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Thread: counter balance weights

  1. #1
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    counter balance weights

    Hi group, I'm in need of an education in the area of aileron counter balance weights. Every application I've looked at has the weight located in a fixed position opposite the aileron and on its chord line. Could this counterweight be relocated into the wing and connected by linkage? Naturally the weight would need to rotate on a bearing and top quality rod ends would be a must as there could be no loose clearances. Engineers please dumb it down a little for me and if possible give me an example of how I could give this idea a fair test. Steve

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    Moderator Dana's Avatar
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    Re: counter balance weights

    Can't see any reason why it wouldn't work, but if the balance weights are to prevent flutter, you'd have to be very careful to make sure there's absolutely no play in the linkage between the aileron and the weight.

    -Dana

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    Super Moderator orion's Avatar
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    Re: counter balance weights

    This was actually common on some WWII airplanes as well as in some of the early jets. The balance works the same way as if the weight was attached - in other words, you figure out how much weight you need depending on how much of an arm you have from the point of rotation.
    "To live is to learn; to learn is to live" (author unknown)

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    Registered User Rick McWilliams's Avatar
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    Re: counter balance weights

    Aileron balance weights are usually installed outboard on the wing or tail to reduce the chance of flutter. The first bending mode has the largest motion at the tip. Still some airplanes have an elevator stick bob weight in other places in the control system.

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    Registered User wally's Avatar
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    Re: counter balance weights

    I thought the control stick bob weight was to give some more feel in pitch when pulling Gs. That is so you get feedback that you are putting more load on the elevator or flying stab with more G load. So you don't pull the tail off.
    Wally

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    Registered User Dan Thomas's Avatar
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    Re: counter balance weights

    Quote Originally Posted by wally View Post
    I thought the control stick bob weight was to give some more feel in pitch when pulling Gs. That is so you get feedback that you are putting more load on the elevator or flying stab with more G load. So you don't pull the tail off.
    Wally
    Yup. To keep from pulling the wings off, too. Airplanes that have bobweights usually also have mass balances on the surfaces. The Cessna Citation has a bobweight right at the base of the control column, under the floor, attached firmly to the column itself and with the weight forward of the column, that pulls forward on the column when pulling G's. It does nothing for flutter resistance; the cables aren't nearly rigid enough.

    Dan

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    Re: counter balance weights

    Guys,

    Without checking my notes (so stay cool if I'm not "spot on")

    IIRC, Control surface weights are for dynamic stability. It addresses the issue of stopping things from getting "worse" but doesn't necessarily go all the way to flutter.

    If you have an unbalanced control surface with the CG (of the, say elevator), aft of the hinge line and you fly into a vertical gust, ;ushing the tail up, the acceleration upward would cause the elevator to drop, pitching the nose down, this is destabling. You'd like the elevator staying right where it was. Same thing with ailerons.

    The purpose of these weights is to limit addition motion in "dynamic" situations. This can improve flutter resistance, but that's a related benefit and involves other factors like airframe elasticity.


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    Marc Bourget

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    Registered User wsimpso1's Avatar
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    Re: counter balance weights

    Control surface balance pretty well prevents flutter in lttle airplanes. Our speeds are not transonic and structures are generally pretty stiff, so most flutter modes are just not an issue. Now if your wing or tail is really kind of flimsy, you might get into some issues. The Zenith line had some problems that appeared to be wing stiffness related. There are a couple well known videos - one of a Comanche stabilator and another of a sailplane wing. In the Comanche line, I do not know if that event set Vne or drove a beefed up tail. I wonder about the same thing in the sailplane.

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    Registered User Hot Wings's Avatar
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    Re: counter balance weights

    Found this interesting, thought some others might as well.

    Stanford engineers put a damper on 'aeroelastic flutter'
    Conventional wisdom and practices yield conventional results. If that is good enough for you:
    Problem solved.

  10. #10
    Registered User Dan Thomas's Avatar
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    Re: counter balance weights

    Quote Originally Posted by Marc Bourget View Post
    Guys,

    Without checking my notes (so stay cool if I'm not "spot on")

    IIRC, Control surface weights are for dynamic stability. It addresses the issue of stopping things from getting "worse" but doesn't necessarily go all the way to flutter.

    If you have an unbalanced control surface with the CG (of the, say elevator), aft of the hinge line and you fly into a vertical gust, ;ushing the tail up, the acceleration upward would cause the elevator to drop, pitching the nose down, this is destabling. You'd like the elevator staying right where it was. Same thing with ailerons.

    The purpose of these weights is to limit addition motion in "dynamic" situations. This can improve flutter resistance, but that's a related benefit and involves other factors like airframe elasticity.


    Onward and upward

    Marc Bourget
    Those weights are to prevent flutter. Dynamic stability is designed into the entire airplane.

    Dan

  11. #11
    Registered User JamesAero's Avatar
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    Re: counter balance weights

    Is it necessary to be 100% balanced or is 70% acceptable? The plane I'm building (Star-Lite) had some problems with flutter and the correction was an elevator counterweight balanced at 70%. I don't know if this was to save weight or if the other 30% had no additional benefit.

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    Registered User Yogi's Avatar
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    Re: counter balance weights

    I'm building a Fauvel AV 361 sailplane. Wood and fabric. The plans show a balance for the elevator but none for the ailerons. The ailerons are cable controlled. Should I be concerned ? Yogi
    Cut corners are usually still sharp , and , can cut you back!

  13. #13
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    Re: counter balance weights

    Quote Originally Posted by Yogi View Post
    I'm building a Fauvel AV 361 sailplane. Wood and fabric. The plans show a balance for the elevator but none for the ailerons. The ailerons are cable controlled. Should I be concerned ? Yogi
    If properly designed? No. Many aircraft, even faster ones don't have balanced ailerons. Most of the wooden and plastic gliders don't have them and many of them have a VNE close to 300 km/h, the only noticeable exception being the LS3a/LS6 (both have a vibration damper with negative flaps)
    Aude somniare

  14. #14
    Registered User Yogi's Avatar
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    Re: counter balance weights

    Thanks Auto
    Cut corners are usually still sharp , and , can cut you back!

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    Registered User dino's Avatar
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    Re: counter balance weights

    Quote Originally Posted by wsimpso1 View Post
    Control surface balance pretty well prevents flutter in lttle airplanes. Our speeds are not transonic and structures are generally pretty stiff, so most flutter modes are just not an issue. Now if your wing or tail is really kind of flimsy, you might get into some issues. The Zenith line had some problems that appeared to be wing stiffness related. There are a couple well known videos - one of a Comanche stabilator and another of a sailplane wing. In the Comanche line, I do not know if that event set Vne or drove a beefed up tail. I wonder about the same thing in the sailplane.
    wsimpso1

    If this is the video you are refering to YouTube - How to break a gliderīs wing then it was deliberately initiated in a DG100G by jabbing the controls. You can hear the pilot calling out the airspeed which if my German is correct reached 130 km/hr. My guess is that they chose to investigate a stable, non catastrophic low speed flutter that could be terminated at the pilots discretion.

    Certification flight testing to 1.3*Vne is a moment of truth for sailplane designers not to mention test pilots. At least 2 sailplane designer/test pilots I know of have had to hit the silk while verifying their designs.

    Dino

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