JAS 39 Gripen. Wake Vortex Penetration Flight Testing

Hans-Erik Hanson, Saab AB, Flight Test and Verification, Linköping, Sweden

Abstract

JAS 39 Gripen is basically a highly unstable a/c in pitch in the subsonic Mach region. Pitch static stability is obtained by means of AoA vane feedback. For optimum control during demanding control tasks, a blend of proportional vane signal and complementary filtered AoA was selected. However, the good control characteristics were achieved at the cost of some sensitivity to vortex transients.

The very robust AoA MLL function in the control laws entice pilots to sustained max commands below corner speed and thereby expose the a/c to a vulnerable condition for Wake Vortex disturbances. After a number of Wake Vortex related incidents in SwAF service there was an obvious need for a new control law.

A new flight control system edition was developed and tested in large batch simulations of more than 12000 wake vortex passages to prove that the new flight control system edition had a significant effect on transient reduction during a Wake Vortex passage.

Flight test represented spot checks in the flight and maneuver envelope, to verify the Wake Vortex simulation model and to verify the significant improvement of the new flight control system edition.

Flight testing, behind another Gripen fighter, was limited to a max entry maneuver of 5 g and AoA 15 to 20 deg at FL 200 and M0.7. Flight conditions for the generating fighter were also limited to those values.

The flight testing with the new flight control system edition was successfully performed. The results indicated small transients at passages of the wake vortices. Max AoA always stopped below MLL AoA-limit with the WVTR-function activated. After the first flight test period there was an optimization update of the control law. The final FCS-edition was then cleared for squadron service and thereby significantly lowering the risk for wake vortex related incidents during squadron service.

Propeller wash and jet streams can also upset the AoA vanes. Flying in the propwash from a C-130 to closing distances within AAR positions was therefore also performed. Good results were obtained.

This paper describes the Gripen Vortex penetration and propwash flight test program performed during 2005 and 2006 at Saab in Linkoping, Sweden.