In-Flight Simulation of Wake Encounters Using Deformed Vortices

During the decay process the shape of wake vortices changes significantly which has an influence on the encounter characteristics, hence on the encounter hazard. In order to evaluate the influence of vortex deformation on the wake encounter hazard, in-flight simulations with the DLR research aircraft ATTAS were carried out. For a realistic analysis of wake encounters flow fields of matured vortices were generated with large eddy simulations. These flow fields were used for the determination of histories of induced forces and moments acting on the wake encountering aircraft. The force and moment histories were then fed into the equations of motions of the non-linear six degree-of-freedom in-flight simulation of the DLR research aircraft ATTAS. In order to compare different stages of vortex deformation, encounters were simulated in flight with wavy vortices and vortex rings. The most important benefit of the in-flight-simulation is the realistic environment, which enables a realistic assessment of pilots’ encounter acceptance. The analysed scenario was of a wake encounter during final approach. The encounter conditions correspond to separation distances of about 4 nm and 5 nm behind an aircraft of the “heavy” category. During the encounters the ATTAS was flown under manual control. Altogether 31 encounters were simulated in flight, 9 with wavy vortices and 22 with vortex rings. The analysis of the results shows that with increasing vortex deformation the encounter impact acts increasingly in all degrees of freedom. Apparently, pilots assess roll-dominated encounters as more dangerous, which is why the unacceptability threshold is higher for encounters with vortex rings than for those with wavy vortices. Besides this unacceptability boundary, a harmlessness boundary can be determined, which seems to be independent from the degree of vortex deformation. This is a most important outcome of the flight test described in this paper. Although, due to the very limited number of flight tests performed, no clear correlation between the pilot ratings and objective measures could be assessed, an acceptance boundary for the roll control ratio could be determined below which the effects of vortex deformation seem to be negligible