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Recent testing of the OH-58 series helicopter operated by the U.S. Army has revealed the occurrence of an unanticipated right yaw under certain low speed mission conditions. The Army has referred to the right yaw characteristic as loss of tail rotor effectiveness (LTE). The following is a discussion of low speed flight characteristics which can result in an unanticipated right yaw if appropriate attention is not paid to controlling the aircraft. These characteristics are present only at airspeeds less than 30 knots and apply to all single rotor helicopters.

DEFINITION OF UNANTICIPATED RIGHT YAW

Unanticipated right yaw is the occurrence of an uncommanded right yaw rate which does not subside of its own accord and which, if not corrected, can result in the loss of aircraft control. The term ``loss of tail rotor effectiveness'' is misleading. The tail rotor of the OH-58 and the 206 series helicopters has exhibited the capability to produce thrust during all approved flight regimes.

LOW SPEED FLIGHT CHARACTERISTICS

Four aircraft characteristics during low speed flight have been identified through extensive flight and wind tunnel tests as contributing factors in unanticipated right yaw.

For the occurrence, certain relative wind velocities and azimuths (direction of relative wind) must be present. The aircraft characteristics and relative wind azimuth regions are:

1. Weathercock stability (120 to 240 degrees)
2. Tail rotor vortex ring state (210 to 330 degrees)
3. Main rotor disc vortex interference (285 to 315 degrees)
4. Loss of translational lift (all azimuths)

The aircraft can be operated safely in the above relative wind regions if proper attention is given to controlling the aircraft. However, if the pilot is inattentive for some reason and a right yaw rate is initiated in one of the above relative wind regions, the yaw rate may increase unless suitable corrective action is taken.

WEATHERCOCK STABILITY (120 to 240 DEGREES)

Winds within this region will attempt to weathervane the nose of the aircraft into the relative wind. This characteristic comes from the fuselage and vertical fin. The helicopter will make an uncommanded turn either to the right or left depending upon the exact wind direction unless a resisting pedal input is made. If a yaw rate has been established in either direction, it will be accelerated in the same direction when the relative winds enter the 120 to 240 degree shared area of Figure 1 unless corrective pedal action is made. The importance of timely corrective action by the pilot to prevent high yaw rates for occurring cannot be overstressed.

TAIL ROTOR VORTEX RING STATE (210 to 330 DEGREES)

Winds within this region, as shown in Figure 2, will result in the development of the vortex ring state of the tail rotor. The vortex ring state causes tail rotor thrust variations which result in yaw rates. Since these tail rotor thrust variations do not have a specific period, the pilot must make corrective pedal inputs and the changes in yaw acceleration are recognized. The resulting high pedal workload in tail rotor vortex ring state is well known and helicopters are operated routinely in the region. This characteristic presents no significant problem unless corrective action is not timely. If a right yaw rate is allowed to build, the helicopter can rotate into the wind azimuth region where weathercock stability will then accelerate the right turn rate. Pilot workload during vortex ring state will be high; therefore, the pilot must concentrate fully on flying the aircraft and not allow a right yaw rate to build.

MAIN ROTOR DISC VORTEX (285 to 315 DEGREES)

Winds within this region, as shown in Figure 3, can cause the main rotor vortex to be directed onto the tail rotor. The effect of this main rotor disc vortex is to change the tail rotor angle of attack. Initially as the tail rotor comes into the area of the main rotor disc vortex during a right turn, the angle of attack of the tail rotor is increased. This increase in angle of attack requires the pilot to add right pedal (reduce thrust) to maintain the same rate of turn. As the main rotor vortex passes the tail rotor, the tail rotor angle of attack is reduced. The reduction in angle of attack causes a reduction in thrust and a right yaw acceleration begins. This acceleration can be surprising, since the pilot was previously adding right pedal to maintain the right turn rate. Analysis of flight test data during this time verifies that the tail rotor does not stall. The helicopter will exhibit a tendency to make a sudden, uncommanded right yaw which, if uncorrected, will develop into a high right turn rate. When operating in this region, the pilot must anticipate the need for sudden left pedal inputs.

LOSS OF TRANSLATIONAL LIFT

The loss of translational lift results in increased power demand and additional anti-torque requirements. If the loss of translational life occurs when the aircraft is experiencing a right turn rate, the right turn will be accelerated as power is increased, unless corrective action is taken by the pilot. When operating at or near maximum power, this increased power demand could result in rotor rpm decay.

The characteristic is most significant when operating at or near maximum power and is associated with unanticipated right yaw for two reasons. First, if the pilot's attention is diverted as a result of an increasing right yaw rate, he may not recognize that he is losing relative wind and hence losing translational lift. Second, if the pilot does not maintain airspeed while making a right downwind turn the aircraft can experience an increasing right yaw rate as the power demand increases and the aircraft develops a sink rate. Insufficient pilot attention to wind direction and velocity can lead to an unexpected loss of translational lift. The pilot must continually consider aircraft heading, ground track, and apparent groundspeed, all of which contribute to wind drift and airspeed sensations. Allowing the helicopter to drift over the ground with the wind results in a loss of relative wind speed and a corresponding decrease in the translational lift produced by the wind. Any reduction in translational lift will result in an increase in power demand and anti-torque requirements.

RECOVERY TECHNIQUE

If a sudden unanticipated right yaw occurs, the following recovery technique should be performed:

1. Pedal - full left; simultaneously, cyclic - forward to increase speed.
2. As recovery is effected adjust controls for normal forward flight.

   CAUTION

   COLLECTIVE PITCH REDUCTION WILL AID IN ARRESTING THE YAW RATE BUT MAY CAUSE AN EXCESSIVE RATE OF DESCENT. THE SUBSEQUENT LARGE, RAPID INCREASE IN COLLECTIVE, TO PREVENT GROUND OR OBSTACLE CONTACT, MAY FURTHER INCREASE THE YAW RATE AND DECREASE ROTOR RPM.

   THE DECISION TO REDUCE COLLECTIVE MUST BE BASED ON THE PILOT'S ASSESSMENT OF THE ALTITUDE AVAILABLE FOR RECOVERY.

3. If spin cannot be stopped and ground contact is imminent, an autorotation may be the best course of action. Maintain full left pedal until the spin stops, then adjust to maintain heading.

The various wind directions can cause significantly differing rates of turn for a given pedal position. The most important principle for the pilot to remember is that THE TAIL ROTOR IS NOT STALLED. Thus, the corrective pedal position to be applied is always in the normal direction of OPPOSITE PEDAL to the turn direction.