Crab,
You can't tell the cubic feet of air needed to hover from the weight of the helo. Need to know the disk size, too.
You have the momentum thing sort of right, but the real calculation is the mass of air times the speed change you make. A helicopter can be held up if it takes a small amount of air and imparts lots of speed to it, or a large amount of air with little speed. Its identical to a scuba diver's fin size. Big fins move a bunch of water, so the diver needs to pump his legs more slowly. Small fins take more strokes per minute to move the diver the same way.
We could hover a helicopter with a cannon shooting bullets downward, if the bullets weighed enough and the cannon shot enough of them per second. If the velocity of the bullets were high, fewer of them are needed to lift the helo.