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Aviation History
1955
1955 - 1747.PDF
870 FLIGHT BRISTOL OLYMPUS . . . BOI.1 Two-spool High-compression Turbojet. This "Flight" drawing reveals the principal con- structional features of the world's first aircraft gas turbine to hove two independent axial compressors in series. The whole layout is logical and efficient and the overall diameter was kept below that of many other types of turbojet—some of which were axials—of very much lower thrust. Most important of all, however, is the fact that this engine has since been developed to give power undreamed of at the time of its design. HIGH-PRESSURE COMPRESSOR LOW-PRESSURE COMPRESSOR .7 .'IATE <ORY Df Siddeley Python turboprop. As already stated, the Bristol de- signers had previously employed an axial compressor followed by a centrifugal stage at the high-pressure end to achieve approxi- mately the same result. Most of the early Bristol gas-turbine pro- jects had been of this type, the centrifugal unit being inserted partly for geometrical reasons in order to turn the airflow efficiently to meet the needs of the heat exchanger, but mainly owing to the flat characteristics of centrifugal compressors over a wide range of operation. This was particularly important at the high-pressure end of the compressor, where axial blading would be hard put to it to function efficiently over the required range of incidence. Retro- spectively, it is also now seen that design thinking of 10 years ago tended to over-estimate the probable efficiencies of centrifugal units and under-estimate what could be achieved with the axial. With the requirement of a pressure ratio of 9:1 forming a challenging target, the Bristol designers investigated the per- formance of a low-pressure axial compressor succeeded by a high- OLYMPUS DATA Olympus 1BOI.1 Olympus 1BOI.1 fl Olympus 101 ... Did. (in) 40 40 40 Loth (in) 121 124 125 DryWt. (Ib) 3,600 3,520 3,650 MaximumConditions Thr'«t (Ib) 9,140 9,750 11,000 h-pr.p.m. 8.000 8,500 l.f.C 0.83 0.766 0.79 CruiiingConditions Thr'tt (Ib) 7,330 8,000 h-pr.p.m. 7,640 s.f.c. 0.81 0.75 Note.—All the above figures are based on I.C.A.N. static conditions pressure centrifugal, the two units being mounted on separate shafts. The pressure ratio of the axial was chosen at 5:1, in 10 stages, this being as high as it then seemed reasonable to expect from a single unit without serious deterioration in flexibility; the pressure ratio remaining to be achieved by the centrifugal unit was thus 1.8 :1. During the latter part of 1946, the design of this first of all split compressors crystallized out; but as it did so a number of engineering difficulties arose. To start with, considerable difficulty was experienced in meet- ing the diameter limitation, most of the design studies giving an overall diameter of approximately 48in. In addition, in order to achieve the tip speed necessary for a pressure ratio of 1.8:1 within the restricted diameter, the centrifugal compressor bad to be designed to run at high r.p.m. This resulted in a turbine assembly in which the high-revving h-p turbine was doing con- siderably less work than the low-rewing 1-p unit. Even with a two-stage 1-p turbine, this resulted in diameter incompatibility. The problem, when attacked from various angles, eventually led to a different form of compressor. As a first step, it was logical to reduce the diameter of the centrifugal compressor, but this merely aggravated the turbine troubles. A step-up gear was another possibility, but the trans- mitted power of over 4,000 h.p. caused this idea to be rejected out of hand. A better solution seemed to be to put more work into the h-p compression, and this was achieved by adding three or four axial stages ahead of the centrifugal. Widi the same work split, this resulted in the diameter of the centrifugal unit being no greater than that of the first stage of the h-p axial. This change made it possible to expand the diameter of the h-p turbine and
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