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Aviation History
1936
1936 - 1408.PDF
May 28, 1936 Supplement to ^$032 (gK&Fff FLIGHT ENGINEERING SECTION No. 124 (VoI5™egXI1) Hth Year May 28, 1936 AIRCRAFT PERFORMANCE IN NON-STANDARD ATMOSPHERE The Effect of Varying Atmospheric Conditions May Usually be Disregarded in Performance Estimates, but in some Special Cases it May become Very Important By Lt.-Col. J. D. BLYTH, O.B.E., A.F.R.Ae.S., M. I.Ae.E. IN the course of the design of any form of power-driven craft it is usual, at some period during the proceedings, to make an estimate of the performance of the craft in question. In the case of a ship the calculations are simplified by the fact that, for all practical purposes, the density of water is constant; but in the case of an aero plane considerable complications are added by the fact the craft depends for its support upon a medium whose characteristics not only vary with the altitude but also can, and do, alter continually at any given height. It is plainly impracticable to make performance esti mates to cover all possible atmospheric conditions at every height, since such a course would involve an infinite number of calculations ; and to obviate the confusion which would arise if every individual or group of individuals were to refer their calculations and measurements to an atmosphere defined by a set of constants selected by themselves, an international standard atmosphere has been adopted. This standard atmosphere has been defined completely by assigning the values 760 mm., 150 C, and 0.002378 slugs /ft.3 to the pressure, temperature, and density respec tively at zero altitude, and by the assumptions that air is a perfect gas, obeying Boyle's and Charles' laws, that the value of gravity is not affected by altitude, and that the temperature gradient is — 1.980 C. per 1,000 feet up to the lower limit of the Isothermal atmosphere. It is to the conditions obtaining in the standard atmosphere so defined that performance estimates are referred and flight test data reduced for purposes of comparison. At any point in so rigidly defined an atmosphere it is necessary to know only one of the constants H (height), P (pressure), T (temperature), or P (density) for the deter mination of the value of either or all of the three remaining so that the value of H may be determined from the value of either P, T, or p. If to each value of H so determined a name is given indicating the means of determination it will be seen that in standard atmosphere the " tape-line " height, the " pressure " height, the " temperature " height and the " density " height are all equal. Use is made of this fixed relationship in the calibration of the standard aneroid, an instrument sensitive to changes in atmospheric pressure but calibrated to read altitudes. The calibrations are so arranged that when an altitude of, say, 10,000 feet is recorded.the actual meaning is " The pressure is now equal to the pressure at 10,000 feet in standard atmos phere." It does not mean that the tape-line height and the density height are, of necessity, also 10,000 feet, since they may in point of fact be widely divergent. Imagine the case of an aeroplane flying at a height of 10,000 feet in standard atmosphere, so that the aneroid reads the true tape-line height. Approaching a storm centre the barometer falls one inch, while the temperature rises io° C. The atmosphere is no longer standard, and if by some means the pilot maintains his tape-line height of 10,000 feet, the aneroid reading will show a pressure height of over 11,000 feet, while the density height, in round figures, will be 12,500 feet. If, on the other hand, the pilot maintains his aneroid height of 10,000 feet, the density height will be about 11,500 feet, and the tape-line height rather less than 9,000 feet. It will be seen, therefore, that the actual air performance of a machine at a given height may be actually inferior to its estimated performance at a greater height in standard atmosphere ; not on account of any inaccuracy in making the estimate, but owing to the effect of varying atmos pheric conditions. In the majority of cases the effect may
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