Optimization of a main landing gear locking mechanism using bifurcation analysis

A key part of the main landing gear (MLG) of a civil aircraft is its locking mechanism that holds the gear in the deployed or down-locked state. The locking is driven by a spring mechanism and its release by the unlock actuator. This paper considers this mechanism in terms of its stability and the locking and unlocking forces required for down-locking. To study this an analytical model was developed. The equations, consisting of geometric constraints and force/moment equilibriums, were derived using the coordinate transformation method. Using numerical continuation to solve these equations, the effect of the unlock force on the MLG retraction cycle was analyzed. The variation of a fold bifurcation point, which indicates the transition between the locked state and the unlocked state, gives further insight into the required unlock force that governs the sizing of the unlock actuator. Moreover, some important information, such as the critical position for the lock-links’ stops, the unlock position and the unlock force, are discussed using the bifurcation diagrams for the MLG retraction/extension cycle. Then, the effect of three key geometry parameters of the locking spring (the spring stiffness, unstrained spring length and spring attachment point) on the critical over-center angle and the unlock force are investigated. Finally, an optimization of the critical unlock force is carried out with a constraint on the initial over-center angle. The results show that the spring parameters have significant effects on the MLG’s retraction performance. A 37% reduction of the required unlock force is obtained through optimizing for the gear considered here.