Some do. The CH-47 has two.
The two rotors spin in opposite directions.
Planes have large wings, Helicopters have rotors
Helicopters have rotors airplanes have propellers
Those are the helicopters designed by Aerospatiale (which was later merged into Eurocopter). IIRC, the first model to use it was the Dauphin.
James B. Rorke has written: 'Hover performance tests of full scale variable geometry rotors' -- subject(s): Rotors, Testing, Rotors (Helicopters)
An enclosed tail rotor is safer and resistant to fouling.
Helicopters don't just have 3 blades. It depends on the size and weightlifting capability. Small helicopters such as a Robinson R22 have only 2 blades, the military Merlin has 5 and the Chinook has two rotors with usually 3 blades each.
Helicopters do not have any wings. Airplanes have wings Helicopter has rotors. A helicopter has at least two sets of rotors. One rotor creates lift ( Main Rotor on the top ) & the other prevents the helicopter from spinning in circles( Tail Rotor).
Because rotors are noisy and helicopters isually fly close to ground
R. E McFarland has written: 'Establishment of a rotor model basis' -- subject(s): Models, Aerodynamics, Rotors (Helicopters), Rotors
Robert B. Taylor has written: 'Helicopter rotor blade design for minimum vibration' -- subject(s): Rotors (Helicopters), Stability of helicopters
William T. Yeager has written: 'Loads and performance data from a wind-tunnel test of model articulated helicopter rotors with two different blade torsional stiffnesses' -- subject(s): Wind tunnels, Rotors (Helicopters), Blades