## 1 Introduction
[PERSON] [20] studied the effects of kinematic parameters on average lift force experimentally and found that a proper combination of wing structure, installation angle of the wing, and power input can be beneficial for higher averaged lift force. [PERSON] et al. [21] developed a boring-hole designed flapping wing rotor model. Through the experimental data, they found that there exists a critical angle of attack for a perforated wing to increase its maximize mean lift by 35%. In order to further understand the FWR flow mechanism of coupled flapping motion and rotary motion, numerical simulations by CFD were performed. [PERSON] et al. [22] analyzed the unsteady aerodynamic behaviour of a FWR by CFD, at a low Reynolds number, and proposed that the lift and rotational moment will increase with the increase in pitching amplitude and Reynolds number. [PERSON] et al. [23] studied a FWR of aspect ratios of five by CFD method and proposed that a higher Reynolds number and mean pitching angle can strength the leading edge vortex and provide a higher lift and thrust. [PERSON] et al. [24] used a charge coupled device (CCD) camera and the SURFER software to capture and fit the three-dimensional (3D) flapping motion of the wing surface. The 3D flapping motion is then sliced into the flapping motion on 2D cross sections before calculating the flapping wing's lift and drag through 2D quasi-steady CFD. The calculated results show a similar trend with their experiments, which validates their methods. [PERSON] et al. [25] also investigated the three-dimensional (3D) flow fields around a flapping wing with a span of 20 cm through both, COMSOL fluid-structure interaction (FSI) simulation and 3D stereo photography. They obtained similar trends for the numerical and experimental lift force data, and found the simulated stream lines, flowing over the 3D wing, agree well with the experimental smoke traces. [PERSON] et al. [26] employed a quasi-steady method to compare the efficiency and lift of the flapping wing, rotary wing, and flapping wing rotor. The rotary wing showed the greatest efficiency, while the flapping wing rotor can obtain both relative high lift and relative high efficiency at the same time. In the current study, a FWR, driven by an ultrasonic motor, has been designed and manufactured to investigate the effect of flapping frequency variation in down-stroke and up-stroke on the aerodynamic forces. Subsequently, the kinematics of the flapping wing motion is decomposed into the variation of rotary, stroking, and twisting angle, where the elastic deformation of the wing is neglected in the CFD numerical model. The CFD model, which used Fluent software to calculate the lift curves of the FWR, was built and subsequently validated by comparing with previous research results by [PERSON] et al. [23]. The study then focused on the effect of different flapping frequency of the FWR, in down-stroke and up-stroke on the lift, by experiment and CFD simulation. The usual motion of constant flapping frequency (CFF) in a flapping cycle provides a baseline reference for evaluating the effect of variant flapping frequency (VFF) in up-stroke and down-stroke on FWR performance. In addition, the influence of the installation angle of the FWR on the rotational speed and lift is studied.