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Aviation History
1943
1943 - 0648.PDF
262 FLIGHT MARCH IITH. RELATION OF HEIGHT TO PRESSURE particular indicated height, that is to a fixed pressure reading at the aircraft, p will remain unaltered throughout. Suppose first that P, the pressure at sea level differs from 1013.2 nib. If for example it is 1003 mb., this corresponds to the standard pressure for 300ft. and the altimeter will read this height when at sea level. This can lie allowed for by making a zero correction to the altimeter itself, which can be done by providing the means to move either the dial or the hands till the instrument reads correctly. Next let the temperature at the aircraft be different from standard. The variables are now only t and h, all other quantities being considered constant. Therefore from equation (4) log (1 + Lhjl) is a constant, which implies that h/t is constant, or in logarithmic form • log h — log t — a constant This means that if the temperature at the aircraft differs from the standard at the indicated height, the difference of the logarithms of the true and indicated heights will "be the same as the difference of the logarithms of the actual and standard temperatures in degrees absolute. This relation clearly lends itself admirably to " slide rule " treatment, as is illustrated in the accompanying chart. Scale B shows heights in thousands of feet. The adjacent scale A shows the standard temperature for any height. Scale C is to be used as a scale of actual tem- peratures. The method of operation is very simple, and may be illustrated by an example If, at an indicated height of 20,000 feet, the actual temperature is —35 degrees Centi- grade proceed thus :— Against 20,000 feet on scale B read the standard tem- perature —24.6 degrees on scale A. Join 20,000 feet on scale B to the actual temperature — 35 degrees on scale C. From — 24.6 on scale C join to the intersection of the line already drawn with the reference line and produce to the height% scale, where the true height 19,100 is read. Lapse Rate Assumed Constant This method of correction assumes that the lapse rate has remained constant, or in other words that the ground temperature has varied from standard in conformity with the aircraft temperature. Since in any case the actual lapse rate is not likely to be known, and if it were, no very simple method of correction for an alteration in lapse rate is available, the above assumption is probably the safest one to make. It is, however, worth while to consider the effect of a change of lapse rate, as an example of the limitations to which the possibility of height correction is subject. If only h and L vary we must have 1 P / Lh\-j log i 1 -j- — 1 = a constant .. (5) For small changes in L a first approximation to this con- dition is dh — h'1 dLJ2t, which indicates that the correction required is roughly proportional to the square of the' -so -20 Bo-- K30 25 30 2O 10 o H-10 •20 W-30 IND. HT 20.000 FT.PRESSURE 465.4MB STD.TEMP-24.6°C. -35 -35 -Jl -29 -27 -25 -23 -21 -19 -17 -15 TEMPERATURE CENTIGRADE CORRECTIONS FOR TEMPERATURE : Scale A is standardtemperature in deg. C. Scale B height in thousands of feet. Scale C is temperature at aircraft, deg C. height, and is therefore of most consequence at high altitudes. The effect of change of lapse rate is illustrated by the graph in the lower right-hand corner of the accompanying chart. The graph, which is drawn for an indicated height of 20,000 feet, is calculated from the accurate relation (5) above. The relation between temperature at the aircraft and true height is to be read on the oblique line corre- sponding to the lapse rate actually existing. If the lapse rate is 1.8 degrees per thousand feet and the temperature at the aircraft —-*) degrees Centigrade, the true height is seen to be 19,500 feet ; while for the same temperature and a lapse rate of 2.4 the height will be 19,950 feet. When the humidity of the air varies, it will affect height readings in a twofold manner. In the first place, the value of W, the weight of unit volume of air, will be altered. If it can be assumed that this will affect the sea level pressure and the pressure^*at aircraft height proportionately, the change in W will be taken care of by the zero adjustment ot the altimeter for P. But variation of humidity is also one of the chief causes of change of lapse rate, which has already been discussed DIRECTORATE OF AIR FORCE WELFAREr HE importance of welfare work as a contributory factortowards the general efficiency of the R.A.F. has been so fully appreciated by the Air Council that, twelve months ago,a Directorate of Air Force Welfare was formed in order to develop and co-ordinate welfare throughout the R.A.F. Air Comdre. H. Peake was appointed Director, with GroupCapt. Finlay Crerar, C.B.E., as Deputy Director, the former being responsible to the Director General of Personal Services,.Ait Vice-Marshal D. Colyer, D.F.C. Despite the serious shortage of officers when the present warbegan, three were detailed specially for welfare work, and the present directorate was formed in'January, 1942, with resultsthat have demonstrated it to be a valuable contribution to the striking power of the R.A.F. The directorate is divided into six branches, three of whichdeal with organisation and three with entertainment. The first branch is in direct contact with all stations at home and over-seas, and is concerned with matters of policy. The second handles exclusively W.A.A.F. welfare. The third deals withthe problems of individual airmen. The fourth looks after "live entertainment," such as dramatics, concerts, gangshows, Ensa and so forth. The fifth branch deals with cinema entertainment, both static and mobile, and the sixth has chargeof broadcasts, in collaboration with the B.B.C., including the preparation of recorded programmes for re-broadcast over 29stations overseas. An important aspect of the directorate's general policy isthat of trying to get the best man for any particular job irre- spective of rank; it 'does not want all the organisations at•stations to be done by the officers. Some of the best concert parties, for example, are being run by A.C.c's. who have civi-lian cxiHTjciice in this kind of work.
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