You guys need to read up on flight theory, and EricH needs to read up about buoyancy.
In order to make a craft fly you don't necessarily need to generate more thrust than weight. Furthermore, a helicopter isn't exactly a giant propeller on top of a body. It is a rotating wing. A quadrotor relies on conservation of momentum to stay aloft - push air under it to force it upwards. A wing however relies on bernoulli's principle where fast moving air over the top of a wing results in a lower pressure which "sucks" the craft upwards.
The maximum takeoff weight of a 747 is ~980000lbs however its engines only generate ~67000lbs times 4 = 268000lbs How does it fly?
By this ratio, a robot weighing 120lbs.. if shaped like a 747, would only need 33lbs of thrust to fly which is quite possible!
Even though it is possible, to achieve it within the confines of FIRST rules makes it extremely difficult.