mech fan blade rpm limit?

It doesn't engage and disengage like a clutch on an air-conditioning compressor, think more of a clutch that is slipping, like engaging and disengaging a centrifugal clutch on a go cart. It doesn't shock the system. But, I would put a blue anodized flex fan in there because all the guys on here that suggested a clutch fan uses less horsepower and makes less noise probably don't know what they're talking about.