Competitiveness of air taxis regarding door-to-door travel time: a race through Germany / Deutscher Luft- und Raumfahrtkongress 2020 ; G. Schuh, M. Spangenberg, J. Hoh, M. Freitag (Fraunhofer Institute for Production Technology IPT, Germany), E. Stumpf (Chair and Institute for Aerospace Systems, Germany)