Study On The Abnormal Vibration Characteristics Of The Tail Structure Of Single Rotor Plant Protection UAV

In modern society,with the rapid development of Chinese agricultural modernization process,more and more new plant protection machinery are joining in a lot aspects of agricultural production.Single-rotor plant protection UVA is quickly recognized by the market due to its high efficiency and strong adaptability.The tail structure is an important part of the single-rotor plant protection UVA,which plays a role in balancing the counter torque generated by the main paddle and changing the flight direction in the flight process of the UVA.When a plant protection company in Guangxi developed a certain single-rotor plant protection UVA,it was found that the tail structure of the UVA would appear irregular violent vibration during operation,which has significant impact on the uniformity of pesticide spraying and flight stability and reliability of the UVA.In order to solve the problem of abnormal vibration of the rear structure of the singlerotor plant protection UVA,in this paper,the abnormal vibration of the UVA tail structure was studied,through a series of experimental test and finite element simulation analysis to find out the cause of abnormal vibration in rear structure,provide an important basis to solve the problem of the tail structure abnormal vibration.The specific study contents and conclusions of this paper are as follows:(1)In order to accurately obtain the rotor speed of the tail rotor when the throttle is in different positions,the rotor speed was tested,and the maximum rotor speed was 6120RPM,and the calculation showed that the excitation frequency range generated by the tail motor from start-up to acceleration to the maximum rotor speed was 0-102Hz.In order to avoid the test error of a single set of test equipment,according to the vibration characteristics and vibration noise variation characteristics of the UVA tail structure during operation,two test systems were built to collect vibration signals of the tail structure at different rotating speeds of the tail propeller.After analysis and processing,the main frequency of abnormal vibration of the tail structure was about 101Hz.(2)After the analysis of the experimental data,it is found that the vibration intensity of the tail structure is closely related to the tail blades,when the tail blades for variable pitch,the tail of the structure of the vibration strength changed,but the main vibration frequency did not change,and the tail blades to exist,the tail of the structure of the abnormal vibration,when after tear down the tail blades,tail structure vibration are faint.Therefore,it is preliminarily judged that the abnormal vibration of the tail structure may be caused by the resonance of the tail structure caused by the motor excitation,and the tail rotor has the effect of amplifying excitation.(3)According to the tail structure parameters of the single-rotor plant protection drone,the finite element model of the tail structure is established,and then using ANSYS Workbench application type automatic control mode solver to solve before 5 order natural frequency,and extract the top 5 order modal vibration mode cloud.Using the natural frequency test method of single point excitation and single point response,the natural frequency of the tail fin structure is obtained.The frequency difference between the natural frequency obtained by modal analysis and measured by test is calculated through the correlation of the natural frequency,and the results show that the frequency difference of each order is less than 10%,indicating that the finite element model of the tail structure is reasonable and can correctly reflect the vibration characteristics of the tail structure,which has reference value for engineering application.In addition,the composite UAV tail structure model can provide model support for enterprises to develop new models,greatly reducing the company’s research and development costs,shortening the research and development cycle,and improving economic efficiency.(4)Through experimental test and modal simulation results indicate:the tail motor produces the highest excitation frequency,the structure of the wing main vibration frequency,modal analysis and inherent frequency test for the second order natural frequency are all about 101 Hz,and the tail is sudden severe vibration,which can determine the tail of the structure of the abnormal vibration is caused by the tail motor reaches the highest speed of the tail caused the incentive structure resonance,and the bending degree of the motor shaft has a serious impact on the vibration of the tail structure.The greater the bending degree of the motor shaft,the greater the excitation generated during high-speed rotation,and the more severe the vibration of the tail structure.(5)Based on the above research results,two solutions are proposed for the abnormal vibration of the tail structure:The first is to make the natural frequency of the tail structure avoid the excitation frequency of the tail motor by changing the structure of the tail.Through modal analysis of the second mode cloud chart we can see that the weak point of vibration is the tail skid and the tail skid diagonal brace,and we can change the thickness of the tail slide and tail skid diagonal brace to increase the natural frequency of the tail structure.After modal analysis and calculation,when the thickness of tail skid and tail skid diagonal brace increases from 3mm to 4.6mm,the natural frequency of the second order of the tail structure increases by 38.56%,effectively avoiding the excitation frequency of the motor.The second is to change the excitation of the source.It is found that the bending of motor shaft of variable pitch propeller has a significant influence on the abnormal vibration of tail fin.Therefore,the fixed-pitch propeller motor is selected to replace the variable-pitch propeller motor,in order to reduce the excitation to the tail structure from the vibration source.In addition,the vibration test of the tail using the fixed-pitch propeller motor is carried out,and the test results show that there is no severe vibration of the tail structure.