Analysis And Application Of The Unsteady Aerodynamic Theory Of The Flapping Wing Aircraft

Flapping wing aircraft is an unmanned aerial vehicle designed according to the movement posture and flight mechanism of birds or insects.Compared with traditional fixed wing and rotary wing aircrafts,flapping wing aircraft have huge advantages in vertical take-off and landing,flight efficiency and maneuverability.In recent years,the unsteady aerodynamic mechanism of flapping wings has gradually become clear.Several methods of calculating the unsteady aerodynamic characteristics of flapping wings have been established.On this basis,a comprehensive analysis of the research status of aerodynamic calculation methods is made in this paper.A derivation about the computational fluid dynamics method(CFD)and the three-dimensional unsteady aerodynamic calculation method(UAM)is carried on in detail.Based on the above two methods,the two practical problems of the landing strategy of the bird-like flapping-wing aircraft and the flapping mode of the hummingbird flapping-wing aircraft are deeply explored.The main work of this paper includes:(1)This paper analyzes and summarizes the status of aerodynamic mechanism and aerodynamic theory of flapping wings at home and abroad,and compares the advantages and disadvantages of the four unsteady aerodynamic calculation methods:Theodorsen's method,quasi-steady method,CFD method and UAM method.(2)According to the flapping-wing flight rules of small and medium-sized birds,a flapping-wing mechanism with a single driving source is designed to achieve"flapping-twisting" two degrees of freedom motion,and an experimental prototype is manufactured.The prototype weighs 310g and the total wingspan reaches 112cm.In the flapping wing mechanism,the crank length is 3 1.8mm,the main connecting rod length is 185mm,the auxiliary connecting rod length is 183mm,and the main spar rod is 500mm.The maximum chord length of the wing reaches 20cm.(3)An experimental platform for motion and aerodynamic forces measurement have been established.The movement attitude data of the wing is collected,and the experimental aerodynamic data of the prototype in this movement mode is measured.Based on the three-dimensional UAM and CFD models,the theoretical aerodynamic forces of the flapping-wing mechanism under the experimentally measured motion data is calculated respectively.The results show that the aerodynamic forces by the three methods are in good agreement,especially the difference in average lift between UAM and CFD methods is only 1.3%,which verifies the reliability of the UAM model.(4)A parametric study is carried out by employing the UAM method to analyze the effect of variations of the pitching angle on the aerodynamic lift and drag forces.According to the study,it is revealed that an optimal pitching amplitude and change rate of pitching angle near the end of up-stroke play a critical role for the lift force.The kinematics of motion of a flapping wing can be improved by a quick reduction of pitching angle in up-stroke to achieve maximum lift force.As the forward flight velocity decreases during landing,the aircraft should continuously increase the wing pitching angle up to 75° to gain maximum lift.In the same time,the increased pitching angle results in increased drag to decelerate the aircraft motion.(5)An investigation is made into the aerodynamic lift and efficiency of a flapping wing in parallel motion compared with conventional insect-like "fan-sweep" wing motion.Numerical simulation of aerodynamics of a flapping wing without slit is performed using CFD method in the two types of flapping motion and compared in a range of pitching angle and equivalent stroke amplitude.It is found that the aerodynamic force and efficiency of both types of flapping motion increased with the stroke amplitude at the same pitching angle and the results were comparable.Compared with the fan-shaped flapping,the parallel flapping results in a higher aerodynamic efficiency especially in smaller pitching angle in the whole stroke range,but a greater lift in a limited range.(6)The study of the flapping wing in parallel motion is extended to analyze the effect of wing slit with different width on the aerodynamic performance.Compared with the non-slit wing,the flapping wing with a slit can produce much greater aerodynamic lift force(>20%)in the same aerodynamic efficiency.A small slit(1/50 wing chord length)results in better aerodynamic performance compared with larger slits(1/25 and 1/10 wing chord length).