Anti-torque

August 10, 2009 by Tim McAdams

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.

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17 Responses to “Anti-torque”

  1. Kris Says:

    Dear Tim, Have you seen this unusual solution: http://www.youtube.com/watch?v=eMoUvlYXBc0

  2. Avi Weiss Says:

    Tim;

    While tail rotors are the predominant anti-torque systems in use, a discussion on anti-torque systems wouldn’t be complete without discussing counter-rotating (both coaxial and displaced) rotor systems, and NOTAR.

    Would also be interesting to understand rotor failure modes in a tilt-rotor (though technically that is a “powered lift” class aircraft, and not strictly helicopter).

    -avi

  3. Gary Underland Says:

    My experiance with airplanes is large ,helicopters,small. But we toured a lot of Canada with our Bell 47 on floats (heavy blades). As you realize, a free water start needed attention. Without outside help our machine made about 2 complete revolutions before we had cotrol. We got it down to 90 degrees with an outboard motor or zero with a moving boat tow alongside. Hey, it wasfun.

  4. Paul Herndon Says:

    I have only ridden in a helicopter once, as a passenger, and until this article, had no idea how they worked. Now I do. Thanks.

  5. Joe Connell Says:

    Your discussion about torque should include those helicopters not requiring anti-torque provisions such as the H-21 Shawnee, H-46 Sea Knight, H-47 Chinook, H-43 Huskie, and the Russin Kamov series helicopters. All of these aircraft were equiped with counter-rotatiing main rotors.

    It has been estimated between 10 -15 percent of total generated power is spent counteracting torque…

  6. Ehud Gavron Says:

    While the whine-fest is going on, the official FAA answer is that the tail-rotor counteract “the torque of the main rotor.” In actuality the torque is produced by the engine and transmission. Perhaps you could cover what produces the torque while you’re discussing the anti-torque methods (tail-rotor, NOTAR, counter-rotating MRs, multiple MRs, etc.)

    Cheers,

    Ehud
    Tucson AZ

  7. Don Pointer Says:

    I fly a gyroplane. It’s a lot simpler, no torque to counter and infinitely safer. Helicopters and things with wings are dangerous things.

  8. Mike R. Says:

    Great Article on anti-torque rotors. In response to a couple of the comments here….There are a lot of differenty types of helicopters with different rotor systems…and that’s ok…but, I don’t think this article was meant to cover “all” of those different models flying…just the ones that use tail rotors!….

    Thanks Tim for taking the time to educate those less fortunate…those that have never had the pleasure of learning about the Anti-Torque rotor under actual flight conditions!!!!

  9. Tim McAdams Says:

    Mike R, you’re right.

    Unfortunately, I do not have the space to expand on all the details about a subject. However, all the comments are right on and I plan to write about those subjects in future blogs. Please keep the ideas coming as it gives me some insight into what readers are interested in.

  10. Fred Says:

    Why do large helos make a whoop whoop noise from the rotor blades? Is the same noise heard inside the helo cabin as on the outside of the cabin?

  11. Ehud Gavron Says:

    That sound is called “blade flapping” and is exacerbated by maneuvers which cause more load on one blade or the other. High speed turns, descents, and low-G operations unload the rotor and allow much more flapping (the blades moving up and down and “chopping” the air down). The larger the rotor blade the more sound it makes, so the large helos necessarily make more sound than the small one.

    Noise abatement procedures in many airports call on pilots to reduce this as much as possible. Often the simple answer is “don’t do rapid maneuvers.” One can also help reduce blade flap by raising collective pitch to add load to the rotor blades and therefore prevent the amount of flappage.

    The same noise is heard inside the cabin, albeit differently, as the noise is brought through the rotor’s already-loud downwash, the body of the aircraft cabin, and the insulation of the headset over the ears :) (In larger helos there’s also the interference of the helmet.)

    When flying the small Robinson helicopters in doors-off operation, when blade-flapping sounds occur, it’s extremely clear. A little up-collective and the sound-issue (not a flight issue) goes away.

    Ehud
    P.S. In some airports, the traffic pattern is so tight that rapid maneuvers are required (e.g. KTOA right traffic takeoff 11R but must stay east of PCH at all times due to noise abatement… requires rapid turn and climb, and a rapid turn on the north side to return for landing). Yes, that’s right — noise abatement restrictions and “do not overfly our neighborhood” rules make it harder on the pilots, and create more noise for the remaining neighborhoods under the flight pattern path.

  12. Bob Kuriger Says:

    Tim,

    I”ve never heard a satisfactory answer to the question of why blade flapping makes the sort of noise it does. Would you care to give it a try?

    Thank you,
    Bob Kuriger

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