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Aviation History
1947
1947 - 1002.PDF
580 FLIGHT JUNE 19TH, 1947 SONIC SPEED WARNINGS Thus with this new K.B.B. instrument all the pilot has to bother about is to see that the normal needle indicating his airspeed does not overlap or pass the red needle indicat- ing the safe limit. From an installation viewpoint the instrument is the same size and shape as, and is inter- changeable with, the standard airspeed indicator. The mechanism of the instrument is divided into two quite separate elements, each of which actuates its own indicator needle. Airspeed is registered by a conventional sensitive airspeed indicator mechanism, and the limiting safe speed is indicated by an adaptation of a sensitive altimeter mechanism. The former comprises a phosphor-bronze capsule sub- jected internally to dynamic pressure (J/oV2), the expansory movement being transferred through a multiplying mec- hanism to turn the indicating needle. A yoke carried by thu capsule terminates at its head with a block to which is anchored a bi-metal link; this in turn carries a further block from which projects a rocking lever. This latter is pivoted on the axis of a shaft which carries a toothed segment meshing with a pinion on the nan* staff. Refer- ence to the cut-away drawing and schema^nc diagram will make clear the operative sequence. Compensation for three variables is incorporated. The Schematic diagram of maximum safe A.S.I, operativesequence for each function. bi-metal link just mentioned compensates for the effect of temperature change on the elastic qualities of the capsule. A push-rod connecting the capsule with a beryllium-copper spring bearing against a series of screws is arranged to compensate for the natural inability of any normal capsule to give true conformity with |/>V2. The screws are pre-set so that the degree of spring tension affords the requisite degree of compensation to capsule movement at any given condition. The third compensation is embodied to allow the first 150 knots (or m.p.h.) range of pointer movement to be large relative to the remaining angular sweep: a tie rod from the rocking lever pierces a spring beam at Jts foot, the effect being that, after the 150-kt point, the tie rod pulls up against the spring beam, so introducing a resistance to pointer movement for a given degree of capsule effort and thus permitting a closer spacing of the measuring scale. The limiting safe-speed mechanism comprises a normal phosphor-bronze capsule evacuated to give pressure / height response. Carried on the capsule is a bi-metal strip—to give temperature effect compensation—on which a rocking lever bears, this being a cantilever from an anchor block on a layshaft. Projecting in the opposite direction from the layshaft block is a second rocking lever; this contacts a third rocking lever mounted off the shaft carrying the toothed sector by which the red pointer is driven. A shaft carrying a cam, against which the end of the lay- shaft effectively bears as a follower, terminates at a pinion meshing with the toothed periphery of the sub-dial on which the Mach number scale is printed. (This sub-dial lies behind the main dial and is visible through a window cut out in the left-hand side of the instrument face). The camshaft is pre-set from the rear of the instrument case, the appropriate Mach number setting being made when the instrument is installed. Rotation of the cam alters the linear, or axial, position.of the layshaft, and, by so doing, alters the effective arm of the segment rocking lever so that the red pointer takes up the requisite position on the dial, and, at the same time, is set to cover the appropriate Mach-number range. A screw-stop is embodied whereby the maximum limit of red pointer travel can be pre-st according to the desired maximum safe speed of the aircraft. The R.A.E. specification for this instrument called for accuracy tolerances as follows: — kt. or m.p.h. kt. or m.p.h. 80 — 200 + 2 200 — 500 + 3 500-700 ± 5 r On the altimeter mechanism the specified requirement was for ±3 m.p.h. limiting speed indication at any point throughout the red-pointer range. Both of these require- ments are more than satisfied. The second of the Kelvin instruments concerned with high airspeed is a Mach meter. Just as the normal air- speed indicator is concerned with indicating the fatio of the aircraft's speed relative to air at rest, so the Mach meter is concerned with indicating a ratio, only in this instance the ratio is that between the aircraft's speed and the speed of sound at any particular altitude at which the machine may be flying. The Mach meter is, of course, essentially a test instru- ment. For ordinary purposes a pilot is not interested in the fact that he is flying at a particular Mach number; such Schematic diagram of linkage motion in Machwhereby the operation may be followed. meter information is only of use if related to something else, as it would be in the case of test flying. Unlike the maximum safe-airspeed indicator already dealt with, in which an airspeed and altitude mechanism perform separate functions, the Mach meter airspeed in- dicating mechanism is subjected to the modifying influence of an altimeter mechanism. The airspeed capsule is internally subjected to dynamic pressure £/A72, the capsule carrying a push-rod whose head bears against a beryllium-copper spring set by a series of screws for similar compensation as already described for the maximum safe A.S.I. The push-rod carries a cantilever arm with which a rocking lever on a layshaft is kept in (Continued at foot of page 585)
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