| Compared with traditional aircraft,flapping wing has many aerodynamic advantages,especially high lift performance at high angle of attack.It is due to the unique mechanism and phenomenon of unsteady flow field that flapping wing shares including Wagner effect,clap and fling effect,leading and trailing edge vortex delayed shedding effect and Kramer effect.In order to accurately and rapidly simulate the effects of the above phenomena on the flow field and aerodynamic force,it is necessary to derive the unsteady aerodynamic method(UAM)based on the potential flow theory.With this method,it is convenient to analyze the aerodynamic characteristics of flapping wing/flapping wing rotor and understand the influence of various flapping modes on their aerodynamic results.On this basis,combining the ADAMS software and the UAM program,this paper analyzes the favorable effect of the wings’ high-frequency flapping on flapping wing structure’s deceleration performance during the process of short-range landing,which provides a new idea for realizing short-range landing.The idea of flapping wing rotor stems from the combination of flapping wing and rotorcraft,which has both the characteristics of delayed stall of flapping wing aircraft and high efficiency like rotorcraft.The ultrasonic motor has the characteristics of large torque,light weight and no inertia.Based on the above advantages,this paper proposes a variant speed flapping wing rotor model driven by the ultrasonic motor.Given the different voltage inputs,the ultrasonic motor will rotate at different speeds.Therefore,a control system composed of photoelectric sensor and control circuit board is built up to realize the fast switching of different frequencies between the upstroke and downstroke stages.Then,combined with the unsteady CFD model and experimental aerodynamic measurement,the influence of variant flapping frequency on the flapping wing rotor’s lift is evaluated and analyzed.Finally,based on the force measuring system and a flapping wing rotor driven by the electromagnetic motor,the influence of bow wing on the aerodynamic characteristics of flapping wing rotor is evaluated.The main work of this paper includes:(1)The research progress of the flapping wing / flapping wing rotor is demonstrated with some key technical problems are summarized.Then,the research status in the area of unsteady aerodynamic calculation,landing flight mechanics and structural design of the flapping wing / flapping wing rotor is summed up.At last,the necessity and significance of the researches in this paper are described.(2)Based on Ansari’s method,the unsteady aerodynamic formula for cambered wing is derived.A novel unsteady aerodynamic method(UAM)is proposed to avoid the vortex penetrating the airfoil surface,which can be used in the aerodynamic calculation of the thin airfoil.The 2D UAM model is then extended to a 3D UAM model.The reliability of the UAM model is verified by comparing its calculation results with the 2D/3D experimental results and CFD results in the previous studies.(3)By coupling the above three-dimensional UAM model and ADAMS software,the landing process of a seagull-like flapping wing prototype is analyzed.Firstly,the flapping mode of the prototype is obtained by the size of the prototype’s structure.After that,the deceleration effect of the landing speed is studied under different combinations of the flapping frequencies,tail-plane deflection angles and flapping starting time.It is found that the optimal combination that can meet the landing requirements occurs when the flapping starting time is 40.5 s,the tail-plane deflection angle is and the flapping frequency is 3 Hz.Under which case,the forward velocity and descending velocity before landing are reduced to 0 m/s and 2.09 m/s respectively.Then the prototype is scaled 0.5,1,5,10 times in length and the relationship between the required flapping frequency to reduce the descending velocity by 50% and the scale ratio is studied.Finally,it is found that the minimum flapping frequency required to achieve 50% landing deceleration can make the dimensionless lift to weight ratio close to0.51.(4)Combined with the current research focus--"ultrasonic motor",a variant speed flapping wing rotor is made.By using the speed control of ultrasonic motor and its characteristics of light weight and high torque,the flapping wing rotor shares different flapping frequency during the upstroke and downstroke process to achieve the improvement of the average aerodynamic lift.At the same time,a measurement circuit including the load cell,signal amplifier,signal acquisition card and other components is built to measure the aerodynamic force of the flapping wing rotor.Then,an unsteady CFD sliding mesh model is established for aerodynamic calculation of the flapping wing rotor.The results calculated by this unsteady CFD model are compared with the results in previous study to verify its reliability.By analyzing the pressure distribution and vortex shedding process around the flapping wing rotor,the flow field difference of the flapping wing rotor under variant flapping frequency and constant flapping frequency is found,which assists the design and modification of the structure.Finally,through the power curve of the ultrasonic motor and the dimensionless lift efficiency curve,it is found that the greater output power corresponds to the greater aerodynamic force;Given the same output power,the aerodynamic force generated by the flapping wing rotor under variant flapping frequency is greater than that generated by the flapping wing rotor under constant flapping frequency;The optimal installation angle to produce the maximum lifting force is no matter under constant flapping frequency or variant flapping frequency;In variant flapping frequency cases,the Strouhal number of flapping will be closer to 0.3 and have greater lift efficiency.(5)The traditional planar wing will generate nearly symmetrical deformation during the flapping process of the flapping wing rotor,while the bow wing uses fish line to fasten the leading edge and the trailing edge of the bow wing so that smaller deformation is found during the downstroke and larger deformation occurs during the upstroke.In order to study the influence of bow wing on the flapping wing rotor’s lift,based on the force measuring system and a flapping wing rotor driven by the electromagnetic motor,the lift generated by the planar wing and bow wing at different installation angles are measured respectively.The results show that the bow wing will achieve the maximum lift at compared with the planar wing at installation angle.In addition,given the same flapping frequency,the maximum lift of bow wing could be 37 % higher than that of planar wing. |
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