Mechanism and Improve Methods of Rudder Lock for Mechanically Controlled Aircraft

Rudder Lock is a threat to flight safety, which should be carefully considered in the airworthiness certification. Since general aircrafts which are equipped with mechanical control system and controlled by unprofessional pilots becomes more and more popular, researches on rudder lock are still necessary. In this paper, a mechanically controlled medium sized transport is studied as an example. By analyzing the steady straight sideslip flight test data and flow field numerical simulation results, the mechanism of rudder lock is analyzed. Two aerodynamic improving methods including double-triangle dorsal fin and reducing set-back hinge balance are proposed. The flow field numerical simulation results show that double triangle dorsal fin can improve the stall characteristic of vertical tail, but the strong vortex will move the center of pressure forward, which worsens the rudder lock. Reducing set-back hinge balance can improve rudder lock effectively, while the hinge moment and rudder force will be increased, therefore the gearing ratio and tab should be carefully designed