Newton’s third law of motion says, for every action, there is an equal and opposite reaction. So, when a helicopter’s rotor system spins in one direction, the fuselage wants to spin in the opposite direction (since this is a rotational force it is called torque). To prevent this engineers put a small thrust-producing rotor on a moment arm (the tail boom) to create a rotational force (torque) that is equal, but opposite, to the force trying to spin the fuselage. Its technical name is an anti-torque rotor, however it is often referred to as a tail rotor.
A set of pedals in the cockpit change the pitch of the tail rotor to vary the amount of thrust produced. Although they control yaw, they function differently than rudder pedals in an airplane.
As long as the main rotor rpm stays constant, so will the tail rotor’s. In fact, if you turn the main rotor by hand the tail rotor will also turn. This is because a system of drive shafts and gearboxes directly connect it to the main rotor transmission. Depending on the helicopter’s design, the tail rotor will spin 3 to 6 times faster than the main rotor.
When viewed from above, most main rotor systems spin counterclockwise (CCW). Sometimes people refer to this as the American direction and clockwise (CW) as the European direction. This is not really accurate as some models built in Europe also turn counterclockwise. For example, Augusta (based in Italy) manufactures models that spin CCW and several Eurocopter models (EC135, EC145) do as well. However, the most popular helicopter with a CW turning rotor system is the Eurocopter Astar.
The rotor system’s rotational direction makes very little difference to gravity or air, but it does change things a little for the pilot. When a pilot increases power (raising the collective control) the torque applied to the fuselage increases. In a CCW turning rotor the pilot must add left pedal to increase the tail rotor’s pitch, and therefore thrust, to keep the nose straight. Likewise, decreasing power requires right pedal input. Right pedal reduces the pitch and thrust allowing excess engine torque to turn the fuselage. In a CW turning rotor just the opposite is true.
Pilots who routinely switch between airframes with different rotor directions, have to remember which one they are in as over time collective movement and the associated pedal movement become automatic. Even if they forget, it is not that big of a problem as it is fairly easy to just react to yaw direction with the necessary pedal movement. Spend enough time switching airframes and eventually it becomes an automatic response again for each airframe.
When an engine fails the torque goes away. As part of the entry into autorotation the pilot must neutralize the tail rotor thrust. With a CCW turning rotor this means pushing almost full right pedal and for a CW turning rotor it’s left pedal. In a hover this must be done quickly as the unnecessary tail rotor thrust will start spinning the helicopter. In forward flight, the pilot will experience a yaw to the left as airflow over the vertical fin helps hold the tail straight.
Coming next is more on tail rotor emergency maneuvers and different types of anti-torque designs.
Tags: Tim McAdams