The greatest design strength of helicopters, the horizontal main rotor blade that lets the aircraft take off and land vertically, is also its greatest design issue: as the main rotor spins, it generates enormous amounts of torque, which, if left unopposed, will cause the rest of the helicopter to spin as well, rendering it completely uncontrollable. To balance out this torque, the vertical tail rotor spins in opposition of the main rotor to cancel out the rotary effect.
Like the main rotor, the tail rotor consists of several small airfoils, or blades, usually made from aluminum cores covered in carbon fiber composite materials and can be made with both symmetrical and asymmetrical construction. Tail rotors are powered by the helicopter’s power plant via shaft and bearings and rotate at a speed proportional to the main rotor’s. In both piston and turbine-powered helicopters, the main rotor and tail rotor are mechanically connected through a freewheeling clutch system, which allows the rotors to keep turning and providing lift and thrust in the event of engine failure.
Alternatives to the tail rotor design do exist. McDonnell Douglas has developed the NOTAR (NO TAil Rotor) system, which uses a variable pitch ducted fan driven by the helicopter’s powerplant, mounted inside the fuselage ahead of the tail boom. When exhaust passes through the tail boom to the end, it is expelled out one side, fulfilling the same role as a tail rotor. Other solutions involve multiple tail rotors operating in opposition of one another to cancel out their respective torque. The tandem/transverse rotor system, typically seen on heavy lift and transport helicopters, uses two non-overlapping main rotors that turn in opposite directions.
Coaxial rotors use two rotors that are mounted on the same axis, but spin in opposite directions. Finally, intermeshing rotors turn in opposite directions, and rotate within each other’s planes without colliding thanks to mechanical linkage that prevents them from trying to occupy the same space at the same time.
Like all aircraft parts, tail rotors need regular inspections and maintenance. Many parts are considered life-limited, meaning that they are replaced after a certain number of flight hours, regardless of that part’s condition.