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Aviation History
1922
1922 - 0776.PDF
affect seriously the drag measurements. This method was therefore abandoned in favour of a hit-or-miss method, where a pin inserted in a scale was either knocked out or left untouched by the pendulum. With several swings of the pendulum we found that, the top of the swing could be located to at least -^ in., which gave very fair accuracy in these measurements. " Lift forces are converted into absolute coefficients by calculating the maximum speed of the pendulum. Drag forces are calculated from an integration of the speed of the pendulum throughout the swing. " Initial Drag.—-It is obvious that the length of swing of the pendulum is a measure of the combined drag of the model and pendulum. It is now necessary to subtract the resistance of the pendulum. To do this in principle it is only necessary to detach the model and replace it by a mass equal to that of the model, with its centre of gravity on the same radius of swing as that of the model. This mass must offer no air resistance. The height of swing of the pendulum must then be observed. " This problem developed the method of holding the model in a frame. The frame is tubular, and the mass takes the form of lead weights inserted in the arms of the frame. After a time all models were tested in this way on account of observations when pressure plotting, which indicated that air-flow at the ends was affected by end support. " The angle of the model is measured from a protractor fitted on a stand under the vertical position of the pendulum. " A light arm carrying a recording pencil is mounted crossways on the bottom of the pendulum. This records lift on a piece of paper held on a suitable frame to the side of the path of the pendulum. ' " Advantages of Method.—The air through which the mode passes is still, and there can be no question of turbulence or steady flow. Wall effect, which may be great in an ordinary wind tunnel, is greatly reduced. The cost of running the apparatus is very small. The apparatus is cheap, and the power required is one-man power. The accuracy of the f measurement appears high. Repeat experiments usually agree to about i per cent. " Disadvantages of Method.—The motion is not steady, but accelerated. The motion is not rectilinear. iThese two facts may cause the apparatus to give erroneous results, but it should be observed that when tests are made on forms tested by the N.P.L. the results are in very fair agreement. Only on some thick sections the results are not the same ; on the other hand, Handley Page effects are very obvious. " Description of Method of Carrying out Test.—The model is placed on the frame and run at varying angles for lift; the usual increments of angle are by 2. The model is then set at o° incidence and run for initial drag. The model is then removed and replaced by weights, and the pendulum drag measured. The model is then replaced and the weights removed to check for initial drag, and if any discrepancy is noted the experiment is repeated. This test for initial drag is most important, and should be carried out with the greatest care. The model is then run through at varying angles for drag measurements.' " Pressure Plotting.—This can be carried out satisfactorily on this apparatus. When the necessary holes and passages have been made in the model they must be connected to a small tube carried up the pendulum. This tube is coupled up by means of a thin, flexible rubber pipe to a small inclined U tube filled with alcohol. If the tube is small the necessary mass of alcohol is reduced, and the inertia effect of this .H 0 DECEMBER 21, 1922 mass*also reduced. The pendulum is swung in the ordinary way and the motion of the alcohol noted. With practice this can be read to ^fa in., with a possible error of ± -j^o in. " A correction has to be applied for the centrifugal force of the air in the tube down the pendulum. When this correction is applied the results are in agreement with those obtained at the N.P.L. on the same section. The work is a little tedious, but a section can be plotted in one day and invaluable results obtained. These results show our want of a satisfactory theory. Progressive modifications of form do not produce the expected alterations of pressure, and no general conclusions can so far be stated. " Movements of Centre of Pressure.—So far no attempt has been .made to measure this quantity, as time and money have not permitted, but it does not seem impossible to devise an apparatus to carry out this work if necessary. "Further Possibilities.—With .the pendulum it is possible to observe to a certain extent the effect produced on the air by the passage of any body under test through it. For this it is necessary to create a smoke cloud and swing the model through it. " Wing-tip vortices," he continued—" first, I think, sug gested by Lanchester and later developed by Prandtl-—can be observed. These vortices exist, but appear to be not quite as assumed by theory. " The vortex is non-existent at the angle of no lift, and increases in size and probably in velocity up to the burple point, where the motion changes. With square-ended planes the centre of the vortex occurs on the line of the wing tips, but apparently below it. As far as can be observed the whole of the rotational effect is below the wing. " With tapered wing tips the centre of the vortex comes in nearer the centre of the model, approximately one-sixth of the span, and the vortex obviously increases in size and apparently decreases in speed of rotation. " With a biplane two vortices are formed which rotate separately and finally round one another ; here the vortices appear smaller. '' The vortices persist for a long period after the passage of the model. They have been observed for 800 or 900-chord lengths. " Further Smoke Observations.—When a model passes through the air a distinct kick is given to the air at con siderable distances from the model ; this is especially notice able on the upper side, where the effect is visible up to 8 ft. from the model. What the nature of the kick is is not clear, but it looks like the first swing of a very heavily damped vibration. This may have a bearing on tunnel-wall effect. " Finally," he concluded, " I should like to try and impress on you the effects that making experiments such as these have on anyone who undertakes them. One becomes impressed by the unsatisfactory nature of usually accepted theories when applied to observed facts. " Turning to the practical side, the experiments, whilst at times apparently giving illogical results, continue to indicate the possibility of increasing the aerodynamic effi ciency of the forms we use. Gliding angles of 1-18 are attainable, and probably much higher figures. " The Handley Page effect shows the possibility of multi plying lifts by 3 ; unfortunately, it multiplies drag by 9- Does this not indicate that if we do right we can reduce the drag forces ? To do this we must do something new and not copy. Every one who can should take this work up and experiment. " The present aeroplane is wasteful of power, and for progress we must improve it." 1 _7 PERSONALS Married Lieut.-Col. CUTHBERT EUAN CHARLES RABAGLIATI, late K.O.Y.L.I. and R.F.C., youngest son of Andrea C. F. Rabagliati, M.D., Whinbrae, Ben Rhydding, and grandson of the late Duncan McLaren, M.P., Edinburgh, was married on December 4 at St. Ethelburga's, Bishopsgate, to CLARISSA CATHERINE MELVILL DE HOCHEPIED LARPENT, only daughter of the late John Melvill, ninth Baron de Hochepied, and of the Baroness de Hochepied. Flight-Lieut. EDWARD THORNTON, R.A.F., only son of the late Edward Thornton, Minister in H.M. Diplomatic Service, and of Mrs. E. D. L. Harvey, was married on December n, at St. Saviour's, Chelsea, to MARJORIE GABRIELLE, only daughter of Mr. W. R. PIKE. To be Married The marriage will shortly take place between RORY ALWYN MACNAMARA SCRASE DICKINS, late 60th Rifles and R.A.F., eldest son of Mr. and Mrs- Alwyn Scrase Dickins, of Heronsgill, Hoi sham, and BRENDA BAILLIE, of Tismans House, Rudgwick, Sussex, only daughter of Mr. and Mrs. SOUTHBY HEWITT. Deaths Capt. JOHN EVELYN HARRISON DAKIN, late Squadron- Commander, R.A.F., who died suddenly on December 8, in South Africa, aged 34, was the son of the late Joseph Harrison Dakin, of Brittany Lodge, Edwardes Square. Flying Officer BERTRAND REGINALD HARRIS, R.A.F., of 56, Clifton Gardens, Maida Vale, W., died of pneumonia on December 10, at the Royal Naval Hospital, Chatham. Flying Officer Harris, whose age was 27, had been twice mentioned in Despatches. Item Lieut. AVIATEUR WILLY COPPENS, D.S.O., M.C., Air Attache to the Belgian Embassy, left London on December 3, for Baghdad. 776
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