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Center of Gravity "CG" Computations - 11/22/2007 5:27:36 PM   
RC Specialties


 

Posts: 262
Joined: 12/23/2006
From: Zachary, LA, USA
Status: offline 		I have not found a thread that specifically talks about balancing and CG computations

I thought I would start this thread to discuss easy ways to compute changes to a CG without it being a trial and error method. It can easily be done using mathmatics with perfect results.

You can compute exactly the weigh to add to a desired location and the CG will be exactly where it should.

Do you want to move your servos to the tail. You can compute what effect it would have and how to compensate before actually making the change.

Calculate where to place servos batteries, etc. to limit the amount of ballast weight needed.

Let's talk about those CG problems!



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RE: Center of Gravity "CG" Computations - 11/22/2007 7:37:57 PM   
KitBuilder



Posts: 250
Joined: 2/17/2007
From: Palm Harbor, FL, USA
Status: offline 		I was reading an article the other day about just this from an older RCM issue... I will scan it and post it here after the holidays as I'm away from home.. In a nutshell. (if memory serves) it had to do with taking the distance from wheels to CG on the plans, put the nose of the craft on a scale... and correlating the distance calculation to weigth on the scale to be equal... The author said this was very very accurate and was the only way he balanced his planes. He stated the old "fingertip" method was very poor as fingers are wide and will affect balance point greatly.
Mike

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RE: Center of Gravity "CG" Computations - 11/22/2007 7:48:45 PM   
Bad_Daddy



Posts: 527
Joined: 1/27/2007
From: Tracy, CA, USA
Status: offline 		Aircraft SuperCalculator = http://www.geistware.com/rcmodeling/cg_super_calc.htm
Conventional AC CG Calculator = http://adamone.rchomepage.com/cg_calc.htm
Canard AC Calculator = http://adamone.rchomepage.com/cg2_canard.htm
Engine Offset Calculator = http://www.lcrcc.net/offset_calc.htm

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RE: Center of Gravity "CG" Computations - 11/22/2007 10:06:09 PM   
RC Specialties


 

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Joined: 12/23/2006
From: Zachary, LA, USA
Status: offline 		Bad Daddy,

These references are great but probably a little complex for many modelers. If you work with the basics that most moderlers are used to, the same principles can be applied. As an example, a couple of years ago I bought a GeeBee Y 120. After I had it all together, I took an accurate weight of the aircraft. I then put it on the balancer, finding the balance point on the wing cord. When doing this I use the bubble level and made sure it was level and marked the balance point on the wing. For a warbird, I leave it a half bubble nose heavy.

I used the end of the prop shaft as the datum line. I then measured back to the marked balance point on the wing. Using the weight times the arm gave me the moment. The arm being the current CG. You then take another measurement to the desired arm (CG). You can then pick the best location for the ballast and compute how much weight it will take to move the CG to the desired location.

Once a current CG location is established you could then use the same process for locating servos, batteries, etc.

I look forward to hearing your points KitBuilder and your thoughts Bad Daddy.

Jim

< Message edited by RC Specialties -- 11/23/2007 12:23:54 AM >

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RE: Center of Gravity "CG" Computations - 11/26/2007 5:44:57 AM   
themadmax


 

Posts: 92
Joined: 10/6/2004
From: Port Isabel, TX, USA
Status: offline 		Jeez!...you guys make this sound so scientifically difficult! With the fuel tank empty, measure back from the leading edge 1/3rd the chord...that's your "safe" balance point. 1/4 chord, the safest. Most RC Aircraft are nose heavy, to begin with, and once you add 6-8 oz of fuel. or whatever, 3"- 4" forward of the CG, they just become more nose heavy. I've only seen one a/c in 30 years of flying that had a tail-heavy problem. A Nose-heavy condition only adds to the dynamic stability of the a/c while detracting from the instability that makes an a/c more aerobatic. The further aft you move the balance point, the better for aerobatics...to a point. Too far aft and a stall becomes irreversible

Those of you considering putting your servos at the tail, shouldn't. It increases the pitch stabaility. (Not good for aerobatics) and creates CG problems for what?...now you've got long servo leads, (GLITCH!@#), another connection to go bad...what was the good point, again?

If in doubt as to where stuff should go, build the airplane to a point where you can bolt on the wing and the engine. Then put the servos and battery such that your balance point is between 1/3 - 1/4 chord. The covering, cowl, wheels won't change things much and you can always move the battery out to the end of the engine mount, worst case lol

KISS, (Keep it simple ------!)

< Message edited by themadmax -- 11/26/2007 5:53:04 AM >

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RE: Center of Gravity "CG" Computations - 11/26/2007 8:08:12 PM   
scratchonly


 

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Remember that about 80% of the paint/covering is behind the CG.

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RE: Center of Gravity "CG" Computations - 11/26/2007 8:46:36 PM   
RC Specialties


 

Posts: 262
Joined: 12/23/2006
From: Zachary, LA, USA
Status: offline 		THEMADMAX,

I understand the simplicity aspect of your comments. There are those that want the CG to be exactly where it should. A well balanced plane flies its best. The point here is that once a known CG location is determined, you can determine the effects of any changes without putting the plane on and off the balancer a dozen times. Some models have the servos in the tail. It is easy to determine what changes are required with a calculator. Likewise, you can determine what will happen to your CG if you install a different engine. All without actually modifying the aircraft. Don't you agree it can be beneficial?

Jim

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RE: Center of Gravity "CG" Computations - 11/27/2007 12:54:38 AM   
themadmax


 

Posts: 92
Joined: 10/6/2004
From: Port Isabel, TX, USA
Status: offline 		Stick your fingers under the wing @ 1/3-1/4 chord, if it doesn't balance there, you've got to move something! Calculating sounds like math and I've never considered RC as something I want to do math in, except to figure how what my wing/power-loading is. I fly "real" airplanes so I know how to do weights and balances computations but one can't exactly jack a "real" plane up at the desired "finger point" to make sure it's not tail-heavy...with most RC aircraft, you can!

KISS

< Message edited by themadmax -- 11/27/2007 12:56:24 AM >

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RE: Center of Gravity "CG" Computations - 11/27/2007 1:32:26 AM   
RC Specialties


 

Posts: 262
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From: Zachary, LA, USA
Status: offline 		In a real airplane you make sure you don't exceed your forward and aft CG, don't you? A model is not much different if you want optimum performance. Your method will work, but there is an alternative.

Jim

< Message edited by RC Specialties -- 11/27/2007 1:47:06 AM >

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RE: Center of Gravity "CG" Computations - 11/27/2007 3:22:21 AM   
SoCalSal


 

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From: spring valley , CA, USA
Status: offline 		If anybody is interested I have a excell spreed sheet all set up to figure out the correct CG of any aircrafte full scale or otherwise. Just drop me a note and I will forward it to you. You will need Microsoft excell to open it. But once you have the spreed sheet you can change any of the cells from metrics or pounds and ounces and it will work just the same. Simple to use and costs you nothing but about 20 minutes of your time and it can be used over and over. And the most important thing it is very very accurite.

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RE: Center of Gravity "CG" Computations - 11/27/2007 1:12:07 PM   
RC Specialties


 

Posts: 262
Joined: 12/23/2006
From: Zachary, LA, USA
Status: offline 		SoCalSal,

Thank you for the input. I am sure some will find it useful. I will send you a PM. I would like to take a look myself.

Jim

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RE: Center of Gravity "CG" Computations - 11/28/2007 5:01:46 PM   
JRFisher


 

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From: Pittsburgh, PA, USA
Status: offline 		Are the ailerons included when measuring the wing chord?

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RE: Center of Gravity "CG" Computations - 11/28/2007 5:10:15 PM   
RC Specialties


 

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Status: offline 		JR,

Yes, the ailerons are included if you are to measure the chord at their location.

Jim

< Message edited by RC Specialties -- 11/28/2007 5:18:47 PM >

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RE: Center of Gravity "CG" Computations - 12/10/2007 4:55:32 AM   
Mitsu1



Posts: 543
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From: Dumas, AR, USA
Status: offline 		I used somebody's online CG calculator when I built my most recent Katana, since I didn't have documentation from the designer on the proper CG.. The calculartor I used had blanks to plug in all relevant info, ( root chord, tip chord, span, sweep, etc.) ... and a blank to plug in what percentage point you wanted it balanced. I wanted a very aerobatic aircraft, and knowing the balance range is generally 25 - 33%, I plugged in 34%. I wanted it near tail heavy. I hit the "compute" buton, and it said that point twould be 6, 3/4" from leading edge at root. I proceeded to do a very tedious and tidy installation of batteries , etc that would give me a balance exactly at that 6, 3/4" position, only to find that that calculation was way off .. to the conservative. The plane was way, way nose heavy still. I had to trial and error my way through an additional 2" to get to what I assume was the true 33 -34% position. What's really strange is that I cross checked the numbers by using a second calculator, which didn't give the same point exactly, but within a 1/2" of it.

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RE: Center of Gravity "CG" Computations - 12/10/2007 11:36:02 AM   
RC Specialties


 

Posts: 262
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From: Zachary, LA, USA
Status: offline 		Mitsu1,

I have not used a calculator, so I can't comment. It makes it a tough when you don't know the CG point. The ranges you mentioned should be close depending on wing design. I have not worked with an unknown. Maybe others can comment.

Jim

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