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Aviation History
1912
1912 - 0263.PDF
MARCH 23, 1912. A programme of investigation, chiefly concerned with the properties of balloon and aeroplane fabrics, will be carried out as approved by the Advisory Committee for Aeronautics. The Friction of Air Currents in Pipes. (Dr. Stanton and Mr. Pan/tell).—The experiments on the distribution of velocity in the cross section of a pipe through which air is flowing, together with the measurement of the fall of pressure in the axial and radial directions, referred to in the Report for 1910, have progressed sufficiently to enable the relation between the shearing stress and rate of change of distortion in the fluid to be determined. Following the nomenclature of Osborne Reynolds, who first called attention to the difference between the shearing stresses in "laminar" and in " eddying" motion, this shearing stress is here called the " mechanical viscosity." Thus if F is the average shearing stress on the surface of any cylindrical portion of radius r of the fluid coaxial with the pipe through which it flows, and -• the mean axial velocity dv of the fluid at this radius, then, writing F = fa —, the quantity determined by the experiments was /u', the coefficient of mechanical viscosity, expressed as a function of the velocity of the fluid and the dimensions of the pipe. Experiments were made on pipes of two kinds. 1. Pipes in which the surface was artificially roughened in such a way that the friction varied precisely as the square of the speed. 2. Pipes of smooth bore in which the friction varies as a power of the speed appreciably less than two. In the case of the artificially roughened pipes it was found that the distribution of velocity across the pipes was such that if a curve was plotted whose ordinates were the ratios of the observed velocities to that of the centre filament, and the abscissae the ratios of the corresponding radii to the external radius of the pipe, then this curve up to within a relatively small distance from the boundary was a parabola, and expressed the motion at all speeds and in all pipes. From this relation it follows that for pipes in which the friction varies as the square of the speed :— 1. From the parabolic form of the velocity curve fx.' is constant across the pipe for any given speed of flow. 2. Since the rate of distortion is proportional to the speed of flow and the shearing force is proportional to the square of the speed of flow, fi' is proportional to the speed of flow. 3. For two pipes of different diameters since the shearing forces are the same at the same speeds of flow and the rates of distortion are inversely proportional to the radii, M' must lie proportional to the radius. The expression for (*' is therefore of the form KV,. L where Vc is the velocity of flow, L the linear dimension of the pipe and K a constant depending on the roughness of the pipe. In the experiments on smooth pipes, the identity in the velocity- curves at varying speeds and diameters was found not to exist, and the: expression for the mechanical viscosity was much more complicated, being given by fi = CV^L/I -.- I, where c is the kinematical coefficient of viscosity of the fluid. A paper on the results of the experiments has been published in the Proceedings of the Royal Society. A Vol. 85, 1911. The following researches, of which a detailed account is given in the Report of the Advisory Committee for Aeronautics, have been carried out during the year :— I. Wind Channels.—a. The determination of the forces on, and the efficiency of, aeroplane wing forms, including investigations into the distribution of pressure over the surface. This forms part of a definite scheme of research of some magnitude undertaken for the Aircraft Factory, and will be continued during the coming year. Among other questions, the effect of separate variation of the upper and lower surfaces of an aerofoil has been studied. b. Determination of the flow of air round aeroplane wing forms by means of smoke trails. This work is being undertaken with a view to determining the direction in which improvements in wing forms may be expected, and has rendered assistance in the prosecution of the previous research a. c. Measurement of the lift and drift of a model Paulhan girder. d. Determination of the resistance of wires and ropes. These tests were made on wires and ropes ranging from 0*04 to CT25 inches diameter for smooth wires, and from O'l to o-5 for stranded wire and hemp ropes. The air resistance of the stranded ropes was found to be approxi mately the same as that of small square plates, the area being considered as the product of the diameter and the length. The smooth wires gave a resistance approximately 20 per cent, lower. The effect of vibration of the wire in any direction was found to be negligible, so far as the aerodynamic resistance is concerned. 2. Experimental Water Channels.—a. Determination of the moments on 12 model dirigibles when inclined to the current. b. Photographs of the flow of water round model dirigibles in a tank specially constructed for the purpose. The experiments in the water channels showed clearly that even for an elongated fish-shaped airship model the relative velocity of flow near the tail is con siderably less than in the main stream. 1: Investigation of the eddying motion in the rear or plane and curved surfaces. These experiments demonstrated the existence of two distinct types of flow for the same surface at different velocities. 3. Whirling Table.—A new dynamometer has been constructed to enable the range of the experiments to be increased. 1 -or experi ments with fairly high values of slip it has been found necessary to augment the factional resistance of the whirling table by the provision of a rope brake, while arrangements have been completed to enable higher propeller speeds to be attained. The research into propeller efficiency has been continued, special attention being given to the effect of blade area. Tests on tht Mechanical Strength of Ba/hum Fairies,—The laws of compound stress for balloon fabrics have been investigated as far as concerns single fabrics or fabrics composed of two layers applied to each other warp on warp. It was anticipated from theoretical considerations that the breakdown stress would be slightly lowered by the presence of a stress in the direction at right angles to the principal stress by an amount depending chiefly on the fact that each short length of yarn from one cross yarn to the next is inclined to the plane of the fabric ; the elastic deformation of this angle alio modifies the result. To check this theory six rets of five bags of a double parallel fabric (No. 1,007 made by the Continental Tyre and Rubber Co.) were tested under various ratios of compound stress in the apparatus previously described. The results agreed with the theoretical prediction within the comparatively large variability which is always exhibited by fabrics. Great simplication can l>e introduced into strength tests on fabric for dirigible balloon con struction by the use of the following empirically simplified form of the laws of compound stress :— For single or for double parallel fabrics breakdown under com pound stress is reached a little before the maximum stress attains a value equal to the breakdown strength of the material as determined by simple tensile tests. For cotton the weakening effect may be taken as 5 per cent, for equal stresses (the case of a spherical balloon) and 2i per cent, for a 2 : t ratio of stresses (the usual approximate condition in an elongated dirigible balloon envelope). This does not apply to double diagonal fabrics. Preliminary tests on wounded specimens of fabric under compound stress have been made, using a cross shaped specimen which gives results of sufficient accuracy for ordinary purposes. From these tests the following preliminary conclusion has licen deduced :— For single or for double parallel fabrics the factor of safety for tearing under a 2 : 1 ratio of stress appears to approximate to the value determined in simple tension by the method previously described. This docs not apply to double diagonal fabrics. The case of double diagonal fabrics is being further investigated. A report on some preliminary work on the time curve for the deterioration of unprotected rubber-proofed fabrics after exposure to the weather was presented to the Advisory Committee early in the year. An extensive scheme of investigating the rate of deterioration of eight typical balloon fabrics derived from various sources was then undertaken. The specimens for exposure were placed under cover at night, being run in and out from a door in the gable-end of a roof of the chemical laboratory on to a lead-covered roof flat by means of a specially constructed trolley and rails. One half of the specimens were exposed on both wet and dry days, and the other half on dry days only. The specimens were tested for both tensile strength and permeability to hydrogen at regular intervals, and in most cases a clearly marked difference between the two scries could be found in the results. A full account of the results will be given in the Report of the Advisory Committee for Aeronautics for the year 1911-12. The extent to which water adheres lo balloon and aeroplane fabrics has been investigated. The materials were either sprayed with water from fine jets under constant pressure, or they were dipped into water and allowed to drain for a short period. It was found that the results from the two methods were in fair accord ; the increases of weight observed vary from about 9 to 30 per cent, of the weight of the fabric. A full-size balloon valve has been tested for tightness to hydrogen, special apparatus being set up for the purpose of determining the rate of escape of gas from the valve both when tightly closed and when a portion of the pressure on the joint was removed. As balloons are sometimes exposed to very low temperatures, a number of fabrics were tested when frozen to temperatures of from —io'C. to —25"C. ; at these low temperatures the fabrics were either («) crumpled or (/<) carefully protected from crumpling. It was found that rubbered materials were practically unchanged by either treat ment, but that varnished silks if crumpled at the very low temperatures might show fairly large leakages ; exposure for a short time to a temperature from 15'C. however, caused the minute cracks in the
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