1 23 CEAS Aeronautical Journal An Official Journal of the Council of European Aerospace Societies ISSN 1869-5582 CEAS Aeronaut J DOI 10.1007/s13272-013-0096-6 Optimum number of engines for transport aircraft employing electrically powered distributed propulsion Hans-Jörg Steiner, Patrick C. Vratny, Corin Gologan, Kerstin Wieczorek, Askin T. Isikveren & Mirko Hornung