Why should a helicopter necessarily have two propellers ?

Why do all helicopters have two properllers?

A nucleophile must necessarily have

A nucleophile must necessarily have

A nucleophilic reagent must necessarily have

Why there are two propellers in a helicopter?

Why should a Carnot cycle have two isothermal two adiabatic processes?

Two physical quantities are having the same dimensions. Should their units be necessarily the same? Give an example supporting your answer.