Bell patent
nodrama,
That Bell patent for an anti-flail device only looks effective if you have a failure of the flex coupling itself. It would not appear to be helpful if you experienced a shaft structural failure between the couplings, due to some event like an impact. If the shaft is used on a military aircraft drivetrain, damage due to a projectile impact is something you must seriously consider.
And more importantly, some sort of driveline de-couple device would be especially desireable, in order to isolate the driveshaft failure. And to prevent the flailing shaft from beating a hole through any adjacent (flight critical) structure. Such a de-couple device would not really be helpful for a conventional rotorcraft like a CH-53 or CH-47, but for a tilt-rotor like the V22 (which doesn't have such a device), or even Sikorsky's X2 compound, it would be a very effective improvement to the drivetrain's fault tolerance.
Regards,
riff_raff