When the MD 520N first came out, I did a flight test on it for an article. As it was directly comparable at the time with the MD 500, a pencil and fag packet calculation showed that it was heavier, slower and also had one or two odd directional handling characteristics in the transition phase between the hover and cruise flight and back again. I grant that it was externally a lot quieter, and if you wanted to stick your tail into a tree, safer. It seemed a lot of effort for not a lot of benefit. Rather that someone had an idea, spent a lot of money and didn't bin it before doing a cost/complication benefit analysis.