I always wonder why people are not trying to build these with helium balloons for lift. This would free up a lot of power for propulsion via the rotors. I suppo

I always wonder why people are not trying to build these with helium balloons for lift. This would free up a lot of power for propulsion via the rotors. I suppose you'd need a very large balloon to lift the entire thing, but why not, say, one half the size, for some of the lift? This way it wouldn't float away either. Anyone know the reasons this is not being done?