Reduction of Wake Vortex Transients in the JAS 39 Gripen

Abstract

JAS 39 Gripen is a modern aerodynamically unstable multi role combat fighter. It has delta canard configuration with all moving canards. It has very high demands on agility, precision, flying qualities and carefree maneuvering. For high precision and performance during demanding control tasks a blend of pure angle of attack vane signal and a complementary filtered angle of attack was selected for feedback at a cost of some sensitivity for wake vortex transients at high load factor and high angle of attack. The very robust carefree maneuver limit function enables the pilot to sustain max commands and thus expose the aircraft to wake vortex passages and their transients. After a number of wake vortex related incidents in the Swedish Air Force service a flight control system control law change was necessary. A wake vortex model was developed and validated with available flight test data from earlier wake vortex passages. The wake vortex model was then used to find the actual problem areas in terms of flight conditions, load factor and angle of attack at the wake vortex passage. A flight control system change was developed using the wake vortex simulation model. The change was tested with extensive batch simulations with favorable results and small transients at passage of a wake vortex. The flight control system change was implemented in the flight control system and flight tested. The flight test validated the batch simulation results and showed small transients at passage of a wake vortex from another aircraft. Flight tests represented a spot check in the flight envelope. The wake vortex simulation model was again checked with flight test results. The flight control laws update is implemented in the JAS 39 Gripen Flight Control System and it will significantly reduce the risk for wake vortex related incidents.