| Rotary-wing aircraft are widely used in military and civil fields,and their spatial structure is also developing in a more compact direction.In this paper,the influences of viscous effect and aerodynamic interference between rotors on the aerodynamic performance of a micro coaxial rotor are analyzed qualitatively from the aerodynamics.In addition,numerical simulation and experimental results are verified by each other,and the important aerodynamic parameters of rotor system are optimized.Therefore,better aerodynamic layout scheme is obtained,and some theoretical basis and experimental data are provided for the optimization of rotorcraft.Based on the mature theory which is used to analyze the rotor aerodynamic performance,the modified blade element-momentum theory is used to calculate the rotor aerodynamic performance considering viscous effect at low Reynolds number,and the theoretical calculation formulas of rotor aerodynamic force and torque are obtained.At the same time,the influence of viscous effect on aerodynamic characteristics of airfoils is clarified.Additionally,aerodynamic models in hovering and wind gust are established,and the aerodynamic interference between the upper and lower rotors is qualitatively analyzed.A set of method for numerical simulation is established to analyze the characteristics of flow field for the micro rotor,and the lift-drag coefficient,distribution of tip pressure,streamline and velocity vector of the airfoil are also obtained to analyze the influence of Reynolds number on the characteristics of lift-drag and distribution of flow field.In addition,test rig used for the measurements of aerodynamic performance for the micro rotor was built,and the obtained experimental data were compared with the simulation results,which verified the accuracy and reliability of simulations and the tests.A new test rig was built for the measurements of aerodynamic performance for a micro coaxial rotor based on the improved one designed for micro-single rotor.Mainly,the rotor thrust and power consumption at different rotor spacing and rotational speed were measured,and the measurement errors were all less than 2%.At the same time,the influence of spacing ratio on the distribution of flow field is analyzed in detail by using the validated method of numerical simulation.Based on the experimental and simulation results,the optimal aerodynamic layout of a micro coaxial rotor is obtained.In order to test the wind-resistant ability of a coaxial rotor,low-speed wind tunnel was used for the wind blowing test,selecting the typical secondary wind and tertiary wind in natural environment.Combined with the numerical simulation results,the wind-resistant performance of the coaxial system was preliminarily explored by analyzing the influence of magnitude and direction of wind gust on the aerodynamic performance for micro coaxial rotors.The results show that:(1)The airfoil has good lift-drag ratio and hover performance.(2)Aerodynamic performance of the airfoil decreases at low Reynolds numbers.(3)Spacing ratio h/r=0.38 is the best aerodynamic layout of the micro coaxial rotor.With the increase of spacing ratio,the coaxial thrust and power consumption decrease and are gradually independent of rotor spacing.(4)Higher power-loading are obtained with lower rotational speed for the coaxial rotor.(5)Contribution of the upper rotor to the total thrust of a coaxial rotor is greater than that of the lower rotor.(6)In the environment of vertical wind gust,aerodynamic performance of the coaxial rotor decreases,and the performance decreases with the increase of wind speed.In the environment of horizontal wind gust,the coaxial rotor has better wind-resistant performance,and the performance is improved with the increase of wind speed. |
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