a transmission. The throttle setting must maintain enough engine power to keep the rotor speed within the limits where the rotor produces enough lift for flight. In many helicopters, the throttle control is a single or dual motorcycle-style twist grip mounted on the collective control (rotation is opposite of a motorcycle throttle), while some multi-engine helicopters have power levers. In many piston engine-powered helicopters, the pilot manipulates the throttle to maintain rotor speed. Turbine engine helicopters, and some piston helicopters, use governors or other electro-mechanical control systems to maintain rotor speed and relieve the pilot of routine responsibility for that task. (There is normally also a manual reversion available in the event of a governor failure.) Anti-torque pedals The anti-torque pedals are located in the same place as the rudder pedals in an airplane, and serve a similar purpose—they control the direction that the nose of the aircraft points. Applying the pedal in a given direction changes the tail rotor blade pitch, increasing or reducing tail rotor thrust and making the nose yaw in the direction of the applied pedal New designs known as 'NOTAR' have forgone the use of a tail rotor and instead use an air stream to provide the anti-torque control. This air stream is generated in the fuselage by a small fan or turbine, and directed out of the rear of the tail-boom through vent holes. Internal control vanes can vary this flow, allowing for control of the yaw axis. NOTAR systems have the key benefit of being safer due to no spinning tail rotor. The removal of the tail rotor also removes the associated drag forces, providing a potential increase in efficiency. Flight conditions There are three basic flight conditions for a helicopter: hover, forward flight and autorotation. Hover Some pilots consider hovering the most challenging aspect of helicopter flight. This is because helicopters are generally dynamically unstable, meaning that deviations from a given attitude are not corrected without pilot input. Thus, frequent control inputs and corrections must be made by the pilot to keep the helicopter at a desired location and altitude. The pilot's use of control inputs in a hover is as follows: the cyclic is used to eliminate drift in the horizontal plane (e.g., forward, aft, and side to side motion); the collective is used to maintain desired altitude; and the tail rotor (or anti-torque system) pedals are used to control nose