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Aviation History
1935
1935 - 1577.PDF
JUNE 27, 1935. FLIGHT. SUPPLEMENT c four 112-lb. bombs and two 250-lb. projectiles. In certain tests, which could be called qualitative rather than quanti tative, the fitting of external items which might affect the handling qualities is essential. A navigation light, for example, would, perhaps, cause tail flutter, or the head and shoulders of an observer might set up eddies during a high-speed dive, with similar results. Ballast is used in spinning and diving tests, but an observer is carried during handling trials. The trials made by the firm's pilot preparatory to de livery for official test are, in the main, similar to those to which the aircraft is subjected on its arrival, so a descrip tion of the latter should suffice. In all probability the machine is flown to Martle- sham, assuming that it is a landplane, by the firm's test pilot, and it is likely that im mediately on arrival it will be weighed with the crew and equipment in position. The centre of gravity can be esti mated from the weights on wheels and tail skid, and this may be compared with the results of the official e.g. de termination, thus providing data of use to the pilot who puts the machine through its first handling tests. Subse quently, the '' drained '' or "light" weight (taken when the aircraft is stripped of all equipment, and the fuel and oil tanks are empty) is measured, the tare e.g. ascer tain e d , and the aircraft weighed with its tanks full. The capacity of these latter can, as a result, be checked. During all its tests the air craft is accompanied by a list giving the weights of '' fixed '' and "movable" equipment. In the former category are the parts of the machine which, although not integral, are essential to its safe operation ; these include, of course, the instruments. " Removable " items might actually be classed as military equipment— torpedoes or bombs with their racks and releases, guns and ammunition, wireless equipment, cameras, and electrical, pyrotechnic, and oxygen gear. "Bare," "tare," and "gross" weights are then deter mined ; the definitions of these terms, taken respectively, are: the weight of airframe and engine with no fixed or removable equipment, but with radiator water; the bare weight plus the weight of fixed equipment; and the weight in flying trim with full military equipment and test instru ments in place. Each member of the crew with his para chute, be it of the " observer's " or " seat pack " type, is assumed for these purposes to weigh 200 lb. Weighing being completed, the official determination of the e.g. is made. The airscrew is examined to ensure that it con forms to the design drawings and is finally weighed. It is customary during an early flight to determine the "position error" of the A.S.I., for, owing to interference from parts of the aircraft with the pitot head, a true read ing is not obtained until a correction factor has been applied. Accordingly the machine may be taken over the speed course, which, at Martlesham, is a stretch of the local railway line. If the machine is a multiseater an " air log," which, suspended below the machine, measures the true airspeed, may be employed. Among the most arduous but necessary of the trials of a military aircraft are "partial climbs," which determine to climb only to the Typical kit worn by test man's oxygen mask and aneroid strapped the best rate of climb at a given height and the corre sponding climbing speed. Calm weather is, of course, essential. The machine is climbed at various forward speeds, through 1,000ft. intervals at various heights up to its service ceiling (the height at which the rate of climb has dropped to 100ft. per min.) until the best one is obtained. Partial climbs'' are usually followed by trials to determine "full climb and level speed," which are com bined in one test. In the case of machines incapable of climbing above 20,000ft. it is necessary for the pilot service ceiling, and types capable of flying at greater altitudes than this are pro vided with oxygen for the crew. Should it not be desir able to take a machine above, say, 20,000ft., it is easily pos sible in these days to predict its performance above that height by working on the results obtained at lower alti tudes. At least two climbs are made (the climbing speeds pre viously determined are closely adhered to), usually by differ ent pilots. On the first descent, level speed readings are taken at even thousands of feet, and at odd thousands after the second climb has been made. If pos sible, two barographs are taken and the pilot equips himself with a statoscope (a specially sensitive form of aneroid for recording minute variations in pressure) and a stop-watch, in addition to the normal dash board instruments. To give some indication of the high powers of concentration re quired in a test pilot, the fol lowing readings are required to be taken at every thousand feet on a "full climb"— height, time to height, air temperature, indicated a i r boost, mixture setting, water temperatures, oil pressure and temperature, and radiator position. Level speeds are measured over a period of at least three minutes at constant height, the actual readings being taken at the end of that period. From the partial climbs, checked by full climbs and level speeds at various heights, the full performance chart is built up. It is essential, of course, that an aeroplane designed for use not only from aerodromes of vast dimensions but rather, if necessary, from fair-sized meadows, shall require short landing and take-off runs. The length of these runs is measured, if possible, on days when the wind is light, and, as the condition of the aerodrome surface and the human factor enter into the question, it is not always easy to obtain accurate results. Owing to the variety of methods employed by various pilots to take an aeroplane off the ground, and the take-off characteristics of individual aircraft, it is difficult to make a ruling, but one condition is always imposed—that the machine must cross the " screen "—an imaginary horizontal line at a certain height above the ground—at a speed which enables it to main tain climb ; this, of course, prevents the pilot zooming it to stalling point. Trials are made by two pilots, and the mean of at least six different measurements taken. Dis tances, times, air speed, and wind velocity are measured, while a camera shows the height cleared during the take off over the screen. The stalling speed—the lowest gliding speed which can pilots. Note the left-hand writing pad and the special to the other's knee. speed maintained, r.p.m.
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