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Aviation History
1955
1955 - 0272.PDF
FLIGHT DOUBLE MAMBA PROGRESS Development History of the ASM D.I: Full Production at Ansty THE problems of control of turboprop aircraft are manyand complicated. Particularly so, one might suggest,in the case of Naval aircraft, which need absolute pre- cision of control at all stages of carrier approaches and land- ings. And particularly so, where engine-stalling is possible, in the case of axial-flow engines. A recent visit to the Ansty and Parkside, Coventry, factories of Armstrong Siddeley Motors, Ltd., enabled us to see for our- selves the tempo of Double Mamba super-priority production at present. Behind this production lies a background of solid develop- ment progress, more especially aimed at the intricate difficulties of engine control and handling. The development history of the Double Mamba to date shows no radical changes in design. Problems similar to those experi- enced on all engines have been encountered, and have been overcome. It was in September 1948 that development running of the Double Mamba began. One year later, the unit made its first flights, powering the Fairey GR.17. As running time increased and periodic-check intervals were extended, development snags, as in any aircraft or engine, became more evident. The first main problem encountered was that of "scuffing" and wear on the reduction gears, more especially on the satellite- gear units. To overcome this, phosphating or "Parkerizing" of all gears was introduced; in addition to curing the scuffing and wear, this also increased the load capacity. During 1950, a variation in r.p.m. due to oil-transfer leakage, mainly on the rear airscrew shaft, was countered by a tighten- ing-up of clearances. Improvements in the airscrew itself, made PRIMARY VALVE SECONDARY VALVE THROTTLE THROTTLE LINKAGE DELAY INPUT VANE UNDER-SELECTION STOP TO P.C.U. OVER-SELECTION STOP by the Rotol company, also helped to overcome the r.p.m. wander. An engine synchronizer for speed trimming up to ±180 r.p.m. was introduced at the beginning of 1952, and another modification made at about that time involved a re-staggering of compressor blades. This latter change, made before the engine went into production, was intended primarily to improve its power. Over- heating of the combustion chambers subsequently occurred, how- ever (presumably due to the resulting change in mass flow) and improvements were made to the design of the chambers. An impression of the changes made may be obtained from the sketch on the opposite page. The original primary air tubes, of curved "walking stick" type, proved liable to burning (as fuel adhered to the outside of the bends, the inside became hot), and so a change was made to a long, square-corner type. Vibration troubles led to an alteration in welding technique, and a shorten- ing of the vaporizer tube. Overheating was noticeable primarily in the outer casing of the combustion chamber (mainly opposite the four large mixing holes), where hot-spots were occurring. "Film cooling," by which a film of cool air was passed along the inner surface of the flame- tubes, was therefore adopted, in the form of three cooling bands. This deflector-type of film cooling was used in preference to a previous type, incorporating a "stepped-cylinder" flame-tube with corrugated spacing-strip, for reasons of simplicity, increased air space and ease of manufacture. Other modifications included the use of a precision "low flow- number" jet (particularly advantageous at low temperatures), the replacing of the locating dimples between the end of the flame- tube and outer chamber by a continuous corrugated locating strip (to overcome fretting), and the use of a slightly "ovalized" tube. The flame-tube material now used is Immaculate 5, an austenitic stainless steel with 25 per cent nickel which has proved to be easily workable. In May 1952 the Gannet's first carrier deck-landing trials took place, and in June the Double Mamba completed its 150-hr type test. Further improvements were made during 1952-53 to the reduc- The Mk 2 anticipator (engine control unit) not only temporarily selects a lower r.p.m., in the case of throttle advancement, in order to give a simultaneous coarsening of pitch and hence increase of thrust, but also restricts the rate of fuel-supply increase during a slam acceleration. Advancement of the throttle rotates the primary valve and feeds oil to the delay vane, which then operates the secondary valve and the flow-control lever. The same movement causes hydraulic pressure to move the speed vone and to move the piston to the under-selection stop; the piston, connected to the p.c.u. (or c.s.u.) then gradually returns to the equilibrium position.
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