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Aviation History
1948
1948 - 1856.PDF
55° FLIGHT NOVEMBER 4TH, 1948 Pointing the Limit A New Smiths Instrument to Show Actual and Maximum Safe Air Speed THAT flight at speeds approaching that of sound caninvolve unpleasant and, in some instances, activelydangerous reactions* on the aircraft is now very well known, and to give the pilot some indication oi how close to thecritical speed he may be flying, machmetexs have been developed. These instruments do not indicate air speed as such, they merelyindicate the Mach Number, that is, the percentage of the speed of sound at which the aircraft is flying, at the obtaining height.For example, if the aircraft was flying at 540 m.p.h. at 16,oooft., it would be flying at 75 per cent, of the speed of sound at thataltitude, and the pointer of the machmeter would indicate a value of 0.75. The air speed indicator cannot well be eliminatedfrom aircraft instrumentation, but the installation of a mach- meter does encroach on the space available in the instrumentpanel. Furthermore, the pilot has to memorize the limiting Mach Number of the aircraft. To obviate the extra instrument, and to contribute to easingthe pilot's job, Smiths Aircraft Instruments Ltd. have developed a maximum safe air speed indicator which, in addition to pro-viding the ordinary I.A.S. information of the conventional indicator, also tells the pilot the maximum speed for which theparticular type of aircraft is cleared. Although the instrument may' be obtained calibrated inm.p.h., knots or k.p.h., and in 1 and i£ sweep versions the method of presenting the information is essentially common toall; actual air speed is indicated by a conventional pointer, but the maximum safe air speed is indicated by a second pointershaped with a broad arrowhead. So long as the pilot does notallow his speed to mount to such a point that the I.A.S. needleoverlaps the S.A.S. needle, the aircraft will be kept within thesafety range. It is as simple as that. The cut-away drawing illus-trates the somewhat complicated viscera of the instrument, butto aid understanding of the basic movements, a conventional-ized diagram is also given. At the rear of the instrument is theberyllium-copper capsule which actuates the I.A.S. pointerthrough an orthodox multiply- ing mechanism. This capsule,incidentally, is brazed, and the manufacturing technique is suchthat the capsule is virtually without histeresis and is capa-ble of withstanding an overload equivalent to 800 m.p.h. without damage. Capsule movementequivalent to a speed range between 40 and 150 knots is un- restrained, but beyond the 150-knot point, movement is governedby a calibrating spring. This permits of an open-scale cali- bration up to 150 knots to facilitate accurate low speed reading. Before going on to describe the safe air speed movement ofthe instrument, we might with advantage first consider the basis upon which this element operates. It can be stated that,if the height at which the aircraft is flying is known, then the speed of sound at that height is also known. (The capsule mustbe calibrated to some standard scale—usually I.C.A.N.—and this scale will incorporate a temperature gradient relative to thepressure and density gradients.) Thus, by using an aneroid capsule to actuate the safe air speed pointer, the desired end canbe achieved. However, the use of an ordinary I.A.S. scale of indication does introduce difficulties in that it means that theaneroid capsule must be so designed that its expansion/contraction characteristics conform to the dial scale used. Although the speed of sound in air is dependent solely ontemperature, for purposes of instrumentation an indication of Mach Number can be given by the difference in dynamic pressure Here is shewn the single- sweep dial calibrated in knots, the I.A.S. and S.A.S. pointer forms, and the window for Mn setting. and static pressure expressed as a ratio of absolute static pressure. The formula for giving dynamic head is : •(i) where p1 P y p V = absolute dynamic pressure= absolute static pressure = ratio of specific heat = i.)= density = velocity for This formula is usually expanded in the form '• 1 + vPwhere a- — — and a = P With the upper part of the case cut away to ease portrayal, the complex intestines of the instrument can here be discerned. The viewpoint is approximately 90 deg. to that of the diagram on the facing page, with which the above drawing may conveniently be followed. The connections at the rear of the case are for top, static pressure and, bottom, dynamic pressure. However, (i) can also be rearranged and written (iii) p1=p\i +0.2-J letting—2= M- where M ='Mach Number This is expanded by anomaly thus : (iv) />j = p i -f 0.7 Ms -j- 0.175 M* + °-OI75 jW + • • • )which can be condensed as : (v) Pl ~P = (0.7 M2 + 0.175 M* + 0.0175 M* + ... ) It can be observed that in rearranging (ii) into (iii) p has beeneliminated, and as this is the factor in which the quality of temperature is vested, the required result can be derived frompressure ratios alone. What it amounts to is that for the purpose of indicating Mach Number, the ambient temperature need notbe considered, since the temperature factor is common to both pressures and thus cancels out, leaving Mach Number as thederivative of the pressure ratios. The limiting safe speed for any given aircraft type will havebeen determined during its development-testing, and thus, when '• 18
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