Loss of tail-rotor effectiveness
Encyclopedia
Loss of Tail-rotor Effectiveness (LTE) occurs when the tail rotor of a helicopter
is exposed to wind forces that prevent it from carrying out its function—that of cancelling the torque of the engine and transmission. Any low-airspeed high-power environment provides an opportunity for it to occur.
. Note that a full autorotative landing isn't necessary—the mere entry into autorotation will eliminate the torque, and then the spin or yaw will reduce through friction, particularly with the buildup of forward speed. At that point power recovery (while maintaining airspeed) can be successfully accomplished.
Helicopter
A helicopter is a type of rotorcraft in which lift and thrust are supplied by one or more engine-driven rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forwards, backwards, and laterally...
is exposed to wind forces that prevent it from carrying out its function—that of cancelling the torque of the engine and transmission. Any low-airspeed high-power environment provides an opportunity for it to occur.
Three wind directions conducive to LTE
- Main-rotor vortexes pushed into the tail rotor by wind. This can occur with wind coming from 10 O'clock on North American (counter-clockwise) rotors and from 2 O'clock on clockwise rotors. The wind pushes the dirty air and vortexes generated from the main-rotor into the tail-rotor preventing the tail rotor from having clean air to propel.
- Wind from the tail (6 O'clock) can cause the helicopter to attempt to weathervane into the wind. The winds passing on both sides of the tail rotor make it teeter between being effective (providing thrust) and ineffective (not providing thrust). This creates a lot of pedal work for the pilot to eliminate unintended yaw.
- Wind from the tail-rotor side (left on North-American with counter-clockwise main-rotor systems, and right on clockwise main-rotor systems). The wind going through the tail-rotor causes an actual stall conditionStall (flight)In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases. This occurs when the critical angle of attack of the foil is exceeded...
as it decreases the effective airspeed of the air through the tail-rotor. This condition will cause an unintended yaw that may develop into a spin. Recovery from this condition may be difficult if no airspeed is available, and will require entry into an autorotation (thus removing the torque of the engine and transmission).
External Factors
Environmental factors which make it easier to occur include higher operating density altitudes or temperatures, and high winds. A high gross weight can also create an LTE conducive situation.Indications of the onset of LTE
Many factors affect the onset of LTE, but no matter which way it occurs, the clues are present:- An environment of low airspeed, and a demand for power.
- An unintended yaw that may even be opposite of pedal input.
Recovery
Recovery is initiated by increasing airspeed, using the vertical stabilizer to reduce yaw, or if uncorrectible by application of speed or tail-rotor thrust, entry into autorotationAutorotation
In aviation, autorotation refers to processes in both fixed-wing and rotary-wing aircraft. The term means significantly different things in each context....
. Note that a full autorotative landing isn't necessary—the mere entry into autorotation will eliminate the torque, and then the spin or yaw will reduce through friction, particularly with the buildup of forward speed. At that point power recovery (while maintaining airspeed) can be successfully accomplished.