• This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn more.

"CG" vs. Balance point

Flite Risk

Well-known member
#1
Very early into my time in this hobby (about 2 years ago I think) my first build was a chuck glider to RC plane hack.
My biggest problem was figuring what everybody kept calling "cg". It didn't make much sense to me, I didn't have the knowledge or terminology then (probably still dont).

Some thing I have learned,
"A nose heavy plane flys poorly, a tail heavy plane flies poorly once"
-J. Bixler

I knew I wanted the plane to nose down. With enough nose weight the fulcrum could be under the tail. What I have since learned is, what I meant to be asking was, "where is the balance point?" from there you can
*effect the center of gravity of a plane by loading and weight distribution.

*effect the center of gravity. that's to say the "cg" is moveable the balance point is fixed.

So the whole point of this post is to hopefully change the lexicon or mindset of the hobby a little bit..
don't be so quick to say the words center of gravity.

So you put a new bird together, the first thing you do is 'check cg' by balancing your plane on your finger tips. On The dots/bumps/divots on the wings or balance point.

Cool if its a arf, bnf or speed build kit, those planes have the balance point marked, but if its a scratch build like my glider was, you have to know where the "balance point" is not the "cg" is because the "cg" is relative to the balance
point. <---- THATS IT !¡!
THATS IT ¡!¡
Thats what I am talking about. When a model is checked for cg, the The dots/bumps/divots on the wings are not actually the "cg" but the balance point.
I wish especially in FT videos the "balance point" was mentioned just before the "cg" and it was stressed "these are the balance points."

Some helpful videos / formulas could be, How to find the balance point of an airframe.

Very tired there's probably. Tons of grammar errors. Sorry, good night.
M T H. :)
 

Hai-Lee

Old and Bold RC PILOT
#2
There are many sites which offer the free usage of CG point calculators. Ideally it does not matter if you call it the balance point or the CG point because ideally they are one and the same for a properly setup model aircraft.

a properly setup or balanced model will fly at the lowest possible speed, (for the design), but if the CG point is not the exact same as the balance point the aircraft will still fly as long as the control surfaces can generate adequate extra lift, (positive or negative), to re-establish the balance. Most models used to be offered with a RANGE of, or for, the CG balance points. your setup within this range would be selected by the builder/owner to give the flight parameters desired.

When flying an unbalanced model the behaviour of the model, at low speed, will be determined by the loss of lift on the pitch control surface or control when the surface stalls. A nose heavy plane will lose its, Negative or downwards force from the elevator and will tilt forward, or nose down. This increases the airspeed and the elevator control will regain its lift and the problem averted. Whereas a tail heavy plane when it loses its positive, upwards, lift the plane will pivot upwards leading to a rapid increase in lift, drag, and a equally sudden loss of airspeed. This loss of speed causes the main wing to then stall and the plane suddenly becomes an airborne brick!

Don't worry about the name as it describes the same point. It is ensuring that the plane flies properly we all should be concerned about!

have fun!
 

Merv

Legendary member
#3
I’d like to suggest that CG & balance point are the same thing. The question is does the model balance where it should for the way you want to fly. For most planes you want the CG at 25-30% of the wing area. A forward CG will be more stable, an aft CG will be more acrobatic. Too far forward and you’ll have a flying pig, too far aft and you’ll have a flying squirrel. Neather are much fun.

Once you become comfortable with your piloting skills, try moving your balance point aft. You’ll open up a whole world of possibilities you never knew your plane was capable of. I’d recommend moving the CG no more than 1/4” (6mm) at a time. Be sure to test the stall with each change. The stall will change. That is why you are changing CG, to change how the plane will stall. With any new plane, I start at 25%. When I get the plane trimmed then I will move the balance point back till the plane fly's the way I want. Just enough stall to have some fun but stable enough to be controllable.

My Versa wing fly's great at 30% and will become a squirrel at 31%.
 
Last edited:

TEAJR66

Flite is good
Mentor
#4
The CG is a matter of design. Lots goes into determining (calculating) where the CG will be, or even the range of a CG. Balancing the plane at the CG or within the range of CG will help the plane fly as it was designed. Here are a couple great resources for figuring out CG.
https://www.flitetest.com/articles/easy-aircraft-design
https://rcplanes.online/index2.htm

For most of the foamies that I trash around, these processes are never even touched. Keeping in mind the rules of thumb seen in the second link, Trainer Design section, you can whack together something that fly's well enough. Great for playing around.

This link might help with terminology.
https://www.rc-airplane-world.com/balancing-rc-airplanes.html

In the end, whatever works toward your enjoyment is the way to go.
 

quorneng

Elite member
#7
I would love to know how the CofG - which is defined as the point around which the resultant torque due to gravity forces vanishes - and the "Balance Point" can be different.;)
It is the relationship between the Centre of Pressure - another imaginary point that is taken to be where the sum of all the aerodynamic forces are considered to act - and the CofG that determines how, or if, the plane will fly.
 

FoamyDM

Building Fool-Flying Noob
#8
@TEAJR66 - Thank you for the links. @Flite Risk for the thread in general. I think about 75% of my "complete" projects to date are scratch build and not from pre-made plans. I think the hardest part I've dealt with is finding where the CG is, and keeping throws low.
 
#9
Heres the thing, from what I have seen most RCers take a very casual approach to CG or Center of Mass if you are British. If you are going to take a more formal approach then there is what is called the datum, it is simply a fixed point from which the maths can flow. It is often at the firewall on a small fixed-wing aircraft but it can be anywhere.
If you want to learn to do this mathematically there is a plethora of free information on the FAAs website. just go find what you need and download the PDFs.
 

Hai-Lee

Old and Bold RC PILOT
#12
Heres the thing, from what I have seen most RCers take a very casual approach to CG or Center of Mass if you are British. If you are going to take a more formal approach then there is what is called the datum, it is simply a fixed point from which the maths can flow. It is often at the firewall on a small fixed-wing aircraft but it can be anywhere.
If you want to learn to do this mathematically there is a plethora of free information on the FAAs website. just go find what you need and download the PDFs.
If you want to see a causal attitude then find out what people know of wing incidence angle and its effect upon not only the planes performance but also its balance/CG point.

have fun!
 

Flite Risk

Well-known member
#14
Monkey, I don't see his comment as dogmatic. Just a point of view. While I appreciate yours and everyone's reply to my question I also understand we all take this hobby to different levels. I am just so happy everyone in this community is so willing to share their knowledge or opinions and is friendly and cordial about it.

Thank you all for your Insight.
 
#15
If you want to see a causal attitude then find out what people know of wing incidence angle and its effect upon not only the planes performance but also its balance/CG point.

have fun!
I hope I didn’t come off as a pompous a$$, I know that I often do!
I really don't care how formally or casually someone approaches this, just wanted to provide a source for anyone who might want to approach it formally.
 

Flite Risk

Well-known member
#16
Very early into my time in this hobby (about 2 years ago I think) my first build was a chuck glider to RC plane hack.
My biggest problem was figuring what everybody kept calling "cg". ...............ya'll read this already up top. On to my thought.
M T H. :)
I over engineer everything and most times I think, I think too much.
Sometimes I wish I could just be happy that something works and just use it...........not really
(I suppose thats why I dont like MAC computers).

I thought of my question in new words, or a different way to phrase whats going on in my brain.
OK everybody,

TAKE 2!

So you are scratch building with pizza boxes, chop sticks and burlap, (attempting levity here) instructions are somewhere in the metaphysical world of astrophysics.

How do you know where to place the Fulcrum when balancing the plane?
How do you know when to determine where the Fulcrum is? <--- I think thats the more important of the two.

EXAMPLE:
The Fuse.,Wings, Vert.& Hor. stab's are on. do you find the fulcrum then? before electronics?
Or start adding electronics and then figure out where the fulcrum is?
Or do you gear it all up then, rearrange gear/size the battery based get the aircraft to balance? If so where is the Fulcrum?

You can make a FT simple cub balance with a 6000mAh battery if you put the Fulcrum in the right place. ( I am exaggerating but there is a sliver of truth in that statement * Note I did not say it would fly :)

______________________
................F

_XX_________________xX__
.........F

_X_____________________
.......F

____________________X___
..............................F

{without the periods before the F, the editor justifys the F all the way to the left.}

So above I show 4 objects that all balance based on placement of load relative to the "F" Fulcrum.
At what point in the build process do you learn/figure/know/find out which point to balance the aircraft on?

WxAxM
weight x arm x moment?
Again at which point in the build process do you apply that formula?


I hope TAKE 2 has been more eloquent (or what ever rhymes with eloquent(Primus reference)).
If I'm beating a dead horse feel free to PM me and buy a DJI you wont hurt my feelings ; -)

Cheerios and Cheers !
 
Last edited:

Craftydan

Hostage Taker of Quads
Staff member
Moderator
Mentor
#17
At what point in the build process do you learn/figure/know/find out which point to balance the aircraft on?

It's easy to get overwhelmed with the fluid dynamics, especially if you want to go deeper, and deeper, and . . . well, don't expect to find a bottom to that hole.

Fundamentally, this question is a good place to start and expand out at practical levels . . . *sigh* but first, let me be clear about my terms (it's confusing enough on it's own, being imprecise in terms only makes it worse):

CM = Center of Mass -- The point inside an object where any force acting on that object will cause only linear acceleration. Any other point, a force will cause both a linear and rotational acceleration (move and spin)
CG = Center of Gravity -- the point inside an object where if the sum of gravity forces can theoretically be concentrated to as a force alone. Any other point, the sum would be a force and a torque. In general, CG = CM, so long as gravity is constant.
Balance point -- a point on an object that is in-line with the CG, while the object is in a pre-set orientation (the point of a pencil could be a balance point if you point it straight down, it's just not a very stable balance point). in general, the theoretical summed gravity force acting through the CG passes through the balance point as well.

Notice, none of these have anything to do with an airframe. All solid objects exhibit these points (at least, in a gravity well, they do). SO enter . . .

CP = Center of pressure - the point where the fluid pressures generating lift can be theoretically summed to a force alone. THIS IS A DYNAMIC POINT. While it moves predictably around an airframe, as you move through the flight envelope and change attitude, your CP will move.

That's scratching the surface, but a fair start . . . and I'm tired of definitions. not done, mind you, just tired of them.

Your initial question had some sloppy usage of CG (no fault of yours, you picked up that from others). What I think you meant is where the "Ideal CG" should be (Ideal being a fuzzy word anyways). It's the answer for the question, "where will my plane fly how I like it, if I were to move my CG there?".

So back to the question I cherry-picked -- at what point do you care, and how do you find it?

As soon as you have your general airframe layout, you're ready to make your first estimate.

How?

The cheater's answer . . . you use a CG calculator.

CG calculators take the overall wing and stabilizer shape and their relative placements to guess where the CP will appear in your normal modes of flight -- remember, it's predictable. Once you know where it is, you as the designer can make a good decision of where you want your CG to be for the flight characteristics you want. Want something stable and friendly in pitch? You push your CG a modest distance ahead of your CP. Want something super responsive to the slightest twitch on your elevator? you will probably decide on a spot VERY close to the CP.

This distance between CG (pointing down and doesn't move) and CP (hopefully pointing up, but usually not the same spot) creates a torque on the airframe that your horizontal stabilizer works to balance out. Change the lift in the tail (with the elevator) and you can change the pitch of the airframe, for trim or control. The farther CG and CP are apart, the bigger the change you need from the stabilizer for a given motion. So how much do you want?

In the CG calculators, this imbalance is set using the "Static Margin". The bigger the margin, the more relaxed the plane responds to your commands (keep in mind, it can be so relaxed you can't control it). A higher static margin also pushes the CG more nose-heavy -- when the lift disappears in a stall, the nose will tend to rotate downward. Using a lower static margin will make the plane more responsive and efficient in controls . . . it also increases the pilot workload in maintaining pitch. In tradeoff, the nose will pitch down less in stall, taking longer to recover. Push the CG farther back and it becomes tail-heavy, and the tail drops first. Bad, usually, but if you want that 3D plane to pop into a hover easy . . .

Once you've loaded in your plane's dimensions, and adjusted your static margin to taste (stick with the defaults until you have a taste ;) ) the calculator will then tell you a good guess to put your CG on the airplane -- Mark your plans, and now you start playing the balance game.

After that, a few things to keep in mind (yeah, you probably know many of these):

- Ideal CG is generally forward of the "midpoint" of traditional aircraft, so you have more volume of plane in the tail side of CG than the nose.
- keeping the tail small and light keeps the overall weight down, for lack of matching ballast.
- Motors and batteries are great mass concentrations, and ideal for nudging the CG in a favorable direction before resorting to ballast.
- Tractors make balance easy, pushers make balance hard.
- adding a layer to the skin (paint, tape, ???) uniformly will tend to push the actual CG toward the tail (more surface behind the CG too).
- TAILS ARE ALMOST ALWAYS OVERBUILT. (if they weren't, they'd be demolished with the rest of the plane in the crash)
- A heavy spinner on the nose is worth many times more it's weight in ballast behind the firewall. They aren't always just for pretty.

and my favorite . . .

- Ideal is in the eye of the beholder. Start with the first guess at ideal, and adjust to your taste. An expert pilot may demand more response or efficiency of a neutral CG (close to CP), while a novice pilot may appreciate the ease of a more nose-heavy CG. Most planes have a "happy" range where they fly predictably, and most pilots develop a taste for which side of that range they prefer.

It's your plane, set your CG where you like it best, regardless where Bixler puts his marks ;)
 

Merv

Legendary member
#18
Again at which point in the build process do you apply that formula?

As a non engineer, I don’t apply the formula. I probably should but I don’t.

I build the plane, everything but the electronics, put it on the balance stand, then lay the electronics on the plane, use tape of necessary to hold them from shifting. You’ll need to keep the plane from flopping off the stand. Then move whatever is necessary to make the plane balance.

Ideally the plane will balance at 25% of the wing area, with the battery near the front of the plane. I’ll then reposition the plane on the stand, move the battery until it will balance at 30%. Once I have a good idea where everything is going to go, I’ll install it.

After everything is installed, I’ll do again, this time I’ll mark where the battery is to balance at 25% of wing area. Make the first flight, get the plane trimmed out. Then slowly move the battery aft until the plane fly’s the way I want it, controllable yet with enough instability to have some fun.
 
Last edited:

Hai-Lee

Old and Bold RC PILOT
#19
One thing I will add apart from that there is no problem with me and the statements on this thread. I just like to try and help others not to get too confused.

The addition is; Regardless of the plane, the builder, the designer, or any other factor you MUST be prepared to adjust your balance/CG point to suit the plane as it is before you and how it performs. Rarely will 2 aircraft, even identical models, have the exact same sweet balance spot. I have built a number of the same planes with the same electronics and still found that the balance points can vary slightly from one plane to the next.

Sure there are calculators which are definitely recommended as a starting point but you must know your aircraft and what it is supposed to perform like. Get to know how unbalanced aircraft perform and what changes to the CG you need to make to get it to perform adequately for you. The threads are full of persons how have had CG problems even when using the recommended balance point.

Use all the recommendations by all means but when it comes to your aircraft the balance is up to ypu and it may not be exactly where the suggestions or recommendations place it.

Just a note to hopefully reduce the rigid idea of a fixed balance point and remind people as @Craftydan did that it is all up to you, (the pilot), to TUNE your own aircraft to perform as you wish it!

Have fun!
 

Merv

Legendary member
#20
One thing I will add apart from that there is no problem with me and the statements on this thread. I just like to try and help others not to get too confused.

The addition is; Regardless of the plane, the builder, the designer, or any other factor you MUST be prepared to adjust your balance/CG point to suit the plane as it is before you and how it performs. Rarely will 2 aircraft, even identical models, have the exact same sweet balance spot. I have built a number of the same planes with the same electronics and still found that the balance points can vary slightly from one plane to the next.

Sure there are calculators which are definitely recommended as a starting point but you must know your aircraft and what it is supposed to perform like. Get to know how unbalanced aircraft perform and what changes to the CG you need to make to get it to perform adequately for you. The threads are full of persons how have had CG problems even when using the recommended balance point.

Use all the recommendations by all means but when it comes to your aircraft the balance is up to ypu and it may not be exactly where the suggestions or recommendations place it.

Just a note to hopefully reduce the rigid idea of a fixed balance point and remind people as @Craftydan did that it is all up to you, (the pilot), to TUNE your own aircraft to perform as you wish it!

Have fun!
Couldn't agree more, make the plane fly the way you want it to.
 
Last edited: