FRANKFURT (Reuters) - Lilium, a German start-up with Silicon Valley-scale ambitions to develop a 5-passenger "flying taxi", has raised a second, $90 million round of financing from top tech investors, making it one of the best-funded electric aircraft projects to date. The company, which took in $11.4 million only last year, has emerged as one of Europe's hottest start-ups for attempting to solve hard physics problems for a new category of aircraft capable of vertical take-off and electric po