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Post by Dillzio on Sept 8, 2009 5:05:16 GMT 1
Hi gang,
I've been finding some conflicting information on where the cub's center of lift is.
The consensus on this forum seems to be that it's about 1.8 inches from the leading edge, but on NEcubflyer's youtube videos he measures the thickest part of the wing being 2.25 inches, and he says that THAT is the center of lift.
So, who knows the real deal?
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Post by ginginho on Sept 8, 2009 9:08:12 GMT 1
I believe that NECubflyer is correct in that the CoL is 2.25". However, do you really need this or the Centre of Gravity?? The CoG should be forward of the CoL to give a slight nose heavy setup. This will fly move positively and smoother than if set on the CoL. My B/L has a CoG of around 2" and cuts through the wind nicely under power yet floats around gently when required.
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Post by Dillzio on Sept 8, 2009 10:43:11 GMT 1
Ah, I see. Thanks for the advice.
So I should set it up so that if I balance it on two points that are 2 inches from the leading edges of the wings, the horizontal stablizer should be at 90 degrees to the ground?
Thanks again,
Dill
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Post by ginginho on Sept 8, 2009 12:03:41 GMT 1
Ah, I see. Thanks for the advice. So I should set it up so that if I balance it on two points that are 2 inches from the leading edges of the wings, the horizontal stablizer should be at 90 degrees to the ground? Thanks again, Dill You probably want to go a little further back that 2" if you are running a stock motor, mine pulls ~250W at WOT so has more than enough to overcome the nose heavy attitude. Also different flyers prefer different setups, the closer the CoG is to the CoL, generally the more lift (but less control) you'll get. Try flying it with different CoG's ( a few millimetres can make quite a difference). You probably want to set it on your balance rig (bent coat-hanger!) at 2.25" and see whether it is nose up or down. Adjust the weight so that it's a little nose down hanging there. That'll mean your CoG is a bit in front of the CoL. If you move it on the rig until the horizontal stab is parallel to the ground, then it will be balancing on the CoG. You can of course use this method to adjust the preferred CoG. |
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Post by Dillzio on Sept 12, 2009 3:54:40 GMT 1
OK, I've worked out that I need to add about 1&1/2 ounces to the cowl. This surprises me, because my most significant modification is mounting the battery vertically, and it seems to me that about 3/4 of the batteries thickness is in-front of my CG (2 inches), but even with the battery slightly forward like that it still needs more weight at the front.
Anyway, my main question is as follows:
Where is the best place to mount the counterweight?
It seems to me that where one mounts the weight will dramatically effect how the aircraft flies. Put the weight on the side, and the airplane would have a tendency to bank. Put the weight on top, and you could effect the stability of the aircraft, and compromise it's tendency to stay upright. Put the weight on the bottom, and you will enhance the aircraft's tendency to stay upright, but will have trouble doing loops and flying inverted (I don't have ailerons).
I was thinking perhaps two equal weights left and right, or maybe even just one on the bottom while I'm learning.
I'm mounting my counterbalance inside the cowl. I'm using lead weights that have been drilled out and threaded with a self tapping screw. Then I just screw it to the cowl.
Please, enlighten me. :-)
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Post by patmatgal on Sept 12, 2009 12:26:07 GMT 1
When I had to add weight in the cowl I just used electrical tape on a lead fishing weight and put it under the motor mount (a direct drive motor so there was plenty of clearance) The only problem I found was that one loop of tape around the center of the weight would let the weight eventually slide out, 1 loop around each end of the weight worked fine. If you have the room I would try attaching to the motor mount or maybe the firewall. In so far as getting everything perfectly centered, eyeballing it so everything is close to the centerline is plenty good enough (the Cub is not an aerobatic plane where it matters, just overbuilt and tons of fun to fly)
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Post by Dillzio on Sept 13, 2009 8:36:04 GMT 1
Does anyone know if it matters where the counterweight is?
In terms of top, bottom, left or right of cowl.
thanks
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duck
Squadron leader
R/C Addict
Posts: 219
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Post by duck on Sept 13, 2009 15:31:57 GMT 1
You want the weight centered on the aircraft's roll axis, lateral balance is just as important as CoG balance.
My opinion, is as low as possible, on the centerline.
As for Cog, I have good luck at 1-7/8" back from the LE, For field checks, I balance on the two forward strut eyes.
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Post by Dillzio on Sept 15, 2009 7:08:40 GMT 1
After a bit more experimenting, I found that I can get away with only adding one ounce if I add it to the very front of the cowl instead of mounting it near the firewall. I have opted for two weights either side of the bottom of the cowl instead of just one in the middle. I think with a set up such as this, it may even be possible to fine tune the balance of the plane somewhat by loosening the nuts that hold the weights on, and rotating them so that their mass moves closer to the firewall, and thus reduces how nose-heavy you're making the plane. Pictured is my home-made cowl, but I'll be doing the same thing to the stock one too.  Duck, are you using the stock motor with lipos? Was wondering if your 1-7/8" setup is for a stock or modified cub. cheers |
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duck
Squadron leader
R/C Addict
Posts: 219
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Post by duck on Sept 15, 2009 14:03:40 GMT 1
I used the same CoG for my stock cub, and my brushless setup .
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