Aerodynamics Discuss the physics of flight revolving around the aerodynamics and design of aircraft.

How to determine CG ??

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Old 12-23-2012, 11:15 PM
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CPTOZZY
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Default How to determine CG ??

I was given a Homemade half finished Airframe. The wing is symmetrical and the fuselage is a Profile. I will be adding my own electronics and motor. How do I determinemathmatically where the CG should be on this (or any future scratch designed) aircraft??

Thanks
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Old 12-24-2012, 03:24 AM
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Default RE: How to determine CG ??

Look to the top of this forum for the sticky with the aerodynamic tools. Punch the numbers in and it will provide a safe baseline CG.
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Old 12-24-2012, 04:43 AM
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Default RE: How to determine CG ??

Lots of good tools and info in that sticky .
 A quick way to determine the CG for a standard constant cord wing is to measure the cord from the leading edge to the trailing edge and then multiply that number by  33.3 percent. Then take the result and measure back from the leading edge and put a mark on the wing. This 1/3 measurement will be a good baseline to start with and 27 percent works well too for most planes.
If it's a low wing plane you need to invert the plane to balance it. If it's a high wing you balance it upright.
Once you get it balanced and flying, trim it out for normal flight then invert the plane and see how it behaves with "hands-off".. If it heads toward the deck it's nose heavy so  you can recalculate a lower CG percentage, remark the wing and re-balance it.
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Old 12-24-2012, 06:46 AM
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Default RE: How to determine CG ??

Quote:
ORIGINAL: CPTOZZY

I was given a Homemade half finished Airframe. The wing is symmetrical and the fuselage is a Profile. I will be adding my own electronics and motor. How do I determinemathmatically where the CG should be on this (or any future scratch designed) aircraft??

Thanks

Nine measurements with a yardstick. It takes a couple of minutes. Plug them into any online app that does CGs and click.

This is a good one. http://www.geistware.com/rcmodeling/cg_super_calc.htm

Plug in the measurements then put 15 in the 'desired static margin' field and click. You can use the CG location given for 15. It'll be safe for sure. Of run once for 5% and once for 20% and you'll have a safe range for the CG. It's magic.

If you want to go simple, then go really simple and just balance on the spar. MOST models have the spar located where the CG can safely be. No need to figure anything if you think your model is an average layout. The simple methods work for 'normal looking' models. So use the simplest if that's good enough for you. If you'd prefer to have something that takes the important details (there really aren't too many) into consideration, spend a minute or two measuring and run the geistware app. The measuring is simple to do. They do the figuring for you.







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Old 12-24-2012, 07:01 AM
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Default RE: How to determine CG ??

If you just want to do some sport flying, locating the CG on the wing spar is a good place to start. The spar on most planes, is located at about the 25% of MAC point, which gives a 10% static margin. This results in a good balance of stability and reasonably good aerobatic ability for a novice flyer. If you want to fly more aerobatic maneuvers, you will need a reliable way to adjust the CG location. The factors effecting the plane's stability and aerobatic performance are discussed at my website, www.rcaeronauts.com. Check them out and good luck.
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Old 12-24-2012, 08:08 AM
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Default RE: How to determine CG ??

A suggestion while you've got the sucker on the balance tool..........................

When you have the suggested CG range to go on, you can use that info to help you out later on. See what it takes to balance the plane where the mfg's forward CG is. See what it takes to balance on the mfg's rearmost CG. Then balance wherever you want to maiden and do what you do normally.

When you maiden it, if it's sluggish on the elevator or flies nose heavy to you, you'll already know what it's going to take to move the CG back, and how far you can go safely aft with the CG. etc
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Old 12-24-2012, 02:02 PM
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Default RE: How to determine CG ??

I balance from the wingtips center or slightly forward of center.To far forward gives you your problem! Check out my (Electric rotowing)
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Old 12-24-2012, 04:15 PM
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Default RE: How to determine CG ??

has anyone ever found an on-line CG calculator for biplanes?
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Old 12-29-2012, 05:39 AM
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Default RE: How to determine CG ??

http://www.google.com/url?sa=t&rct=j...55534169,d.cGE

Excel or spreadsheet required.

I've never used it, just recall having seen it before.

Curtis
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Old 12-31-2012, 11:24 PM
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Default RE: How to determine CG ??

Perfect. Based on this calculator, my own calculations look pretty close. I hope it flies
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Old 01-01-2013, 05:42 AM
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Default RE: How to determine CG ??



When doing these caluclations do you include the Aileron in the Chord Measurement?

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Old 01-01-2013, 06:02 AM
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Default RE: How to determine CG ??

Yes, all control surfaces are included.

It's good to make a habit of taking the measurements on every airplane. Often manufacturers give extremely conservative CG locations. You can waste a lot of effort getting the CG far enough forward according to the directions when actually the plane might be perfectly stable with the CG further back.

As you can see from calculators above, you need the wing area, the stab area, the tail arm and the chord of the wing. To get a proper tail arm and chord you need at least an approximate MAC if it's a tapered wing. I have found over 30 years of flying that the formula works every time. I will do maiden flights with the CG farther back than indicated on the plans if the formula tells me it's OK. People get scared away from doing this by the constant repetition of "...a tail heavy plane flies once," but it's not tail heavy if you've done the math.

Jim
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Old 01-01-2013, 08:59 AM
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Default RE: How to determine CG ??


Quote:
ORIGINAL: buzzard bait
I will do maiden flights with the CG farther back than indicated on the plans if the formula tells me it's OK.
Yes, the designers and kits' manufacturers are conservative in order to compensate for any deviations and imprecision's introduced during construction.
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Old 01-01-2013, 09:45 AM
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Default RE: How to determine CG ??


Quote:
ORIGINAL: CPTOZZY



When doing these caluclations do you include the Aileron in the Chord Measurement?


In addition to always including the movable surfaces as mentioned you also include the area within the fuselage. It's not a big deal either way with a skinny fuselage but with some designs it becomes a pretty major difference. The SOP is still to include the area bounded by the fuselage.
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Old 01-01-2013, 12:32 PM
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Default RE: How to determine CG ??


Quote:
ORIGINAL: Lnewqban


Quote:
ORIGINAL: buzzard bait
I will do maiden flights with the CG farther back than indicated on the plans if the formula tells me it's OK.
Yes, the designers and kits' manufacturers are conservative in order to compensate for any deviations and imprecision's introduced during construction.
If someone messes up the wing area, stab area, chord or tail arm they have really messed up the whole airplane.

Errors in alignment or incidence can have an effect on the maiden flight, but if they do, they won't be helped by having a conservative CG.
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Old 01-10-2013, 06:36 PM
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Default RE: How to determine CG ??


Under 4 pound RTF weight. I just take it to the 2' deep Orchard grass & heave it.

Tells me L or R unbalances & COG position for a dead engine landing.
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