Flight dynamics simulation of formation flight for energy saving using LES-generated wake flow fields

Abstract

Wake vortices are an inevitable result of lift generation and can pose a threat to any aircraft, which accidentally encounters the wake of another aircraft. However, wake vortices can also be used in a beneficial way. Due to its rotational direction, the air flows upwards outside of the vortex pair, giving additional energy to any aircraft located in these regions. This method of saving energy is used by migratory birds, resulting in these birds flying in typical V-formations. This study deals with the question, whether it is possible with a standard autopilot (without a dedicated formation flight mode) to keep the aircraft’s position accurately at a desired position in the wake flow field without accidentally encountering those areas of the wake where steady-state flight is impossible, even in the presence of atmospheric disturbances (e.g. turbulence) and fluctuating vortex core positions. For this purpose, simulations were performed applying three-dimensional flowfields generated with large eddy simulations. Here, even with young vortices, the target sweet spot position varies in the lateral and vertical directions with a magnitude of a few metres at a constant distance behind the generator aircraft. Hence, also the vortex-induced forces and moments change continuously while flying at the same relative position to the leading aircraft. Preliminary simulations with an A320 flying in the wake of an A340, utilizing the regular autopilot of the comprehensive DLR A320 flight simulation model without a dedicated formation-keeping mode, show that the autopilot does not accidentally encounter hazardous regions within the wake. This indicates that it could be sufficient for a formation-keeping autopilot for civil transport aircraft to be designed as the outer loop of the regular autopilot.