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Aviation History
1957
1957 - 1726.PDF
816 FLIGHT DOUBLE MAMBA ASMD.8 . . . lent figure for a piston-engined aircraft.After the issue of the O.R. two tenders were accepted; the Gannet from Fairey and the YB-5 from Blackburn. It has now,of course, passed into history that the Gannet was the aircraft chosen for the Royal Navy and the development of the DoubleMamba engine has since been closely associated with this airframe. The Mamba 3 was chosen as the basis of the first Double Mamba—the ASMD.l—and the independence of the two power sec- tions was made as complete as possible. The accessory gearbox,for example, can be driven through gears and freewheels from either engine. The starting systems too, are independent, althoughit is normal procedure to perform a cartridge start on one engine, to accelerate it, and start the other engine by unfeathering thepropeller—either engine can be started from the other. Discussions between the M.o.S. and firms concerned and pre-liminary design work took place in early 1947, and in September 1948 the first Double Mamba development engine completed atwo-hour acceptance test. Remarkably few changes had to be made to the ASM.3 power sections. The reduction ratio in thecompound epicyclic gearing, which was chosen to suit faster aircraft than the Gannet, was modified and the ratio betweencompressor speed and propeller shaft speed, affected by both reduc- tion and transfer gears, was changed from 0.0846 : 1 (Mamba 3) to0.0961 : 1 (Double Mamba 1). Another obvious change was to the method of mounting; the single engine was supported by a sixpoint pick-up on the centre section, and in the two power sections of the double engine this casing was redesigned and a three pointmounting was built into the gear casing from which the power sections are cantilevered. There are two airframe-mounting balljoints at the lower corners of the engine and a further one centrally above. For installation purposes it was obviously necessary toredesign the exhaust cones and the operational need to be able to use fuel of various densities (aviation kerosine, wide-cut gaso-line and diesel oil) resulted in the replacement of the swash- plate type of fuel pump by one of Dowty design, in which thegovernor would not have to be re-adjusted for each change of fuel used. Destined as the Double Mamba was for the peculiar needs ofcarrier operations, some changes were also made to the engine- speed control system. Instant increases in power are needed inthe event of wave-off, and similarly, it must be possible to decrease power very rapidly at the moment of touch-down. Rapidity ofthrust response to throttle movement in turbine engines of this period was generally rather poor and in any variable speed enginethe problems of jet pipe temperature limits and compressor surge have to be overcome. This led to the adoption of a constantspeed engine with all power changes being made by attention to the fuel flow and propeller pitch. The small loss in cruise efficiency Intake, combustion and cooling airflow paths are shown on this sectional diagram of the ASMD.8. Cooling air is indicated with dotted arrows. A diagonal section has been taken to show the true centre lines of the co-axial propeller shafts and port power section. This enlarged section through the reduction gearing of the ASMD.3 should be com- pared with the drawing above. Note the simplified gearing of the later engine. EXHAUST CONE STAGE COMPRESSOR STARTER _ DRIVE Armstrong Siddeley Double Mamba ASMD.8 turboprop: Overall width, 56.5 in; overall height, 51.1 in; overall length (from pro- peller cone abutment face to end of exhaust cone), 98.6 in; net dry weight (less oil tank), 2450 Ib; shaft horse power at 15,000 r.p.m. (constant speed), 3,600; jet thrust, 710 Ib; fuel consumption 320 gal/hr.
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