Impact of a variable cycle engine on conceptual aircraft sizing

At a prescribed technology level, the optimum value of design bypass ratio for a fixed-cycle military turbofan is restricted by the supersonic flight requirements and engine size constraint. This fixed-cycle turbofan operates at a lower bypass ratio at subsonic flight points, which penalizes the fuel efficiency. A variable cycle engine that permits an in-flight increase in bypass ratio can aid to overcome this compromise by improving the subsonic regime engine-airframe matching. This paper makes use of a selective bleed turbofan as the representative variable cycle concept, and presents a few numerical case studies to quantify the effect of in-flight bypass ratio variation on the mission performance of an engine cycle at different levels of technology and engine size constraint. The net effect is always a saving in mission fuel consumption, and hence in overall aircraft take-off gross weight. However, the magnitude of savings depends upon a mutually coupled interaction between the nature of mission application, engine size constraint and technology level. If their combination is such that it is possible to configure the fixed-cycle turbofan itself for a high value of optimum design bypass ratio, the payoffs of using a variable cycle engine shall decrease. An attempt has also been made to address the issue as to which option has a greater potential; advancing the technology in a fixed-cycle engine or use of an in-flight bypass ratio variation capability. This study is purely conceptual in nature, and assumes mechanical feasibility.