Final answer:
Helicopters have a small tail propeller to counteract the rotational torque generated by the main blades, based on Newton's third law. In helicopters with coaxial rotors that rotate in opposite directions, the torque is neutralized, which provides stability and efficiency in flight.
Step-by-step explanation:
Helicopters utilize a small tail propeller to counteract the torque generated by the rotation of the main lifting blades. According to Newton's third law, every action has an equal and opposite reaction. In the context of a helicopter, as the main blades spin in one direction, they impart a rotational force (torque) on the helicopter body, making it want to spin in the opposite direction.
This phenomenon is due to the conservation of angular momentum. The small propeller at the tail, often referred to as a tail rotor or antitorque rotor, provides thrust that counters this torque, thus preventing the helicopter from spinning uncontrollably.
When a helicopter is equipped with two sets of main lifting blades that rotate in opposite directions, this is known as a coaxial rotor system. The opposite rotation of the two sets ensures that the torque produced by one set is canceled out by the other, essentially neutralizing the overall torque effect and eliminating the need for a tail rotor.
This arrangement, which negates the counteracting forces on the helicopter's body, is preferred as it provides intrinsic stability and can be more efficient in certain flight conditions.
Thus, the correct answer to the question of why helicopters have a small propeller on their tail is to counteract the torque generated by the main blades (Option a).