Rolls-Royce – Compressor Bleed

Rolls-Royce initially adopted still a third approach to solving the high pressure-ratio compressor problem. After experimenting with variable stator vanes, they elected to employ only variable inlet guide vanes, bleeding off flow from the middle stages of the compressor during off-design operation in order to limit the flow entering the rear stages. The principal engine produced with this approach, the Avon, went through several versions. The 16-stage compressor in one version produced an overall pressure-ratio of 8.5 to 1 (for an average pressure-ratio of 1.14 per stage), while a later version produced a pressure-ratio of 10 to 1 (for an average of 1.15 per stage).26 Commercial versions of the Avon powered the ill-fated Comet and the highly successful Caravelle.

In the early 1950s Rolls-Royce developed a new, larger engine, the Conway, that solved the compressor problem in a new way. The Conway, shown in Figure 7, was a two-spool engine, with 6 stages in its low-pressure compressor and 9 stages in its high-pressure compressor. Its overall pressure-ratio was 12 to 1 (for an average pressure-ratio of 1.18 per stage). Like the Avon, the Conway had flow bled off from the middle of the compressor in order not to overload the rear stages. In the Conway, however, the flow bled off from the tip of the low-pressure compressor became bypass flow, adding to the thrust of the engine, in essentially the same manner as in the De Havilland engine from the 1940s discussed earlier. The Conway thereby became the first bypass engine to enter flight service, operating at a bypass ratio of 0.6 – i. e. three-eighths of the total flow bypassed the gas generator. The bypass flow accomplished three things: (1) it provided cooling of the gas generator casing; (2) its lower exhaust velocity reduced exhaust noise, which was becoming an increasing concern in commercial aviation; and (3) it

improved overall propulsion efficiency, gaining more thrust per unit fuel. Rolls tended to emphasize the first two of these in their efforts to sell the Conway, for the bypass ratio was too small to produce a dramatic improvement in propulsion efficiency. Nevertheless, the improvement was there. Scaled-up versions of the Conway, producing more than 17,000 pounds of thrust, powered some of the advanced 707s27 and DC-8s, as well as the Vickers VC-10.

Pointing to the arbitrariness of restricting the designation “fan” to no more than 2 or 3 stages, Rolls-Royce has long argued that the Conway has claim to being at least the immediate progenitor of the turbofan engines that entered service in the early 1960s, if not the first turbofan.28 This underscores the futility of worrying about firsts here. The more important question is how the Conway fit into the evolutionary development of the turbofan. Over time, a sequence of incremental advances to the Conway in “normal design” might well have reduced the number of low-compressor stages pressurizing the bypass flow and increased the bypass ratio, resulting in an engine little different from P&W’s first turbofans. This “gradualist” evolution, however, is a history that might have been, not what happened. The turbofan engines that entered service in the early 1960s and established the turbofan’s dominant place in aviation did not evolve from the Conway, but instead emerged along a very different sort of pathway.