Aerofoil Section Research Articles

The effect of rotation upon the lift and moment of a Joukowski aerofoil

In a recent paper the author has given a number of general formulæ concerning two-dimensional hydrodynamics, including the working out in some detail of a solution of the problem of a rotating cylinder by a method outlined by him some years ago. It was suggested by Prof. G. I. Taylor that the application of these methods and results to the Joukowski and other aerofoil sections would be interesting, and perhaps useful, and the present paper is an account of the results obtained. The solution of the main problem, that of the effect of rotation upon the lift and moment coefficient, necessitated the examination of the force and moment due to fluid pressure acting upon a cylinder in the most general case (the motion of the cylinder being considered as compounded of a translation and a rotation, with circulation about the cylinder superimposed), and the author’s solution has been recently published.

The Interference between the Body and Wings of Aircraft

SummaryTests on certain aircraft having disclosed discrepancies between the characteristics estimated from the figures for the individual components and those observed for the complete model, the present investigation was undertaken to determine the nature and magnitude of the effects of interference between the body and wings of aircraft and to ascertain the factors influencing the interference.The models were made up of standard 3in. x 18in. duralumin aerofoils and fuselages of wood.The aerofoils employed in the investigation were:– (a)R.A.F.-15–a thin section.(b)U.S.A.-27–a section of medium thickness.(c)Gottingen-387–a thick section.The fuselages used were:– a.Fuselage A–of streamline form.b.Fuselage B–of cabin form.c.Fuselage C–of usual open cockpit form.A number of typical monoplane and biplane combinations Of wings and fuselages were tested, and in some cases the effect of fairing between wings and fuselage was studied.The results of the investigation indicate that the interference effects are dependent on the shape of the fuselage, the aerofoil section and the relative position of the fuselage and aerofoil. The better the aerodynamic form of the fuselage, and the thicker the aerofoil section, the greater are the interference effects and the more marked the influence of the relative vertical position of the wings and body on the interference.Interference between wings and body tends, in general, to lower the critical angle and increase the drag of the combination as compared with the individual components. It may increase or decrease the lift.From an aerodynamic standpoint, the best position for the wing is at the top of the fuselage, and the worst that at the bottom.Fillets and fairing may improve a combination having poor characteristics, but have little effect if the arrangement already possesses good characteristics.An explanation of the cause of the observed interference effects is advanced.

Month in the Patent Office

A hollow air screw built up from sheet metal bent along a leading edge and joined along a trailing edge and having no internal stiffeners has a root end piece which is pivotally connected to the boss, whilst the blade is formed widest at the root tapering more or less uniformly to the tip. The sheet metal blade 2 is of aerofoil section and riveted or otherwise secured along the trailing edge. A channel‐sectioned end piece 9 is secured and riveted to the base of the root and bolted thereto is a hinge piece 13, drilled to take a bolt 14 for connection with lugs 15 on the boss 8. The single pin joint may be replaced by a double pin forming universal joint.

Designed for Aerobatics in Germany

OWING to the various competitions in Germany last year one type was neglected, a moderately high‐powered machine for stunt flying. The Raab‐Katzenstein firm has now filled the gap with the “R.K. 26,” which combines high strength factor with speed and rapid climb. This machine is a cantilever biplane with one‐piece wings. Above the fuselage the top wing is carried on a capotage of steel struts and there is one N‐strut towards the wing‐tip on each side. The American N.A.C.A. M‐12 aerofoil section is used, giving low centre of pressure travel, the wings being constructed of wood and fabric covered. The fuselage is built up of wire‐braced steel‐tubes, the pilot's cockpit being in rear of the passenger seat. The empennage is also constructed of steel tubes. The tubular steel undercarriage has a divided circle supported by M struts for the bottom longerons. Four fuel tanks are provided, three in the wings and a special tank for upside‐down flying in the fuselage, with a total capacity of 210 litres (46 gallons).

XLVIII.Vortices behind Aerofoil sections and rotating cylinders

XLVIII.<i>Vortices behind Aerofoil sections and rotating cylinders</i>

Stalled Flying

Stalled flying is a subject which at the present time is occupying the thought and time of aerodynamic experts all over the world. It is still more or less in the experimental stages, but its value to flying, and more especially commercial aeronautics, is now almost universally recognised.It has hitherto been possible to fly an aeroplane at all angles of incidence from the no-lift angle up to the burble point of the aerofoil section employed for the wings. The burble point on the lift curve marks the change-over from streamline to turbulent critical airflow, and at this angle the aeroplane will stall. The machine sinks, and what is more dangerous, becomes unstable laterally so that the pilot loses power over its orientation. A spin then usually ensues.It may be as well to state here that the only way of defining the change-over point from normal to stalled conditions is by means of the stalling angle.

V. An investigation of the flow of air an aёrofoil of infinite span

1. A great deal of attention has been directed of late years to the development of a rational theory of the aёrofoil. Prof. L. Prandtl and others in Germany have applied the principles of the hydrodynamics of a perfect fluid to the aerofoil with remarkable results, whilst investigators in this country have extended this work and have verified experimentally many of the deductions of the Prandtl theory. The assumptions underlying the work of Prandtl are, however, of uncertain validity, and it has become a matter of great importance to add to existing experimental evidence of the fundamental characteristics of the motion of a viscous fluid round an aёrofoil. With this purpose in view an aerofoil section of fairly high lift coefficient was selected, and a model of it tested in the Duplex Tunnel at the National Physical Laboratory, the field of flow being thoroughly explored with a wind-velocity meter. At the same time the theoretical stream-lines corresponding to inviscid fluid flow were determined experimentally, as described in Part II of this paper. The case considered is that of an aerofoil of infinite span, the flow being two-dimensional. A comparison was made of the theoretical and experimental distributions of pressure over the surface of the aёrofoil, as well as of the two sets of superposed stream-lines. The work has provided an experimental verification of the law of Kutta and Joukowsky, that the product of the mean velocity and density of the fluid and of the circulation (according to the hydrodynamical definition of this term) around a contour enclosing the aerofoil is equal to the lift of the aёrofoil (per unit length). It has further shown that the circulation around the aёrofoil is constant within the limits of experimental error and independent of the contour of integration chosen, provided that the contour line does not at any part approach too near to the aerofoil, and also that it cuts the trailing “wake” approximately at right angles to its core. The lowest value of the circulation found (calculated for a contour as close to the aёrofoil surface as the observations permitted) was about 6½ per cent, less than the value corresponding to the lift coefficient; this is hardly outside the limits of experimental accuracy in the neighbourhood of the aёrofoil.

Societies and Academies

LONDON. Physical Society, March 13.—Jas. P. Andrews: The variation of Young's modulus at high temperatures. The variation is found for zinc, silver, phosphor-bronze, lead, and soda glass by a static method, to within about 150° of the melting-point. It varies exponentially with temperature, so that q (Young's Modulus) = q1e-b1t (where q1 and b1 are constants) up to a temperature roughly half-way from absolute zero to the melting-point, and q = q2e-b2t for the remainder.—E. G. Richardson: The critical velocity of flow past objects of aerofoil section. By observations of the “Æolian tones” of vibrators of aerofoil section, critical values for flow past an object of aerofoil section have been obtained of a fluid incident at various angles. The minimum value of VL/v for unsteady flow falls from 60 at o incidence to 45 at 20, and then more rapidly.—J. Brentano: A focussing method of crystal powder analysis by X-rays. For any given angle of reflection, a surface of double curvature can be found, such that it will reflect X-rays coming from one point, to any other definite point. For an element of this surface, situated so as to be distant from the two points by lengths a and b respectively, the relation sin α/sin β = a/b must be satisfied, where α and β are the glancing angles of incidence and of emergence of the X-rays with respect to the surface. An arrangement for crystal analysis based on this relation is discussed.

Critical velocity of flow past objects of aerofoil section

By observations of the AEolian Tones of vibrators of aerofoil section, critical values for flow past an object of aerofoil section have been obtained of a fluid incident at various angles. The minimum value of VL/ν for unstead flow is found to fall from 60 at 0° incidence to 45 at 20°, and then more rapidly.

Helicopters

SummaryThe present article is an account of some calculations on helicopters.Airscrews have been calculated for different conditions according to two theories: (1) the multiplane interference theory; (2) Glauert's vortex theor). According to both there should be no difficulty in designing a screw to give a good lift at a reasonable rate of climb, and the ceiling should also be quite good. When we consider the speed of falling, with the screws free-wheeling, the two theories give widely different results, and the practicability of the helicopter depends largely upon this question being settled. Simple airscrew theory shows that at least moderate speeds should be obtainable by inclining the airscrew axis. For many reasons it seems desirable that the screws should have at least four blades; gyroscopic couples on the whole machine are eliminated; the forces are widely fluctuating during forward motion with only two blades; the stability derivatives and equations are simplified; but the aerodynamic efficiency will be impaired. In general the stability equation is of the tenth degree, and the lateral and longitudinal stabilities are not separable when the machine, in a state of steady motion in a straight line, receives an asymmetrical disturbance.The following notes are the results of an attempt to investigate the theoretical possibilities of the helicopter, and generally to develop some branches of the theory of helicopters. In this country extremely little work on the subject seems to have been published, and the only experiments I have been able to find are those given in the Report of the Advisory Committee for 1917-18 (Vol. II.). Several articles have appeared in “ L'Aérophile ” from time to time, notably by Lamé, Touissaint and Margoulis, and some experimental work has been done by Eiffel. But it is extremely difficult to find adequate experimental results with which to compare any theory ; for instance it is very rare to find results of tests on airscrews working under helicopteral conditions, and also the aerodynamic data of the aerofoil sections used. In the matter of stability I do not know of any experimental work at all.I shall give first the results of my investigations into airscrews for helicopters, and then proceed to the consideration of the dynamics of helicopter flight and the development of the stability equations.

Speed control and stopping devices : Anon. (Safety Standards of the Industrial Board, Pennsylvania Department of Labor and Industry, Electric Code, vol. I, No. 21, p. 59.)

Many small wind turbines operate at high tip speed ratio and this gives rise to situations where compressibility may influence performance. The most important example is runaway, the operating point for an unloaded turbine where the rotational speed of the blades is maximised. Because compressibility significantly increases drag through the action of “shock-stall”, it may provide an inherent mechanism for overspeed protection. This possibility is tested computationally for a turbine with a high optimum tip speed ratio, having an aerofoil section (NACA 0012) whose behaviour in compressible flow is well known. Calculations of turbine performance for differing windspeeds, and hence Mach numbers, show that compressibility effects occur too slowly to prevent overspeeding. However, it is suggested that aerofoils could be designed to maximise the onset of shock stall.