## 3 Experiment Results and Discussion
### Experimental and CFD Simulation Comparison  
It is apparent that, compared with CFF case, the negative inertial and lift forces will be reduced by decreasing the flapping frequency in up-stroke. However, the up-stroke time and the period of a flapping cycle will be longer, means that the average flapping frequency is reduced. The study then focused on the effect of VFF on the lift by comparing the experimental data with CFD simulation results, as shown in Figures 13 and 14. It is noted that the elastic deformation of the FWR wing spar and membrane skin was ignored in the CFD model.  
For the CFF case 1 (3 Hz), the lift force results, from experiment and CFD methods, are compared in Figure 13. For the baseline case 1, a similar trend of the lift curves is found between the CFD and experiment results, with the average lift 218 mN, and 214 mN in a flapping cycle, respectively. The maximum negative lift occurs at mid-up-stroke, while the maximum positive lift occurs at mid-down-stroke by both methods. The peak lift values by experiment is smaller than that by CFD simulation, mainly because a constant rotational speed (\(\dot{\psi}\)) is taken in the CFD model. While, in fact, the rotational speed of the FWR experiment is slightly larger during the up-stroke than the average value (\(\dot{\psi}\)), but smaller during the down-stroke when the maximum lift is produced. Despite the difference in lift peak values between the CFD and experiment results, their general trend and average lift values are quite similar. Thus, the assumptions made in the CFD model is valid for evaluating the effect of variant flapping frequency on the FWR performance in the study (<3 Hz).  
For the CFF case 3 (1 Hz), the average lift value 17.8 mN from experiment is much smaller than the average lift (214 mN) in the CFF case 1 (3 Hz) and the lift (80.5 mN) in the VFF case 4 (3 & 1 Hz), according to the results shown in Figure 13. A similar average lift from CFD is found for the case 3 (19.9 mN) and case 4 (85.3 mN). It is observed that the lift curves in case 3 are similar to the case 4 during up-stroke at 1 Hz flapping frequency, while the lift during down-stroke is similar to the case 1 at 3 Hz flapping frequency. The average lift in case 4 (3 & 1 Hz) is apparently greater than the lift in case 3 (1 Hz), but smaller than case 1 (3 Hz), due to smaller input power and rotational speed during up-stroke motion.  
Figure 12: Force measurement of the FWR model in (**a**) 3 Hz case; (**b**) 3&1 Hz case.  
For the CFF case 2 (2 Hz) and VFF case 5 (3 & 2 Hz), the lift force results are shown as Figure 14 in comparison with the baseline case 1 (3 Hz). The average lift forces for the case 2 from the experiment and CFD simulation are 72.8 mN, and 77.3 mN, respectively; for case 5 (3 & 2 Hz) are 151.7 mN, and 167.5 mN, respectively, which is significantly greater than the lift force 72.8 mN in case 2 (2 Hz) and the lift 80.5 mN in case 4 (3 & 1 Hz), but smaller than the lift force 214 mN in case 1 (3 Hz).  
Figure 14: Comparison between lift curves of the flapping wing rotor measured in (a) 2 Hz case; (b) 3&2 Hz case.  
Figure 13: Lift force comparison between the experiment and CFD in (a) 3 Hz case; (b) 1 Hz case; (c) 3&1 Hz case.