Angular momentum conservation in helicopters...

  1. I had read in a book that the primary reason for the use of tail rotor in a helicopter is to counteract the rotation of the main body generated as a response to the rotation of the main rotor blades to keep angular momentum of the rotor-body system zero.
    Is it true that the rotation of main rotor blades results in the rotation of the main heli body to maintain a zero net angular momentum?
  2. jcsd
  3. SteamKing

    SteamKing 8,599
    Staff Emeritus
    Science Advisor
    Homework Helper

    I don't know about angular momentum, but the tail rotor keeps the pilot from getting dizzy due to the body of the helicopter wanting to spin from the torque to the main rotor.
  4. mfb

    Staff: Mentor

    Without a second rotor (tail rotor, or a second main rotor rotating in the opposite direction), this would happen.

    Air resistance of the main rotor gives a torque, to keep the helicopter stable you need some torque in the opposite direction to cancel this.
  5. A.T.

    A.T. 5,498
    Gold Member

    Here you can see what happens, when the tail rotor fails (at 0:24):
  6. The pilot was lucky.
    When the tail rotor stopped and the helicopter started rotating in a certain direction, did it mean that the main rotor must have been rotating in a direction opposite to the heli's?
    So does it apply to other rotating bodies like the system created by the sun and say the earth? Do they have a non-zero, constant net angular momentum or a zero net angular momentum?
  7. jtbell

    Staff: Mentor

    A common classroom demonstration has a person sitting on a chair on a (more or less) frictionless turntable, holding a bicycle wheel in a horizontal plane. Initially everything is stationary. The person grabs the rim of the wheel with one hand while holding the axle steady with the other hand, and starts the wheel spinning in one direction, let's say clockwise. He and the turntable start to rotate counterclockwise in response, so that the total angular momentum remains zero.

    You can probably find a video of this on YouTube if you search for a while. I couldn't find one with a quick search. I did find several videos in which the bicycle wheel was spinning initially, then the person flips the wheel over and starts rotating in the same direction that the wheel was rotating initially. Same general idea, but with a nonzero initial total angular momentum.
  8. I like these guys:
    Last edited by a moderator: Sep 25, 2014
  9. I'd watched a lecture from MIT in youtube too, demonstrating a lot of experiments regarding rotational dynamics. But I couldn't understand if a system consisting of sun and earth rotating around their common center of mass would have a constant non-zero or a zero L.
    Yes, I've gone through a lot of those SixtySymbols videos, got a lot of fun stuff.
  10. rcgldr

    rcgldr 7,408
    Homework Helper

    The net angular momentum is not zero, since both the main rotor (vertical axis) and tail rotor (horiztonal axis) are rotating, while the main body of the helicopter is not rotating. In order for the net angular momentum to be zero, the main body would have to be rotating opposite of the main (and tail) rotor.

    Imagine a model electric helicopter in outer space, effectively free of any external forces or torques. If the model's initial state is zero angular momentum, then it it's net angular momentum will remain zero regardless of any angular acceleration in the main or tail rotors, as the main body of the helicopter will rotate in the opposite direction so that zero net angular momentum is maintained.

    As mentioned above, the purpose of the tail rotor is to counter the torque related to the main rotor, so that the helicopter body remains stable (not rotating unless the pilot controls it to do so, such as a turn, pivot, or pitch maneuver). During pilot induced maneuvers, angular momentum is not conserved because an external (to the helicopter) torque from the air (in response to torques from the main and tail rotors) changes the angular momentum of the helicopter.
  11. Nugatory

    Staff: Mentor

    Constant, non-zero.
    The easiest way to see this is to imagine that you were looking at the system from outside the plane of the ecliptic. They'll both be rotating in the same direction (clockwise or counterclockwise, depending on whether you're looking from "above" or "below") so their (constant, non-zero) individual angular momenta are adding instead of canceling.
Know someone interested in this topic? Share a link to this question via email, Google+, Twitter, or Facebook

Have something to add?