drive and air smash
How do you find the cg of your own scratch build? I am currently trying to make a edge 540 for 3d and i am wanting to know how do i find the cg of it? when it is done and i can fly it and do 3d then i will post the hand drawn plans . So how do you do it? This is a pic of what i have of it before i cut it out!IMG_0967.JPG 
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Airforce101
Usually - on a traditional wing, not a delta or whatever - your starting point would be about 1/3 behind the wing's leading edge and then adjust based on the flight characteristics. Do a glide test: just gently toss the plane (have someone catch it or make sure you do it over some soft ground / tall grass) with all gear installed (but powered off) and see how it glides. I've seen some videos somewhere on the forum of a scratchbuilder doing right this, to give you an idea.

Squishy did an interesting post on this subject, have a look: http://www.rcpowers.com/community/threads/determining-cog-on-a-new-design.15227/#post-175715
My avatar is what I look like after a "landing"...
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sgf323
I've always done glide tests with no gear installed and control surfaces pinned to neutral. Gear would make it too heavy and easier to damage.
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MacLarry
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How do you find the cg of your own scratch build?...

Yeah, here, try this & see if it works for you:
http://adamone.rchomepage.com/cg_calc.htm

Larry
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MacLarry
Forum member bobdabilduh55 has been designing models using his "Five Easy Pieces" technique and making his plans freely available for some time now. Here are a couple of links to some glide tests he made of his designs:

and


That's how he does it.

Larry
F-35 V2 - Fun & Done, J-20 - Fun & Done, plans reprinted, Cheap-N-Easy - Fun, Done & Salvaged Parts, F-18 V3 - No fun - All done, JXD-385 - Fun Super-Mini Quad, Sci-Fi Jet (Mikey's RC Twin EDF) - Treed - Salvaged parts, Alpha Jet (FRCFOAMIES) - Treed - Salvaged parts, Bixler 2 - Flies nice, Alpha Jet MkII (FRCFOAMIES) - Easy flier, Mig-29 V2 - Scary, parked, Yf-23 V2 (Five Easy Pieces) - In Progress, Slowly (Christian Huber, Germany) - Slow softie, Y600 Tricopter - Stalled, F-35 V3 - Plans Printed, SU-37 Mk2 (FRCFOAMIES) - Plans Printed, Turnigy 9X - OpenTx firmware, Ikarus AeroFly Deluxe Premium Flight Simulator for Macintosh + Ikarus wireless line converter - Awesome!
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whatmovesyou
If you are going to do 3D, you may want to re-think the way you designed your ailerons. They need some area out at the end.
As for Cg, my rule is start at 25% of the wing rather than 1/3. Reason being, you always want to start with nose heavy rather than the plane trying to flip backwards(tailheavy) and usually rotating and flipping causing major damage.
When doing 3D, as you move the CG backwards, increase your surface deflections to keep your plane under control. On one of my 3D planes, my total elevator deflection is 92 degrees, 45 up and 47 down(for inverted stuff).
I like to design and fly unique planes.
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Argonath
You might find this useful to give you an idea of where the CG is on your models if you want to do the 'math' part of it.

http://adamone.rchomepage.com/cg_calc.htm
D.I.L.L.I.G.A.F.
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Argonath
You might find this useful to give you an idea of where the CG is on your models if you want to do the 'math' part of it.

http://adamone.rchomepage.com/cg_calc.htm

From there, you'll need to fine tune the exact spot because even the calc won;t give you a perfect CG on your plane but it will put you within about an inch or less of where it should be on your plane. Once this is done, you can plan out where you're going to lay out your gear and trim it out further from there.
D.I.L.L.I.G.A.F.
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Airforce101
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I've always done glide tests with no gear installed and control surfaces pinned to neutral. Gear would make it too heavy and easier to damage.


Ah yeah, good thinking. But in the end, you need to have some weight to move around to adjust CG.
My avatar is what I look like after a "landing"...
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Airforce101
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Forum member bobdabilduh55 has been designing models using his "Five Easy Pieces" technique and making his plans freely available for some time now. Here are a couple of links to some glide tests he made of his designs:

and


That's how he does it.

Larry


Exactly the clips I was referring to! I remember seeing them on the forum here, but they were hard to find because I didn't remember who posted them: bobdabilduh55 of course!
My avatar is what I look like after a "landing"...
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sgf323
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Ah yeah, good thinking. But in the end, you need to have some weight to move around to adjust CG.


I just tape quarters to the nose until it glides even then I balance the plane between 2 fingers to find the CG.
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