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Aviation History
1917
1917 - 1346.PDF
•weight equals the mass x acceleration due to gravity, and the mass is the volume x the density. The volume is A x Sh, hence - A5P = gBASh. The density equals AP/T where h is some constant. " • PHence — 5P = gk ~ SA or Integrating 5A= -4 °±..gk P h = — —— log, P + a constant. Let h be measured from sea level at which the pressure is P., then to determine the constant we have h « o When P -P.. -log,P) We can change to common logarithms by dividing by •Z. 3026, hence, h •= juT leg Po/P when p is a constant dependent on the units in which h is measured. In feet h «= 221.13 T log P,,/P>, and it will be seen that as the ratio of the pressures only is involved the unit in which they are expressed is immaterial. As an example, suppose there is a uniform temperature of 500 F. which is equivalent to 283° A, and that the barometer stands at 30 ins., to find the height at. which we have the barometer at 27 ins. we have— h =221.13 x 283 x log. 1.mil —2,863 ft. When integrating to obtain equation (1) it was assumed that T was constant, but in actual practice the temperature is arbitrary, and bears no fixed relationship to the pressure. For the actual conditions therefore (1) is not rigorously exact, but the error will be inappreciable, not more than 1 ft. in a thousand, if the mean temperature of the column be used and if h does not exceed 10,000 ft. For heights of the order of 30,000 ft. the error, even if the correct mean temperature be used, may reach nearly 1 per cent. Hence, for great heights the only way is to proceed by steps, deter- mine the pressure at, say, 10,000 ft., and then use that for the starting point for the next 10,000, and so on. In the " Computer's Handbook," published by the Meteoro- logical Office, tables are given showing the factor for all temperatures between 200 and 300 A, by which the pressure at any height must be multiplied in order to obtain the pressure at the point 1 kilom. higher. The effect of the humidity and the variation of gravity is also shown. In equation (1), if h is put equal to 1,000 ft., the ratio P/Po for any assigned value of T is readily attained. The values for a few temperatures are shown below, and by multiplying the pressure at any height by the given factor the pressure 1,000 ft. higher is obtained Temperature: 200 210 220 230 240 250 260 270 280 290 300 , Factor to give pressure 1,000 ft. higher : .95O5 -9529 -9551 -957i -9589 -9605 .9620 .9635 .9648 .9661 .9672 Capt. Tizard has given a table of average densities at each 1,000 ft. up to 20,000. From what has been said above-about the. variations of temperature and the manner in which the pressure at any precise height is dependent on the pressure at the ground and on the temperature between the ground and the height con- sidered, it will be seen that when no temperature observations away from the ground are available, to determine the precise value of the density is impossible. But we can advance a stage beyond, being content with the average value. We might prepare tables giving the average for each month, but this would not be satisfactory, since it would ignore trie conditions of pressure and temperature prevailing at the time, conditions which are important and easily measured. The lapse rate, apart from its daily variation at inland •stations in the first kilometre, is the least uncertain of the different variable elements which define the atmospheric conditions, and on this basis I have prepared a table of corrections in terms of the pressure and temperature at the DECEMBER 20, 1917. surface to be applied to Capt. Tizard's values of the average density. It is too long to go into details, but from some 200 observa- tions made in the southern part of England tables have been prepared showing the fall of temperature between the ground and various heights in terms of the height of the barometer. From these tables by the ordinary statistical method of calculation the most probable value of the pressure and temperature at any point up to 30,000 ft. has been found and from thence the density. The values are given in percentages, so that they may be applicable to any units in which the density is expressed. The mean sea-level pressure in the south-east of England is a little over 760 m.m., or a little under 30 ins. of mercury. The mean temperature is 500 F., io° C, or 2830 A. Thus, if we require the most probable density at 10,000 ft. When the barometer is 29.50 and the temperature at 320 F., proceed thus : 29.50 is .50 in. below the pressure mean, the correc- tion is therefore —.012 x .50 = —.006. The temperature is 10° below, and the correction is 4-.013. The whole correc- tion is +.007, and we must multiply the average density by 1.007. For heights oi and above 4,000 ft. this table is probably the best that can be done under the present state of our knowledge, and it should in most cases give the density within 1 per cent., but the following remarks are necessary. Over the open sea or at a coast station, with a fairly strong wind off the sea, the surface temperature at the start is to be used. At an inland station or a coast station With an off- shore wind, the mean temperature for the day is to be used in preference to the surface tenfperature at the time, because the latter is purely local and depends so largely on the time of day. TABLE II. Percentage additions to be made to the density at different heights to allow for variations from the mean of the surface pressure and the surface temperature. Height . Surface 2,000 ft 4,000 6,000 8,000 10,000 12,000 14,000 Pressur e differenc e in ins . a t surface . + -033 . +.026 + .020 + .016 + .013 + .012 + .011 + .011 emperatur e differenc e L degree s F . at surface . H .9—.0020 -.0018 —.0017 —.0016 - .0015 -.0013 —.0012 -.0011 Height . 16,000 ft. 18,000 20,000 22,000 24,000 26,000 28,000 30,000 Pressur e j 4 H 4-1 difierenc e in ins . a t surface . h.012 -.013 K015 -.017 - .020 -.025 K033 I-.044 emperatur e H —- —— —- 4- differenc e 1 degree s F . at surface . c . 0009 .0008 .0006 .0005 .0003 .0002 . 0000 .0002 The correlation coefficients between the lapse rate and the height of the barometer on which Table II is based range as a rule from . 30 to . 50 ; they are not large enough to make fehe esUmate very reliable, but they are sufficiently large to make it worth while to take the surface pressure into account. There are so many variable quantities involved that the question is very complicated, and it is perhaps of more im-' portance for long-range artillery fire than for aeronautics. The humidity.—The humidity is an important matter for . aviation, inasmuch as fog, clouds, rain and snow depend upon it. But the value of the humidity is only of use for the forecasting of fog, and this matter has been so fully dealt with in a lecture given here on February 28th, 1917, by Major Taylor, that I have nothing further to say about it. Instead, therefore, of wasting time on the subject I will refer to his remarks, published in Aeronautical Journal Vol. XXI, p. 75, and also to a paper by Capt. Cave, published in the same volume, p. 301, in which the dangers due to rain and snow are mentioned. :...'_' • . /. (To be continued.) , • "-.-v1., ^ ,.- Gallant Rescue of Seaplane Pilot. IT was announced in the London Gazette of December 14th, that the King has been pleased to award the Albert Medal in Gold to Nicholas Rath, Seaman, R.N.R., and the Albert Medal to Richard Knoulton, Ordinary Seaman, R.N., and George Faucett Pitts Abbott, Deckhand, R.N.R. (Trawler Section), in recognition of their gallantry in saving life in the following circumstances :— " On September 14th, 1917, a seaplane came into collision with one of the masts of a shore wireless station and remained wedged in it, the pilot (Acting Flight Commanedr E. A. de Ville) being rendered unconscious and thrown out of his seat on to one of the wings. The three men above mentioned, at once climbed up the mast for 100 ft., when Rath, making use of the boatswain's chair, which moves on the inside of the mast, was hoisted up by men at the foot of the mast to the place, over 300 feet from the ground, where the seaplane was fixed. . He then climbed out on the plane, and held the pilot until the arrival of Knoulton and Abbott, who passed the masthead gantline out to him. Having secured the pilot with the gantline Rath, with the assistance of Knoulton and Abbott, lifted him from the plane to the inside of the mast and lowered him to the ground. The three men were well aware of the damaged and insecure condition of the mast, which was bent to an angle Where the seaplane had become wedged. One of the three supports of the mast was fractured, and, so far as the men knew, the mast or seaplane might at any time have collapsed." 1346
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