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Old Dec 18, 2002, 10:14 AM
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rakm's Avatar
Pensacola Regional, Florida, United States
Joined Jan 2002
442 Posts
Getting CG forward on a swept wing design

Home designed powered swept wing that flys very well. The only problem is I have to place the battery packs all the way to the rear to get the cg correct. What needs to be done in the design to get the battery packs forward? Do I need to increase the root cord? Reduce the sweep? Thanks for any help.
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Old Dec 18, 2002, 12:46 PM
Ascended Master
Sparky Paul's Avatar
Palmdale, CA
Joined Oct 2000
13,499 Posts
Reducing the sweep moves the c.g. and therefore the battery pack forward.
If changing the root chord reduces the sweep...
Adding a tail will move the c.g. forward also.
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Old Dec 18, 2002, 01:50 PM
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East Anglia, UK
Joined Sep 2002
29,703 Posts
ER what?


First of all are the wings swept forward or back?

If you reduce sweepback, all other things being equal it will move the CG *back* with respect to the wing center of lift, at least. So although Sparky is right that the CG moves forward as sweepback is reduced, because some wing mass will move forward, its not going to do what you automatically think 'moving CG forward' would do. Thats because the wing is moving forward faster than the CG is, so to speak!!!

If the aircraft is nose heavy, reduce sweepback. Or add a tail!
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Old Dec 18, 2002, 05:18 PM
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rakm's Avatar
Pensacola Regional, Florida, United States
Joined Jan 2002
442 Posts
The wings are swept back. Looks like a small Zagi (28 inch span). Thanks for the advice. It looks as though the answer is to reduce the sweepback. I'm assuming this will also reduce the stability as it's taking away some effective dihedral? Will try a few different angles and get the best trade-off. Right now it's very stable/forgiving and don't really want to loose that.
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Old Dec 18, 2002, 07:55 PM
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Cincinnati, OH
Joined Aug 2002
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Better question is, "If it flys well now, why worry about the aft battery location?"
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Old Dec 23, 2002, 12:14 AM
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exempt's Avatar
Providence, RI
Joined May 2001
380 Posts
I had almost the same problem with my home made Zagi style wing, but with mine it's tail heavy (i think) when I tried to launch it, it just did a backwards loop?? I dunno, I guess somethings wrong.
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Old Dec 23, 2002, 06:32 AM
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Robbie d's Avatar
Australia, SA, Windsor Gardens
Joined May 2001
1,933 Posts
you can calculate the cg of any wing easily using nothing but a ruler and a pencil. First measure your root and tip chords.
The red lines on the diagram represent the root chord and the light blue lines are the tip chords. You draw the chord lines top and bottom. Then draw two intersecting lines (the dark blue ones) from the tip of each line and where they cross is the c of g.
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Old Dec 23, 2002, 12:03 PM
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Sparky Paul's Avatar
Palmdale, CA
Joined Oct 2000
13,499 Posts
Not quite.
That procedure locates the mean aerodynamic chord.
You must then compute where the c.g. is on that chord, and project that location to the root chord.
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