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Aviation History
1957
1957 - 1728.PDF
818 DOUBLE MAMBA ASMD.8 . . . which results from this arrangement was found to be relativelyinsignificant. (These problems were discussed at length in an article on Double Mamba Progress in Flight, March 4, 1955).In this new engine Rotax twin-breech starting was used on the Mamba for the first time and Lockheed propeller brakes, requiredin carrier operation to prevent auto-rotation, were introduced into the gearbox main diaphragm. Some of these problems werealso to be encountered with the Mamba 3s installed in the carrier- borne Sturgeon and the Breguet 960. Another refinement madeon the Double Mamba was the introduction of propeller synchro- nizers to prevent stroboscopic shadows. Direct application of the single Mamba power sections to thedouble engine has made it possible throughout the development of the ASMD series to do performance development on singleMamba units and to do mechanical work on the double engine. The design power of 1,270 s.h.p. was achieved on a Mamba 3engine in 1950, and the efficiency of the double unit was such that twice this power was achieved on the Double Mamba 1. Development on double and single engines then proceeded sideby side. Problems encountered were relatively few but, as with all engines in which progressively greater powers are achieved,various troubles were encountered and corrected as the design progressed. After the inlet guide vanes had been adjusted to givethe optimum performance and mass flow, compressor fatigue failures associated with aerodynamic flutter were encountered, andthis led to the adoption of steel blades for the first four and last two stages. As experience was accumulated, development progressedwith the method of welding the outer flame tubes; the shape and spacing of the combustion chamber dilution ports was altered toobtain better turbine inlet temperature distribution and the com- bustion chamber vaporizing tubes—the "walking sticks"—werere-designed. This was part of general Armstrong Siddeley develop- ment. The radiused walking stick handles were changed to a typewith the two right-angle bends, and lateral turbulence pins were introduced into the parallel section of the tubes. Other changesin the materials and design of die combustion chamber were made to accommodate the higher sulphur content permittedin the specification of JP4 wide-cut fuel. While development was continuing on the power sections ofthe engine, work was also being done to develop the reduction and transfer gear trains. Breakdowns had occurred on the high-speed portions of the epicyclic gearing, to the teeth of the sun and satellite wheels. This was found to be due to minute imperfec-tions in the geometry of the teeth as they came into mesh, resulting in impact instead of rolling loading. A change in the tooth profileimproved matters, but problems affecting the satellite gear bear- ings were not entirely solved when the double engine camealong. Due to high tooth parting loads and the configuration of the gear trains, the thrust and roller bearings are heavily loaded, anda good deal of development work was required to bring the gear- box to a high standard of reliability. The ASMD.l passed its official 150-hour type test in earlyJuly 1955, and went into production in March 1952. Some 19 150-hour tests were carried out at various modification stagesuntil production ceased in December 1955. Large numbers of Double Mamba Is are now in service as the ASMD.l Mark 100.Inevitably, as development of the Gannet progressed, the all-up weight of the aircraft increased and the engine manufacturerswere asked to provide additional shaft horse-power from the Double Mamba. The logical way of doing this was to take morework out of the gas stream by introducing a three-stage turbine; a step which provided the engine manufacturers with the oppor-tunity to reduce individual stage loadings. The three-stage turbine engine, using ASM.5 power sections, was designated ASMD.3.The fuel flow was no greater than that of the D.I but the shaft horse-power was increased and specific consumption was im-proved. No alteration was made to any part of the engine forward of the compressor, but with the additional turbine stage an annularcombustion chamber—offering various structural and thermo- dynarnic advantages—was introduced. This step gave the engineadditional stretch, since for a given envelope size an annular com- bustion chamber offers higher power and greater heat release andpreserves the operating parameter of C.H.U./cu ft/hr/atmosphere. Another change that was made to the engine at this stage wasthe introduction of a flood-feed oil-cooled lubrication system instead of a total loss, micro-pump system using air-blast cooling.As an oil cooler was in any case required in the reduction gear lubrication circuit, it was logical to increase the size of this unitslightly to cater for main bearing oil. The engine in this form went into production in January 1956,following the official type test in July 1955, at more than 2,740 s.h.p. and 820 lb thrust. The engine was certified at 2,800 s.h.p.and 900 1b thrust, after allowances had been made for intake and let pipe losses. There have been nine 150-hour development tests.Later in the life of the Gannet airframe, the installation of certain new equipment led to a request for the thrust line of the FLIGHT engine to be lowered in relation to the centre line of the powersections, so that the whole engine could be raised within the air- frame. Additional power was also sought. By this time theASM.6 Mamba was already available in single form and the only remaining step was to develop an entirely new reduction andtransfer gearbox and air-intake assembly. Although this was a major proposition, it was possible to make provision for handlingmore power than would be provided by two ASM.6s, and during re-design, the reduction gearing was stressed to handle 4,400s.h.p. (2x2,200 s-h.p.). Originally conceived as the ASMD.4, this engine had ASM.6s with eleven stage compressors as thepower sections; the additional blading being a zero stage giving a mass flow increase from 2 x 17.5 lb/sec to 2 x 21 lb/sec. While this D.4 version was being developed, work continuedon single engines—particularly with Nimonic 100 rotor blades- bringing the ASM series up to eight, and the power up to1,950 s.h.p. This power section was married to the strengthened reduction gearing and the double engine then designated ASMD.8.An alternative development that was studied by Armstrong Siddeley while work was beginning on the D.4 three years ago,was a project design for single and double Mamba 7s, rated at 2,430 s.h.p. and 4,200 s.h.p. These represented a fairly majordeparture from the single and double Mamba series up to that date. They were to have had 12-stage compressors and four-stageturbines, the two last (power) stages of which were free; although there was undoubtedly considerable development potential in abasic re-work of this nature, it was realised that the single and double Mamba 8s would achieve nearly the same result in termsof immediate performance increase for less development effort and for a lower weight, and the project engines designated P.160and P.156 remained shelved. In general, the ASMD.8 follows the lay-out of its predecessors,except for a lowering of the thrust line which reduces the offset between the propeller centre line and the power section from llinto six. As before, the power sections function independently. Both are self-contained, so that in the event of the failure of one powersection, it is possible to remove it from the reduction gear assembly and substitute a sound unit. As a direct result of the design of theengine and the intended method of operation, the Double Mamba possesses what is probably a unique method of measuring enginelife. This is, of course, normally expressed in hours run, but for Reduction and transfer gear train of the Double Mamba 8. the typical single-engined operation of the Double Mamba inNavai service, each power section is fitted with a life-recording revolution counter. The hours of operation can be deduced fromthe fact that the engine operates at a constant r.p.m., and as a working basis one million revolutions represents one hour's run-ning time. This enables the pilot to make equal use of the port and starboard power sections of the double unit. Mechanical Design. The common reduction gear casing is inthree sections; the front cover, an intermediate diaphragm, and an air duct unit into which are cast the intakes to the two powersections. The front cover, cast in D.T.D.748 light alloy, houses the co-axial propeller shafts and their bearings and the two low-pressure Rotax air starters, with the attendant gearing and engage- ment mechanism. The front cover and intermediate casing 'sextended upwards on the centre line of the engine to accommodate the two internal-excanding-shoe propeller brakes operating on thetransfer gearing. These are operated by ensine oil supplied from a spring-loaded hydraulic accumulator. Bolted to the rear faceof the intermediate diaphragm is the main Hi-duty alloy casting air duct unit carrying the three mounting feet. This main casingcontains the idler gears and aircraft accessory drive gearbox, which by a system of freewheels, permits either power section to supplythe full accessory load of 200 b.h.p. The gear train can be followed from the sketch on this page-
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