Straight-line path-following control for roll-out phase of the equipped-skid aircraft

The paper deals with straight-line path-following control design for the aircraft equipped with skid landing gears. First, a simple yet accurate on-ground aircraft model is established, which takes into account the effects of the aerodynamic and ground forces. To improve the directional stability of the aircraft during the roll-out phase, a novel skid with variable friction coefficients is proposed. Second, the path-following problem is converted to the attitude control problem by constructing a guiding vector field that generates the commanded course, and then an improved error function is proposed to manage the trade-off between the convergence rate and the strong lateral maneuvers. To achieve a good performance in path following, the incremental nonlinear control allocation is applied to make full use of three available actuators (nose wheel, variable friction skid, and rudder). The expected path here is the runway centerline so as to avoid runway excursions. Finally, the effectiveness and robustness of the path-following control are validated on different initial conditions. Results show that the proposed skid structure and control scheme are propitious to enhancing the resistance to crosswind. Moreover, the maximum lateral displacement during the path-following process decreases, especially in the low-speed region.

Properties Analysis of Disk Spring with Effects of Asymmetric Variable Friction

The primary objective of this fundamental research is to investigate the mechanical properties of the disk spring when the friction at the contact edges is asymmetric and varies with the load. The contact mechanics study shows that the static friction and static friction coefficient on fractal surfaces change depending on the normal load. In this paper, a fractal contact model based on the W-M function is used to explore the connection between the static friction and the normal load. Subsequently, taking into account the asymmetry of the contact surface at the edge, the variable static friction coefficient is brought into the existing model to obtain an improved static model of the disk spring. Different fractal dimensions, frictional states and free heights are considered under quasi-static loading condition, the relative errors between this paper and the method using Coulomb friction are also calculated, and experimental validation was performed. The static stiffness and force hysteresis of the disk spring for different forms of asymmetric variable friction are discussed. It is shown that using the variable friction model can improve the computational accuracy of the disk spring model under small loads and help to improve the design and control accuracy of preload and vibration isolation equipment using the disk spring as a component.