Chao Yang

Active flutter suppression of a two-dimensional airfoil based on sliding mode control method

Flutter, a dynamic instability of aircrafts, may degrade the safety of the structures. Active flutter suppression (AFS) is a considerable solution to this issue. Sliding mode control (SMC) method, a nonlinear control strategy, is applied to AFS of a typical two-dimensional airfoil in this work. The airfoil has a trailing-edge flap utilized for flutter control. The system involves a two-DOF (degree-of-freedom) motion whose equations are constructed by using quasi-steady aerodynamic forces. First, the AFS system is designed by state feedback SMC method to suppress the flutter. Then, a sliding mode observer (SMO) is incorporated for further improvements. Finally, classical Runge-Kutta (RK) algorithm is utilized for simulations. The results indicate that the SMO can estimate the system states fast. Combining with the SMO, the controller could suppress the flutter at 14.8% above the open-loop flutter boundary.

Robust aeroservoelastic stability margin analysis using the structured singular value

A new framework based on structured singular value (μ) analysis is introduced to evaluate the robust stability margin of an SISO aeroservoelastic system with the structural and actuator uncertainties considered. The essential of the proposed method is to extend the nominal gain margin concept based on open-loop analysis to robust stability margin by introducing an extra uncertainty. This newly developed stability margin framework is performed on a large aspect ratio wing model with GVT data to evaluate the considered uncertainties. Three main results are obtained from the current work: (1) The measurement of robust stability margin, compatible with the classical nominal margin evaluation, is practical to associate the nominal aeroservoelastic system and the perturbed one. (2)The results show that the aircraft may go critically unstable with only 0.9% frequency variations, though gain margin of the nominal aeroservoelastic system exceeds 6dB. (3) The sensitivity of gain margin is defined and calculated numerically to decide the most important uncertainties affecting the stability margin. The result shows that the damping ratio of the elastic structure can be omitted to save computation time of μ because of its relative small sensitivity.

Control Surface Efficiency Analysis and Utilization of an Elastic Airplane for Maneuver Loads Alleviation

Flexible effect and multi control surfaces bring new challenges to maneuver load analysis and alleviation for modern aircraft. The load alleviation ability evaluation inclusion of flexibility effect becomes the key of control surface utility. Aeroservoelasticity state-space model of flexible aircraft is deduced based on the modal method and the rational function approximation of unsteady aerodynamics. Rigid-body and elastic modes are included in generalized coordinates. The mode displacement method is used to calculate incremental airplane dynamic loads. The control surface zone is divided into a number of elemental chordwise strips and every one is used as one control surface. The alleviation efficiency is assessed one by one for all control surfaces. It is found that the efficient deflection is down for inboard control surfaces and up for outboard ones and the alleviation percent per unit area of outboard zones is higher than inboard ones. Finally an alleviation scheme is carried out by use of an inboard control surface and an outboard one. The result indicates the load peak value and steady-state value both decline and the load oscillation is suppressed. The assessment method can provide useful reference and a basis for the control surface utilization and control scheme design.