The Aerodynamic Performance Research On Backswept Blade Of HALE Uavs

The aerodynamic design of high altitude long endurance(HALE)unmanned aerial vehicles(UAVs)propeller has an important influence on the comprehensive performance of the whole aircraft.However,there are some shortcomings in the current design of propeller,whose highspeed performance is not good,and when the flight speed deviates from the design point,the aerodynamic efficiency of propeller drops sharply.These problems make the fixed pitch propeller system difficult to meet the design requirements of fast climbing and fast maneuver of the HALE UAVs.In order to solve this problem,this paper applies the backward swept design to the aerodynamic design of propeller blades,so that the UAV can not only cruise at low speed,but also maneuver and climb at a faster speed.The research work of this thesis includes the following aspects:First,geometric modeling and parameterization research of swept propeller were developed.In this paper,the modeling method was proposed that the blade stack line was swept along the direction of the twist angular of airfoil elements on the promise of keeping the blade surface area constant.This modeling method could ensure that blades do not bend during the sweep operation,avoiding the influence of surface area changing and blade bending.Moreover,the parametric research method of blade swept was developed in order to find the optimal sweep angle.According to the above method,five blades with different sweep angles were generated.To ensure the same tip compressibility,these five blades were required to have the same tip Mach number,and that decided the rotation rate of each swept blade.Second,the CFD(Computational Fluid Dynamics)numerical simulation method for viscous flow around swept blade propeller was developed.In order to verify the effectiveness of the swept blades in weakening the compression of the blade tip,the numerical simulation method must be able to capture the shock wave accurately.It was found in this study that besides the mesh density,different turbulence models had different ability to capture shock waves,the Realizable k-ε turbulent model was considered to have better shock capturing ability by comparing with others.Moreover,the method that steady state computation of the viscous flow around the propeller based on the MRF(Multiple Reference Frame)model and periodic boundary condition was selected by comparing with other numerical simulation methods of rotating machinery.Finally,the computational method was verified by grid independence test and results comparation with strip theory.Third,the research revealed the reason for the decrease of the efficiency after the fixed pitch propeller deviated from the design point,and the influence of the backward sweep angle on the aerodynamic performance of the propeller and revealed the flow mechanism of the swept blade propeller.The reason of efficiency droop after design point of the fixed pitch propeller was due to the decreasing of the angle of attack caused by the increasing velocity of the UAVs.Through the comparison of the computational results of different swept propellers and un-swept propeller,it was found that the swept blade propellers had greater thrust and postponed the decrease of efficiency.The results of the parametric research showed that the optimal sweep angle was 40 degrees.The aerodynamic performance of swept blade propeller was influenced by the angle of attack,the three dimensional effect at the blade tip,the intensity of the shock wave and the shock-boundary layer interactions.