Motors for rotors

[tom_olsen]

I've been looking at engines for a UL heli design. There is a dizzying number of options to consider from water cooled, air cooled, pull start, push start , snow mobile, motorcycle, airplane, weed eater... you get the picture. For single seat, high HP/Weight ratio that meets UL requirements, what works and what doesn't? Are all engines equal? and if not, can they be converted to be?

[tom_olsen]

I have a Rotax 640 but no specs. Does anyone know where to find spec sheets. Also, does anyone know if the 640 can be run safely in a vertical orientation?