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Old Apr 22, 2015, 09:11 AM
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CG for 3.7 M Swift S-1

I have an old Rosenthal Swift, and have been trying to get it ready to fly. No much has been accomplished so far, and am trying to figure where to place the tail servos to end up with as light a model as I can. There are no instructions or drawings with the glider, so not sure where to start. I'd guess the balance point is somewhere near the fiberglass wing tube, but not sure of that either. Anyone know where a good balance point might be? The servos are in the wing for the ailerons and spoilers, and the wing and stab has been fiberglassed with 0.75 oz cloth. Nothing yet has been done to the fuse for the landing gear, rudder, or elevator controls.

Any help or insight would be appreciated.

Larry Fitch
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Old Apr 23, 2015, 01:56 PM
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Do a search for mean aerodynamic chord calculators. Plug in the numbers and find where 33% MAC is. Set your CG there. The Swift should have minimal decalage, 0-0.5 degrees. That should get you a safe maiden flight. Expect some up trim. The correct CG will need to be determined through flight testing. Worked perfectly for my 1:3 Fox (and many other planes).
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Old Apr 24, 2015, 09:44 AM
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Which calculator?

MAC calculators I've been able to find require "CG" as an input, and CG calculators require "MAC" as an input. I look a while longer, but no luck so far. As I don't know either value, not sure how this is going to work. Thirty percent of the root chord is about at the rear edge of the wing rod, so likely will start there. At 3.7 meters, I think the model is small enough to test launch at a large grass field to see how it glides, and if it's really tail heavy, similar to a TD glider.
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Old Apr 24, 2015, 11:19 AM
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Success?

With the help is the web, I've calculated MAC to be 10.8727", so 33% MAC is 3.588". As the leading edge of the Swift is straight, this should also be the location for the Center of Gravity, 3.588" back from the leading edge at the root. Or am I missing something again?

Larry Fitch
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Old Apr 24, 2015, 11:24 AM
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Go to http://www.nasascale.org/howtos/cg-calculator.htm

For the "Enter MAC Balance Point" parameter, enter 33, then hit calculate.

The bottom box of the output is what you want and gives the distance back on the root chord to set the cg.

Usually what I do is calculate for 30 and 35% MAC then put the cg in the middle. This gives me an idea of the range I'm dealing with.

Note that this calculator only works for wings with a simple taper. Multipanel MAC calculators are available but usually need to be downloaded. I always check any recommended cg with a calculator.
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Old Apr 24, 2015, 11:27 AM
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You are correct.
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Old Apr 24, 2015, 12:09 PM
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FYI - the multipanel calculator I like and have been using for years is from tracfoil. You can download the English version for free at:

http://tracfoil.com/cm/indexE.html
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Old Apr 24, 2015, 05:17 PM
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Thank You

k2k, Thank you very much for your support in helping me sort through this MAC/CG determination. I now have the range for 30--35% as well as the exact number for the 33% balance point. Now I just need to make that a reality for my little Swift. I hope your information helps others as well.

Thanks Again,
Larry Fitch
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Old Apr 25, 2015, 04:58 AM
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I have a Top Models 3.7m Swift. The instructions say to set the CG from 80 to 90mm from the leading edge.

So your 3.5 inches is in the range.

Cheers
Mark
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Old Apr 25, 2015, 08:33 AM
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Thanks

mstone,
Thanks for the conformation. How do you like your Swift? Do you aerotow it, or just fly it on the slope? I hope to do both. Can you share the all up weight for your Swift ready to fly?

Sincerely,
Larry Fitch
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Old Apr 27, 2015, 09:26 AM
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My Swift is unflown

I was going to chuck it off a cliff 2 weekends ago but the weather was not good.

The Swift is setup for Aerotow as well.

I haven't weighed the planes as a whole but I would guess it is about 6kgs

Cheers
Mark
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