Well, electric ultralights are still largely prototypes (although a quick Googling makes it look like production units may be available soon). They're definite

Well, electric ultralights are still largely prototypes (although a quick Googling makes it look like production units may be available soon). They're definitely not quiet though -- electric motors are quieter than gasoline, but at that size they still make noise. I can't speak as to the radar signature of ultralights. An ultralight that could carry a 50kg payload would be at least the size of a small powered hang glider trike, if we assume negligible weight allowance for the control system and a payload capacity equivalent to the weight of an average human pilot.