Loss of Tail Rotor Thrust and Critical Azimuth

Losing tail rotor thrust in a helicopter can be deadly. The tail rotor provides yaw / heading control and prevents a helicopter from spinning out of control, particularly at low speeds. Unfortunately, there are a few phenomena that can reduce tail rotor thrust in exactly these low speed conditions where it’s most needed. This article reviews the role of the tail rotor and explains phenomena that can reduce its effectiveness in low speed flight.

• Background
• Phenomena
  • Tail rotor vortex ring state
  • Critical azimuth
  • Main rotor vortex interference
  • Weathercock stability

Background

Tail rotors provide heading control for helicopters. For American helicopters, engine torque spins the main rotor counterclockwise (CCW), viewed from above. If not for the tail rotor, this torque would spin the fuselage the opposite direction (CW). (All the directions here and after are opposite if the main rotor spins CW, which is the case for many helicopters manufactured outside the US.)

Pilots use pedals in the cockpit to control tail rotor thrust. Pushing her left pedal down, a pilot causes the tail rotor to increase thrust and turn (yaw) the helicopter nose left. Pushing the right pedal decreases thrust (and can even create thrust in the opposite direction), which turns the nose right.

This control of tail rotor thrust is critical. Without it, the helicopter would spin (yaw) uncontrollably. The intent of this article is to explore phenomena that cause temporary loss of tail rotor control. In the next section, we devote a subsection to each phenomenon.

Phenomena

Tail rotor vortex ring state

All rotors that create thrust also emit trailing vortices—pockets of whirling air. This is true for wind turbine rotors, helicopter main rotors and tail rotors. These vortices are self-propelled in the direction opposite to rotor thrust.

In the case of a helicopter tail rotor thrusting to the right, the vortices travel left. This is shown in the diagram below. A problem arises when the airflow relative to the tail rotor keeps the vortices close to the rotor. For example, if the vortices move 50 feet per second (fps) to the left, but the helicopter is hovering in a wind of about 50 fps from the left (a condition that should be avoided). The vortices emitted from the tail rotor effectively can’t move away and just “hang out” in the vicinity of the tail rotor.

When the vortices are compressed around the rotor, it becomes inefficient. The airflow is very turbulent, making it hard for the rotor blades to create lift. This condition is called vortex ring state. Tail rotor thrust is mostly lost so that the helicopter starts spinning nose right. Adding left pedal doesn’t help; the turbulent air renders the tail rotor mostly ineffective.

It’s worth noting that this flow condition around the tail rotor doesn’t have to be due to wind. The situation is essentially the same if the pilot is in left lateral flight at 50 fps, without any wind. It’s also similar if the pilot is yawing the helicopter nose right at a high yaw rate, so that the tail rotor is moving leftward at the same speed. Often it’s a combination of these scenarios that triggers the problem, e.g. a pilot yawing nose right into a condition with wind from the left.

Critical azimuth

Another problem can occur with wind or flight in the opposite direction (wind from the right). The pedals control tail rotor thrust by feathering (pitching) the blades of the tail rotor. Left pedal moves the leading edge of the blades right (trailing edge left) so that they “dig into” the air more and create more thrust. The more wind from the right (or flight speed to the right), the more left pedal that is needed to obtain a given amount of thrust.

With enough wind, the left pedal will become fully depressed. At that point, no more thrust can be created. Additional wind will remove thrust and cause the nose to turn right. Angles for which this is worst (where full left pedal is required with the lowest wind speed) are called critical azimuths. Again, these azimuths will typically be around 90 deg (wind from the right side or helicopter moving right).

An often missed point about this critical azimuth condition is that it self-corrects. As the nose turns right, the helicopter heads into the wind. As the flow changes from right-to-left to head-on, tail rotor thrust increases. The helicopter will be able to hold a new heading with much less left pedal. Right pedal will be required as the flow condition improves, otherwise the tail rotor will attempt to push the helicopter back to the critical azimuth.

Main rotor vortex interference

As mentioned earlier, all thrusting rotors emit vortices. Main rotor vortices are self-propelled downward, but also are moved by the relative wind. Depending on the location of the tail rotor relative to the main rotor, there's a range of wind speeds and directions where main rotor vortices significantly affect tail rotor thrust. In particular, with wind from the front-left side, main rotor vortices can induce substantial right-to-left flow through the tail rotor. This is akin to the critical azimuth condition—it will require more left pedal and possibly cause the pilot to "run out" of left pedal.

For the technical reader, a detailed analysis of this phenomenon in both CFD and wind tunnel experiments was performed in "Main Rotor Wake Interference Effects on Tail Rotor Thrust in Crosswind."

Weathercock stability

Helicopters are directionally stable only when flow is head-on, like flying forward or hovering in a headwind. In this condition, the helicopter will naturally return to its initial yaw state after a disturbance. This is referred to as weathercock stability, because the helicopter naturally wants to turn into the wind like the weathercock device that measures wind direction.

In aft flight (or hovering with a tailwind), helicopters have negative directional stability. Any yaw disturbance gets magnified by the helicopter—if the nose is pushed right, it will naturally accelerate further to the right. The pilot (or software control system) has to work hard to keep the helicopter facing the same direction. So, while this phenomenon itself doesn’t represent a loss of tail rotor effectiveness, it can cause a lack of yaw control and lead to other phenomena described above (e.g. losing yaw control hovering in a tailwind could turn you to an azimuth where the other problems occur).

Pedal curve

Much of the phenomena above can be understood by examining pedal curves below. For a particular helicopter, these show the required pedal position in hover, as a function of the wind direction, at various wind speeds. Of course, with 0 wind speed the direction is immaterial, and the pedal curve is flat as shown below. As the wind speed increases, the pedal at 90 deg azimuth (wind from the right) moves left, as discussed above. With enough wind, usually above 40 kt, full left pedal will not be enough.