# Design and Verification of Large-Scaled Flapping Wings for High Altitude Environment
## 3 Results
### Effect of Film Materials and Number of the Ribs  
The takeoff weight is critical for the vehicle, so it is necessary to reduce the weight of the flapping wing. Therefore, on the basis of increasing the internal area of the wing, as determined above, the film material and the number of wing ribs will be discussed. The planform of the wing, the position of the oblique rod, and the airfoil are guaranteed to be consistent. Without additional materials, six or seven wing ribs are combined with polyester or linen wing film material, respectively. The weight of all four wings in Figure 12 is around 120 g.  
From Figure 13, it can be concluded that the solid and dashed lines show opposite trends for the flapping wing aerodynamic performance in the range of \(Re\) and \(St\) studied in this paper. The reduction in the number of wing ribs results in a significant increase in the lift coefficient and a slight decrease in the thrust coefficient. The wing membrane material affects the aerodynamic performance to a small extent, but the linen material creates a slight increase in the downward pitching moment. Figure 14 shows the lift, thrust, and pitching moment of the last four wings for different wind speeds, attack angles, and flapping frequencies. The solid black line indicates the 163 g wing before weight reduction. In terms of lift, the performance of all four wings deteriorates after weight reduction. Comparing these four wings, the flapping wings with six wing ribs are better than those with seven wing ribs. While the material of the wing film has little influence on the lift, the flapping wings of linn material produce greater lift than that of polyester film. For flapping wings, lift generation is mainly related to wind speed and an effective angle of attack. In comparison, the increment of flapping frequency does not significantly improve the lift, if any, probably due to the flexible deformation of the wing, caused by high frequency, thus changing the effective angle of attack. For thrust, it is obvious that the wing before weight reduction still performs better. In contrast to the effect of the number of wing ribs on lift, the reduction has a slight detrimental effect on thrust. The two materials of the wing film do not play a significant role in thrust. The main factor affecting thrust, while keeping the amplitude constant, is flapping frequency. Increasing the flapping frequency will effectively improve thrust at the cost of power consumption. Figure 12: Four types of flapping wings with the internal area of the wing increased for the experiment: (**a**) 6 wing ribs + wing film material of polyester, (**b**) 6 wing ribs + wing film material of linen, (**c**) 7 wing ribs + wing film material of polyester, and (**d**) 7 wing ribs + wing film material of linen. Figure 13: The time-averaged lift coefficient (**a**), thrust coefficient (**b**), and pitching moment coefficient (**c**) of the last four wings as a function of the Reynolds number and the Strouhal number, respectively. When referring to the pitching moment, it can be found that the wing after weight reduction is weakened in absolute values. The combination of seven wing ribs and linen film creates the minimum downward force moment. Linen is about 2.6 g heavier than the polyester film material, which has a slight lift improvement and a certain degree of thrust increment at high speeds. The six ribs are about 7.1 g lighter compared to the seven ribs, with a significant lift and a slight decrease in thrust. The slow-motion video of the flapping motion shows that the six wing ribs are stiffer than the seven wing ribs in the chord direction of the wingtip, and the former deformation is not as pronounced as the latter. During stroke reversal, the chordwise twist of the wing with seven ribs is faster, which results in a greater chordwise twist angle at the wingtip at the initial moment of the downstroke.