The most recent improvement I am aware of, prepared for correction, is that of the British Experimental Rotor Program. The paddle at the end of the blade seems to delay the tip reaching the speed of sound due to the increase in surface area, therefore allowing the advancing blade/helicopter to go faster.
Due to the cr@p that the mill and police strap onto their aircraft I have never been able to achieve VNE straight and level. We live in hope.
