Question

Why there are two propellers in a helicopter?

Answer 1

The reason there are two propellers in a helicopter is because if there would be only one rotor, the helicopter would start rotating in the direction opposite to the movement of that rotor. 
The two rotors rotate in the opposite direction as to cancel the torque produced by one propeller. 
One moves left-right, other in right-left. 
Hope it helps!
HARRY POTTER FOREVER!

Answer 2

HLO MATE HERE U GO

⭐. If there were only one propeller, then according to law of conservation of angular momentum , the helicopter would rotate itself in the opposite direction .