A new conceptual design for the VTDP system was proposed in this section. Its aerodynamic performance was calculated using the numerical simulation approach verified in previous section. The calculated results were compared with those of 16H-1 and X49.
3.1 Results of 16H-1 and X49
In the simulations of 16H-1 discussed in the previous section, the deflection angle of the vertical vanes was fixed. In the simulations discussed in this section, the rotation speed was fixed at n = 6500 rpm, and the deflection angles were varied.
Fig. 16 displays the axial force or thrust at different deflection angles. The black solid line represents the total axial force for the VTDP of the 16H-1, and the red, blue, and dark cyan colors represent the axial force components from the propeller, duct, and vertical vanes, respectively. With the increase in the deflection angle, the thrust of the propeller (red color) increased, the absolute value of the drag of vertical vanes (dark cyan color) increased and its direction was opposite to that of force produced by the propeller. Meanwhile, the thrust of duct (blue color) reduced. As a result, the whole thrust of the VTDP system decreased monotonically.
Fig. 17 shows the lateral force at different deflection angles. The total lateral forces of the system and the vanes increased first and then decreased. The maximum forces occurred near Φ = 40°. The lateral force of the duct decreased with the increase in Φ.
Fig. 18 shows the power variations at different deflection angles. The maximum consumed power occurred at the same deflection angle as that of the maximum lateral force in Fig. 17.
The numerical simulation results for the X-49 were similar to those for the 16H-1. As shown in Fig. 19, the outer deflector completely opened, and the deflection angles of the vertical vane were 50° and 60°. The numerical results are shown in Table 2.
3.2 Conceptual Design for the Deflection System of VTDP
The conceptual design for deflection system is displayed in Fig. 20. In the conceptual design, the duct was prolonged, eliminating the horizontal and vertical vanes of the 16H-1. Two rotatable slices that were parts of the prolonged duct replaced the extra outer deflector in the X-49. As displayed in Fig. 21, the first slice rotated in an anticlockwise direction, and the second slice rotated in a clockwise direction. While operating, the two slices constituted a nozzle that was similar to a vectored thrust nozzle.
Fig. 22 shows the grid at the horizontal central section. Fig. 23, Fig. 24 and Fig. 25 show the variations of the axial force, lateral force, and power with the deflection angle Ψ of the rotating slice. In these figures, “VTDP system” represents conceptual deflection system of VTDP as shown in Fig. 20, the “Propeller” represents the propeller in the VTDP system as shown in Fig. 20(a), the “Duct” represents the duct in the VTDP system as shown in Fig. 20, and the “First rotating slice” and the “Second rotating slice” represent the rotating slices in the VTDP system as shown in Fig. 21.
3.3 Comparison of the Results of the Conceptual Design, 16H-1, and X-49
3.3.1 Force and Power
The lateral forces between the three deflection systems are compared in Fig. 26 where only the maximum lateral forces for the 16H-1 and X-49 occurred at Φ = 40°and Φ = 50°, and the lateral forces for the proposed design at different deflection angles are presented. The maximum lateral force for the 16H-1 was smaller than those of the other two deflection systems. The lateral forces of the proposed design for deflection angles from 90° to 120° were comparable with the maximum force of the X-49, although the maximum lateral force of the proposed design at Ψ = 110° was a bit larger than that of the X-49.
For hovering, a larger lateral force and smaller axial force are preferable. The axial forces for the proposed design at deflection angles less than Ψ = 120° were smaller than that of the 16H-1 at Φ = 40° and X-49 at Φ = 50°, where these angles corresponded to the maximum lateral forces. This indicated that in a range of deflection angles for the proposed design, the axial force was always smaller than those of the other two systems.
The consumed power for the proposed design was the smallest. These comparisons demonstrated that the proposed design provided a high lateral force with smaller values of the axial force and consumed power.
3.3.2 Streamlines and Pressure Contours
The streamlines and pressure contours are displayed in Fig. 26, Fig. 27 and Fig. 28, providing further information for the proposed design. According to the principle of momentum conservation, the forces exerted on the VTDP, including the axial and lateral forces, are determined by the pressure on the VTDP, the airflow deflection, and the mass flux, which can be explained by Fig. 29.
3.3 Further Analysis
The axial velocity and pressure of the free stream were V0 and P0. The velocity increased and the pressure decreased as the airflow approached the propeller. The pressure before the propeller was P'. After the flow passed through the propeller, the pressure increased to P' + ΔP, and the axial velocity increased to V1. The area of the propeller disk was A1, and the area of the outlet was A2. At the outlet, the velocity further increased, and the pressure decreased to P0. The flow from the outlet of the duct was assumed to be deflected completely. The average deflection angle of the flow was Φ, and the velocity was V2. According to the principle of momentum conservation, the overall axial force of the VTDP system was
As indicated by the streamlines at the horizontal central section in the Fig. 26(a)-Fig. 28(a), the 16H-1 caused less airflow deflection than the X-49 and the proposed design. Thus, the 16H-1 yielded a greater axial force and a smaller lateral force, according to the principle of momentum conservation. The deflection devices installed in the X-49 and the proposed design were more effective than the vertical vanes design in 16H-1.
The pressure contours can provide an explanation for the superiority of the designs of the X-49 and the proposed configuration compared to that of the 16H-1. In the momentum analysis, it was assumed that the exit pressure recovered. However, this assumption was not supported by the pressure contours as shown in Fig. 26(b) -Fig. 28(b). For the 16 H-1, a high pressure was present on the left side, balancing the effect of the vertical vanes, which can be seen in Fig. 26(b). In contrast, high pressures were observed on the right side of the X-49 and the proposed design, providing a higher positive lateral force. It is evident that a positive lateral force was beneficial for hovering in the present case.
Another factor for effective hovering is the mass flow. To calculate the flow through a certain surface, two rectangular sections were used to calculate the axial and lateral flow respectively as shown in Fig. 30. In the calculation process, the reference areas of the three different configurations (16H-1, X-49, and the proposed design) were consistent. Table 4 shows the mass flows of the models. The proposed design had the largest mass flow.
The flow analyses, with the aid of the momentum theorem, revealed that the proposed design caused larger flow deflection, a favorable high pressure, and a larger mass flow, which resulted in a larger total lateral force.