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  1. #26

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    RE: help on balancing low wing planes

    The idea of hanging is the plane will slide on the line to show the CG. The hook doesn't always allow this so a pulley should be used instead of the hook. A plumb bob is hung from the center of where the string is attached to the ceiling and will show the CG. In my old shop I had a small block and tackle for hang balancing. When done correctly it works great. You can balance for CG and lateral at the same time. No, the plane doesn't have to be level to start balancing. If the plane is out of balance it will never be level. I haven't bothered with it in my new shop, I have been using a CG machine. I do hang them for lateral balancing once in a while.
    Drinking and driving are illegal, why do bars have parking lots
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  2. #27

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    RE: help on balancing low wing planes

    Bob,

    With regard to the drawing in the post that I was replying to, it does not matter whether the plane is level or not; the cg will still fall directly below the fixed point at the top. I would suggest fine tuning your cg by flying it instead of studying a bubble level.


    Kurt

  3. #28
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    RE: help on balancing low wing planes

    Ah, but that is my point.

    The plane must be level. The point on the hook is fixed, but the aircraft isn't. If you move the string to get the aircraft level, as it should be, you will see that the point of the plumb bob will move off your previous CG point. Try it yourself. If the plane is level and the plumb bob is on the CG mark, it is correct and you have equal moment arms on both sides of the balance point.

    Of course you may wish to move that point fore or aft to obtain the characteristics that you may want in your aircraft. At least you will be in the ballpark for the first flight.

    Graybeard, the hook works. A pulley would allow free wheeling movement and I prefer the friction of the cord against the hook to keep the aircraft from slipping to vertical.

    Bob
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  4. #29

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    RE: help on balancing low wing planes

    Your understanding of physics is wrong. The cg will always fall directly under the plumb bob, regardless if the plane is level or not. There is no force present to prevent the cg from falling directly under the plumb bob. The strings certainly don't.

    Kurt

  5. #30

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    RE: help on balancing low wing planes

    A few pictures are worth a few hundred thousand words:

    Kurt
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  6. #31

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    RE: help on balancing low wing planes


    ORIGINAL: Bozarth

    Does it really matter if the plane is level?

    Kurt
    If you'd actually like to know where the CG is, yes.

    Good flying wit ya today

  7. #32

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    RE: help on balancing low wing planes

    ORIGINAL: Bozarth

    A few pictures are worth a few hundred thousand words:

    Kurt
    Picture 1 is good. You will actually be able to mark where the CG is on the wing.

    Picture 2 is not. The mfg said the CG should be X inches back from the LE. Where is the plumb bob pointing? Where would it be pointing if the plane was actually level, which is how you measure the X inches back that the mfg suggested. And Picture 2 and 3 really screw you up when the airplane wing isn't exactly centered in the fuselage. A high or low wing plane in the Picture 1 and Picture 2 positions are going to be impossible to guess at where the CG would be back from the LE of the wing.

    Picture 3 is not. It's just as hard to guess where to mark the CG as in picture 2.

    You really will not know much of anything when your plane isn't horizontal other than the CG isn't right, and which way it's 'too far'. Good thing is that you can tape weights on the sucker while it's suspended to find out how much weight is needed where. If you were crafty and did the test before installing some heavy components, they'd be taped on the sucker and you'd simply move them until the plane was horizontal. THAT'S the most efficient way to use that rig. Then Pictures 2 and 3 would be what you'd see just before you saw Picture 1.

    Picture 4 is what it is: something you can't see with the rig, and something you can see.

    Good flying wit ya today

  8. #33

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    RE: help on balancing low wing planes


    ORIGINAL: retransit

    Ah, but that is my point.

    The plane must be level. The point on the hook is fixed, but the aircraft isn't. If you move the string to get the aircraft level, as it should be, you will see that the point of the plumb bob will move off your previous CG point. Try it yourself. If the plane is level and the plumb bob is on the CG mark, it is correct and you have equal moment arms on both sides of the balance point.

    Of course you may wish to move that point fore or aft to obtain the characteristics that you may want in your aircraft. At least you will be in the ballpark for the first flight.

    Graybeard, the hook works. A pulley would allow free wheeling movement and I prefer the friction of the cord against the hook to keep the aircraft from slipping to vertical.

    Bob

    When the plane isn't level, the place on the wing the bob points to wouldn't be correctly placed. retransit is correct. If you want the bob to point to where your cg is, the plane has to be level when you mark the CG on that wing.

    When ANYONE refers to the location of the CG on the wing, they're really talking about where the CG's vertical plane passes through the wing. The CG is almost always above or below the wing.
    Good flying wit ya today

  9. #34

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    RE: help on balancing low wing planes

    A couple of quick pictures.

    The actual CG of a plane is going to be somewhere inside the plane. Almost never will it be somewhere along the top skin of the wing. Very seldom will it even be somewhere between the top and bottom of the wing. When the plane is tilted, not a one of our methods of finding the CG is going to work accurately. We'll have to guess or judge from what we see where it might actually be.

    We're actually trying to find out where the vertical plane of the CG passes through the wing. When the wing doesn't pass through the CG, things go bad for every one of our methods.

    The way to make every method work accurately is to have the plane horizontal at the time we mark the CG. Then we will accurately mark 'where the CG is on the wing'.

    Here's a picture of where the CG really is on a model. It's the red spot. And the yellow arrow points to where a plumb bob would be pointing at the wing when that model had quit swinging in the rig or quit tottering on a CG tool. There would be nothing that pointed out where the CG was along the wing for balancing purposes. There wouldn't be a blue arrow. It's your best guess if your CG is located where the mfg suggested it be.

    If the mfg had suggested the CG should be where the yellow points on the wing, you'd really have it forward of that. How much forward? How would you know.
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    Good flying wit ya today

  10. #35

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    RE: help on balancing low wing planes

    It really only takes a couple of seconds to balance the plane level. Then we know exactly where the CG is 'from the LE of the wing' or wherever the mfg suggested.

    If you want a slightly nose heavy airplane then have it balance ahead of the recommended distance. Fly it and decide if your extra distance worked for you. The mfg's suggestion usually is very much on the safe side anyway. They fly a lot better when they are better balanced. And you'll know accurately where the balance is that worked for you.
    Good flying wit ya today

  11. #36

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    RE: help on balancing low wing planes

    Here is a sample of the balancing process. The size of the plane (AMR Giant Stick 50) exceeded the limits of all my existing balancing tools.

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  12. #37
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    RE: help on balancing low wing planes

    "The actual CG of a plane is going to be somewhere inside the plane. Almost never will it be somewhere along the top skin of the wing. Very seldom will it even be somewhere between the top and bottom of the wing. When the plane is tilted, not a one of our methods of finding the CG is going to work accurately. We'll have to guess or judge from what we see where it might actually be"

    I think what is described here is the "Center of Balance". I'd have to ask my aeronautical engineer friend if that is correct.

    As far as Bozarth's deduction that my understanding of physics is wrong, again, why would Great Planes provide a bubble level with their balancer? I know this; when you move the supporting string fore or aft to obtain a level aircraft the point of the plumb bob moves away from its location when the model is hanging nose up or down, This tells me that the CG is moving. And the force present is called moment arm. Nose up or down, the moment arm is unequal.

    The following is from their manual:

    HOW TO USE THE LEVEL
    To assist you in accurately balancing your airplanes, we have included a small, lightweight level vial.You can use this in various ways to help determine when your airplane is truly level, rather than relying on estimation. Although the level vial may be used alone, you may attach it to the 1/2" x 3" plastic strip, with a small piece of double-sided tape (cut from the included 1" square), carefully aligning the vial with one edge of the strip. Attaching the vial to the plastic strip will allow it to be taped to the side of the fuselage along a reference line.You can also place the assembled level on the stabilizer.


    Bob
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  13. #38
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    RE: help on balancing low wing planes

    ORIGINAL: kaboomski

    Here is a sample of the balancing process. The size of the plane (AMR Giant Stick 50) exceeded the limits of all my existing balancing tools.

    That is exactly the reason I went to the method described. A 23 ib. Fleet biplane doesn't do well on a Great Planes balancer.

    Bob
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  14. #39

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    RE: help on balancing low wing planes

    ORIGINAL: retransit
    As far as Bozarth's deduction that my understanding of physics is wrong, again, why would Great Planes provide a bubble level with their balancer? I know this; when you move the supporting string fore or aft to obtain a level aircraft the point of the plumb bob moves away from its location when the model is hanging nose up or down, This tells me that the CG is moving.
    The CG isn't moving. However, you just rotated the plane around it. The top part of the plane, where the bob was originally pointing just rotated forward or aft. The parts of the plane below the CG just rotated the other way.

    The plumb bob pointed at the CG the first time and at it when you moved the strings. It's pointing at the CG all the time, but at different things that're in the way whenever you shift the strings.

    CGs don't move unless we shift the individual parts of the plane around from one place to another in the plane.
    Good flying wit ya today

  15. #40
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    RE: help on balancing low wing planes

    ORIGINAL: jetdeck

    ....i want to know why you have to balance a low wing plane upside down,
    Ever tried to balance a broom in the palm of your hand? The CG of the broom is on top of the balance fulcrum. Its a lot easier to balance the broom by hanging it below your hand.

    if its on the stand upside down, and you want it a tad nose heavy, would the nose be pointing toward the cieling, or toward the floor?.....
    Heavy is heavy. Gravity only comes from one direction and that is going to pull the heavy part down.


    I can't believe this thread has taken so long to discuss. And if you actually read thru it, the two most opinionated people are agreeing with the methods, but the pictures in their heads are completely different. One of them only wants to FIND the CG in a 3-dimensional object, the other one wants to MARK it on a 2 dimensional surface. But they are actually agreeing. By they way they are posting it will lead you to beleive they are not agreeing. WOW.


    Rafael
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  16. #41
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    RE: help on balancing low wing planes

    You're right, the CG didn't move, my mistake. What I should have said was, when you add weight to one end or the other to get the model to balance level, the point of the plumb will move due to the moment arms being equalized. At that time the CG will shift to a new location.

    Bob
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  17. #42

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    RE: help on balancing low wing planes


    ORIGINAL: Rafael23cc

    I can't believe this thread has taken so long to discuss. And if you actually read thru it, the two most opinionated people are agreeing with the methods, but the pictures in their heads are completely different. One of them only wants to FIND the CG in a 3-dimensional object, the other one wants to MARK it on a 2 dimensional surface. But they are actually agreeing. By they way they are posting it will lead you to beleive they are not agreeing. WOW.
    The written word is about the worst communications us humans flog each other with.

    What at least one of the most "opinionated people" has for motives is to provide enough detail that every beginner who comes through this swamp doesn't leave with just wet feet.

    The point is for everyone to have a chance to understand most of this isn't an individual's opinion, and to understand how to find the mfg's suggested CG location with sufficient accuracy.


    Good flying wit ya today

  18. #43
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    RE: help on balancing low wing planes

    Here's the link and it took me all of an 1/2 hour including looking for the parts I knew I had somewhere http://www.rcuniverse.com/forum/m_708060/tm.htm This is absolutely the most easiest thingI have usedto balance a plane with and when you're done throw it in a drawer. Don't try to figure it out, build it use it and then say WOW where you been all my life.O.k. graybeard what are you waiting for.So easy even a caveman can do it
    If what you believed to be true was false would you want to know the truth?

    "You shall know the truth, and the truth will set you free".

  19. #44

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    RE: help on balancing low wing planes

    Hi!
    Come on guys!
    All you need to do is putting your two index fingers (or long fingers) under the wing where the supposed C of G is and notice if the airplane sits level or is ponting forward or tillting backwards. Simple as that!
    Don't complicate things that don't need to be complicated.
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  20. #45

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    RE: help on balancing low wing planes

    My name is Tony and I need help locating parts for my fs racing kingkong 4wd, Gotten no response from heat hobby about buying a front lower suspension ket. some one please helpe!

  21. #46
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    RE: help on balancing low wing planes

    ORIGINAL: jaka

    Hi!
    Come on guys!
    All you need to do is putting your two index fingers (or long fingers) under the wing where the supposed C of G is and notice if the airplane sits level or is ponting forward or tillting backwards. Simple as that!
    Don't complicate things that don't need to be complicated.
    Not that simple if your fingers aren't long enough or your wife refuses to help you finger-tip it. Besides, that method does not allow for movement of batteries or weight while it is suspended, unless of course you have three long arms.

    Bob
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  22. #47

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    RE: help on balancing low wing planes


    ORIGINAL: jaka

    Hi!
    Come on guys!
    All you need to do is putting your two index fingers (or long fingers) under the wing where the supposed C of G is and notice if the airplane sits level or is ponting forward or tillting backwards. Simple as that!
    Don't complicate things that don't need to be complicated.

    Agreed. Please, somebody tell me the value of knowing the EXACT location of your cg, assuming it falls somewhere between 25 to 35% back along the mac? And after you tell me, I will simply ask you "how does it fly?"

    Kurt

  23. #48

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    RE: help on balancing low wing planes


    ORIGINAL: Bozarth

    Please, somebody tell me the value of knowing the EXACT location of your cg, assuming it falls somewhere between 25 to 35% back along the mac? And after you tell me, I will simply ask you ''how does it fly?''

    Kurt
    No problem.

    I do it for every airplane I design, build from a kit, or assemble as an ARF. I run the numbers to find out where I wish to locate the CG in order to have a safe maiden. OK, that's actually the reason just for my designs. The reason I run the numbers for a kit or ARF is also based on experience however. I don't really trust the present ARF 'designers'. Their documentation often has fatal flaws in it and I've seen way too many that were screwed. I run the numbers on them and use that number. So anyway.....

    I have found out from experience almost every model I like flies the way I like when the static margin is 5% or thereabouts. I can run the numbers ahead of time and find out EXACTLY where that would be. Do I need it exact for any other reason? Nahhhh. But why not use science to my advantage. Why not cut out the fiddly fiddling around and come out of the gate running. Could I use the ARF recommendations? lol..... I do, when they match the numbers I ran.

    As for everyone else.... if they choose to start with a recommended CG location, actually flying with it makes sense. If they want to use the mfg's recommended CG, then it doesn't take a genius to accurately adjust for it, and it most certainly doesn't take any longer to do it accurately than it does to do it sloppy. Almost everyone will benefit from flying the maiden with a correct CG. It's easily done accurately for sure. So why 'assume'. Find out where it is suggested and put it there. Almost everyone is going to approach the maiden flight with confidence knowing they've got a safe CG and it is where it's supposed to be. They won't need you to ask how it flies. They'll know ahead of time and it will fly safely, without any nose heavy or tail heavy problems.

    But you're correct about one thing..... The CGs don't ever have to be in only one exact spot. They've got a range that is safe and reliable. But you really do need to know where that range is, and it's good to use a balancing technique that is accurate enough to be worth the time you spend employing it.

    With every model of mine I've built in the last 50 years, I could tell you BEFORE I flew it, how it was going to fly. And every one did. At least how their pitch stability was going to be, which is about all the CG is going to help with.
    Good flying wit ya today

  24. #49
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    RE: help on balancing low wing planes

    Well said!

    Bob
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  25. #50

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    RE: help on balancing low wing planes

    da Rock (I still don't know your real name),

    I applaud your diligence. But it falls under the TLAR method if you apply science to part of the equation, and experience to the other. Specifically, how are you determining what the coefficient of lift is for your wing and tail? Or the wing's pitching moment coefficient? My guess is you are estimating. Which then leads to an estimated SM, which then leads to an estimated desired CG location. Which I would bet, falls somewhere between 25 to 35% of the MAC.

    How do you learn of these coefficients for ARFs? I bet you estimate based on your experience. And I bet you know how it should feel when you fly it. And I bet you recognize there is a trade-off between maneuverability and stability, etc.
    There is no "correct" cg provided it is within the static margin.

    Measuring a cg location to the nearest 1/16" means nothing. How do you like how the plane flies?

    Measuring the cg to the nearest nth degree is like measuring how much hair the barber cut off. What matters is do you like your haircut.

    Kurt


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