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Aviation History
1920
1920 - 1252.PDF
DECEMBER 9, 1920 AIRSHIP PILOTING* By Major G. H. SCOTT, C.B.E., A.F.C. (late R.A.F.), A.M.I.MEC.E. Introduction I FEEL it a great honour to have the privilege of addressing the members of the Royal Aeronautical Society on the subject of Airship Piloting, especially in view of the interest you have taken in furthering and generally assisting in the development of all types of aircraft. Although the airship has hitherto not occupied the thought and brains of Aeronautical Engineers to the same extent as the aeroplane and seaplane, I feel sure the confidence and support of the Society will lead to a more general and scientific interest being taken in lighter-than-air craft, which is bound to result in more rapid progress in the near future. I hope the discussion to follow will provide the foundation for solving some of the problems that will have to be faced, when piloting the airships over routes to various parts of the world, where totally different atmospheric conditions are likely to be encountered. Once these problems are solved, I feel that the future of the airship as a mode of fast long- distance transport is assured. In the early days of sea-going ships, the captain of the ship worked the helm, and by his own personal observation and skill at the helm, navigated his ship in unknown seas, but his progress was slow and the distance covered comparativelv small. As the size of the ship increased, the captain handed over the helm to one of his men, and turned his attention to the navigation and the general working of his ship. As the ship grew bigger still, and the speed and range increased, he rele- gated more duties to his junior officers and turned his attention to the control and co-ordination of the whole. It is the same with the airship. In the early days when airships were small, the pilot himself worked the controls, but as the size increased he turned his whole attention to the navigation and general piloting of his ship, his crew working the controls. Thus the captain 'of the large airship of the future, although having an intimate knowledge of the detail working of the airship, will merely control and direct, and his chief stuGy will be the weather, and the best method of using it, to make a quick, safe passage. It is this side of piloting that I will chiefly consider in this Paper, and will deal with it under four heads— (a) Aerostatics of Airships. (b) Aerodynamics of Airships. (c) Weather with regard to Airships. (d) Navigation. Aerostatics Before discussjpg the piloting of airships, it is necessary firstto appreciate the statical conditions which govern "their performance. (a) Lift of Hydrogen In modern airships the gas used to give the buoyancy or liftis hydrogen. This gas has a weight of only 1 /15 the weight of an equal volume of air at the same temperature and pressure. That is, at normal temperature and normal atmosphericpressure, 1,000 cubic feet of hydrogen weighs approximately 5 lbs., whereas 1,000 cubic feet of air weighs 75 lbs. • So that under usual conditions pure hydrogen has a buoy-ancy or hft in air of between 70 lbs. and 72 lbs. per 1,000 cubic feet. In practice it is impossible to obtain or maintain purehydrogen in the airship, it is therefore necessary to know the degree of purity. This is determined by means of a purity meter, which givesthe relative densitv between the air and "the hydrogen under test. ' J 5 The relation between air and pure hydrogen is known, so that a relation can be calculated between the hydrogen tested and pure hydrogen. For convenience, it is assumed that the impurity is air, and the " purity " is expressed as the percent- age by volume of hydrogen in a mixture of hydrogen and air, which would give a similar density to that of the mixture of gases tested. Thus a purity of 90 per cent, means a density corresponding to a mixture of 90 per cent, pure hydrogen and 10 per cent, pure air at the same pressure and temperature. This figure has to be applied in calculations of the lift of hydrogen, and the true lift is obtained by multiplying the hft of pure hydrogen by its per cent, purity" (b) Effect of Barometer on the Lift of Hydrogen As an airship has a fixed maximum volume, it is usual to deal with the lift of hydrogen for a fixed volume; 1,000 cubic feet is usually taken as the unit of volume. With alterations of barometer, the density of the hydrogen and the density of the air displaced, and therefore the lift of the hydrogen, varies. For the purposes of calculation, Boyle's Law is sufficiently accurate. Thus PV =K, or the density is proportional to the pressure, that is, the lift of a fixed volume of hydrogen is directly proportional to the pressure of the barometer. It is assumed that the same pressure exists inside and out- side a gas bag. No correction has, therefore, to be made in the case of temperature. (c) The Effect of Temperature on Lift of Hydrogen The density of air and hydrogen vary with temperature, and assuming a constant pressure the density varies according to Charles' Law, or the density varies inversely as the absolute temperature, therefore the lift of hydrogen varies inversely as the absolute temperature. Taking temperature, purity and barometric pressure, the lift of hydrogen is given by BL = ^. x P X K where L = Lift per unit volume of the gas. B = Barometric pressure. T = Absolute temperature. P = Percentage purity of the gas. K — Constant, depending upon the units used. % ' Superheating In the foregoing consideration of the lift of hydrogen it is assumed that the hydrogen and air are under the same pressure and temperature. In practice this is reasonably correct with regard to pressure, but with regard to temperature very big differences can be experienced. Thus when an airship is flying through a warm sun, the temperature of the gas is raised above that Of the surrounding air. This condition is termed superheating, and in British practice is measured by the number of degrees Fahrenheit between the temperature of the air and the temperature of the gas. Thus a io° superheat means the gas is heated 10° F. above^the surrounding air. The Effect of Superheating If an airship contains a known quantity of hydrogen, that is, a known weight of hydrogen, and the temperature and pressure of the hydrogen are the same as that of the air displaced, then the lift of this fixed weight of hydrogen will remain constant whatever the pressure and temperature. Thus in a rigid airship where the gas bags are not full, the ship can rise or fall, and as long as no gas is lost and no super- heating takes place the hft will remain constant. In practice a rigid airship is seldom full of hydrogen, and I will deal with the effect of superheating on an airship under this condition. The volume of the gas in the airship depends upon the barometer and the temperature of the gas, and is given by V = J X K where Tx is the gas temperature. Now this is the volume of air displaced, so that the lift of this gas is given by LKF where T2= air temperature, or L = --' 1 x K. Paper read before the Royal Aeronautical Society. Thus the lift of the gas is increased on superheating and is calculated by taking the lift without superheating and multi- plying by the absolute temperature of the gas the absolute temperature of the air Or the increase in lift due to superheating = lift without superheating multiplied by the degree of superheating (i.e., difference between gas and air) absolute air temperature of air. This quantity is known as the false lift. There, is an error in the above method owing to the fact that the weight of 1,000 cubic feet of hydrogen varies with the temperature, and it is therefore not correct to assume that the lift is directly proportional to weight Of air displaced. In the above the temperature of the gas is only taken into con- sideration to obtain the new volume of the gas 1254
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