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Aviation History
1913
1913 - 0577.PDF
MAY 3i, 1913. I7DCBTI SCIENTIFIC INSTRUMENTS, THEIR DESIGN AND USE IN AERONAUTICS.* By HORACE DARWIN, M.A., F.R.S. BEING THE FIRST WILBUR WRIGHT MEMORIAL LECTURE. (Continued from p^e 574.) In the magnetic method of damping, the force varies as the velocity and the true mean is obtained. With liquid and air damping, the force varies as the square of the velocity, unless the movement is extremely slow, when it varies nearly as the velocity. Speed of Aeroplanes. The speed of the aeroplane through the air is usually gi ven by reading the position of some form of index on a scale graduated in miles per hour. A Pitot tube and a manometer are often used. The principle of the Pitot tube is very simple (Fig. 3). If the open end of a tube faces the wind, the air wants to pass down the tube, and if the tube is closed at the other end, the air pressure is increased in the tube and this increase of pressure is a remarkably accurate means of measuring the velocity of the wind. This method is used in Dines' Anemometer, and for measuring the velocity of the air in the wind channel at the National Physical Laboratory. In 1903 Dr. Stanton read a paper before the Institution ot Civil Engineers (Proc. Inst. C. E.,vol. clvi. p. 78) proving the accuracy of this method of measuring air velocity. Dr. Glazebrook tells me that improvements have recently been made which give even more satisfactory results. The delicate measurement of the air pressure necessary for the most refined work is made by the Tilting Water Gauge designed by Professor A. P. Chattock and Mr. J. D. Fry. This is a laboratory instrument of the highest order of precision, and is far too delicate and accurate to be used on a flying machine. It is a difference of pressure that has to be measured—the increase of pressure in the tube, above the air pressure outside—and a second tube transmits this pressure (the static pres-ure) to the manometer. It is found by experiment that changes in the sue of the opening of the Pitot tube, or the thickness of the tube, or the bevelling of its edge, make little or no difference in the pressure. With the static opening it is different, and its design is important. In the design now adopted at the National Physical Laboratory the pressure obtained is almost exactly what we should expect from theoretical considerations. This is an advantageous simplification, and this form of Pitot tube should be used for all the most refined measure ments. But the static tube can be so made that it will give a pressure blow the true sutic pressure, and the Royal Aircrmft Factory hare made use of this and have increased the manometer readings by 20 per cent, in order to give a more open scale. The tubes transmitting the pressure can be earned a considerable distance to allow the manometer to be placed in a convenient position for reading; this is often of great imjxirtance. If it U found advisable to have a large amount of damping in the manometer it is best to have long tubes of large diameter. This gives the correct form of damping. Short tubes of (mail diameter will give the same amount of damping, but in this case the damping force will vary, as the square of the velocity of the air in the tube, and the reading will not necessarily be the true mean. For the same reason it is inadvisable to cause damping by throttling the passage of the air by closing a valve, or by means of letting it pass through a small hole in a plate. If a Pilot tube speed-meter gives the correct speed when flying near the ground level, it will not be correct when flying at a great altitude. The error is caused by the change in the density of the air. As you mount, the air becomes less dense because the atmo spheric pressure is reduced, and more dense because the temperature falls. In the following table the actual speed is assumed in all cases to be ico miles per hour. The temperatures given in the last column are taken on the usual assumption that there is a fall of f' F. for every 300 ft ri-e above the earth's surface. Speed readings Speed readings tern-Height in ft. 0 1,000 2,000 3,000 4,000 5,oco temperature constant. 100 98-3 96-5 94-7 93'° 91- 3 perature failing with height. too 986 97'1 9S-7 943 929 Assumed temperature SO' V. 47" y. 43 F. 400 F. 37" F. 33° F. These conections were made in the air kpced measurements at the Military Aeroplane Trials in 1912 when obtaining the gliding angles of the various competing aeroplanes. The simplest form of manometer is a U-tube containing a liquid (see Fig. 3). The difference of the level of the liquid is then a measure of the difference of the air pressure in the two tubes. F*or use on an aeroplane this has two drawbacks : the scale is not open enough to read the speed easily and accurately and tilting of the aeroplane causes an error. Mr. Short of the Royal Aircraft Factory has designed a manometer which overcomes both these objections. It is in effect a U-tube manometer, and he uses two liquids of different densities and which do not mix, and thus obtains a more open scale (Fig. 4). One tube is placed inside the other, and this € SUCTION l»Illl«K | " IB V V »-- B- - 70— »- 55 •- 0- L w ~"*tC «« - - 70 M SO 40 -0 'HtllUtl y-mttsvac • SINCll LIQUID TV»C ' COMCtNTKIC CAUC AIK SPEED INDICATOR Fig. 4. 599
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