Without rotors, helicopters cannot achieve flight. In fact, helicopters necessitate two rotors, though the configuration of those two rotors varies. Today, helicopters benefit from two different designs, those of which are single rotor and coaxial rotor variations. The single rotor design is the most common, but its name is deceiving. Single rotor helicopters actually have two rotors, but one of them is located on the tail. Coaxial rotor helicopters, on the other hand, do not have a tail rotor. Instead, they have two main rotors that lift the aircraft and steer as well.
While the large main rotor does a majority of the work to lift the aircraft, the tail rotor on a single rotor helicopter makes sure this work does not go to waste. The tail rotor is responsible for counteracting the torque produced by the large central rotor. As the main rotor spins to lift the aircraft, it generates a torque imbalance over the entire helicopter. Meanwhile, the tail rotor balances the forces generated by the main rotor, allowing the pilot to adjust the direction of the nose when the helicopter is hovering. Tail rotors are typically powered by the same driveshaft at the main rotor, so that they can sync up. Furthermore, they may be built onto the tail in a traditional fashion or built into the tail in a fan-type configuration, called a fan-tail or fenestron design.
There is another design, however, that lacks a second external rotor and is not a coaxial design. In this design, which is called the NOTAR system, a jet of air is sent through a vent on the tail of the helicopter to create a boundary layer of air flowing along the tail boom. This low-pressure air changes the direction of airflow around the tail boom, creating ample thrust opposite to the motion generated by the torque effect of the main rotor. At the same time, a rotating vented drum at the end of the tail boom offers directional control.
Regardless of the configuration of the tail rotors, all helicopters have a torque imbalance that is managed by varying tail rotor designs. When helicopters were first invented, designers and engineers focused on ways to create an aircraft that could hover while also being stable. According to Newton’s third law of motion, every action has an equal and opposite reaction. That being said, when the rotor spins in one direction, there must be an equal and opposite force in the other direction. As such, to generate ample lift, torque must be applied to the main rotor, meaning that engineers must account for the equal and opposite reaction somehow.
To solve this problem, early helicopter designs were equipped with multiple rotors spinning in opposite directions. This is the configuration used in the coaxial design today, solving the torque issue without the need for a tail rotor. In these scenarios, half of the torque turns one rotor in one direction and the other half of the torque turns the other rotor in the opposite direction, rendering the overall torque vector zero.
Though this design garnered popularity in its early application, Russian–American aviation pioneer Igor Sikorsky settled on the single tail rotor design as the primary arrangement for helicopters. While this design is popular, it is not the optimal configuration for flight. There are actually a few significant drawbacks to tail rotor helicopters that are not present on coaxial or dual-main-rotor helicopters.
To begin, tail rotors consume about ? of the engine power, making this a major power drain for the entire aircraft. Since the tail rotor is not tasked with lifting the aircraft, it tends to be fragile. Though this may benefit the lightweight characteristics needed of any aircraft structure, many helicopter crashes are caused by the tail rotor striking something and breaking. Moreover, tail rotors are quite unreliable when it comes to accurately controlling aircraft. As a result of the various forces in the air around helicopters, keeping the aircraft steady on one bearing can be difficult. To counteract these forces, the tail rotor must speed up and slow down at a precise rate.
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