Research On Aeroelasticity Of All-movable Control Surface With Freeplay

In modern aircraft structures,freeplay nonlinearities generally exist in the all-movable control fin,folding and driving structures.This is severely threatening the safety of flight,and increasing the forecast difficulties of the aircraft aeroelasticity steady boundary.How to solve these problems has gradually been the worldwide research hotspot and challenge.In this thesis,the analytic modeling,theoretical calculation and wind tunnel test study for an all-movable control surface with freeplay nonlinearity.Then an analytical and testing method for nonlinear aeroelasticity with freeplay is constructed,which may be used in engineering practice.The work in this thesis are summarized as follows:First,an identification modeling method based on a Hammerstein model is put forward to obtain both linear and nonlinear part of the system simultaneously.In this method,the cumulative coherence function is considered as the criterion,and base functions of the nonlinear part is effectively chosen to reduce the number of identified parameters.The modal information of the linear part is acquired by the conditioned reverse path spectral method,and orthonormal base functions are constructed.Both linear and nonlinear part of nonlinear systems could simultaneously be parameterized by using orthonormal base functions.The model parameters of nonlinear systems are calculated through Least Squares Algorithm and Error Eradication Least Squares Algorithm.The validity of the identifying method is verified via an example of a two-dimensional airfoil with the 5th nonlinearity of stiffness in the pitch direction.Second,an identification modeling method of the nonlinear aeroelastic system is set up.A nonlinear aeroelastic system is divided into three linear subsystems by using switching points of freeplay.Its nonlinear part is represented by a Hammerstein model.The iterative sequence of switching points is established,and solved by a method synthesized of non-iterative and iterative algorithms.Then all model parameters including switching points of freeplay are achieved.The identification method is proved effectively by an example of a two-dimensional airfoil with freeplay.Third,a mechanism is designed for the limit cycle oscillation in the wind tunnel test.It is based on a thought,that is,the rudder’s bending stiffness and torsional stiffness are not coupled in the flutter wind tunnel test.Component parts of the mechanism are decoupling pin,fitting with tabs,shaft,bearing pedestal,rotary leaf spring,upper-freeplay leaf spring,lower-freeplay leaf spring,freeplay trim and pedestal with spring.The working principle of the mechanism is for the following reasons: It is easy for an all-movable rudder with a thin rotary leaf spring to flutter in the wind tunnel test.During fluttering,the rudder’s torsional movement could make the rudder collide the upper-freeplay and lower-freeplay leaf spring.Thus,the phenomena of the limit cycle oscillation is generated.The gap between the rotary leaf spring and the upper-freeplay leaf spring could be exactly adjusted via the freeplay trim.Similarly,the gap between the rotary leaf spring and the lower-freeplay leaf spring could also be exactly adjusted via the freeplay trim.The different thicknesses of the rotary leaf spring are at different approximation level of the freeplay nonlinearity.The design of the rotary leaf spring avoids test failure due to static aeroelasticity divergence.Forth,the flutter wind tunnel test is conducted to show that the theoretical analysis agree very well with test data.A set of technical criteria is summarized by obvserving the static aeroelasticity divergence in the limit cycle oscillation test,that is to say,the all-movable control surface with the rotary leaf spring avoid of the static aeroelasticity divergence.Then the thickness of the torsional leaf spring is adjusted to further tests,and successfully obtain the limit cycle oscillation.It is a verification of the mechanism’s effective and design thought’s feasibility,and aslo the theoretical analysis agree very well with test data.