Dennis Vechtel

Flight Simulator Study on the Influence of Vortex Curvature on Wake Encounter Hazard using LES Wind Fields

A flight simulator study has been carried out to evaluate the performance of modern flight control systems encountering curved wake vortices. During the decay process the shape of wake vortices alters significantly which has an influence on the encounter characteristics and thus on the encounter hazard. To analyse most realistic wake encounters, flow fields of matured vortices have been generated with large-eddy-simulations. These were used for the determination of induced force and moment histories during the encounter. The force and moment histories have been implemented into the equations of motions of a 6 DoF flight simulation. For the sake of comparison encounters have also been simulated with straight vortices as they were mostly used for many other investigations. The most important goal of the study was to analyse the difference between these ideally straight vortices and vortices with a more realistic shape regarding encounter acceptance. The simulator study was conducted in an A330 motion-based full-flight simulator with pilots-in-the-loop. The analysed scenario was a wake encounter during final approach. The encounter conditions corresponded to a heavy-behind-heavy situation for Instrument Flight Rules (IFR) operations. The aircraft was flown either manually (in normal law) or with autopilot engaged. Altogether 93 encounters have been simulated, 38 with straight and 55 encounters with curved vortices. For encounters under manual control the simulator study revealed a potential risk of pilot induced oscillations (PIO) during encounters with curved vortices. With autopilot engaged not even one encounter with curved vortices was classified to be unacceptable. Although significant aircraft response was experienced the autopilot never disengaged automatically in any encounter. Altogether about 12 percent of the encounters were not accepted by the pilots. This is indeed a significant number, especially as the analysed scenarios can be regarded to be realistic situations which can occur in reality even if the applicable separation distances are applied.

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.