Shawn: I'm not sure I understand your point about F/W. The issue with rotorcraft and IFR isn't merely an issue that they're single engine. (My hazy memory recalls an IFR STC for a B407 at some point in the past.) Rather, the limiting factors probably relate more to:
- the additional requirements for handling qualities (meaning you probably need SAS or autopilot) in Appendix B (which don't exist for F/W, since their H/Q are generally acceptable for IFR w/out an AFCS); and
- the related 1309 requirements for system reliablity/redundancy ... electrical, hydraulic, and SAS/autopilot, gyros, etc. ... "fly to destination with a failed system under IFR, yada yada"
These requirements add space, weight, power, and integration penalties which may be significant in many single-engine rotorcraft.