Scariest
Some may refer to it as a Zeppelin, but the Piasecki Helistsat looked scary to me from the outset. One day the phone rang in the SA pilots office and the pilot who had hired on to fly it wanted to talk to anyone. He got connected to the pilot in our office who was our strongest in the area of structures/ dynamics ( a former USMC aviator with an Engr MS in that area who came to us from SA Engr Dept. ). The caller was given a whole list of things to look into. Later events seemed to say that some basics were missed. I say that based on the published info which said the landing gear wheels started to shimmy, which then kicked off a mechanical stability ( ground resonance ) situation. With all of the military history of that subject, on that machine, the V-leg gear vs the Straight-legged gear etc, one would have thought that the importance of the landing gear would naturally have been a primary design issue.