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.
An enclosed tail rotor is safer and resistant to fouling.
James B. Rorke has written: 'Hover performance tests of full scale variable geometry rotors' -- subject(s): Rotors, Testing, Rotors (Helicopters)
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.
Because rotors are noisy and helicopters isually fly close to ground
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).
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
No, birds cannot hover in place like helicopters. Helicopters use rotors to generate lift and stay in one spot, while birds rely on flapping their wings to stay airborne and move forward.