Hi and welcome to Arturia forums.
FYI: This forum is for audiofuse. There is a forum for each software further down the page.
Tha said, then you control the speed using the LFO rate on the Prophet the factory bank 55-Helicopter sound. That is if it's the correct preset. The Modwheel controls the depth of the LFO.
EDIT: I see the post have been moved to here by a moderator. EDIT END.