Keith, no challenge involved, only education. You said
Twin engine helicopters usually have a clutch on one engine. only the engine that is to be started first needs to have a clutch
I'm just enquiring what multi engined helos have this sort of set up. On the 76 you can start any engine first (with rotor brake on if you don't wish the rotor to turn, no clutch required, the N1 spins and the N2 doesn't), electrics run off the engine and hydraulics from the main gear box. You could also, given certain stipulations, stop the rotor with the rotor brake and leave an engine running On the 212 and 412 you may start either engine first and once again electrics run off the engine and hydraulics from the main gear box, but you can not start an engine and have the rotor stopped. All three aircraft only have sprag cluches (free wheel units) and no other clutch system. Only one I've flown with a centrifugal clutch was the good old 47. The H-34 (going back 41 years so memory is rather dim) had some sort of fluid coupling run off the hydraulics to spin the rotor up following engine start where upon positive mechanical engagement was made through a sprag clutch set up (I think I have it right). With reference to words we use, every ground school I've under taken has used the words "sprag clutch" interchangeably when referring to the free wheel unit (47 aside). Very interested in which helos (61, Blackhawk, Puma?) have the system as you describe with a clutch system on one engine and how it works. Thanks in advance.