Originally Posted by
soarbum
While reading about the engineering challenges and additional height associated with the Defiant X double rotor, I wondered to myself why they didn't go for a conventional main rotor and articulate the tail rotor (a bit like the Valor but horizontally) so that it could be configured like a standard heli at low speed but as a pusher at high speed. It would probably also require either one or two winglets to counter the effect of retreating blade stall at high forward airspeed. Just when I though to myself "Eureka", I of course found that something similar has been tried already.
https://en.wikipedia.org/wiki/Piasecki_X-49_SpeedHawk
What worked against Speedhawk was that it turned out to offer no significant advantages (including speed, as it turns out) over a conventional helicopter and was a lot heavier.