Question

why should a helicopter necessarily have two propellers??​

Answer 1

Answer:

to mentain its balance in air and avoid rotating at a place

Answer 2

Answer:

Explanation:

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.