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Aviation History
1936
1936 - 0196.PDF
SUPPLEMENT TO FLIGHT 84A THE AIRCRAFT ENGINEER JANUARY 23, 1936 FURTHER EXPERIMENTS ON STATISTICAL MEASUREMENTS OF TURBU LENCE. By H. C. H. Townend, D.Sc, of the Aerodynamics Depart ment, N.P.L. K. & M. No. 3655- (10 pages and 4 diagrams.) November 24, 1934 Price od. net. In some recent measurements of turbulence use has been made of a method in which cinematograph records of the movements of small masses of heated air have been used to obtain the velocities. Analysis of the films involved the determination of the displacements of large numbers of such masses from their mean position. The possibility of inaccuracy introduced by the personal factor in reading such films has been tested in the present pipe experiments by using a photo-electric cell to detect the position of the masses instead of measuring them from photographs. The analysis was thereby made automatic. The results show good agreement with those obtained by film analysis, and indicate that the method is reliable for making measurements of this kind. THE USE OF SPLIT FLAPS AND OTHER DEVICES FOR FACILITATING LANDING. By S. B. Gates, M.A. Communicated by the Director of Scientific Research, Air Ministry. R. & M. No. 1659. (12 pages and 16 diagrams.) April, 1934. Price is. 3d. net. The evidence of recent experimental work from many sources on the characteristics of split flaps and other devices for increasing gliding angle and maximum lift co efficient is collected and summarised. It is rotated to the practical problem of giving the pilot an easily controlled means of spoiling the cleanness of his aircraft and decreasing its stalling speed, when he comes in to land. The evidence, so far as it goes suggests that the problem is soluble, cither with a split flap or with a slotted flap, without disturbing the action of conventional ailerons. A few rough indications which may lie helpful to designers are suggested :— (1) A trailing edge flap is the most suitable device for reducing the landing space of a clean, heavily loaded design. (2) A flap chord' of something like 20 per rent, of the wing chord will piobably be necessary, and some means of balance is probably essential. (3) If the trailing edge flap covers the tailplane, the trimming difficulties which would otherwise follow from the large change in load distribution on the wing appear to lie avoided. (4) If the flap is carried right across the body o a low wing monoplane, the wing root conditions should be'careftilly attended to in order to limit the turbulent wake of the flap, and its effect on the tailplane. TURBULENCE TESTS OF THE R.A.E. WIND TUNNELS. By R. G Harris, M.A., D.Sc., F.R.S.E., and A. Graham, M.Sc, D.I.C. Com municated by the Director of Scientific Research, Air Ministry. No. 1662. (6 pages and 6 diagrams.) November 20, 1034. R. & M. Price 6d. net. The drag of spheres was measured over the following range :—Reynolds number: 1 X 10s to 6 x 103. Sphere diameter : 3 in., 6 in., 9 in. Ventilation of five-foot tunnel : open and closed. The Values of K at which k„ is 0.15 are :— Sphere Diameter. Bin. ... 6m. ... 3 in. ... Five-foot Tunnel (Open Jet). 2.30 x 103 2.36 X 103 2.79 X 103 No. 1 Seven-foot Tunnel. 1.84 x 103 1.90 X 105 No. 2 Seven-foot Tunnel. 1.76 x 10° 1.88 x 10* These results are slightly dependent on (a) the history of the fio ventilation id (J) NOTE ON BOUNDARY LAYER FLOW. By H. B. Squire, M.A. Com municated by the Director of Scientific Research, Air Ministry. R. & M. No. 1664. (12 pages and 15 diagrams.) 'February 13, 1935. Price od. net. ]n the flow of air past a streamline body it is known that the aerodynamic forces arc closely connected with the behaviour 0* the layer of air near the surface. The sudden changes iu flow which occur when a wing stalls or a sphere passes through the critical Reynolds Number are closely related to alterations of the flow in this boundary layer. A simple account of recent developments in boundary layer theory was required which could be related to the practical aerodynamic problems in hand In this note the flow along a fla* plate and the effect of pressure gradient on the transition of the boundary laver from laminar to turbulent and on break-away are reviewed. Finally, modern theories of the effect of turbulence and variation of Reynolds plumber on maximum lift arc considered. SLOTS AND INTERCEPTORS IN SPINS. By S. B. Gates, M.A., H. B. Irving, B.Sc, R. P. Alston, B.A., A. V. Stephens, M.A. Communi cated by the Director of Scientific Research, Air Ministry. R. & M. No. ififx). (29 pages and 20 diagrams.) October 20, 1934. Price 2S. od. net. This is a comprehensive report on several lines of research—in flight, on the N.P.L. spinning balance, and in the R.A.E. Free Spinning Tunnel—upon the effect of automatic slots in developing spins. Some information on interceptor slot control is also included. In spinning attitudes each slot lifts and advances its wing tip, the forces depend ing mainly on the local incidence and speed at the centre of the slat. The net effect in a spin depends on the balance between these forces, and also on the influence of the added inertia of the slots; and indirect effects depending on the wcathcrrock stability and on the inertia distribution without slots. The change produced by adding slots cannot be predicted in a particular case without an intimate knowledge of the spinning characteristics without slots. The pro-spin yawing moment produced by slots does not usually exceed 10. A rough classification of slot effect is as follows : — Spin without slots. Good design, spin at <45°. Good design, moderately flat spin. Poor design, moderately flat. Possible effect of slots. Spin steeper, or impossible. Spin flatter. Possible spins •<— Steep or flat, stable. Intermediate, unstable. Example. IIIFlandplane. Bristol Fighter. This covers most of tie designs tested in the Free Spinning Tunnel. Either slot may tend to close under inertia forces if it is on the wing which is down in the spin. If the outer slot closes the effect should be favourable. If the inner slot closes it will probably do so only when it has ceased to supply an anti-spin moment. Thus, if Slots Close'in a spin the effect should not be unfavourable. A slot has occasionally been observed to close in a spin, without altering the motion. Bristol Fighter interceptors do not effectively close the outer slot in a spin and when coupled to the ailerons are rendered ineffective by the adverse yawing moment of the ailerons. In general, if interceptors are fitted it is probably better to couple them with the rudder. THE AIR DENSITY RFEECT IN SPINNING. By S. B. Gates. SLA., and A. V. Stephens, M.A. Communicated by the Director of Scien tific Research, Air Ministry. R. & M. No. 1663. (7 pages and 4 diagrams.) October 25, 1934. Price gd. net. It is possible in spinning mode! tests to represent the effect of a change ,'n air density, or height of the manoeuvre, by changing the mass of the model, keeping its nmtre of gravity and radii of gyration the same. This note summarises recent experiments of this kind at equivalent heights ranging from 5,000 feet to 15,000 leet, and contains a theoretical analysis of the effect of this change in air density. Experimental results so far have always shown that recovery is easier at the lower height, but the analysis shows that a reversed effect is possible. No simple con clusion can be drawn from the analysis, which suggests however as a rough general rule that recovery wil. be easier at a lower height if the bodv tail unit is efficient and there is no tendency to spin flat, and vice versa. The difference in applied yawing moment necessary to make recoverv impossible has been observed to be of the order of 10 when the equivalent height is changed from 5,000 feet to 15,000 feet. This is quite sufficient to account for an easy recovery near the ground and a more difficult one higher up. ROLLING AND YAWING MOMENTS ON HALF WINGS WITH VARIOUS MODIFICATIONS OF WING TIPS. D. H. Williams, B.Sc, of the Aero dynamics Department, N.P.L. R. & M. No. 1665. (6 pages and 15 diagrams.) July 5, 1934. Price is. net. During the past few years, various devices have been tried on wings with a view to delaying the stall or to flattening the top of the lift curve avoiding especially the sudden drop m hft observed aL. the stalling angle of some aerofoils. The work described below was the outcome of a suggestion that experiments should be made on wings to find the effect of modifications of the tips of the wings on the form of the rolling moment curve. Instead of measuring lift and drag on an aerofoil in the usual manner, a half wing only was mounted in a 4ft. wind tunnel with the root of the half wing at the wall of the tunnel and rolling and yawing moments about wind axes were measured on the half wing. Curves arc given in this report for a slotted biplane for swept-back and swept- forward wings and for wings with swept-back and swept-forward tips. Sweep-forward of the whole wing lessens the drop in the rolling moment curve above the stall and sweep-back increases it. Swept-forward and swept-back tips have similar effects. The best result was obtained for a wing with the outer third swept-forward 30° with a sharp angle in the leading edge. A swept-forward wing with curved leading edge did not give such a good curve as one with the sharp bend. A DISTRIBUTION METHOD OF STRESS ANALYSIS. Bv Professor J. F. Baker, M.A., D.Sc., M.I.Struct.E., Assoc. M.Inst.C.E., and A. J. Ockltston, B.E. (N.Z.). R. & M. No. 1667. (82 pages and 42 dia grams. March 30, 1935. Price 3s. 6d. net. Scant attention has been given in this country to a method of determining the stresses in frameworks, described some years ago by Professor Hardy Cross which should be of considerable use to aeronautical engineers. Cross illustrated his method of analysis by determining the bending moments in rectangular building frames with perfectly stiff joints so constrained that no translation of the joints was possible. The better known methods of dealing with this problem, such as slope deflection or minimum strain energv, imagine the structure cut at such sections as would make it simply stiff and necessitate the solution of equations which set out the relations between the angles through which the rut ends must turn, or the reactions necessary to bring about these rotations, to make the structure continuous again. The Cross method, however, not only keeps the joints stiff but, in the first place, fixes them in space so that the structure can be considered as made up of members enoastered at their ends. The application of the external load system produces end moments in the members and also an external moment at each joint since it is fixed in space. One joint is then released, that is to say the external moment is removed and the joint is no longer in equilibrium. It immediately rotates until a new equilibrium position is reached, and in so doing modifies the end moments in the adjoining members. This joint is now fixed again in its new position and another joint is released which moves into a new equilibrium position, further modifying the end moments. This distribution method appears to be complicated but when a number of examples have been worked out and the physical significance -1 the various steps grasped, the full simplicity of the method will be appreciated. It has one great advantage over the more usual methods in that simultaneous equations, apart from the groups of three used in determining the various factors, are avoided and the behaviour of the frame under any load system can be visualised at every stage. The investigator is thus able, with confidence, to make slight alterations hi the norma! procedure to suit particular cases. He is able to carry his work to any desiied degree of accuracy and is not tempted, as in other methods, to make simplifying approximations the full significance of which cannot be gauged. While the distribution method can be used on a highlv redundant structure without considerable labour it enables » solution to be obtained in manv cases where other methods arc impossible.
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