# A Study on Combined Variable Geometries Regulation of Adaptive Cycle Engine during Throttling
## 4 The Demonstration of Variable Geometries Modulation Schedule
### The Demonstration at Subsonic Cruise  
The parameters variation tendency at double bypass mode during subsonic cruise is similar to the above-mentioned variation tendency during the supersonic cruise. Therefore, this section mainly analyzes the variation tendency of cycle parameters and performance parameters at triple bypass mode during subsonic throttling. During subsonic throttling at the triple bypass mode, HPRS first increases and then decreases as T\({}_{\text{H}}\) decreases, which is shown in Figure 24. At the beginning of throttling, T\({}_{\text{H}}\) declines slowly. Turning up A\({}_{8}\) leads to the increase of LPT expansion ratio. Therefore, the increased LPT expansion ratio and rapid upward total bypass ratio lead to the unobvious increase of LPT output work, which results in a break in low pressure rotor work balance. Further, an increasing trend of LPRS occurs because of the excess of LPT work. Due to the constant LPRS, less fuel should be injected, which leads to T\({}_{\text{H}}\) reducing slowly. Meanwhile, high pressure compression work declines, resulting from the lower VSV\({}_{\text{CDFS}}\). Therefore, HPRS rises because of the excess of HPT output work for lower high pressure compression work. Further, at the end of throttling, T\({}_{\text{H}}\) declines significantly. Low pressure rotor compression work reduces, resulting from a decline in VSV\({}_{\text{Fan}}\). Meanwhile, increased LPT expansion ratio and smooth upward total bypass ratio lead to the increase of LPT output work. As a result, the increased LPT output work will lead to a break in the low pressure rotor work balance because of the excess of LPT output work for less low pressure compression work. An increased trend of LPRS occurs owning to this break. Hence, for a constant LPRS, less fuel should be injected. Finally, T\({}_{\text{H}}\) declines significantly. Therefore, the high pressure rotor work balance is broken as the HPT output work resulting from rapid upward T\({}_{\text{H}}\) decrease, which leads to HPRS reduction. Figure 23: The performance comparison during supersonic cruise. Figure 24: Variation of HPRS and T\({}_{\text{H}}\) during subsonic throttling. As illustrated in Figure 25, total bypass ratio rises during throttling. At the beginning of throttling, the decline of CDFS flow capacity by turning down VSV\({}_{\text{CDFS}}\) will increase B\({}_{2}\). Then, for a constant VSV\({}_{\text{CDFS}}\), the decrease of CDFS suction capacity caused by downward HPRS can increase B\({}_{2}\) continuously. Meanwhile, B\({}_{1}\) first increased and then decreased as HPRS varied, as mentioned above. As VSV\({}_{\text{Fan}}\) changes a little, B\({}_{2}\) changes inconspicuously. Consequently, due to the combined effects of B\({}_{1}\), B\({}_{2}\) and B\({}_{3}\), the total bypass ratio rises. According to Figure 26, the main nozzle exit total pressure and total temperature reduce significantly. Therefore, reduced total pressure and total temperature and increased total bypass ratio can decrease specific thrust through the above-mentioned theory analysis. As shown in Figure 27, main nozzle thrust obviously decreases, while the third bypass nozzle thrust changes inconspicuously. Figure 26: Variation parameters of main nozzle exit during subsonic throttling. Figure 27: Nozzle thrust during subsonic throttling. Figure 25: Variation of bypass ratio during subsonic throttling. Based on the analysis of cycle parameters and performance parameters as stated earlier, the most remarkable variable-cycle-characteristic of ACE is to keep airflow constant during throttling, which is different from the conventional invariable turbofan. As total bypass ratio and total pressure ratio of ACE are higher than the conventional invariable turbofan, thermal efficiency and propulsion efficiency of ACE is higher. Consequently, it is necessary to compare the performance during throttling to further prove the variable-cycle-characteristic.