Numerical Simulation Study On Propulsion Efficiency Of Flapping-wing Aircraft

The flapping-wing aircraft is a new type of aircraft that is different from the traditional fixed-wing and rotorcraft,and it adopts the same flapping-wing method as natural birds and insects,which has the advantages of flexibility and concealment,and has broad application prospects in the military and the civil.In this paper,through a one-time optimized iterative flight test of the flapping-wing aircraft drive part,the wing part and the tail part,a medium-sized flapping-wing prototype capable of flying in the air for about 8 min with a wingspan of 206 cm was produced,and the flapping-wing prototype is a prototype to study the propulsion efficiency of the flapping-wing aircraft.The simulation model was constructed by measuring the airfoil,chord length,maximum flap amplitude and flapping frequency of the flapping-wing prototype,and numerical simulation was performed by Fluent software.The propulsion efficiency of the flapping-wing aircraft is the ratio of the useful power to the input power,the useful power is the power generated by the thrust of the flapping wing,and is the relation between the thrust coefficient,the input power allows the flapping-wing aircraft to perform the ups and downs of the plunging and the pitching motion around the axis,and is the sum of the power generated by the lift and the power generated by the torque is the relationship between the lift coefficient and the moment coefficient.In summary,the propulsion efficiency of the flapping-wing aircraft is related to the relationship between the thrust coefficient,the lift coefficient and the moment coefficient,and the magnitude of the three coefficients can be used to determine the propulsion efficiency in the corresponding situation.The simulation process includes the construction of the airfoil motion model,the division of triangular unstructured grid,the compilation of DEFINE CG MOTION macro,the setting of dynamic mesh in airfoil and flow field,pressure-based transient solver,the calculation model of k-? turbulence and configuration of solution parameters and analysis and processing of result data,and to study the propulsion efficiency under different pitch amplitude angles(?0),phase angles of different plunging and pitching motions(8)and different Strouhal number(St),from the simulation results can be obtained the thrust coefficient,lift coefficient and moment coefficient of the flapping-wing aircraft under the above situation.The results showed that when St=0.3,?0=20°,and 8 are between 75° and 105°,the propulsive efficiency of the flapping-wing is not much different,about 0.38,and the phase angle determines the correspondence between the angle of attack of the flapping-wing aircraft and the position of the motion.When St=0.3,6=75°,and ?0 are between 15° and 25°,the propulsive efficiency is basically the same,and ? is between 0.38?0.41.When ?0 is small and its propulsion efficiency is also low,and a certain degree of pitch amplitude can effectively improve the propulsive efficiency of the flapping-wing.When a0=20°,?=75°,St are between 0.1 and 0.4,the relationship between propulsion efficiency and St number is negatively correlated.When St is 0.3,the flapping frequency is 6 Hz,and its aerodynamic performance is much higher than the case where St is 0.1 and 0.2,when the flapping-wing has good aerodynamic performance and the flapping frequency is high,the flapping-wing aircraft can withstand more loads.Based on the analysis results in different situations,the optimal combination parameters of the flapping-wing aircraft are St=0.3,?0=20°,?=75°,and the propulsion efficiency is 0.375.