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Helicopter pendulum behavior

  1. Jul 21, 2006 #1
    Hi, my first post to this forum:
    I'm curious whether the hanging mass of the fuselage beneath a helicopter rotor can be said to exhibit pendulum behavior.

    Specifically, in the case of a helicopter or other rotorcraft with a so-called teetering rotor, no torque is present at the rotor head because of the hinge arrangement. Control is by vectored thrust in which the total lift is perpendicular to the plane of the rotor . The rotor is tilted by cyclic control, which causes the thrust vector to move off of the Center of mass. The aircraft rotates about its CG, as do all rigid bodies.

    Seems to me this is like a weight on a string, but the pivot point(rotor head) is not fixed in space.

    Because the rotor head is not a fixed point, it has been argued that the mass, assumed to be a point mass at some distance below the rotor, lacks the attributes of a true pendulum.

    However, it seems to me that during certain maneuvers, say rapid back and forth turns( "S" turns), that the body of the helicopter would lag in its its response relative to the rotor and there is a 180 phase shift at a certain frequency of turning similar to the phase shift often seen in resonance conditions. This hints at the weight acting like a pendulum.

    In other words, when the helicopter reaches the far end of a left turn the mass below the rotor is banked maximally to the left, but the rotor is now banked to the right in order to commence the next right turn.

    The body would appear to swing back and forth like a pendulum, but I have not been able to properly model this behavior on paper, or in my mind.

    I'd like to think that the pendulum period could be calculated in the same way as it is for any pendulum- based on the distance from the rotor head to the CG.
    Helicopter engineering books sometimes state that the idea of a helicopter as a pendulum is a myth, but I need to know why, if indeed thats true.

    Yet, a suspended "sling load" under a helicopter IS considered a pendulum.
    I'm confused as to how to compare these cases because the helicopter has a relatively large mass compared to the sling load, while the helicopter rotor can be considered to have zero mass compared to the body of the helicopter in this argument.
    Can anyone get me started on how to sort this out?

    Last edited: Jul 21, 2006
  2. jcsd
  3. Jul 22, 2006 #2


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    Your "S-turns" aren't apt to be executed with a "bang-bang" cyclic movement without creating a stall condition of some sort --- gotta be some collective games getting played as well, or you're going to find out why there's a piece called the "Jesus nut."

    "Slung pendulum?" You betcha --- gets to be too much like a pendulum, the load gets "punched off" in one big hurry; look at the center of mass of bird and load, conserve its momentum, and figure the load on the rotor.
  4. Jul 22, 2006 #3


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    With no offense to Fred or Bystander, some of us fixed-wing guys still consider the 'roundy-winged' ones as slightly insane. Why would one voluntarily hit the blue wearing a collection of spare parts flying in loose formation? :biggrin:
  5. Jul 22, 2006 #4


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    Admittedly, I am not going to go through the dynamics equations here, but you might want to take a look. It is pretty much what you are looking at:

    http://www.pages.drexel.edu/~ihm22/Advisor_Deliv/Helicopter%20Dynamics/Inv_Pend_Heli_Comparison.doc [Broken]

    I will say that the sling load pendulum argument holds weight to me as an acceptable simplification. I have seen many sling loads that have had a periodic motion to them in straight and level flight, even when aerodynamic loading is not constant.
    Last edited by a moderator: May 2, 2017
  6. Jul 22, 2006 #5
    I would say it's more like a body moving in the direction of the force applied to it. Imagine one of those large city busses with the hinge in the center. I can't see the chopper acting as a pendulum during maneuvers. Maybe if you came out of a steep turn and then had a computer constantly adjust the rotor head to be parallel/level with the horizon you would see pendulum effects. This is all speculatory blather on my part by the way. :biggrin:
  7. Jul 22, 2006 #6
    Fred, that inverted pendulum document looks interesting, but I will have to wait to read the it on another PC. This one can only read older doc formats.

    Yes, that's more or less what I'm getting at.
    That the pilot could excite pendulum behavior by controlling the rate of rotor tilt and the lift.

    In frame A below, the helicopter is shown deccelerating as it moves from left to right. The stick is being pulled back, but the pilot maintains constant altitude. A camera moving with the helicopter will show the mass slowing down, rotating and translating in x and y.

    In frame B, the same thing is shown from the viewpoint of a camera tied to the rotor head. The mass appears to swing out ahead.
    Thae aircraft is rotating about the cg while experiencing acceleration in x and y.
    I didn't draw a frame C, but I think the restoring force would be from the force couple formed by gravity acting on the mass and the upward component of rotor thrust as the rotor is levelled at just the right rate to maintain the pendulum behavior.

    Attached Files:

    Last edited: Jul 22, 2006
  8. Jul 22, 2006 #7


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    Not offending me a bit --- I could never figure out whether it was motion sickness or burnt JP whatever --- puked my guts out about four hours afterward every time I ever had to ride one of the damned things.
  9. Jul 22, 2006 #8


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    I love the smell of JP8 in the morning. It smells like....victory
  10. Jul 23, 2006 #9


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    To be honest (****, that's a bizarre concept): I have just finished my first version of a logo for the 2013 World Police & Fire Games, to be held in Calgary. There's a contest to choose one. Aside from the fact that I'm just starting to do graphics as a side-line, and this will be a way to show my stuff, the first place prize is 1 hour on the Search and Rescue jet boat, and 1 hour in HAWC1--the flagship of the Calgary police force helicopter squad. I'm going to send in as many designs as I can just for a chance to get my mitts on that thing. :biggrin:

    edit: If you're curious, HAWC1 is an MD520 NOTAR. Don't know about the boat.
    Last edited: Jul 23, 2006
  11. Oct 27, 2010 #10

    This is my first post here, . . I surfed my way here from google search Helicopter plus Pendulum.I also see that this thread is about 4 yrs old, so I hope you guys are still out there.

    I'm not a Physicist but I do technology concepts, thats sort of like guessing with out numbers and when you feel your getting warm you go out and look for an engineer to check your
    design for, "proof of concept".

    I am interested in "Helicopter Pendulum" but in the rescue line type swings.

    After seeing a flash flood documentary several years ago, I have made it a pet project and have talked to a couple of pilots about it.

    The thing is one pilot says that it doesn't happen often enough to warrant the FAA, to ambitiously seek solutions and if you did find a mechanical solution for it, no one would be interested in the paying cost.

    I have my doubts about that, . . as the way I see it, helicopters will be increasingly used in rescues of all kinds and I believe that th, "uncontrollable swinging" of the rescue line will increase in occurrence.

    I hope this post is appropriate for this thread as I do see you have mention slung or sling,
    and I assume you are speaking of a load on the line.

    Anyway my first question for you, is would you happen to feel that the occurrence rate is known or where might I find that kind of information.
  12. Oct 28, 2010 #11


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    Welcome to PF, Jople.
    With no offense intended, I don't exactly understand your question. Are you referring to the oscillations of a rescue basket, or the increased load upon the chopper, or what? The entire scenario is certainly worth discussing, but you need to be a bit more specific as to your questioning.
  13. Oct 28, 2010 #12
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  14. Oct 28, 2010 #13


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    I must say, that is probably the most descriptive visual aid that anyone has ever provided on PF. It appears, then, that you are asking about side-loading of the rescue basket. That doesn't occur under normal circumstances. Either strong crosswinds or a river current as depicted in your link are necessary for that. A good pilot can compensate for either, but it takes some extra concentration.
    By the bye, I've changed my attitude toward whirly-birds now that I've been a passenger. I'd love to drive one.
  15. Oct 29, 2010 #14
    Thanks for your response and glad to have furnished you with an illustrative graphic that was able to provide you with the necessary information.

    I realize that, as you say, "A good pilot can compensate for either, but it takes some extra concentration.", I have had that explained to me by a pilot some time back.


    Here is some commentary I have gathered;

    Pilot Comment

    The problems are:
    1) Winds - winds acting on the helicopter and wind acting on the people
    on the end of the cable - remember the cable is quite a long way beneath
    the helicopter and the wind there is very probably significantly
    different that the wind on the helicopter. In no wind, the down wash from
    the helicopter rotor will be affecting the people on the cable for quite
    a distance, but with any wind speed, (and remember it can be from nearly
    any direction relative to the direction the helicopter is pointing), the
    downwash may be exacerbated or nullified.
    2) Gusts of wind - the wind is not always constant in either strength or
    direction. This means the helicopter may always be correcting it's
    position and attitude (depending on the automatic flight control system
    fitted to the helicopter).
    3) Starting position relative to the helicopter - if the person being
    picked up is not directly below the helicopter, then on lift-off from
    the surface, there will be a pendulum effect started.

    The weight of the people on the line is relatively small in comparison
    to the weight of the helicopter.
    The rescue hoist is not on the center line of the helicopter, but
    displaced to one side by a significant amount.

    Pilot Comment
    Whenever a helicopter pilot has an external load that starts to swing
    (Human or non-human cargo), he will begin to slowly turn and the
    swinging will cease.
    Initiating a shallow turn will remedy the pendulum effect by decreasing
    the ability for the load to swing by using centrifugal force. Even at a
    slow forward speed with a shallow turn to one direction, it will create
    a G Force on the weight, increasing the relative weight on the end of the rope and diminishing the swing.
    Simple problem, simple fix.


    The above may be true, however, I tend to believe from the example I have viewed in a news documentary a few years back, and is a good bit like that, as is demonstrated, in the graphic illustration, . .that a pilot may not always have the time and space to make the compensation.

    I'd like to get to the data, and am thinking about the possibilities that the FAA, may have a report log, if they don't they should.

    If you do get up there, . . happy flying.
  16. Dec 22, 2010 #15

    This is my first post here. If you would like to do some practical experiments along these lines I have several remotely piloted helicopters of different sizes that I will offer up. (I am always looking for an excuse to fly)! They have payload capacities of up to 20 lbs. Some years ago I had to perform a payload test on one. It was a windy day and I suspended 20 lbs. under the heli from a line about 4 feet in length. Immediately a wind gust pushed the heli backwards so I corrected forward. The weight then shifted backwards and an oscillation started. Out of curiosity I allowed the oscillation to continue. It reached an equilibrium with both the helicopter and the suspended weight swinging directing out of phase with an amplitude of about 1 foot off center (total). By timing my cyclic corrections I was able to minimize the swinging motion until the next big gust of wind started the process all over again. I hope this helps.
  17. Dec 22, 2010 #16
    Hi Heliman,

    Glad to have your response, . . I do have an interest in developing some type of
    demonstrative experimentation and am attempting to organize an R&D project.

    Please allow me to get a little information about you.

    Do you have any experience in R&D applications or proposals ?

    Are you a Pilot.

    Where are you Located.

    At my best I'm a technology concept developer.

    I am a free lancer but have an interest in Search and Rescue and Emergency Response.

    I am a member of CCR, DHLS, but am not getting any traction, but have joined a couple of organizations lately where I am hoping to find ways to move forward by communicating in conversation with others of the same interest, etc.

    Have an interest in Wildfire control systems, and am from the North East, . . New Jersey, . .

    South Jersey.
  18. Dec 22, 2010 #17
    Hi Jople,

    I don't really have any experience with R&D proposals. I am located in Ohio. I have been an unmanned helicopter designer and pilot for going on 30 years now. All of my work to date has been for my own uses.
  19. Dec 22, 2010 #18
    OK, . .

    So, I innovated a concept or two for Helicopters, my main interest is in solving problems, and I started with sea search and rescue situations. . . then I had the opportunity to see the SAR variations via the TV documentaries and realized the scope of future possibilities.

    I generally consider myself an artist but tried my hand at technical innovations and
    creating new products, living near the coast and meeting fishermen, I took up an interest
    in rescue.

    Are you in a related field as an occupation ? Freelancer or hobbyist ?

    Are you a model builder ?
  20. Dec 22, 2010 #19
    My day job is as an engineering manager in the electric power industry. I also have a side business that involves unmanned helicopters. The helicopter stuff all started out as a hobby many, many years ago when the equipment was marginal at best.
  21. Dec 22, 2010 #20
    Sounds good, I'd like to make some moves forward with this as a project, . .
    Unmanned fits the bill to.
    I am developing a plan.
    If you will have further interest, I believe there are good possibilities and opportunities,
    it is only a matter of making it happen.
    Will be glad to stay in touch as per PM.
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