel in America, and to test it in a closed tunnel. The airscrew, which was kindly lent for the purpose, by arrangement withthe N.A.C.A.. was a 3 ft. diameter 2-bladed metal airscrew of medium pitch. The correction on the velocity observed in a closed tunnel was usuallyless than 4 per cent., except tor the highest thrusts obtained near static conditions, where it rose to 1 j per cent. This correction, over most of therange, is not much greater than the experimental errors. Comparison with the American results shows excellent agreement on thrust over the entirerange, while the torque is slightly higher, so that the maximum efficiency is about 11 per cent, lower than that obtained in the America; test. R. & M. nil. "A general theory of the autogyro."—H. Glauert. * See Experimental Researches on Air Propellers, X.A.C.A. Report No. 14.The question is discussed in R. & M. K.l3.i: On the advantages if an open jet type of wind tunnel for airscrew tests.—H. Glauert. M.A.. andC N. H. Lock. M.A. o t R. it M. 662. Some notes on the theory of an airscrew working in awind channel.—H.A.E. THE DETERMINATION OF THE ELASTIC MODULI OF A MILD AND A MEDIUM STEEL.—By H. E. Smith. B.Sc. and H. L. Cox, B.A. R. & M. Xo. 1138 (M. 53). (7 pages and 3 dia- grams.) June, 1927. Price M. net. The general object of the research was to determine as accurately as possiblethe elastic constants of certain materials, particularly of the standard Aeronautical Research Committee research materials composed of a mildand of a medium steel. If the materials studied proved to be elastic-ally isotropic, the work wouldinvolve only the determination of Young's modulus and Poisson's ratio : but. although in ordinary test work the materials are usually assumed electri-cally isotropie. there is in many eases very little evidence that this assumption is justified. Therefore, quite apart from the importance of an exact know-ledge of the elastic constants as such, it is of interest to see whether the materials are truly isotropic. or, if aeolotropic. what form of elastic aeolotropyobtains. The values of four of the moduli were determined for both steels.There is in the case of the medium steel no choice but to assume the material isotropic : while in the case of the mild steel, evidence of aeolotropy is slight,and not entirely reliable. THE CONNECTION BETWEEN LIFT AND CIRCULATION FOR AN INCLINED P'LAT PLATE.—By A. Fage, A.R.C.Sc, and F. C. Johansen, B.Sc. R. & M. Xo. 1139 (Ae. 308). (7 pages and 1 diagram.) January, 1928. Price iSd. net. The relation which connects the lift experienced by a unit length of atwo-dimensional body moving through a fluid of density with a definite circulation was derived by Kutta and .loukowski on the assumption that themotion was irrotational.' Messrs. Bryant and Williams have demonstrated experimentally (R. & M. 989) that this relation also holds with good accuracywhen an aerofoil is advancing through a viscous fluid such as air. provided that the contour line around which the circulation is measured does notapproach at any part too closely to the aerofoil, and also that it cuts the trailing wake approximately at right angles to the direction of motion of theaerofoil. Further. Prof. Taylor has shown theoretically in an appendix to the same paper that the Kutta-Joukowski relation holds for the Kirchhoff-Rayleigh discontinuous motion behind an inclined flat plate, if the contour be chosen so that the part which crosses the wake is some distance behindthe plate and also at right angles to the general direction of the motion. In addition, he states that, even should the contour cross the wake close tothe plate, provided it were perpendicular to the general direction of the current, the circulation would differ only slightly from that which correspondswith the lift. In the present paper it is shown that the Kutta-.Toukowski relationshipbetween lift and circulation holds very closely for an inclined flat plate in an air stream, provided that the contour around which the circulation istaken is large and that it cuts the wake in a straight line perpendicular to the direction of the undisturbed motion. Also, the total strength of thevorticity at the front of the plate can be determined from observations of pressure on the plate. THE DETERMINATION OF THE HORSE-POWER HEIGHT FACTOR OF ENGINES FROM THE RESULTS OF TYPE TRIALS OF AIRCRAFT —By J. D. Ooales and A. L. Lingard. Communi- cated by the Director of Scientific Rssearch. Air Ministry R. & M. Xo. 1141 (Ae. 310). (8 pages and 7 diagrams.) October. 1927. Price 6i. net. The height horse-power factor of engines enters into all performancecalculations of aircraft, and the average values of the power factor have here been determined for heights in a standard atmosphere from the type-trial reports of n large number of aircraft. The method of obtaining the power factors of an engine in a standardatmosphere appears to yield reasonable values for this factor, but it gives no clue as to the dependence of the factor upon pressure or density respec-tively. This is because temperature, pressure, and density in a standard atmosphere can each be expressed as definite functions of either one or bothof the others. 834*
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