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Loss of tail-rotor effectiveness
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Loss of tail-rotor effectiveness
"Loss of tail rotor effectiveness" shall not be taken literally. It has been used in the early 1980s by the US Army to designate unanticipated yaw of a helicopter and does not mean that at any time the efficiency of the anti-torque rotor is diminished. Such a behavior has never been demonstrated.
This phenomenon remains poorly understood, and literature on the subject provides little light on how it occurs. It is true that flight mechanics struggles to explain a helicopter rotating rapidly to the right with the pedals fully extended to the left. Complex aerodynamic phenomena that cause rapid variations in tail rotor thrust have only been identified, and it has been recommended that the areas in which they occur be avoided.
Accidents characterized by a loss of yaw control on a fully operative aircraft are numerous. An analysis by Airbus Helicopters has shown that they mostly occur close to the ground, where the maneuver usually recommended for recovery is not applicable. Airbus Helicopters explains the phenomenon based on the pedal curve. This explanation assumes that the pilot only counters the yaw start with a limited amplitude control and does not reach the control stop. Objective data (videos, recorders) collected during such accidents are beginning to become more numerous and confirm this limited amplitude.
Airbus Helicopters' explanation covers not only the majority of events in the opposite direction to the rotation of the main rotor, but also the rarer cases in the direction of this rotation and even accidents occurring on NOTAR (NO Tail Rotor) aircraft for which it is difficult to incriminate the interactions between the main rotor and the tail rotor or the vortex regime of the tail rotor.
At the end of the 1970s, the US Army records a series of similar accidents on its OH‑58 helicopters, a military version of the Bell 206. At low speed, during a right turn, the yaw rate (rotation around the vertical axis of the aircraft) increases without any action from the pilot and the tail rotor appears incapable of stopping it.
A complex theory is proposed, involving an alternation of stalls and recoveries of the tail rotor assumed to be at its maximum pitch. The phenomenon is then called " tail rotor stall ", without this theory being experimentally validated.
A new procedure is defined by the US Army which consists, when the yaw movement to the right is detected, to add foot to the right, in the direction of the turn (presumably to recover from the tail rotor stall) and cyclic stick to the front-right. A working group, bringing together different components of the US Army with the participation of Bell Helicopter Textron, is also set up to understand the problem and remedy it. Its findings are released in 1984.
How unanticipated yaw (UY) occurs is not explained, showing that it is not well understood. A curve measured in a wind tunnel giving, as a function of the wind direction relative to the helicopter, the value of the yaw moment created by the tail rotor—at constant main rotor and tail rotor collective pitches—is used to identify the wind directions giving rise to the most significant changes, estimated to be most conducive to the occurrence of UY. Three areas are thus highlighted at low speeds, which pilots are recommended to avoid as much as possible:
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Loss of tail-rotor effectiveness
"Loss of tail rotor effectiveness" shall not be taken literally. It has been used in the early 1980s by the US Army to designate unanticipated yaw of a helicopter and does not mean that at any time the efficiency of the anti-torque rotor is diminished. Such a behavior has never been demonstrated.
This phenomenon remains poorly understood, and literature on the subject provides little light on how it occurs. It is true that flight mechanics struggles to explain a helicopter rotating rapidly to the right with the pedals fully extended to the left. Complex aerodynamic phenomena that cause rapid variations in tail rotor thrust have only been identified, and it has been recommended that the areas in which they occur be avoided.
Accidents characterized by a loss of yaw control on a fully operative aircraft are numerous. An analysis by Airbus Helicopters has shown that they mostly occur close to the ground, where the maneuver usually recommended for recovery is not applicable. Airbus Helicopters explains the phenomenon based on the pedal curve. This explanation assumes that the pilot only counters the yaw start with a limited amplitude control and does not reach the control stop. Objective data (videos, recorders) collected during such accidents are beginning to become more numerous and confirm this limited amplitude.
Airbus Helicopters' explanation covers not only the majority of events in the opposite direction to the rotation of the main rotor, but also the rarer cases in the direction of this rotation and even accidents occurring on NOTAR (NO Tail Rotor) aircraft for which it is difficult to incriminate the interactions between the main rotor and the tail rotor or the vortex regime of the tail rotor.
At the end of the 1970s, the US Army records a series of similar accidents on its OH‑58 helicopters, a military version of the Bell 206. At low speed, during a right turn, the yaw rate (rotation around the vertical axis of the aircraft) increases without any action from the pilot and the tail rotor appears incapable of stopping it.
A complex theory is proposed, involving an alternation of stalls and recoveries of the tail rotor assumed to be at its maximum pitch. The phenomenon is then called " tail rotor stall ", without this theory being experimentally validated.
A new procedure is defined by the US Army which consists, when the yaw movement to the right is detected, to add foot to the right, in the direction of the turn (presumably to recover from the tail rotor stall) and cyclic stick to the front-right. A working group, bringing together different components of the US Army with the participation of Bell Helicopter Textron, is also set up to understand the problem and remedy it. Its findings are released in 1984.
How unanticipated yaw (UY) occurs is not explained, showing that it is not well understood. A curve measured in a wind tunnel giving, as a function of the wind direction relative to the helicopter, the value of the yaw moment created by the tail rotor—at constant main rotor and tail rotor collective pitches—is used to identify the wind directions giving rise to the most significant changes, estimated to be most conducive to the occurrence of UY. Three areas are thus highlighted at low speeds, which pilots are recommended to avoid as much as possible:
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