# Design and Verification of Large-Scaled Flapping Wings for High Altitude Environment
## 3 Results
### Effect of Platforms and the Function of the Additional Materials  
After complicated fabrication, the first to eighth wings in Table 2 were obtained. The eight wings were installed in the experimental set-up for wind tunnel experiments to investigate the effect of planforms and the effect of the additional materials (Figure 9). Figure 10 shows the effect of dimensionless parameters, i.e., the Reynolds number and the Strouhal number, on the force and moment coefficients. The solid and dashed lines each represent the effects of _Re_ and _St_. The five different colors refer to some typical types of flapping wings and allow a clear comparison of the role of the planform and additional materials. A raise in _Re_ will reduce the lift coefficient, thrust coefficient, and downward pitching moment, which is opposite to the effect of _St_. By comparing the curves of different colors, it can be found that the elliptical planform has the best lift coefficients and the minimum downward pitching moment in all _Re_ and _St_ ranges, but at the cost of thrust. The attachment of the additional materials also significantly enhances the lift and thrust coefficients in the full _Re_ and _St_ ranges. Figure 9: Flapping wings used in experiment: (**a**) basic planform + 7 wing ribs + additional material, (**b**) increasing the external area of the wing + 7 wing ribs + additional material, (**c**) increasing the external area of the wing + 7 wing ribs + additional material + oblique rod setback, (**d**) increasing the internal area of the wing + 7 wing ribs + additional material, and (**e**) elliptical shape + 6 wing ribs. Figure 11 shows the results of experiments conducted with the first eight wings in Table 2 under the experimental parameter configurations in Table 1. The lift increases monotonously with increasing wind speed, angle of attack, and flapping frequency. Additionally, it is found that the thrust decreases with the increment of wind speed and attack of angle, which is opposite to the effect of frequency on thrust production. As for the pitching moment, it decreases with the increment of the above three parameters. Figure 11: The periodic-averaged lift (**a**,**g**), thrust (**b**,**h**), and pitching moment (**c**,**i**) of the first eight wings as a function of wind speed (V) and flapping frequency, respectively, plus the influence of the angle of attack on the corresponding coefficient (**d**–**f**). Figure 10: The time-averaged lift coefficient (**a**), thrust coefficient (**b**), and pitching moment coefficient (**c**) of the first eight wings as a function of the Reynolds number and the Strouhal number, respectively. Comparing the force and moment performance of the first to the eighth flapping wings, the lift performance of the sixth and eighth flapping wings is more prominent compared to the other wings, indicating that the elliptical shape and increasing the internal area of the wing contribute to the generation of lift. For the effect of the planforms of flapping wings on the lift, the elliptical one is slightly better than increasing the internal area of the wing; increasing the external area of the wing with an oblique rod setback, as shown in Figure 9c, is worse in comparison, and the planform shown in Figure 9b has the worst lift performance. The internal area of the flapping wing is mainly related to the generation of lift, so it is necessary to ensure that it has sufficient area, while the chord torsional stiffness of the wing needs to be moderate. Considering the function of the additional materials, as shown in Figure 11a,d,g, it is obvious that the lift of the solid line is always higher than that of the dashed line, which indicates that the periodic-averaged lift of the flapping wing is generally improved substantially when additional materials are attached. To be specific, the lift increases by at least 50%. In terms of thrust, the first, sixth, and seventh wings perform significantly better than the other wings.