Final answer:
A single main rotor helicopter tends to drift in the direction of tail rotor thrust, known as translating tendency or tail rotor drift, due to the reactive torque countered by the tail rotor as explained by Newton's third law. In dual-rotor helicopters, the opposite spinning of the two sets of blades cancels out this torque.
Step-by-step explanation:
During hovering flight, a single main rotor helicopter tends to drift or move in the direction of tail rotor thrust. This is often referred to as translating tendency or tail rotor drift. The single main rotor produces lift and, due to the conservation of angular momentum, would cause the helicopter's body to rotate in the opposite direction if not countered by the tail rotor. According to Newton's third law, for every action, there is an equal and opposite reaction. Thus, as the main rotor spins in one direction, the body of the helicopter will try to spin in the opposite direction due to the reactive torque.
The tail rotor, which provides thrust perpendicular to the main rotor's rotation axis, counters this torque and stabilizes the helicopter. However, the thrust from the tail rotor also pushes the helicopter in the direction opposite to where the tail rotor is pointing, leading to the translating tendency. When helicopters have two sets of lifting blades rotating in opposite directions, this cancels out the reactive torque, which is why dual-rotor helicopters do not require a tail rotor.