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Aviation History
1914
1914 - 0521.PDF
THE MEASUREMENT OF AIR SPEED. By A. P. THURSTON, D.Sc., F.R.Met.S., Associate Fellow Ae.S Concluded from page 499.) THE Chft and Ogilvie air-speed indicators are described in detail because they have proved their worth and deserve to be better known. They are elastic diaphragm instruments belonging to diss (/). In struments of this type utilise the hydraulic or Brahmah Press principle for increasing the magnitude of a force. They consist of two parts, a pressure head and a diaphragm recording device. The Pitot tube is connected with one side of the diaphragm and the static tube with the other. The force due to the difference of pressure between the Pitot and the siatic tubes is multiplied by the ratio of the diameters of the diaphragm and the Pitot tube. A powerful force may therefore be obtained for operating the indicating device. Clift Air-Speed Indicator.— This instrument is shown in Figs. 3 and 4- The " head," which is shown in Fig. 3, consists of the readings do not appear to be affected by rapid charges of velocity. In conducting an exhaustive test of any instrument the ascending and descending readings should be plotted against the true readings. Any lag due to friction ot the parts, fatigue, or to hysteresis is thereby revealed. This has been done for ClitVs air-speed indicator, and the results are plotted in Fig. 5. The maximum difference between a-cending and descending readings is 3 per cent., or \\ per cent, on either side of the mean. As, however, the greater accuracy, in flying machine work, is obviously required on the descending scale, the instrument is graduated so that the maxi mum error on this scale does not exceed \ mile per hoar. tamaBzaBzaggL, « »• H H M • 1 ? ! i 1 W 01 Curt AIRS'CIONNMCATM NMI 1 J u Off it viioci >ccnr>M«i TX:- — 0* :• -- »•* jy Fig. 4.—Sectional elevation and plan of Clift's air-speed indicator. Fig. 5.—Chart showing test of the Gift air-speed indicator. Figs. 7 and 8.—Above is a dia gram of the indicator mechanism of the Ogilvie air-speed indicator, and below, a diagram of the Morris air-speed indicator. *• usual Pitot tube and a static tube provided with a conical cap to increase the suction in the lateral holes in the tube under the cap. A disc of the same diameter as the base of the cone is mounted on the tube so as to leave an annular opening communicating with the lateral holes. The indicating instrument contains a cylindrical chamber divided into two parts by a very light flexible diaphragm of specially prepared leather. This diaphragm, which weighs about JV oz., carries at US centre a small aluminium disc fitted with a central bo >s. This boss contacts with a collar on a light spindle which is guided on both sides of the diaphragm. The upper or dial end of this spindle presses against the free end of a triangle cantilever or leaf spring. The dynamic and static tubes of the " head " are connected by tubing to opposite sides of the diaphragm. Any difference of pressure between the two sides of the diaphragm is therefore transferred through the spindle to the cantilever spring, which is thereby deflected. The movement of the spring is transmitted by a rocking lever having an adjustable radius arm in contact with the spring to a multiplying mechanism, such as is used in aneroid barometers, whereby a hand or indicator is given a rotary movement over a dial which may be graduated to read either pressure or velocity. The multiplying mechanism consists of a rocking quadrant rack, which is provided with a slot engaged by a pin projecting from the rocking lever and meshes with a pinion on the spindle carrying the hand. A spiral or hair spring normally restores the hand to the zero position. The instruments are graduated individually in the works for various air pressures, which are read by a delicate tilting water gauge capable of reading to within o-oo2 ins. of water. The velocity is given by the equation v = 57 7 "» h. Where v = velocity in feet per sec. and h = corresponding difference of level in water gauge in inches. The width of the annular gap has been found to affect the con stant in the above equation, but as great care is taken in adjusting this width the variation of this canstant is negligible with the instruments placed on the market. These instruments have been tested between considerable ranges of temperature with satisfactory results. The inertia of the moving parts is so small that the The Ogilvie Air-Speed Indicator.—This instrument is shown in Figs. 6 and 7. Fig. 6 is a photograph of the instrument used by Messrs. McLean and Ogilvie in their successful flight up the Nile to Khartoum. The Ogilvie air-speed indicator is characterised by extreme simplicity. A cylindrical chamber is divided into two parts by a stretched indiarubber diaphragm. The indicating hand is carried by a stout spindle which is pivotally mounted in the centre of a light metal frame bridging across the centre of the Fig. 6.—Head and details of Ogllvie's air-speed indicator. upper chamber. A second stout spindle is pivoted in close proximity to the first spindle and with its axis at right angles to it. These spindles act as drums or rollers to a thin silk or other thread, one end of which is connected to the centre of the diaphragm. This thread (see Fig. 9) passes upwards between the spindles and over the second spindle to the lower end of the indicator spindle. It then makes two ascending anti-clockwise turns on the firtt spindle and its end is connected thereto. The head is of the usual combined 521 »
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