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CG On flying wing? - 7/31/2012 8:33 AM   
sagehillspilot



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From: Reva SD, SD, USA
Status: offline 		How do you find the CG On a flying wing??

It is homemade with a wingspan of 56 1/2 inches. where do i start?


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RE: CG On flying wing? - 7/31/2012 12:05 PM   
up_in_smoke


 

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RE: CG On flying wing? - 7/31/2012 12:55 PM   
edsplane


 

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Status: offline 		The calculator used in the previous message, is limited to a single-panel wing and does not include the effects of the wing tips. If your flying wing is similar to this simple configuration, the calculator results should get you within about 10% of the actual CG location.  If your flying wing is multi-panel, the true results can be very different. If the wing shape is more complex and/or you want a closer estimate, I suggest that you read the description of a multi-panel wing analysis, including wing-tip effects, at the RCAeronauts website. 

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RE: CG On flying wing? - 8/1/2012 5:03 AM   
Lnewqban



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quote:

ORIGINAL: sagehillspilot

How do you find the CG On a flying wing??

This is why and how:

http://adamone.rchomepage.com/index5.htm

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RE: CG On flying wing? - 8/1/2012 1:43 PM   
edsplane


 

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Status: offline 		 The links suggested in this discussion give some good info about conventional airplanes, ie. with tails, but the flying wing is a special case. Normally, the horizontal tail(HTail) influences the location of the Neutral Point(NP) by moving it rearward.  The NP is somewhere between the Aerodynamic center of the wing and that of the HTail. But, your plane doesn't have an HTail, so the NP of the plane is at the NP of the wing, which is normally very close to 25% of the wing Mean Aerodynamic Chord(MAC).

This being the case, the CG must be located a safe distance forward of the NP. For typical sport flying, a Static Margin would be about 10%.
So, since the NP is at about 25% of the MAC, putting the CG at 15% of the MAC would give you a 10% static margin. My recommendation is that you start test flying the plane with the CG at 10% of the wing MAC, ie.a 15% static margin, which should be a little nose-heavy. Then, when you are more familiar with flying the plane, move the CG rearward in 1 or 2% increments until you are comfortable with the way it flys. I wouldn't go much lower than a 10% static margin.

In order to be confident in what you're doing here, it is very important that you calculate the Neutral Point location accurately, which depends on the HTail area, shape, efficiency and some other important factors. It is equally important that the MAC is correct. For example, any calculation must include the effects of wing tips. If a CG Calculator doesn't use these factors and use them correctly, then you are risking the safety of your plane. You can read a more complete description of the analysis, at the RC Aeronauts website.

Good luck


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RE: CG On flying wing? - 8/3/2012 6:28 PM   
BMatthews



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Status: offline 		Sage Hills, you've given us little to go by. But the simple facts in the online calculator posted in the first link is a good basis for where the CG should go once you know the MAC of your wing.

There's one other factor you need to allow for if this is a swept Zagi or similar style wing. And that's the amount of wing twist you need to use. A good way to get that is using the Panknin twist formulas in a spreadsheet. You can read about and download this handy piece of work at;
http://www.b2streamlines.com/Panknin.html

Now if your foam wings, assuming that's what you used, do not have any twist in them it can be faked by using outboard elevons and simply reflex them upwards a little. Doing so sort of replicates the twist you need for a swept wing model to be stable.

But that's a poor way to do it if this is a thermal flying glider where you want to get all the efficiency you can. For that you want to use a proper graduated wing twist.

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