Aerodynamic Design of Unmanned and Scaled Supersonic ...

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K. Yoshida and Y. Makino ruled body for the “0-2nd Configuration” which was defined in the conceptual design phase described in the following section as a test case [20]. Figure 13 shows the designed fuselage configuration, compared with the area-ruled body based on linear theory of the “0-2nd Configuration”. The influence of the nacelle under the wing leads to the non-axisymmetrical feature on the fuselage geometry. Therefore, this concept is very effective to reduce the interference drag of such a complicated configuration with relatively twin large nacelles. We also investigated an optimum nacelle configuration using the optimum design system. Since the design space of the nacelle was relatively small, that is the forward part and total length of the nacelle had to be fixed, we can not obtain remarkable drag reduction effect for the optimum designed nacelle configuration. However, comparing the initial nacelle and the optimum nacelle, we found some important knowledge of reducing the drag. Furthermore, we investigated the effect on the nacelle position, that is, streamwise station and spacing of two nacelles. We found the optimum spacing of the nacelles [21]. Consequently, it was difficult to reduce the interference drag between the airframe and nacelles drastically even though applying the CFD-based optimum design method because of relatively large nacelle configuration. In genral, the principle to reduce the interference drag was to decrease the cross sectional area of total configuration. One of the best ways was to embed the nacelles into the wing under the limitaiton of structural design constraints. 3.2 Design Process and Results Our aerodynamic design process consisted of two phases. The first phase was the conceptual design phase. The main objective was to compromise aerodynamic performance with some constraints required in developing a practical aircraft such as wing position, aerodynamic tail-volume, intake geometry, front part of nacelle configuration, and embedded nacelle configuration. The next design phase was aerodynamic optimum design phase. We focused on the non-axisymmetrical area-ruled body design, including the optimum nacelle design. In addition, the non-axisymmetrical area-ruled body concept has large possibility to be extended for reducing sonic boom. This is investigated in our fundamental research activity [25]. Design results at each process are described in the following sections. 1) Conceptual design phase We designed a fundamental configuration with two large nacelles as a first step. This configuration had a simple straight body with a Karman ogive cone and a warped wing by Carlson’s method. The planform was selected considering drag characteristics using Carlson’s method according to the same selection rule as the NEXST-1 airplane. The remarkable difference was to adopt supersonic leading edge at outer wing as stated above. This Fuselage radius Linear area-ruled Body Non-linear optimized area-ruled Body (r/L) 0.03 0.02 0.01 0.00 0.0 0.2 Initial Upper Side Lower 0.4 x 0.6 Axisymmetric *0-2nd Configuration Non-axisymmetric ΔC Dp =-0.0006@M=1.7 Figure 13. Comparison of optimum arearuled body design 0.8 1.0 x/L 14
K. Yoshida and Y. Makino configuration was called “0-1st Configuration” of the NEXST-2 airplane. Figure 14 shows a wind tunnel test model for the “0-1st Configuration”. It had a flow-plug just after two nacelles and several total pressure measurement tubes placed near exit station of the nacelle. We were able to control the inflow entering into the intake by changing the plug position. We obtained useful experimental data using this model, as mentioned later. In the conceptual design phase, the “0-1st Configuration” was the start point. As the next step, we designed from the “0-2nd Configuration” to “0-8th Configuration” considering an area-ruled body concept based on linear theory, some improvements of nacelle configuration and its position, optimization of intake geometry, and aerodynamic tail-volume. This design process was not straight-forward but a process of trial and error approach in reducing aerodynamic drag within several constraints required in practice. 2) Aerodynamic optimum design phase (a) Baseline configuration design The “0-8th Configuration” was only designed from the aerodynamic viewpoint. To develop a practical experimental airplane, it was necessary to compromise the following fields: airframe aerodynamics, structural constraints, flight dynamics, intake aerodynamics, and propulsion system performance. As the first step, we designed the “1st Configuration” using present CFD-based aerodynamic design method, maintaining several practical design constraints required by industries. In the aerodynamic design of the “1st Configuration”, the NLF wing design at inner wing was conducted using the CFD-based inverse design method and a modified target pressure distribution. Figure 15 shows the estimated laminar region using our compressible e N code. In this estimation, we selected the transition criterion of N=10 as a reference. This lower criterion than the N=14 for the NEXST-1 airplane was assumed by taking account of the influence of embedded nacelle on the upper surface such as acoustic disturbance. Two nacelle configurations and their positions were optimized under the design condition of intake and diverter, using the CFD-based optimum design method [21]. In addition, a nonaxisymmetrical area-ruled body was designed using the same method [20]. However, we did not obtain the remarkable drag reduction effect of the concept. We considered that present small Flow-plug Figure 14. W/T model for “0-1st” Configuration of NEXST-2 airplane y/l 0.5 0.4 0.3 0.2 0.1 0 -0.1 NEXST-2: Estimated Transition Position M=1.7 , α=0.5°, H=15km Wb0803aGeometry contours Estimated laminar region N=2 4 6 8 10 0 0.2 0.4 0.6 0.8 1 x/ Figure 15. Transition Analysis on NEXST-2 (1st Configuration) 15
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