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Aviation History
1947
1947 - 0193.PDF
FEBRUARY 6TH, FLIGHT 137 Science is Measurement Sensitive Altimeters Ranged to 70,000/t ; True Air Speed Indicators for Cabins THE title of this article was afavourite dictum of Lord Kelvin,pre-eminent scientist in his day and one of the greatest scientific figures of all time. This great man founded the ' now famous firm of Kelvin, Bottomley and Baird, and on a recent visit to their f Basingstoke factory we were able to see •' that the postulates laid down by the founder are still a live force to-day. Of all forms of progression, flying necessitates perhaps a wider use of, and greater reliance on, instrumentation than any other. Not only is the modern air- craft equipped with a veritable host of instruments, but the organization of aviation—flying control and so on—is based to a large extent on the portrayal • of conditions by means of instruments. Kelvin, Bottomley and Baird's model factory at Basingstoke was opened in 1936 as the company's specific tiome of aircraft instrumentation, and during the war it achieved the notable output of wAl over half a gaillion aircraft instru- ments of all types, in addition to repair- ing and returning to service somewhat over 100,000 further instruments. When thinking of such instruments as altimeters, engine-speed indicators and air speed indicators, one is very much apt to take them for granted. They have, so to speak, been with us for so long and are thus not accorded the glamour which attaches to newer and, perhaps, more colourful devices. It is, however, instruc- tive to review their development. Origin- ally the sensitive altimeter had an upper limit of 20,000ft, and then this was in- creased to 35,000ft and, subsequently, to 50,000ft. They are now obtainable ranged up to 70,000ft, and, with this, are accurate to within a tolerance error of less than one per cent whilst operating at temperatures of — 40 to + 50 deg C. Engine-speed indicators have also had quite a considerable degree of develop- ment. At first the common range of E.S.I, calibration was up to 3,500 and 5,000 r.p m., but the advent of gas tur- The maximum safe air speed indicator showing red limit pointer, Mach num- ber setting scale and ordinary air-speed pointer. bines has necessitated instruments ranged up to 20,000 r.p.m. A further development was brought about by the wider use of multi-engined aircraft coupled with the need to keep intrument panels as compact as possible. Thus was born the dual E.S.I., which indicates on one dial the speed of two engines—and also shows when the engines are running synchronously. This basic instrument in a suitably modified form is also being used on helicopters to indicate engine and rotor speeds. Safe I.A.S. As with other instruments, air speed indicators have also had their operating range markedly increased; at first a maximum speed of 160 m.p.h. was the limit, but A.S.I.s are now available cali- brated up to 700 m.p.h. A new develop- ment to the standard A.S.I, embraces indication by a second, red, pointer of the limiting safe speed of the aircraft at whatever height the machine may be fly- ing. The maximum safe speed at which the particular aircraft can be flown is pre-set, in terms of Mach number, on the instrument, and so long as the normal air speed pointer is kept below the figure indicated by the limiting red pointer, the safe speed will not be ex- ceeded. This instrument should be a great boon to pilots of all high-speed aircraft; its development is most timely. Another variant of the ordinary A.S.I. has been produced as a cabin fitting for passenger aircraft. Since it is intended for laymen, true air speed is required, for it is patently useless to give indicated air speed and then tell passengers that such-and-such a correction is required as the aircraft is flying at, say, 15,000ft. The cabin indicator is, therefore, equipped with automatic correction for atmospheric density and also includes correction for temperature based on I.C.A.N. conditions. It is unlikely that the factual conditions encountered on any one day will accurately match the I.C.A.N. standard; however, as an over- all case, the cabin indicator should register a speed within about 3 to 5 per cent of the actual true air speed, and that is quite faithful enough for such a purpose. These instruments are, in- cidentally, fitted in the staterooms of the King's Flight Viking aircraft which are to be used on the Royal Tour. Machmeters are air speed indicators, in their peculiar way, but they are not of very much use to the ordinary pilot: they are essentially test instruments, and very important instruments, too. The K.B.B. machmeters developed for the D.1I. 108 (although, of course, ap- plicable to any other aircraft) have a range of from 0.3 to 1.0 Mil over an altitude range of o to 50,000ft and are accurate throughout the operational range to a standard of 0.01 Mn. During the war a device was evolved at Farnborough which effected a marked reduction in the number of bombers which were lost. Called the Air Mileage Unit it was given to Kelvin, Bottomley A maximum safe air speed indicator being calibrated against reference sub- standard;. and Baird to develop, and it is now used on civil airliners to record, as a check on fuel consumption, the number of miles flown, la brief, the principle of opera- tion is that a fan air pressure equivalent to the pitot pressure causes an output shaft to rotate at a speed proportional to the true air spe%d. Of all the devices shown to us at Basingstoke, perhaps the most intriguing and, certainly, one of the most import- ant in its likely effect on future aviation, was the radio-sonde. This has been developed in collaboration , with the British Meteorological Office and the Telecommunications Research Establish- ment in order to provide accurate infor- mation of upper air conditions. Slung from a parachute, in turn suspended from a balloon, the instrument is of chronometric type and contains meteor- ological elements for testing atmospheric pressure, temperature and humidity. Each element actuates an arm making contact with a rotating helix, and the time interval between each contact and a fixed reference contact gives a measure of the respective quantities tested. A signal is transmitted at each instant of contact by a radio transmitter attached to the chronometric unit, and is auto- matically recorded by a ground receiver and recorder. The recorder thus gives a continuous chart of the meteorological quantities throughout the flight of the sonde. Tracking by a radar ground station with an effective range of the order of 100 miles additionally supplies information as to velocities and direc- tions of upper-air winds. This latest form of radio-sonde is said to be superior to any of the types now in use, and preliminary trials have demonstrated that it will enable accurate forecasts to be made about 36 hours ahead. D
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