Hughes make a very large, tip driven helicopter with 2 blades and no tail rotor in the early 50s. Only one built, as an experimental flying crane for the US Army. I think it was the XH-17. Main rotor diameter was about 130 feet (and you thought that Bell built big 2 bladed machines!).I'm told that NR was so low you didn't need a tach, you just counted seconds between blade passings. Is that the one you're thinking of?