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Aviation History
1927
1927 - 0057.PDF
JANUABY 27, 1927 11 THE AIRCRAFT ENGINEER SUPPLEMENT TO FLIGHT Typical values of the mechanical properties at atmospheric temperatures of the hot-rolled rod so far determined are as follows. Pure 6 per cent. Magnesium. Aluminium. Tensile— Diameter of testpiece, inches ... 0-564 0-564 Maximum stress, tons/sq. in. ... 15-7 18-3 Yield stress, tons/sq. in. ... ... (slight and indefinite) Limit of proportionality, tons/sq.in. 1-60 2 40 Elongation, per cent, on 2 in. ... 12-5 20-0 Reduction of Area Modulus, lbs. /sq.in. 6-25.•. 10G U 30. ; 10° Fatigue ( Wiihler lest*)— Endurance Limit— Range of stress on basis of 20 million reversals, tons sq. in, ... ±4-5 ±6-5 Impact— Energy absorbed, ft. lbs J ' Hardness— Brinell. 500 kg. load, 10 mm. dia. ball—period of application, 30 sees. 3-8 39 y -1 4-6 50 STRESSES IN A STIFF JOINTED POLYGONAL FRAME UNDER A SYSTEM OF LOADS PERPENDICULAR TO THE PLANE OF THE FRAME By J. F. BAKEK, B.A. Presented by Professor A. J. SUTTOX PIFPAKD, M.B.E., D.Sc. R. & M. No. 1039 (Ae. 228). (9 pages and 7 diagrams.) February, 1926. Price 9rf. net. In two recent papers,* by Professor Pippard, solutions have been given for the stresses set up in a stiff jointed polygonal frame under the action of load svstems lying wholly in the plane of the frame. Certain cases have arisen in which the load system is not confined to forces acting in this plane and, in the present paper, formulae are developed for the case in which the loads act in directions perpendicular to the plane of the frame. As in the earlier papers, the frame is of regular polygonal form, made up of members equal in length, cross-section and elastic properties, and rigidly connected at their extre- mities so as to form a single stiff ring. • 11. & M. 820 and li. A M. Dili. THE ACCURACY OF THE VORTEX THEORY OF AIR SCREWS IN THE LIGHT OF RECENT EXPERIMENTAL WORK AND ITS APPLICATION TO AIRSCREW DESIGN. By H. GLACEKT, M.A., and C. N. H. LOCK, M.A. R. & M. No. 1040 (Ae. 229). (16 pages and 9 figs.) June. 1926. Price 9d. net. The vortex theory of airscrews as developed in previous papers* is essentially a modification of the earlier inflow theory which had been built up as a semi-empirical system on the basis of Froude's momentum theory and Drzewiecki's blade element theory. The present paper deals with the accuracy of this theory when checked against the available results of wind tunnel experiments on model screws. A brief general discussion is given of the nature of the theory and of the points in which it differs from the earlier inflow theory. The experimental data may be classified as follows :— (1) Total thrust and torque of an airscrew. (2) Pressure distribution over the airscrew blades. (3) Total pressure head behind an airscrew employed to determine the thrust grading curve. (4) Airflow round an airscrew including the determination of the torque grading curve from observations of velocity and direction just behind the airscrew. (5) Certain special experiments. The accuracy of the experimental results is considered and is found to be satisfactory except in one or two points of minor importance : A slight uncertainty of the interference of wind tunnel walls, especially near zero advance ; an uncertainty in the determination of thrust grading from measurements of total pressure head near the tip of the airscrew blades. The comparison between theory and experiment is classified under the various headings given above. The general agreement is very satisfactory, and all the serious discrepan- cies on total thrust and torque occur (a) for small velocity of advance ; (b) for airscrews with a small number of blades ; (c) for airscrews of high pitch ratio. The agreement on total thrust is satisfactory except for a slight discrepancy near zero thrust which is greater for a two-bladed airscrew than for a four-blader, and a tendency for the calculated thrust curves to be steeper than the observed. The agreement on total torque is satisfactory except for screws of experimental pitch ratio greater than 1 -2 for which the calculated torque becomes larger than the observed. In comparison with experiments which determine the variation of force, pressure and velocity along the blades, the agreement is generally good except that there is a tendency for the theory to over-estimate the thrust and torque near the blade tips. * K. & M. No. 786.—An Aerodynamic Theory of the Airscrew. By H.Glauert. K.A.B. 1 . & M. No. 869.—Notes on the Vortex Theory of Airscrews. I'.y H. Glauert, it.A.E. AN INVESTIGATION ON WING FLUTTER. By R. A. FEAZER, B.A., B.Sc. R. & M. No. 1042 (A. 4). (22 pages and 5 diagrams.) February, 1926. Price Is. net. The present paper extends earlier theoretical work on flutter. The main underlying assumptions are, firstly, that a certain proportion of each wing moves under an elastic constraint in roll about some axis inboard of the interplane struts ; and secondly, that twist does not develop. The dynamical equations are obtained on a more exact basis than has been considered expedient hitherto, and the motion is analysed in some detail, with reference to the illustrative case of a particular biplane. With the rolling axis selected very close to the interplane struts the motion is stable at all speeds ; but as the axis is brought farther inboard, flutter develops over a range limited by an upper and a lower critical speed. The motion at general flying speeds is compounded of a "higher frequency" and a "lower frequency" oscillation. The former becomes rapidly and markedly unstable in the range defined by the critical speeds, whereas the latter is increasingly stable. The instability at a flight speed only 9 ft. per second in excess of the critical value is such that after the first 4 or 5 sees, following a small initial disturbance, the motion is no longer amenable to a small oscillation theory. An evaluation of the separate terms in the moment equation indicates that the twisting deflection which the rolling motion tends to induce at any instant is comparatively small. Unless twisting instability develops concurrently, and damps the motion in roll, the results of the present theory will not be impaired. If the centre of mass of the aileron be very near to the hinge, flutter of the type considered will almost certainly be avoided. Three immediately useful lines of further development are suggested. The first is a re-examination of the theory to allow for neglected terms ; the second is an extension to cater for wing twist : and the third a controlled experimental check of the theoretical work in a wind tunnel. PHOTOGRAPHS OF THE FLOW ROUND A MODEL SCREW WORKING IN WATER, ESPECIALLY IN THE "VORTEX-RING STATE." By C. N. H. LOCK, M.A.. and H. C. H. TOWSESD, B.SC. R. & M. No. 1043 (Ae. 230). (5 pages and 10 diagrams.) May, 1926. Price 9d. net, The modern theory of the action of an airscrew is based on the assumption that any blade element passes through the air in a helical path and reacts on the fluid in the same way as a similar element of an aerofoil moving in a straight path. Over a certain range, including moderate values of the thrust, 466
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