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Finding the model's correct CG

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Finding the model's correct CG

Old 04-15-2022, 04:22 PM
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karolh
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Default Finding the model's correct CG

As a scratch builder I've always used one of the oneline CG calcuiators to find a starting point CG for my models and have been quite happy with the results. In recent times however I've been hearing more and more about the accuracy of the Vanessa balancing rig in finding the proper CG, and am wondering how close are the results from both methods. I would certainly appreciate hearing from anyone who has used both methods as a comparission.
Old 04-16-2022, 07:37 AM
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allanflowers
 
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It seems to me that the CG calculators tell you where the CG should BE. The Vanessa rig measures where it IS on the model (you then can add weights to shift it to where ever you want it to be).
Old 04-16-2022, 10:25 AM
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speedracerntrixie
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I agree with Allen. The big question for each model is where the CG should be. Too many times I see models that are balanced too far forward and results in an airplane that flies poorly and overall more difficult to fly well.
Old 04-16-2022, 12:23 PM
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karolh
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I've always used an initial CG location as a starting reference point and then flight trim the model to my likeness. I guess here it's pretty much a case of is the glass half empty or is it half full, as I figure each method has its merits.
Old 04-16-2022, 05:29 PM
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BMatthews
 
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As the guys mentioned already the Vanessa won't tell you where the CG NEEDS to be. It's just a replacement for your fingertips to show where it's balanced at the moment. It doesn't replace the initial calculations you do with the online CG calculators.

It's also not really any better than your finger tips or using sticks with soft rubber pads on smaller and simpler models. What it does best is hold up larger bulky models well so you can work around them and adjust the CG by adding or removing weight or shifting parts while the model is held suspended. Another good time for a Venessa rig on even more compact models (like under 5ft or 1.5m span) would be if they are biplanes or have troublesome wing struts as on some scale models. But of course if you go to the trouble to make one or two different size rigs to cover the whole range of sizes then you might as well use it. The other "of course" issue is that you need to hang the model in the rig from a ceiling hook. Not everyone is in a situation where they can easily drill and screw a hook into the ceiling, Renters or finished ceilings and such. For those cases perhaps a set of double doors can be opened and a bar set across the tops of the doors or some other open area with ledges can be used?

In the end though for smaller size simple models it's hard to beat just using our fingertips. There's no setup time and they don't take up any room...
Old 04-17-2022, 09:12 AM
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karolh
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Thanks to you all for your responses, which makes it all much clearer to me now.
Old 05-04-2022, 01:07 AM
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Joseph Frost
 
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With scratch build models I normally build a smaller 30-50% scale size prototype as a hand chukker or sling shooter to make sure my guess CG is spot on.

CG proto, R.

Sorting out the balance.




Easy way to find the CofG.
Old 05-04-2022, 04:01 AM
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karolh
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Your way sounds quite intresting but rather time consuming and a bit of an overkill to me. My methods though certainly not as elaborate as yours have always given me a fairly good CG starting point for my maiden and trimming flights.
Old 05-05-2022, 12:39 AM
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Joseph Frost
 
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It all depends how much you value your model, when I put few hundred + hours into the project I want to make sure I won't screw up on the first go.
I had a few very stressful moments in the past with no idea about the CoG, but only guessing always being so tail heavy that the only way to keep them afloat was vertical hover, bring it down as low as possible and let go! LOL.

Hovering 15.5Kwatt "Triptych at 15.5 kilo AUW, just before slamming it down with minimal damage.

Lot safer to be high.
Old 05-05-2022, 03:13 AM
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karolh
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There is no alternative to altitude....lol.

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