# A Study on Combined Variable Geometries Regulation of Adaptive Cycle Engine during Throttling
## 1 Introduction  
Nowadays, the design of the next generation affordable aircrafts includes all-weather, long-range and multi-mission objectives, among others [1; 2; 3; 4; 5]. These objectives lead to new requirements for aircraft engine design. On one hand, the newly designed engine should have turbojet features such as higher specific thrust in order to qualify for thrust-stringent objectives such as non-augmented supersonic cruising and transonic climbing. On the other hand, it should also have the turbofan features of lower specific fuel consumption (SFC) to compete in fuel cost objectives such as long-range reconnaissance. Clearly, to achieve these conflicting goals in an engine, the variable cycle engine (VCE) is undoubtedly an ideal propulsion device [6; 7; 8]. VCE promises to provide high propulsive efficiency with a high bypass turbofan while retaining the ability to produce high specific thrust with a low bypass turbofan. With respect to VCE technology developments and guidance for future developments, a detailed general structure and working mode transformation have been investigated [9; 10; 11; 12]. Then, performance simulation of VCE was performed to optimize parameters and to control variable geometries [13; 14]. One of the greatest challenges during the design phase relates to how to optimize the various cycles and then to match them to the capabilities of the components [15]. Meanwhile, based on the public references so far, VCE cankeep airflow constant during subsonic cruise throttling by modulating variable geometries, which can efficiently decrease spillage drag and increase installed thrust [7; 16; 17]. This is the most remarkable variable cycle characteristic of VCE. However, there are still some issues that need to be resolved during throttling. For example, with the exception of studies that focus on the subsonic cruise [7; 16; 17], study on variable geometries combined modulation is overlooked. However, it is required to avoid spillage drag during supersonic cruise throttling. In these works, during subsonic cruise throttling, total airflow could not be maintained at 100% and it declined appropriately during throttling in reference [7], which led to spillage drag and decrease of installed thrust. Furthermore, when the airflow could be maintained completely, since variable geometries regulation is optimized directly instead of through theory analysis, it resulted in a negligible improvement in performance [16]. Although low pressure rotation speed (LPRS) remains constant based on theory analysis, SFC increases along with thrust, reducing to 87%, which results in SFC degradation when compared with conventional turbofans [17]. As a new concept for VCE, the adaptive cycle engine (ACE) consists of a typical double bypass VCE surrounded by a third bypass duct and a front fan with unchanged inlet guide vanes [18; 19; 20]. At present, the analysis of adaptive cycle engine is still in its preliminary stage. With the support of the Adaptive Engine Technology Development (AETD) plan, the adaptive cycle engine has made great progress. The core engine of ACE has been tested by the General Electric Aircraft Engine Group (GEAE) [19]. [PERSON] analyzed the matching mechanism on an adaptive cycle engine [20]. In essence, the ACE is a triple bypass variable cycle engine as shown in Figure 1 in this paper. The fan system of ACE, which is divided into three sections, includes the front fan stage, a second fan stage with variable inlet guide vanes and a core driven fan stage (CDFS). Both the front fan and the second convertible fan are driven by low pressure turbine (LPT). Meanwhile, the CDFS and the high-pressure compressor (HPC) which both have variable inlet guide vanes, are driven by the high-pressure turbine (HPT).