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Where to put CG on a flying wing?

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Where to put CG on a flying wing?

Old 04-20-2004, 10:21 PM
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Ryans Rebel
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Default Where to put CG on a flying wing?

I have an old combat model that has a good wing one it. I am planning on making a flying wing out of it. I've already removed the tail. I am getting ready to locate the engine. Where should the CG be??? On a conventional aircraft I would say 25%-35%, but I would guess it should be much less on a flying wing. Any ideas? What else should I consider when taking a wing off of a conventional model and making it a flying wing?

Ryan
Old 04-20-2004, 11:03 PM
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Default RE: Where to put CG on a flying wing?

First off I hope it's a full symetrical airfoil. Obviously this wing is a plank style wing in that it has no sweepback. That means you get your stability from the airfoil shape. A symetrical airfoil will be fine with the elevons reflexed a couple of degrees. But a regular cambered airfoil will require a crazy amount of up trim to achieve stability. And possibly wider elevons to help locate the "reflex" further back from the airfoils point of maximum camber.

Anyway if it passes that test then you should probably start with it about 17% to 18% back. You may then be able to move it back to 20 or 22% but it'll be starting to get really hairy at that point. 25% is generally considered as the neutral stability point for a symetrical airfoiled flying wing. With full span elevons you won't need much elevator travel volume, probably 30 to 50%, in the mix but the normal amount of aileron travel.

Best of luck.
Old 04-30-2004, 01:19 PM
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david.eschenbrenner
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Default RE: Where to put CG on a flying wing?

I hate to say it but you have a reciepe for disaster. All wing aircraft have to be designed with the proper sweep twist & spanwise lift distribution. If you do not follow the rules when it comes to an all wing you will have your hands full at the contols. The cheapo special foam wings with twin speed 400 engines suffer from awful stall & death spiral characteristics. I have flown several all wings, some with awsome flight qualities & some with awful qualities & I would recomend that you read more about the design architecture of flying wings before you get carried away.
To answer your question about CG location would be impossible. there are to many unknowns. I can tell you that the CG band is very small. A CG to far aft is a crash waiting to happen & a CG forward is a hand full
Old 04-30-2004, 01:38 PM
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Default RE: Where to put CG on a flying wing?

David, your ideas do not allow for plank style flying wings. Many of this type have been built and flown successfully with no sweep or twist. But the airfoil choice becomes critical with the plank style. The shape of the airfoil replaces all that other stuff.
Old 04-30-2004, 02:22 PM
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david.eschenbrenner
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Default RE: Where to put CG on a flying wing?

Mr. Matthews Does the plank wing have yaw issues?
Does the plank have an elliptical lift distribution?
Old 04-30-2004, 06:21 PM
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Default RE: Where to put CG on a flying wing?

Built a combat wing with a TLAR airfoil. Symetrical, double tapered no sweepback. Fairly sharp LE at the root, fairly blunt at the tips.You can crank a right angle turn at full throttle without tipstalling and lands very nose high and slow on landing without tipstalling Sometimes you get lucky. CG at 12%
Old 04-30-2004, 06:40 PM
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Default RE: Where to put CG on a flying wing?

ORIGINAL: david.eschenbrenner

Mr. Matthews Does the plank wing have yaw issues?
Does the plank have an elliptical lift distribution?
Oh yes, I've never seen or heard of a plank style flying without a vertical fin or rearward placed tip fins. And the wing on a plank style model generally uses very little if any washout since washout would act like up elevator in the plank wing's case. Lift distribution is elliptical in a way similar to conventional wings. Typical swept flying wings that use the washout to achieve pitch stability actually drive the tip airfoils into very low or even negative lift coefficients so the lift distribution looks like that bell curve you mentioned rather than a steep ended elliptical shape. However since they are either not swept at all or only marginally swept due to leading edge tapering the plank style relies on highly reflexed positive Cmo type airfoils like the Eppler 186.

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