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Aviation History
1951
1951 - 0597.PDF
30 March 1951 369 DEVELOPED MAMBA Modifications to a Sound Basic Design Increase Power by 25 per cent "CEW who read the first announcement of the Armstrong •*- Siddeley Mamba on September 8th, 1946, could have fully appreciated how advanced and enterprising was the conception of this compact turboprop. All, however, must have been impressed by the frontal area and weight, which were remarkably small for a power unit giving in the region of 1,000 s.h.p. It was the first of the straight-through units with axial-flow compressor, and considering that relatively little was then known of the design and construction of axial compressors for aircraft gas turbines, the Mamba was most successful from the very beginning. Stepping- stones in its development career were the first flight in a test-bed aircraft on October 14th, 1947; the 150-hour civil and military type-test on February 21st, 1948; and the maiden flight of the Balliol—which thus became the first single-engined aircraft to fly solely on the power of a turboprop—on May 17th, 1948. We cannot resist a digression here to quote some "popular" figures worked out in connection with the Mamba type-test. It seems that the turbine and compressor made 123,600,000 revolu- tions; each turbine blade travelled 100,000 miles at an avera'ge speed of 600 m.p.h.; and the engine swallowed 2,000 tons of air and 10,000 gallons of kerosene. The hours and conditions, expressed in terms of air miles in an appropriate aircraft, give an equivalent of twelve Atlantic crossings, or a return flight between London and Paris every day for three months. Other development highlights for the Mamba include a 500-hour sealed endurance run, under A.R.B. supervision, completed on August 25th, 1948, and the first flight of the Apollo airliner with four Mambas on April 10th, 1949. It was at about the same time that full details were released of the Double Mamba illustrated overleaf. This unit has kept up the record of its individual "halves," and in June of last year became the first turboprop, as sole power unit of the Fairey 17, to land on and fly off an aircraft carrier. The Double Mamba was described in Flight of March 31st, I949- So much for general history and background. Inside the Mamba engine proper, development has been going on while flight and power plant experience has been gained. A most comprehensive review of improvements, and of teething troubles and their solu- tions, was given by Mr. W. H. Lindsey in his November, 1948, R.Ae.S. lecture on the development of the Mamba. In this he described a method employed and since adopted as standard practice to increase compressor throughput without major modi- fications to the engine. The question arose when, as is usually the case with aircraft engines, a need was felt for more power for certain prospective applications. A considerable amount of compressor research by the Armstrong Siddeley company had led to the conclusion that the flow of air through the compressor could be increased very simply, without altering the stator casing, by removing the last two stages and replacing them, at the front end of the compressor, by two other stages with longer blades. After a few minor changes in angle, this was found to give very satisfactory results. It not only achieved the desired increase of 25 per cent in the mass flow, but also im- proved the compressor efficiency by 3 per cent (from 84 to 87 per On the basis of this simple alteration it was found possible to increase the power output of the Mamba by over 25 per cent with- out increasing either the overall diameter or the weight. Moreover, very little change had been called for in the major components. The original Mamba 2 incorporated this compressor modification, and it has been used for all subsequent aircraft applications, in-cluding the Double Mamba. The special Flight drawing published with this article illustrateswhat may be called the 1951 version of the Mamba turboprop—the ASM.3. A data table is also provided. In addition to the altera-tions to the compressor, several other minor modifications have been made since the power unit was first described in Flight ofMarch 18th, 1948. So far as the compressor is concerned, the first three and last two rows of rotor blades are now of steel. The first three rows- including, of course, the two with longer blades—were altered to overcome a tendency towards aerodynamically excited flutters which would have led to fatigue failure, and also to provide a blade more resistant to erosion and to damage by small foreign bodies entering the air intake. These blades are mounted in aluminium- alloy discs by means of fir-tree-root serrations of the same form as those employed for the turbine blades, and the resultant com- bination of steel blades and aluminium discs came out no heavier than the aluminium blades and steel discs which they replaced. The last two rows of compressor blades, referred to above, were changed to another material for a different reason; under extreme Simple rearrangement of Mamba compressor to increase throughput conditicns the blades on the outlet end of the compressor were working near to the temperature limitation for aluminium, and it was therefore, considered desirable to provide a bigger safety- margin. Steel blades were, therefore, substituted. The compressor stator casing has been cleaned-up by fitting the stator blade rings into grooves cut in the solid material of the casing itself instead of in grooves formed by bolting-in half rings as on early Mambas. To preserve a proper balance of performance it was necessary to modify the turbine to suit the new compressor, and the desired result was achieved by the simple expedient of increasing the length of the blades—the discs remaining unchanged—and altering the stationary parts to suit. In the course of normal development, modifications have also been made in the combustion chambers. One improvement has been to move the igniter plugs and spray jets from the side of the chamber into the nose. Flame propagation now takes place in a region of comparatively stagnant airflow, and it has been found Most important installation of the Mamba is in t/ie Apof/o. The small diameter and clean enclosure of the power units are seen here to advantage.
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