## 5 The SRVL Strategy  
Given the flying performance analyses, the SRVL strategy can be customized. This section designs the piecewise SRVL strategy, and an NDI-based control architecture to support the strategy implementation. Generally, the SRVL has three in-air stages: the descending and deceleration stage, the stability adjustment stage, and the touchdown preparation stage, as shown in Figure 15.  
Figure 14: Control logic of the STO strategy. 1. Stage I: Descending and deceleration  
In this stage, the aircraft enters the landing window from the last cruise way point. As the airspeed is still fairly high, the aerodynamic control surfaces have enough control efficiency for attitude control. Therefore, the aircraft is driven by attitudes for flight path following. For deceleration, the propulsion system is idle to cut off the energy import, and the aerodynamic drag dissipates the kinetic energy. Therefore, in this stage, the thrust vector nozzle can be adjusted for flight mode transition. The airspeed that divides the descending and deceleration stage and the stability-adjusting stage is named the stage-2 window speed, \(V_{2}\). If \(V_{2}\) it is set too high, the aircraft will have insufficient time for an attitude adjustment in the stability-adjusting stage, and in the descending and deceleration stage, the thrust vector nozzle has to deflect rapidly for mode transition, which brings about the rapid inertia change. These all increase the risk of controllability loss. However, an under-evaluated \(V_{2}\) may also cause stall and controllability loss. Thus, \(V_{2}\) requires a cautious selection that can be instructed by the aforementioned trajectory stability. In the preliminary design, the flight path angle was set as \(\gamma=-4^{\circ}\), and according to the level-1 flight quality specification (analyzed in Section 3.2), the last cruise waypoint speed, which is also the stage-1 window speed, \(V_{1}\), was set as \(V_{1}=24\) m/s and \(V_{2}=20\) m/s. 2. Stage II: Stability Adjustment  
In this stage, the aircraft is required to decelerate from \(V_{2}\) to touchdown speed, \(V_{3}\), which is also the stage-3 window speed. This work desires to verify the designed controller for its stability augmentation ability. Thus \(V_{3}\) was set as the stall speed, \(V_{T}=14\) m/s. To acquire enough deceleration efficiency, the pitch angle was assigned a default value in advance for the angle of attack control. The default value was set according to safety landing altitude specifications. Empirically, the stage-2 window altitude, \(h_{2}\), was set as 15 m, and the stage-3 window altitude, \(h_{3}\), was set as 5 m, which means that the aircraft was required to decelerate to touchdown speed with the altitude decreasing by less than 10 m. Given the flight path angle \(\gamma=-4^{\circ}\), the forward flight range was easily obtained as 143 m. The forward flight speed changed from \(V_{2}\)= 20 m/s to \(V_{3}\)= 14 m/s, which needs a deceleration of \(\dot{V}_{xg}\leq-0.713\) m/s\({}^{2}\). To let the aircraft be bound to adjust the expected attitude in this stage, a safety margin of 50% was reserved. Thus, the deceleration was required to be \(\dot{V}_{xg}\leq-1.0659\) m/s\({}^{2}\). According to the performance analysis in Figure 7, a more than 5 degree pitch angle was required. 3. Stage III: Touchdown preparation  
In the touchdown preparation stage, the descending rate is controlled by the direct lift coming from the thrust vector nozzle and lift fan. The forward flight speed is governed  
Figure 15: SRVL strategy. by the backward thrust component of the thrust vector. The descending rate and the pitch angle needed to be set in advance and were designated as \(\dot{h}=-1\) m/s and \(\gamma_{3}=-4.08^{\circ}\) by this work. According to the performance analysis results in Figure 9, it was still inside the feasible zone that \(\gamma_{3}=-4.08^{\circ}\) when \(\theta_{3}=5^{\circ}\).