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Magnum CG - ?????

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Old 10-07-2005, 11:56 AM
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Default Magnum CG - ?????

Okay guys, help me out here. I have read the comments from Magnum fliers about CG placement, and from the what I saw it seemed the range of 135 - 145mm from the LE at the root was a good spot.

I was sitting over tea at lunch, fiddling with the last details on my Magnum hoping to put it up on Monday. So, wondering about the CG in relation to the MAC I grabbed a sheet of paper from our plotter and laid out a wing panel. I then looked at the tip and decided to extrapolate the LE outward, and intersect a line that seemed to bisect the swept tip area about in half. So now I have what I considered a good representation of the wing planform with a straight cut tip.

Now.. I used the standard method of determining MAC, drawing lines forward and rearward from the root and tip, equal to the other's chord length respectively. I then drew the lines that intersect at the MAC. So far it looked like it made sense. Then, I calculated the 25% and 30% locations on the MAC, and extended them back to the root. Here's what I got:

25% MAC: 188mm from LE at root
30% MAC: 200mm from LE at root

So - either I did something wrong here, or you guys are all flying with way forward CG's... but you'd think someone would notice that.

Anyone have thoughts on this?

MJD
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Old 10-09-2005, 08:43 PM
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Default RE: Magnum CG - ?????

You are right... we are flying with "forward" CG's, at least we are when you compare things to straight wing designs. I don't quite know why, but this seems to be the case for most delta and high sweep wing designs.

If you take a look at a Diamond Dust for instance, and you determine the MAC the same way, you end up thinking that the plane will be nose heavy... yet it flies just fine with the (most rearward) CG as indicated on the plans.
Must be something about delta wings...?

But rest assured, if you balance your Magnum at 135-140 mm, it will fly good.
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Old 10-11-2005, 09:20 AM
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Default RE: Magnum CG - ?????

Yup, I'll believe the 135-140mm measurement as one that works and go with that. Was going to test fly yesterday but got drizzled out.. grrr!

Deltas are a different story with CG relative to MAC AFAIK. The Magnum has generous stab area so I am still wondering why the forward location. Maybe I'll build a chuck glider with the same planform and check it out. I might check the decalage with an incidence meter too to see if there are any clues there. It seems to me with the forward CG (if it is forward compared to the "optimum" position) that induced drag would be increased and thus speed would be affected to some degree. Has me curious is all.

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Old 10-11-2005, 04:16 PM
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Default RE: Magnum CG - ?????

Still the issue vexes me.

I went here: http://www.geistware.com/rcmodeling/cg_super_calc.htm and banged in all the measured values for the Magnum, and selected 15% static margin. So guess what - it tells me 198mm!

Maybe one day I'll start moving my CG back from 140mm until I can't fly it anymore. Then I can build another one and move it ahead a little from there...

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