# Research on the Flight Performance of Biomimetic Moth Based on Flapping Function Control
## 4 Flight Experiment and Result Analysis of Biomimetic Flapping-Wing Aircraft
### Turning Test: Turning Based on Amplitude Difference of Left and Right Wings
When the absolute value of the mid-position angle decreases, the flight trajectory of the flapping-wing aircraft fluctuates violently, indicating that decreasing the absolute value of the mid-position angle can make the flapping-wing aircraft pitch backward and climb, but it will significantly weaken its flight capability. The position distribution of the flapping-wing aircraft is compact when rising, indicating that the propulsive force generated during climbing is weak. In general, decreasing the absolute value of the mid-position angle can make the flapping-wing aircraft climb for a short time, but it will significantly reduce its flight capability, making it difficult to keep flying stably. In summary, changing the absolute value of the mid-position angle during flight can control the flapping-wing aircraft's flight altitude by controlling the flapping-wing aircraft's pitching. Increasing the absolute value of the mid-position angle can make the flapping-wing aircraft pitch forward and dive, and it has almost no impact on its flight capability; decreasing the absolute value of the mid-position angle can make the flapping-wing aircraft pitch backward and climb, and the flight altitude can rise for a short time, but it will significantly reduce the flight capability. There are some issues worthy of further exploration and discussion in the flight altitude test:  
1. **Sudden Change of mid-position angle:** According to the experimental observation, a sudden increase in the absolute value of the mid-position angle will cause the flapping-wing aircraft to pitch forward sharply and then fall rapidly. Similarly, a sudden decrease in the absolute value of the mid-position angle will cause the flapping-wing aircraft to pitch backward sharply and even turn 180\({}^{\circ}\) in the vertical plane;
2. **Reduction of Propulsive Force Leads to a Reduction in Lift Generation:** In the experiment of decreasing the absolute value of the mid-position angle, the flapping-wing aircraft climbs and the propulsive force weakens, resulting in a decrease in the forward speed, and then its generated lift is not enough to overcome its own weight, then the flapping-wing aircraft dives. Try to use the calculation model established in Chapter 3 to analyze and explain this phenomenon: when the forward speed of the aircraft is too slow, the free stream velocity is only affected by the flapping speed, and the direction of the free stream is nearly perpendicular to the wing surface, that is, the angle of attack \(\alpha\) of the flapping motion is approximately 90\({}^{\circ}\), and the lift coefficient is about 0.1 using the empirical formula (4); the lift coefficient is extremely low, leading to a correspondingly small amount of lift. The theoretical analysis results are consistent with the experimental results, indicating that the theoretical analysis is basically correct. This is the main reason for the violent fluctuation of the flight trajectory of the flapping-wing aircraft;
3. **Impact of mid-position angle on Flight Capability:** In Experiment 3 of the lift test, a larger initial mid-position angle brought better flight capability, which is consistent with the results of simulation calculation analysis, while in this experiment, increasing the absolute value of the mid-position angle during flight eventually led to a decrease in flight altitude, which is consistent with the results of mathematical modeling analysis. There are two hypotheses about this. Firstly, when the absolute value of the mid-position angle is small, the direct impact of the mid-position angle on lift is the secondary impact, and the indirect impact on lift by affecting the angle of attack is the main impact; when the absolute value of the mid-position angle is slightly larger, the direct impact is the main impact. Secondly, in this experiment, the increase in the absolute value of the mid-position angle causes a change in the attitude of the flapping-wing aircraft, which was not reflected in Experiment 3.