RE: ducted fan engines
Not even close, typically most fan engines turn less RPM's with a prop than with a fan, at least from what I've seen and read, Not because the engine isn't capable of turning those speeds, but because most guys with a brain realize you best prop them in a range where the props won't explode. The biggest challenge with larger props is not exceeding their safe RPM's. Sure you might get close to 30,000rpm with a 7x8 pylon prop, but you will get that with a Nelson QM40 or JETT QM40 too and not have to deal with the weight issue. Once you start getting up to a 14" prop which you might get some use out of an OS91VRDF you really don't want to exceed probably 16,000 or 17,000 RPM.