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Determining CG

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Old 12-17-2008, 11:46 AM
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the original Chriss
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Default Determining CG

O.K. so I had a good fuselage and good pair of wings from two different planes. Wings mount in same place on fuse as the originals but are a couple inch's wider in chord... and not symmetrically about the tube, more behind than in front.

Seems a somewhat simple question but, where should the CG be? The textbook answer is "at the aerodynamic center of lift" but how do you determine that? It's a wide symmetrical wing, safe to say at the high point? It wasn't there originally...help, I'm confused.

Chris
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Old 12-17-2008, 12:09 PM
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Default RE: Determining CG

If you have not altered the wings, why would their the cg change?
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Old 12-17-2008, 12:31 PM
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I did not change the wings, just put them on a different airframe. Is that it? The CG is determined by the wing and wing only?

Chris
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Old 12-17-2008, 01:32 PM
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Default RE: Determining CG

The attached txt file is actually a zip file. Unzip into a directory, then run CenterOfGravity.htm either from Explorer or by entering the file name in a browser window.

The method used was from an article by Run Van Putte in '92. It has worked well on many types of airplanes, including a flying wing. Notice that for the Integral, the calculated CG is at 8.07 inches or 205 mm from the leading edge. This matches the instructions, and is consistent with the more tail heavy recommendation from most people. I prefer flying close to neutral, and this location is completely safe. There is a very slight drop on an inverted 45 degree line, and down elevator is required for inverted.

Let me know how this works for you!

Ken

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Old 12-17-2008, 02:30 PM
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Very cool, thanks Ken. I'll give that a try tonight. It'll be interesting to try on different models and see if it agrees.

Chris
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Old 12-18-2008, 12:20 AM
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Default RE: Determining CG

Hey Ken.

Good to hear from ya. Are you flying 2 Sundays from now? I think I will be out at Hollister, unless I can get Jon here.. lol Gonna do some new sequence follies.. lol

Anyway, I ran the numbers for the Element, and it comes out to 7.25" I am nearly 2 " in front of that. But, there's no mix in knife edge. Although I am feeling like it is causing the to the canopy pull in ups and downs. 1 piece wings are horrible!! lol .... Would be easy to crank some positive into the wings to try to solve the pulls with 2 piece wings.

Cool program though, thanks for sharing.

Would be cool to have a moment arm calculator by just entering the "on the wheel" weights like this without having to enter a bunch of other junk..

Hope to see ya at the field!!

C
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Old 12-18-2008, 03:56 AM
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Default RE: Determining CG

I believe that sort of computation only gives you the rough indication. I believe the aerofoil also makes a difference, a friend who studied aerodynamics told me that. That is why they call it the AERODYNAMIC CENTER OF LIFT. Hence maybe you guys who fly profile 3D foamies will notice that you can take the CG waaaaaaaay past 30% MAC and it still flies fine. Examples is like the Concorde as it comes to speed of sound the aerodynamic center of lift shifts backward. Interesting?
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Old 12-18-2008, 07:09 AM
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Default RE: Determining CG

Chris, I'm trying to fly whenever the weather allows, and am not travelling over the holidays. Let me know your schedule as the time gets closer. When I tried the Integral a lot more nose heavy, I didn't like the 45 and 60 degree lines, or the excessive elevator for inverted. Snaps were a bit better, but I've finally got the throws about right for decent snaps. It's even continuing the loop on the 1 1/2 snap avalanche instead of flattening out!

I agree that there are factors other than the simple plan form of a wing that determine center of lift. For example, it would be kind of tricky to factor in an eliptical wing tip, and I too have heard that center of lift can move with air speed. However, my experience has been that this formula is a great starting point for those who don't have plans or just want a second opinion. Pattern has always been about the latest in fads. There are a lot of people preferring the CG more forward now, and I generally believe in going along with what the majority are doing. But for CG, I really do prefer closer to neutral.

Ken
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Old 12-18-2008, 11:18 AM
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Default RE: Determining CG

Hello All

I too prefer a neutral CG. I just want to be sure I start well in front of that and work my way back... So, I'll compare what the calculator spits out to the original CG for the plane that the wings came on and go from there. It'll be a while before I get to the point
of flying the plane, probably January-ish.

Thanks again, much appreciated

Chris
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Old 12-18-2008, 11:22 AM
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Default RE: Determining CG

http://adamone.rchomepage.com/cg_calc.htm
http://adamone.rchomepage.com/cg2_canard.htm
http://www.geistware.com/rcmodeling/cg_super_calc.htm

There are 3 different online CG calculators for you.
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