Design and Evaluation of Control Laws for the X-31A with Reduced Vertical Tail

The characteristics of the thrust-vectored X-31A aircraft with reduced vertical tail were investigated as one of the key research objectives within the German-American VECTOR program. This paper describes the design of the flight control laws for this modified configuration of the X-31A using Nonlinear Dynamic Inversion (NDI), as well as the subsequent handling qualities evaluation in the flight simulator. NDI is based on the inversion of model equations, so that control laws are easily adapted to varying aircraft configurations and, as will be shown in this paper, may be generated automatically with the help of advanced modeling tools. Initially, the NDI control laws for the original X-31A aircraft configuration with full vertical stabilizer were developed in an off-line design phase. Next, the aerodynamic database of the simulator model was supplemented with data from wind tunnel measurements with 30 % and 50 % of the vertical tail removed. Based on the updated simulation model, the control laws were automatically re-generated and implemented in the VECTOR fixed-base flight simulator facility. Six experienced pilots assessed and compared flying qualities (for conventional as well as post-stall flight conditions) for full and 50% tail configurations. In general, Level 1 flying qualities were achieved for both configurations. However, with the tail (including rudder) reduced, controllability limits were reached at lower thrust levels due to the deteriorating thrust vectoring control effectivenes