Analysis of Formation Flight for Energy Saving Using LES-Generated Wake Flow Fields

Abstract

Wake vortices are an inevitable result of the 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 benefi-cial 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 to save energy is performed by migratory birds, resulting in birds flying in a typical V-formation. This study deals with the question, whether it is possible with a standard autopilot to keep the aircraft’s position accurately in the optimal region of the fuel saving sweet spot with atmospheric disturbances and fluctuating vortex core positions without accidentally encountering the hazardous areas of the wake. For this purpose, a simulation study was performed applying three-dimensional flow fields generated with large eddy simulations (LES). Here, even with young vortices the target sweet spot position can vary in lateral and vertical direction 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. Preliminary simulations with an A320 flying in the wake of an A340, utilizing the regular autopi-lot of the DLR A320 simulation model without a dedicated formation keeping mode, show that, nevertheless, the autopilot does not incidentally 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 outer loop for the regular autopilot.