A Research On The Key Design Problems Of The X-type Tail-Sitter SUAV

The tail-sitter vertical take-off and landing(VTOL)small unmanned air vehicles(SUAVs)are a kind of vehicles with VTOL ability of helicopter and high-speed cruising ability of fixedwing at the same time.With these abilities,they can take off in narrow space easily so that they attract a lot of attention and have a very wide application prospect.But there are many key technologies restricting their development,such as the obvious design discreteness,the requirement of two design points for propulsion,the low Reynolds number aerodynamics in slipstream and the conversion corridor with coupled variables.In view of the above problems,this thesis proposes a X-type flying-wing tail-sitter VTOL SUAV with one group of propulsions,and it also can vertically take off and land like helicopters and cruise like fixed-wings.On basis of this vehicle,the optimization method for propellermotor propulsion with two design points,the low Reynolds number aerodynamics of X-wing with semi-wings interaction,and the analysis method of conversion corridor and its performance contour in transition mode,are all studied in detail.And the design method based on key problems is built.Then the principled prototype of X-NMRL is produced,and the related flight tests are carried out to prove the effectiveness and feasibility of above solving methods.Then the guidance for design,propulsion selection and conversion design are given.The main issues and achievements in this thesis are as follow:(1)Without systematic design method for the tail-sitter VTOL SUAVs,the improved parameter selection method based on traditional method is proposed,and the design of X-wing VTOL SUAV is finished.The relationship,between wing-load and power-weight ratio in takeoff and landing mode,transition mode and fixed-wing mode,is drawn into the improved design method.And the total weight is obtained by the statistics model between characteristic parameters and component weight.Finally,the parameter selection of the X-NMRL is finished.(2)To reduce the hidden trouble caused by weight redundancy of two groups of propulsions and the mechanism reliability of variable pitch propellers,the fixed pitch propellers are always used for SUAV,but the difficulty is increased to achieve better efficiency under the condition of thrust requirements at two design points.The propeller calculation model,brushless electric motor model,the electronic speed control model and Li-po battery model are constructed,and the optimization for better current consumption is carried out under the condition of matching property and battery discharge ability.Then the effects of battery voltage and propulsion group number on performance are studied,and the criterion are obtained to show the difficulty for design or selection.(3)Using the numerical method verified by wind tunnel tests,the low Reynolds number aerodynamics of X-wing with semi-wings interaction and the longitudinal and lateral stability are studied.The influence of wing-angle and wing airfoil on aerodynamics are analyzed to make foundation for parameters optimal selection.The research shows that X-wing can improve the lift performance in finite span,and change the flow characteristics and pressure distribution on upper and lower wing.The maximum lift coefficient and stall angle of attack are improved for lower wing.The airframe with 50 deg wing-angle will achieve better aerodynamic performance and stall property,and reflexed airfoil will benefit the transition mode to keep control enough.(4)Based on X-NMRL vehicle,the aerodynamic model at every possible angle of attack and velocity is obtained,and the estimation model for wings in slipstream is established according to the different kinetic pressure and immersed area.Combined with the subsystem models,the conversion corridor model construction method is introduced with main parameters of velocity,pitching angle,angle of attack,thrust and elevator deflection.Using this method,the conversion corridor and its performance contour can be obtained.With the analysis on the conversion corridor boundaries and their dominated parameters,the conclusions are drawn to guide the vehicle design and the transition trajectory selection.(5)Based on the study on three key problems of X-NMRL VTOL SUAV,a multidisciplinary and multi-attribute design method is proposed with the consideration of key problems.This method fully considers the characteristics of the propulsion with two design points,aerodynamics in low Reynolds number and conversion corridor analysis.It will ensure the safe transition and the design effectiveness.(6)In rotor mode,the windward area of X-wing is so big that the yaw control will be not enough to control against disturbance.Then two methods to increase yaw control ability,adding the control surfaces in slipstream or adding the propulsion installation angle,are introduced.According to the conclusions drawn from key technology studies,the principle prototype is produced and the flight tests are carried out.To verify the effectiveness and feasibility of above methods for key technologies,the flight tests for propulsion selection and conversion corridor property are designed and finished.The flight tests show the methods proposed for propulsion selection with two design points and conversion corridor analysis are effective and acceptable.And they are meaningful for the preliminary design,transitional strategy and trajectory design,especially for the design of this kind of tail-sitter VTOL SUAVs.