## 3 Experiment Results and Discussion
### Flow Field Simulation and Analysis by CFD Method  
It is understood from a previous study that the evolution of leading edge vortex (LEV) and trailing edge vortex (TEV) is a decisive factor for generating the lift and thrust of FWR. For case 1 (3 Hz), the flow pressure and vorticity, produced by the FWR, are plotted in Figure 15 at eight non-dimensional time step \(\hat{t}=0\), 0.125, 0.25, 0.375, 0.5, 0.625, 0.75, 0.875 in one flapping cycle. As given in Figure 15, the flapping cycle starts at the beginning of the up-stroke \(\hat{t}=0\). In the plots, only the mid-span section of the FWR was taken as the typical section for analyzing the LEV and TEV structures [23]. According to Figure 15, a TEV forms at the lower surface at the end of down-stroke (\(\hat{t}=0\)), while a high-pressure region is formed on the upper surface of the FWR, mainly in the region of chord 0.6 and 1.0. When up-stroke started at \(\hat{t}=0.125\), significantly low (negative) pressure and LEV occurred on the lower surface of the wing section, which exacerbated the generation of negative lift. From \(\hat{t}=0.25\), the LEV started shedding off the section and eventually a weak TEV is formed on the upper surface of the trailing edge. During the second half of the up-stroke motion, the lift increases from negative to small positive values, reaching about 20% of the maximum lift in one cycle. Wake capture (LEV shed from the airfoil during up-stroke) was observed at the lower surface, at the start of the down-stroke, when \(\hat{t}=0.5\). From \(\hat{t}=0.5\)-0.75, the LEV and TEV continued to accumulate on the upper surface of the FWR section until shedding off started at \(\hat{t}=0.875\). The lift reached a maximum of 880 mN in the mid position of down-stroke, at about \(\hat{t}=0.75\), when the strongest LEV and TEV were found on the upper surface. In general, the evolution of L  
Figure 15: Variation of the flow pressure and vorticity during one flapping cycle in case 1.  
lift generation and form on the upper surface most of the time in one cycle, which leads to a positive average lift (218 mN) in a complete flapping cycle. The evolution of LEV and TEV are quite similar for the CFF case 1, 2, and 3. However, several differences of the flow field are found between the above CFF cases and VFF case. Since the difference between \(f_{u}\) and \(f_{d}\) is larger in the VFF case 4 (3 & 1 Hz), the difference of fluid field in up-stroke and down-stroke is more apparent as presented in Figure 16. Unlike case 1, the pressure difference between the lower and upper surfaces and the pressure value is much smaller over a longer period until \(\hat{t}=0.75\) during the up-stroke. Hence, the negative lift keeps in relatively small value (about 10-15% of maximum positive lift) most of the time in a flapping cycle. It is not until \(\hat{t}=0.75\) that the LEV and TEV started to accumulate on the upper surface of the section, with the peak lift (650 mN) occurring at \(\hat{t}=0.875\), when down-stroke started and the strongest LEV and TEV formed. From the results in Figures 13 and 14, it is found that the lift generated in the VFF case 4 and 5 is smaller than the CFF case 1, with higher output power from the motor in a flapping cycle. However, given the same output power, the lift forces generated by the FWR in the CFF and VFF cases are yet to be evaluated and compared. The measured output power against the rotation speed and torque of the motor in standard operating conditions is shown in Figure 17a. The results show that the maximum output power is 1.8 W with associated torque nearly 0.1 Nm. The stalling torque of the motor is 0.16 N-m. From the results in Section 3.2, it was found that the lift reaches maximum value of 214 mN when the maximum power output occurs at the motor speed 180 rev/min.