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!

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

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

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

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 ------!)

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

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

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

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

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.

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

Are the ailerons included when measuring the wing chord?

JR,

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

Jim

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. [sm=confused.gif]

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

Mitsu1- The Calculator that I used said that the Static Margin should be between 5% and 15%. The lower the percentage, the more tail heavy it will be. I plugged in the numbers for my plane and used 10% and the CG came out right on the spar.

JRFisher, ... When you say static margin between 5 - 15%, what exactly are you referring to? When I said normally balance range is between 25 - 33% I was referring to that percentage of the mean aerodynamic chord, measured from leading edge, which as I understand it is an average median point spanwise of a wing panel, be it tapered and swept, or simply constant chord from which to start your measurement. If I understand you correct, 5 -15% of the MAC would seem to be overly nose heavy even for a beginning point, would it not?

Mitsu1- The calculator I used is at http://adamone.rchomepage.com/cg_calc.htm.
It says "For an aircraft to be stable in pitch, its CG must be forward of the Neutral Point NP by a safety margin called the Static Margin, which is a percentage of the MAC (Mean Aerodynamic Chord). Static Margin should be between 5% and 15% for good stability. It goes on to say that "Low Static Margin gives less static stability but greater elevator authority..." This is what most 3der's are looking for.
I think what they are saying is the CG should be 5-15% of the MAC ahead of the Neutral Point.

JRFisher - Ok, I understand what you are saying. Your calculator put you right on the spar when you selectd 10% as the point forward of the neutral point for good stability. The neutral point is as I would have suspected somewhere aft of the spar. Anything aft of the neutral pont would be considered tailheavy. Gotcha

Hi!
I'm with you themadmax!
Why on earht complicate something that doesn't have to be complicated! Just put your fingertips under the wing! That easy!
I have been active in this fantastic hobby/sport competing in pylonracing and scale and many other disciplines for 32 years and I can assure you guys that the worlds best competitors in racing doesn't mess around with unnecessary computation such as finding the right Cof G.
..It's an entirely a cut and try thing! That easy! Just begin with a Cof G of 25-30 % of the cord...fly the plane and observe how it flies ...and turns. If it drops it's nose in sharp 90 degree pylon turn ...it's nose heavy and you mover the battery pack or servos slightly, some centimeter rearward.Then fly again and observe how it behaves.Finding the right C of G by calculation isn't going to help you anywhere ...it's these small cut and try flights that makes you a better flier and let you understand what makes a plane fly better.

jaka,

You point is well taken. However, look at the great info being shared. Thank you, gentlemen.

Jim

I agree with themadmax and Jaka! Why turn something as simple as ballancing a plane, into something so complicated?

Actually I do not ues the "finger" method. I "hang-em" from a single wire from the ceeling, and ballance it front/back/side/side all in one shot.

It is a great discussion though!

The CG has to be measured on the Mean Cord and where this point is depends on the shape of the wing. You cant just measure 1/3 back from the leading edge Model Aircraft News done an article on this exact thing in the April 2007 issue you will find this helpful the article is " Balancing Act" it shows how to balance with the different chord wings they are not all the same.

RC Specialties;

As you can see from your responses most modelers are satisfied with the trial and error method, probably because it's simple, and they're not so inclined to work with mathematics.

But there are some of us who do use math to get answers - and rather simple, just by knowing that for an object to balance about a point, the summation of moments about that point must be equal to zero.

Example: Say you airplane is tail heavy, weight 5 pounds and is off the desired CG by 1 inch. I have a place in the nose that is 8 inches forward of the desired CG, so how much weight do I need to put there? The tail heavy moment is 5 pounds x 1 inch = -5 inch-pounds. You need +5 inch-pounds to balance, so you will need X pounds at 8 inches to balance 5 inch-pounds. Then X = 5/8 pounds or 10 oz of lead.

As a model designer, I use weight and balance calculations during the design process so that the airplane will balance as close as possible at a desired CG without the battery installed - so that the battery can be moved around to fine tune the CG. About 2 years ago, I used this process for my first foamie and it worked out fine. Here is the weight and balance sheet from my notes that you might find interesting. BTW the Kaos has had many flights and is still flyworthy today.

feihu

feihu,

Thank you for the example. I too used to use the trial and error fingertip method. It works, but is not as accurate. Once an existing CG is known, any calculation and/or possibilities can easily be determined. An accurate CG makes a difference in proper glide ratios, etc. I don't have a problem with the trial and error for those that wish to use it. However, I like knowing exactly where to put all my equipment and mount it one time with the least amount of ballast. Hopefully, this thread will share a little knowledge and options for others. Thanks again for your input.

Jim