Flight Test Verification of a Wake Vortices Model

Thomas Lampe, Flight Test and Verification, SAAB Aerosystems, Linköping, Sweden
Yngve Sedin, Aeronautical Engineering, SAAB Aerosystems, Linköping, Sweden
Per Weinerfelt, Aeronautical Engineering, SAAB Aerosystems, Linköping, Sweden

Abstract

A simulation model for encounters of wake vortices has been developed at Saab in Linköping, Sweden. Distribution of incremental forces on the aircraft surfaces is calculated for different locations related to the vortices and also for different relative attitudes of the aircraft. Also, the response of the angle-ofattack and angle-of-sideslip vanes is calculated. For simulation, multidimensional tables are created describing the increments in forces, moments and vane angles as function of relative position and attitude.

The model, represented by table lookup algorithms for these particular tables as well as some scaling algorithms, was in the year 2000 added to an experimental version of a 6-DOF Gripen simulator. The results seemed promising because the response of the aircraft flying through the vortices was, seen from a pilot’s view, realistic. However, it was evident that the model also needed to be verified with and, most likely, adjusted (scaled) to flight test data.

The need for accurate estimates of the aircraft’s trajectory through the vortices requires special attention. Regardless of the ability to accomplish that accuracy in relative position between two aircraft, we still have to determine, also very accurately, where the vortices are, regarding cores and size. Local wind fluctuations and the self-induced motion of the vortices away from the flight path contribute with uncertainties in the range of a couple of meters. From that standpoint, we have too large uncertainties.

Our best solution so far is to use a “closed-loop” verification of the model where we use a subset of the output parameters to identify a two-dimensional position bias that gives us the most likely trajectory. We use the angle-of-attack and angle-of-sideslip vane signals for that purpose.

The result is not only a method for verification of a trailing vortex simulation model but also a tool to map out in detail where we have been in wake vortex penetration tests. With respect to the simplifying assumptions used in the design of the current model, the agreement with flight test data is surprisingly good.