Research On Nonlinear Aerodynamic Elastic Response Of High-Aspect-Ratio Wings

The flutter of wings is a classical aeroelastic phenomenon. The high-aspect-ratio wing will generate large deformation under the aerodynamic load, and the large deformation has great influence on the aerodynamic load distribution. The coupling between aerodynamics and structure will lead to flutters of aircraft. Thus the flight speed will be limited. Considering the unsteady aerodynamic load based on the Wagner function, geometrical deformation and structural nonlinearity between high-aspect-ratio wing and store, the nonlinear response of the wing with store system is studied by Galerkin method. The effects of several parameters on the flutter critical speed and the aeroelastic stability of the nonlinear system are investigated in this thesis. The main works are as follows:1. The aeroelastic equations of high-aspect-ratio straight long wing are deduced. The stability of the balance point is analyzed based on the algebraic criterion of Hopf bifurcation. Two computing methods of aerodynamic load in time domain are contrasted. The effect of wing parameters on flutter and buckling boundary is analyzed. The results show the critical flutter speed and flutter frequency decrease as aspect ratio increases and there is no influence of gravity center on buckling boundary. The responses of the wing system are complex and the ways to chaos are different by the influence of geometrical nonlinearity.2. The aeroelastic equations of high-aspect-ratio long straight wing with store system are established, and the aeroelastic responses are investigated by numerical simulation method. The results show the increase of store mass and location closely to wing tip and 30-40% half chord before elastic axis will result in a higher flutter speed. Also, the critical flutter speed will decrease with the decrease of joint stiffness of store. The oscillation characteristic of the system in some speed region will be influenced by the initial conditions. There is no relationship between buckling boundary and store parameters. When the joint stiffness of store is smaller, the system flutters firstly. But if the joint stiffness of store is larger, the system buckles firstly, and the responses are different after buckling under different joint stiffness.3. Structural nonlinearity such as freeplay nonlinearity and multiple linearity, is leaded into the system at the jointing point of store. Then the influence of the structural nonlinearity on the flutter characteristic and the responses of the wing with store system are analyzed by numerical simulation method. It can be seen that the limited cycle flutter speed can be predicted by the equivalent linearization method based on KBM. The bigger the gap is, the lower the critical flutter speed is. The bigger the initial value of the freeplay is, the bigger the critical flutter speed is. The freeplay nonlinearity makes the amplitude of limit cycle oscillation (LCO) larger. For the same length of freeplay, the central freeplay nonlinearity has the greatest influence on the critical flutter speed.4. The responses of the wing with store system become more complex under the combined influence of structural and geometrical nonlinearity. With the effect of the central freeplay nonlinearity, the response of bending displacement at wingtip is from convergence to LCO, then to chaos via quasi-period oscillation. With the continue increase of flow velocity, the response presents the alternate phenomenon of period vibration and chaos, then LCO occurs but the position of LCO is different at different speed region.5. The effect of material coupling stiffness on critical flutter speed of composite wings is analyzed based on differential quadrature method (DQM). The results show that the critical flutter speed is not monotonous, but there are two peak values. That means with the increase of material coupling stiffness the critical flutter speed increases first then decreases and then increases again then decreases at last. However, for the wing with store system, the critical flutter speed increases first then decreases.