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Thread: Cub CG


  1. #1
    hugger-4641's Avatar
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    Cub CG

    I know there's been a million questions about CG, but humor me with another if you would. I have a 71" J-3 that was given to me crashed, I have resurected it and test flown it and it was nose heavy as crap!! I did balanced it rather hastily, but I did it as I have many other models, I took 1/3 of the wing chord, which as usual, happened to be right on the spar, and balanced the plane just aft of this. I'm not sure what brand of Arf it is, but it looks very similar to the Aeromax cub, both have about 71 1/2" wing with about 10" chord. When I researched the CG for the Aeromax Cub, it says 4.5" from LE. That is almost half the wing chord, none of my other Cubs are balanced that far back. If 4.5" is correct for the Aeromax, and the plane I have needs a similar balance point, then I was almost a full inch nose heavy on the CG. I moved the rx back to the aft of the cabin and made an access panel under the tail section of the fuse to enable moving the battery pack as far back as physically possible. The pack weighs 4oz and is now sitting right under the front edge of the vertical stab. I still had to add 3 1/2 oz of weight under the tail to get the CG at 4.5". I haven't been able to fly it again yet, hopefully this Saturday will cooperate. So now for my questions:

    Does anyone here have the Areomax Cub or another cub with similar wing dimensions and is your CG in a similar spot?
    If so, how much weight if any did you have to put on the tail to get your Cg there?



    Jerry
    AMA -922698 Nomal people scare me, but not as much as I scare them...

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    Steve Landron's Avatar
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    RE: Cub CG

    Don't know if it helps but I just checked the plans for my older Carl Goldberg Cub and the CG is marked exactly 4" from the leading edge (wing Chord 10 3/4"). Wing span 67 1/2" (clipped), normal wing span 76 1/2", Fuse length is 48", model weight is 7lbs. I've had this plane for about 19 years and balanced right at the CG point and have had no problems. I did not have to add any weight to the plane to balance it out, fuel tank is all the way forward, and battery pack is right under fuel tank. The servos’ and reciever are located in main cabin according to plans. Good luck, hope it helps.
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    Cub Brotherhood #34, Mustang Brotherhood #29, Glow Brotherhood #64

  3. #3
    hugger-4641's Avatar
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    RE: Cub CG

    That's closer to what I would expect. This model is about 5 lbs 12oz with a .46 ax installed. With a wing chord of only 10", I was expecting the CG to be closer to 3.5". I also have the 12oz tank all the way foward and servo's installed in the cabin.
    Jerry
    AMA -922698 Nomal people scare me, but not as much as I scare them...

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    RE: Cub CG

    25-33% of the mean aerodynamic chord is pretty common for most planes. Try using an vanessa rig, theres a thread under the tips section for it and it's so simple to use you will wonder how you ever balanced planes before.
    If what you believed to be true was false would you want to know the truth?

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    Steve Landron's Avatar
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    RE: Cub CG

    Jerry, sounds good glad I could help. I'm running an OS .46 with a Pitts style muffler and like I said before the plane flys great. Good luck let us know how it goes this weekend.

    Steve
    Cub Brotherhood #34, Mustang Brotherhood #29, Glow Brotherhood #64

  6. #6
    hugger-4641's Avatar
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    RE: Cub CG


    ORIGINAL: raptureboy

    25-33% of the mean aerodynamic chord is pretty common for most planes. Try using an vanessa rig, theres a thread under the tips section for it and it's so simple to use you will wonder how you ever balanced planes before.

    That's pretty much what I'm accustomed to , but 4.5" is 45% of the chord on this wing, a lot more than I'm used to seing as a starting point for Cg. I've looked at that vanessa rig before, may just have to go ahead and make one.

    Jerry
    AMA -922698 Nomal people scare me, but not as much as I scare them...

  7. #7
    hugger-4641's Avatar
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    RE: Cub CG


    ORIGINAL: Steve Landron

    Jerry, sounds good glad I could help. I'm running an OS .46 with a Pitts style muffler and like I said before the plane flys great. Good luck let us know how it goes this weekend.

    Steve


    Will do Steve, hopefully it will come down in one piece! I'm still a little leary of the Cg being that far back, but it was definitely nose heavy when I flew it before, so we'll see what happens and I'll post the pics, good or bad!

    Jerry
    AMA -922698 Nomal people scare me, but not as much as I scare them...

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    RE: Cub CG

    hugger-4641, If the Aeromax Cub has a 10' cord, then a C/G spec @ 4.5" aft of the leading edge is a mistake. Sometimes incorrect set up information gets printed in an instruction manual. I feel this is the case here. I have a 1/6th scale World Models J-3 Cub ARF with a 71" wingspan and a 9 & 7/8" wing cord (measure yours accurately and compare); specs for this Cub call for a C/G at 2 & 3/4" aft of the leading edge. This is where I balanced it and it flys fine. I believe your airplane is TAIL heavy at 33% of 10" and even more tail heavy @ 4.5" aft of the LE. I would bet at 4.5" it's unflyable!!!

  9. #9
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    RE: Cub CG

    Thanks for the reply Larry. My education and gut feeling definitely agree with you, but it definitely flew nose heavy when I had the Cg at about 3.5", even with the fuel burned off. I may try settng the CG at about 4" and see how much difference that makes. With a 12oz tank full of fuel set all the way foward, I'm probably not going to see a big difference until I get at least half the fuel burned off anyway. If it doesn't survive, I'll know we were both right!
    Jerry
    AMA -922698 Nomal people scare me, but not as much as I scare them...

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    RE: Cub CG

    hugger, Are you balancing it with an EMPTY fuel tank? Empty fuel tank is the correct way.

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    RE: Cub CG

    What did it do in flight that made you feel it was nose heavy? Could the incidence or elevator trim be so far off that it made you think it was nose heavy?

  12. #12
    hugger-4641's Avatar
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    RE: Cub CG

    Yep, balanced with empty fuel tank, but everything else installed as ready to fly. My homade balance rig is a frame made of 2x2 pine with pencils fit tightly into holes on the end of the 2x2 arms. The wings of the plane rest on the erasers of the pencils and I can adjust the width between the arms by several inches for different sized fusealages. This rig has served me well on dozens of planes so far, though I did have to use shorter pencils with this plane so my rig would fit between the struts and the wing. I'm not a 30 year veteran of RC, but I don't consider myself a newbie either, however, this is the first time I've run into this situation.
    The plane had weak elevator authority at lower speeds and when flairing out to land, even though I've got at least 20 degrees of deflection on high rates. It also landed very fast compared to both the smaller and larger cubs that I have. It required quite a bit of trim adjustment between cruise and full throttle. When trimmed at 1/2 throttle, it really wanted to climb when I went WOT. It was also very light on the tail wheel while taxiing and came very close to nosing over when I landed and hit some very minor clumps of grass that normally do not pose any problems for my other .40 sized planes. All of this tells my brain it is nose heavy, but my brain has been scattered before, guess we'll see one way or another !!
    Jerry
    AMA -922698 Nomal people scare me, but not as much as I scare them...

  13. #13
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    RE: Cub CG

    Stab/wing incidence is one thing I haven't examined that closely yet, so you could be on to something there. I'll do some checking and get back to you. Probably be tommorow as I'm headed to the hay for tonight! Thanks again Larry.
    Jerry
    AMA -922698 Nomal people scare me, but not as much as I scare them...

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    RE: Cub CG

    BTY, The $27.99 Great Planes C/G balancer that Tower sell is a great value for the money. Great for small and medium size models, up to about 10-12 lbs or so. Do yourself a favor and retire your pencil and eraser set up. You'l thank me if you do. Sounds like you do have an understanding of how to set the C/G so maybe a good nights sleep will clear your head and the answer will jump out at you tomorrow.

  15. #15
    hugger-4641's Avatar
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    RE: Cub CG

    Ok, the Cub survived! I didn't get any video, but I did get to fly it for a bit just before the Super Bowl kicked off. Here's the verdict so far. I don't have my incidence meter as it is loaned out at the moment, but just looking at it, the wing/ fuse incidence looks right but the horizontal stab looks maybe slightly too negative to the wing. I decided to go ahead and play with Cg before I make any inicidence changes. Cg at 4" from LE, Cub flew like a drunk bat, barely got it back down in one piece. Cg moved to 2 1/2", Cub flew like a slightly intoxicated bat, but got better as the fuel burned off, even better when I got up high and idled down, and even better when I deadsticked it. I believe I have more of a thrust line issue than incidence. I'm used to Arfs having thrustline corrections built into the firewall, but the motor mounts were already installed in this plane so I didn't pay much attention. After looking closer, this plane has no down or right angle built in. I ran out of time to play with it Sunday, but I'm going to set the CG back to about 3 1/4" and add some down and right offset to the motor mount and see what happens. Will probably be Wed or Thur before I can try again, I'll try to get some video next time.
    Jerry
    AMA -922698 Nomal people scare me, but not as much as I scare them...

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    RE: Cub CG

    Your in flight description with C/G @ 4" sounds like very tail heavy, and C/G @ 2 1/2" sounds a little nose heavy with fuel burn off helping, so I would try C/G @ 3" first and then 3 1/4". Also with no down thrust a lot of climb on higher throttle settings would be expected. I believe you are narrowing it down so keep at it. Glad you didn't auger it in when the G/G was @ 4". How "exciting" do you think 4.5" would have been??

  17. #17
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    RE: Cub CG

    Thanks Larry. I think 4.5" would probably have been fatal! With the Cg at 4", I made two circles around my field trying to trim out and decided I better get down before anymore fuel burned off. This may sound strange, but I'm glad it was windy. I had about 8mph winds with light gusting that probably helped me get the the thing back down. My next step was going to be setting the Cg at 3 to 3 1/4" , but I ran out of time about the time I ran it out of fuel and deadsticked it. As soon as the engine was killed it was a totally different plane, so I think I do have a thrust problem as well as maybe propped a little to high. I've got a 12x5 wood prop on right now, which is not what I usually use on a .46, but I'm going to leave it on for now until I work out the other variables, then I'll probably go back to a MAS 11x6.
    Jerry
    AMA -922698 Nomal people scare me, but not as much as I scare them...


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