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Old Nov 16, 2012, 06:35 PM
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FlyBoy20's Avatar
United Kingdom, Wales, Swansea
Joined Aug 2012
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Question
Where is CofG with deltas?

Can't resist the urge to DIY a flying machine, so thought I'd design and build a really simple delta with sheet balsa - no aerofoil..

Having built the airframe, now my problem is where to put the electrics. Obviously weight distribution is critical, but surely also the centre of lift? I've determined the CofG of the naked airframe, and I've made a calculation of where the centre of lift could be - not so simple a task with a complex wing shape!

I reasoned that it should be where the fore-and-aft lift areas were equal in area, so using basic geometry methods for calculating area, I find the two centres happen to fall within 3/4" of each other.

Should I choose the point between the two for siting the electrics to achieve proper balance?
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Old Nov 16, 2012, 06:48 PM
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C₄H₁₀'s Avatar
United States, AK, Fairbanks
Joined Aug 2009
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First, ehh... Don't use balsa the next time around. You can get foam sheeting from Dollar Tree, most craft stores, and many hardware stores. Balsa's good if you're going for a more complex and detailed structure, but for flat planes it's just easier and lighter (and cheaper) to go with bamboo- or carbon-stiffened foam.

As for the CG, this calculator has proven itself many, many times over for my builds.

Got any pics of your creation?
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Old Nov 16, 2012, 07:46 PM
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United Kingdom, Wales, Swansea
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Originally Posted by C₄H₁₀ View Post
First, ehh... Don't use balsa the next time around. You can get foam sheeting from Dollar Tree, most craft stores, and many hardware stores. ..Got any pics of your creation?
This is the poor ol' UK - no Dollar Tree or anything similar over here - just standard foamboard with thick paper covering and really hard glue which destroys a blade with every cut - absolutely hopeless. Hence the balsa...

Will show a picture when it's tarted up a bit, it looks a bit ropey at the moment!

Got some neat U/C on it though..
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Old Nov 16, 2012, 11:50 PM
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Cheese5's Avatar
United States, CA
Joined Jul 2010
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Usually it is 1/3 of the wing. That includes flying wings and deltas too. I'll recommend flying over tall grass for the first flight/glide. Balsa breaks too easily!
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Old Nov 17, 2012, 12:12 AM
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United States, AK, Fairbanks
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Usually it is 1/3 of the wing. That includes flying wings and deltas too.
1/3... of the wing's what? Chord? Area? ... Answer: None of the above. It's usually somewhere in the neighborhood of very roughly 1/3 of the wing's mean aerodynamic chord, but even when stated correctly this isn't really the hard rule that some people make it out to be. Many planes, tailed or not, will require a CG that's very much outside the oft-repeated 33% range. The area of the horizontal stabilizer, or lack thereof, the intended performance and flight envelope, and a couple other factors will determine where a plane's adequate CG range falls. Particularly in forward-swept wings, many canards, and even some pylon racers, the CG range can be entirely outside the wing itself.

I've flown planes where having the CG located at 25% MAC made them ridiculously squirrely, and other planes where the CG can be at 50% and still be flyable.

There's a general mathematical way to calculate the MAC of a wing, along which you can then measure to find the CG point you want. If you click "Show MAC Lines" on the calculator in my link, you can figure out how it works pretty easily.
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Old Nov 17, 2012, 11:34 AM
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Interesting, all of my builds started off at 1/3 from the leading edge of the wing, then moved forward or aft if needed. Trial and error.
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Old Nov 17, 2012, 06:46 PM
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Taiwan, 北市
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Originally Posted by Cheese5 View Post
Interesting, all of my builds started off at 1/3 from the leading edge of the wing, then moved forward or aft if needed. Trial and error.
Your way works well on the wing of Cessna or J-3 Cub kind, but might not for Spitfire one or delta one, either. I located the CoG at 1" from the leading edge on my scratch built Alula, while its wing chord was 8" more. The stock Alula is with 9" chord, and its CoG is just behind the dimple on its wing. It's still 1" or so from the leading edge. But the CoG is still about 30% when referring to the said MAC.
http://www.dream-flight.com/alula.html
http://www.rcgroups.com/forums/showt...65996&page=118
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