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Why do some helicopters have a huge roof propeller and a small one and some just have one big one?
The fancy name for double roof propellers is coaxial contra-rotating rotors. (That is a fancy way of saying they spin in different directions.) Some helicopters use this double rotor instead of a small tail rotor.
Newton's third law says that every action has an equal and opposite reaction, so when the helicopter blades spin in one direction, something must spin in the other to conserve the angular momentum. Most helicopters use a tail rotor to fight against this torque, or they would spin out of control underneath their propellers!
Having two propellers on top of each other but spinning in different directions can also fix this problem.