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Aviation History
1931
1931 - 0776.PDF
52 SUPPLEMENT TO FLIGHT THE AIRCRAFT ENGINEER JULY 24, 193! STANDARD ATMOSPHERE. Standard pressure p0 to be the pressure exerted by a column of mercury 760 mm. (29.921 in.) in height. Standard temperature t0 = +15 deg. C. = 59 deg. F. Standard absolute temperature To = 288 deg. C. = 51S.4 deg. F. Standard specific weight gP = 1.2255 kg./m.3 = 0.07651 lb./ft.s Standard gravity g = 9.80665 m./sec.2 = 32.174 ft./sec* Standard density P = 0.12497 = 0.002378. Standard temperature gradient a = 0.0065 deg. C./m. = 0.003566 deg. F./ft. The Standard conversion factors are: — 1 metre = 39.37 in. = 3,280833 ft. 1 kilogram = 2.204622 lb. Specific weight of mercury at 0 deg. C. - 13595.1 kg./m.s = 848.7149 lb./ft.3 The above values are taken from the National Advisory Committee for Aeronautics Report No. 218. It is necessary, then, to correct the observed perform- ance figures of an aeroplane to reduce them to what they would have been had the condition of the atmos- phere been " standard " all the way up to ceiling. To enable this to be done, it is obviously necessary to carry a thermometer on the aeroplane in some posi- tion where it will record the general temperature of the surrounding air (not in the cockpit), and to carry an altimeter with a dial locked to show zero feet or metres altitude when the temperature and pressure are standard. Initially, this altimeter must have been put into an exhausting box at a temperature of + 15 deg. C, in which the pressure of the air is reduced to 760 mm. by pump, and then locked. It is important to note that performance cannot be reduced to standard atmosphere unless an altimeter such as this, calibrated in an exhausting box and locked, is carried. An altimeter is only an aneroid barometer, or pressure indicator, but is marked in thousands of feet for the convenience of the pilot. For scientific purposes the indications of the ordinary aircraft altimeter with movable dial mean nothing, as is obvious, for, when the pilot sets the dial to read zero height on starting, the aerodrome itself may be several hundreds of feet above sea level. By calibrating the altimeter as described and locking it, it is transformed into a pure pressure indicator, the zero of which means that the pressure is equal to 760 mm. of mercury. The air speed indicator, like the altimeter, measures the pressure of the air, but is graduated in miles or kilometres per hour for convenience. There is a definite relation between the pressure and the speed of flight, and the instrument is calibrated so that it will indicate the correct velocitj- at ground level, that is, at " standard " ground level. At any other values of temperature and pressure the velocity indi- cated by thfr instrument is not true, and must be corrected. In other words, when the aeroplane is high up and the air is of much lower density, the pressure in the air speed indicator piping is insufficient to displace the diaphragm which actuates the needle to the extent necessary to indicate the true velocity. The pressure in the pitot tube is proportional to the density of the air and to the square of the speed of the pitot tube through the air, so that it is necessary to divide the indicated air speed by A / ±- where — is the V Po P« relative density, in order to obtain the correct speed. The relative density is obtained by dividing density at height by density at zero standard feet; these figures may be taken from tables in the N.A.C.A. Report No. 218, quoted above. The relative density at 20,000 standard feet height is 0-5327, so that an indicated air speed of 130 miles per hour at this height would correctto 178 miles per hour. At 30,000 standard feet height the density falls to0-3740, so here, if the true speed were 150 miles per hour, the air speed indicator would show only 91-8 niilesper hour. In addition to this correction for air speed indicator reading, a further correction should be applied which may be important. The air speed indicator will almost certainly be subject to a " position error," due to the proximity of the pitot head to the structure of the aeroplane. As a rule, the instrument in the cockpit gives a lower speed than it should, because of the reduc- tion of velocity at the pitot head due to the strut to which the latter is attached, and to the effect of the wings on the air near the pitot head. There is no method at present whereby the position error may be calculated, and to find the magnitude of the correction experimentally, it is necessary to carry a second pitot and indicator, the pitot being either suspended below the aeroplane or mounted on a long horizontal mast projecting a considerable distance ahead of the wings, so that it travels in air undisturbed by the aeroplane. Usually, the indicated air speed as stated in the pilot's report is stated against altimeter height. It is neces- sary, therefore, to correct the height to standard atmos- phere at the same time as correcting the speed for density, and to draw a speed curve so that the speeds may be finally quoted against regular intervals of standard height, such as 15,000, 20,000, 25,000 ft., etc. When a machine is being put through performance trials, it is usual to carry a recording barograph and a recording air speed indicator. These instruments pro- vide graphic evidence of the performance in addition to the records taken by the pilot. From the nature of the records made by these instru- ments, the data available are not generally of a higher degree of accuracy than that provided by the pilot's observations, assuming, of course, that the pilot's figures for altitude have been taken from a calibrated and locked altimeter and not from an ordinary alti- meter, and provided that the air speed indicator—if not a specially calibrated instrument for test flying—has been calibrated in the usual manner by means of a column of liquid in a U-tube. A greater degree of accuracy may be obtainable from the recording instruments when facilities are available for calibrating them with proper laboratory apparatus. As a rule, the height or speed and time scales which are printed on the sheets of paper used to wrap round the revolving cylinders of these instruments are fairly accurate in themselves, but are difficult to sub-divide, and the mark made by the recording pen is too broad for a delicate analysis to be made. Further, unless the apparatus is in the charge of a responsible person, there is no certainty that the base line of the printed sheet is truly horizontal with respect to the spindle of the cylinder. For these reasons it is generally advisable to consider the recording instruments as independent witnesses only that the required height has in fact been achieved, and that the required speeds have been taken whpn the machine has been flyiig level (i.e., not climbing or diving) for a reasonable length of time, and are not instantaneous readings of the air speed indicator. With this evidence the pilot's recorded observations may be used with confidence for correction of perform- ance to standard atmosphere conditions. CORRECTION OF CLIMB. The data required are as follow: — (a) Height from locked altimeter, feet; (b) Time to altimeter height: (c) Air temperature at altimeter height.
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