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Aviation History
1914
1914 - 0578.PDF
[/OGHT| MAY 29, 1914. THE DEVELOPMENT OF THE AEROPLANE. By DR. R. T. GLAZEBROOK, F.R.S., F.Ae.S. IN the first part of his lecture, the author, with the assistance of lantern slides, gave a brief account of the development of the aeroplane, from Henson's machine, which was designed in 1842 and never constructed, to the Santos Dumont aeroplane of 1906 ; and then proceeded to deal with the work of experiment and scientific research conducted at the N.P.L., demonstrating that an extraordinarily high order of accuracy is attained, and that the results obtained correspond with almost mathematical exactness when the observations are made in either of the two existing wind channels by different observers. The second section of the lecture was devoted to the consideration of laws connecting the forces on the model with those upon the # in terms of the velocity of its centre of gravity along the axes of reference and of the angular velocities of its centres of the machine about these axes. This expression involved a number of constants, quantities which depend on the shape and dimensions of the machine, not on its motion. If we know the energy, values can be found for the forces and moments on the machine—these involve the same constants—and the equations of motion can then be formed. Their solutions can be obtained at any rate in certain Fig. 1.—Variation of lift coefficient of a model aerofoil with changes of speed. model itself—the laws of similarity. The lecturer observed that experiment had proved that the force on a surface due to the wind was not proportional to the square of the speed, but that the lift/drift and the lift coefficient increased with increase in speed (see Figs. I and 2). Lord Rayleigh, however, had called attention to the fact that if K in the expression for wind pressure—KSV2—be not constant, it must depend upon the quantity (LV-i-v) where V is the velocily of the current, L some linear dimension of the surface, and v is the kinematic viscosity of the air. Hence, if the values of K as found for an aerofoil in a given position but for different eo |K5 ; •1 O 1 . _-4/—" * » """"S5 —To* 0 > u z T r 0 Z s b. z u > u 0 4 | i 1 i t , •0 w -J 0 K U & w—- D U. w — Lg. uy Fig. 3.—Variation of lift/drift with LV, where L is the length of chord in feet and V is the velocity in feet per second. values of the velocities are plotted against LV, the points ought to lie upon a smooth curve, and the form of this curve will determine K as a function of LV. This was illustrated in Fig. 3 where the values of the lift/drift ratio are plotted against LV (or rather, for convenience, against log LV) for the series of experiments shown in the preceding curves—the points on the line marked " Full Size Aerofoil" being taken from the results of experiments at the Aero dynamical Laboratory of the University of Paris on full-sized aerofoils of the same design as ths models used in the preceding experiments, which were made to a ^5 scale. It will be seen that the values for the coefficients found from the 50 feet per sec. obser vations in the channel do not differ greatly from those belonging to the actual machine. Dr. Glazebrook next considered the problem of the Stability of Aeroplanes, remarking that it was most complex, and depended on finding an expression for the energy of the machine in any position, * Digest.of the Wilbur Wright Memorial Lecture, delivered before the Aero nautical Society on 20th inst. Fig. 2.— Variations of lift/drift of a model aerofoil with changes of speed. cases of importance, but to utilise the results we require to know the numerical values of the constants just referred to and to deter mine these we must have recourse to the model experiments. By means of the balance the forces on the model can be measured ; these forces can also be expressed in terms of the constants, and the wind velocity, and hence we can find certain of the constants applicable to the aeroplane considered. Further experiments of a somewhat different character are required to determine the values of the rest of the constants or coefficients in the energy expression— the rotary derivatives, as they are called ; but by means of the model experiments all these can be found, and on substituting the values in the equations of motion, the nature of the motion can in many cases be determined by A the solution of the equa tions. In determining the sta bility coefficients a model is supported in the chan nel in various positions relative to the direction of the air current and the forces measured ; the axes or direction of reference are taken as shown in Fig. 4. The angle of pitch is positive when the nose of the machine rises, the angle of yaw is positive when the machine turns to the right, and the angle of banking which properly accompanies this turn to the right will also be positive. Fig. 5 gives the forces and moments which are produced in the plane of symmetry when the attitude of the machine to the wind changes, but without yawing, while in Fig. 6 are shown the forces and moments produced by yawing without altering the angle of pitch, so that the flight is horizontal. The wind speed for which the forces are given is 30 ft./sec. Starting from zero pitch angle the longitudinal force falls as the angle of attack is increased, reaches a minimum at about 8°, and then rises again rapidly. The nor mal force increases regularly as the angle of attack increases, while the pitching moment increases in amount but is negative, that is to say, it tends to reduce the angle of attack. The machine is stable longitudinally, so far as pitching moment on it is concerned, but further investigation is required before the motion can be com pletely determined. Turning now to Fig. 6 we see that as the angle of yaw increases the longitudinal and normal forces are some what reduced though the changes are not at first large, but a con siderable negative lateral force is brought into action, if the machine turns to the right the side force is from left to right, the machine side-slips in the direction in which it is turning. There is a yawing couple N which is negative, i.e., tends to reduce the angle of yaw and turn the nose of the machine into the wind, at the same time a Fig. 4.—Axes of reference. 578
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