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

"CG" vs. Balance point

buzzbomb

I know nothing!
#23
I want to add that I'm not belittling anything. This is serious stuff, that you much more experienced pilots consider when balancing a plane, or trying to design one, or you're just trying to extend your knowledge-base. It just struck me how funny it was at how lost I was. So much to learn.
 

Hai-Lee

Old and Bold RC PILOT
#24
I want to add that I'm not belittling anything. This is serious stuff, that you much more experienced pilots consider when balancing a plane, or trying to design one, or you're just trying to extend your knowledge-base. It just struck me how funny it was at how lost I was. So much to learn.
For a newbie it is wise to either start at the recommended balance point or use one of the plethora of CG calculators as a starting point for your balance. Once the first flight has finished then you can assess the planes handling and re-balance as required.

For a first flight on a home design it is highly recommended that you launch and fly as fast as you can to give the control surfaces a chance to work for you and to allow the plane to fly, (even if poorly). It is hard to adjust the balance point if you are only guessing at the planes performance or the reason it crashed!

I personally start all of my prototypes with a undercarriage, or use a bungee launcher, so I can get a very fast take off and thereby some level of control response immediately it starts to fly.

Just what works for me!

have fun!
 

Tench745

Elite member
#25
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 !
I'm hoping I can add a little something to answer the specific question in a way that doesn't rehash what's already been said.

A "properly balanced" plane will balance, wings level fore and aft, on the ideal CG mark. This is the position that most ARFs and FT planes have marked on them.
If you're designing your own aircraft you can find this "ideal" position from any of the means described by others above.

From there you can adjust the Center of Gravity on the aircraft by moving weight forward and back to get it to fly the way you want/are used to (for the reasons others have stated above).
Rather than redrawing your balance point every time you try a new CG, most people just fudge by balancing "a little nose down," then "a little more nose down," or "a little less nose down." What they're really doing is moving how far the new balance point is in front of the marked "ideal" CG.

One thing I think throws a lot of newbies off, or at least me when I started, is that you shouldn't ever actually be looking at the nose of the aircraft when you balance it. What you are supposed to be looking at is the chord line of the wing.
The chord line is an imaginary line drawn from the leading edge of the wing to the trailing edge.

This line is what defines the angle the wing is set at. If the chord line is parallel to the ground, the wing is balanced on the balance point. The fuselage could be pointing wherever it wants, it's the chord line that shows you whether the plane is balanced "nose down" or not.
 

Hai-Lee

Old and Bold RC PILOT
#26
I'm hoping I can add a little something to answer the specific question in a way that doesn't rehash what's already been said.

A "properly balanced" plane will balance, wings level fore and aft, on the ideal CG mark. This is the position that most ARFs and FT planes have marked on them.
If you're designing your own aircraft you can find this "ideal" position from any of the means described by others above.

From there you can adjust the Center of Gravity on the aircraft by moving weight forward and back to get it to fly the way you want/are used to (for the reasons others have stated above).
Rather than redrawing your balance point every time you try a new CG, most people just fudge by balancing "a little nose down," then "a little more nose down," or "a little less nose down." What they're really doing is moving how far the new balance point is in front of the marked "ideal" CG.

One thing I think throws a lot of newbies off, or at least me when I started, is that you shouldn't ever actually be looking at the nose of the aircraft when you balance it. What you are supposed to be looking at is the chord line of the wing.
The chord line is an imaginary line drawn from the leading edge of the wing to the trailing edge.

This line is what defines the angle the wing is set at. If the chord line is parallel to the ground, the wing is balanced on the balance point. The fuselage could be pointing wherever it wants, it's the chord line that shows you whether the plane is balanced "nose down" or not.
Nice theory but it quickly falls apart when there is a significant difference between the chord line and the incident airflow, (Angle of attack or incidence). As the incidence angle varies the point in front of the wing where the air splits, (to travel over or under the wing), changes and so the effective chord line changes as does the mean centre of aerodynamic pressure.

If the plane is to fly with its fuselage level it is at this attitude that the balance should be set and not the incidence angle of the main wing. In addition the area and lift generated by the control surfaces needs to be considered in any determination of where the craft should balance. Canards and conventional aircraft have a definite lift contribution in balanced flight. Normally the canard provides a measure of positive lift and the conventional tail a small measure of negative lift for most stability. This assumed normal distribution of lift provides a situation where an elevator stall causes the craft to nose down thereby avoiding main wing stall. This balance is lift dependent and not on main wing chord line. You may note that the slightly nose heavy being more stable is equally applicable to canards, conventional, and flying wing models.

The aerodynamic cord line, (from which the lift is generated by speeding up and slowing down the airflow), is measured from where the air splits at the leading point of the wing and where the 2 airflows reunite at the rear of the wing. It is the difference in path traveled that provides the lift.

Just trying to clarify any misunderstandings!

Have fun!
 

Hai-Lee

Old and Bold RC PILOT
#28
Nothing to do with CG or PP but many forget to check balance from center line of the fuselage wing tip to wing tip.
Sadly I am guilty of this one more often than I would like to admit.

You can get away with it mostly BUT if you also get the plane to stall with such an imbalance the result is a vicious wing drop and vertical spiral to its final resting place.

Done it too often!

have fun!
 

Tench745

Elite member
#29
Nice theory but it quickly falls apart when there is a significant difference between the chord line and the incident airflow, (Angle of attack or incidence). As the incidence angle varies the point in front of the wing where the air splits, (to travel over or under the wing), changes and so the effective chord line changes as does the mean centre of aerodynamic pressure.

If the plane is to fly with its fuselage level it is at this attitude that the balance should be set and not the incidence angle of the main wing. In addition the area and lift generated by the control surfaces needs to be considered in any determination of where the craft should balance. Canards and conventional aircraft have a definite lift contribution in balanced flight. Normally the canard provides a measure of positive lift and the conventional tail a small measure of negative lift for most stability. This assumed normal distribution of lift provides a situation where an elevator stall causes the craft to nose down thereby avoiding main wing stall. This balance is lift dependent and not on main wing chord line. You may note that the slightly nose heavy being more stable is equally applicable to canards, conventional, and flying wing models.

The aerodynamic cord line, (from which the lift is generated by speeding up and slowing down the airflow), is measured from where the air splits at the leading point of the wing and where the 2 airflows reunite at the rear of the wing. It is the difference in path traveled that provides the lift.

Just trying to clarify any misunderstandings!

Have fun!
Hai Lee,
I don't entirely follow how what you've said is different from what I have, but don't want to derail or further clutter this thread.
I mostly wanted to point out that many full scale/civilian aircraft will design in an amount of wing incidence so that the aircraft cruises in an attitude that may look "nose-down" which improves visibility for the pilots. A model built off these aircraft would not fly properly if balanced so the fuselage was level.
The wings and H-stab/elevators are the primary surfaces that do the flying, not the fuselage, so look at the wings when balancing.
That's all I was trying to say.
 

Hai-Lee

Old and Bold RC PILOT
#30
As for the fuselage being level I did state that, "If the plane is to fly with its fuselage level it is at this attitude that the balance should be set". I did not say that the fuselage must be level! This reference was to orient the craft as it would be in level flight and that is the best position to ascertain proper balance.

When flying the model and particularly at distance most people will only have the fuselage incidence angle to determine whether the plane is in level flight! The wing incidence angle is normally not visible. Though some of the top or bottom wing may be visible the exact wing incidence angle is almost impossible to determine.

That some aircraft may have a slightly nose down cruise fuselage attitude might be a good thing I doubt that the wing incidence angle is determined by how much visibility a pilot might require. Normally the incidence angle is best determined to give most economical cruise at max AUW with the wing clean, (no flaps or slats deployed). That an aircraft with near empty fuel tanks, or less than max AUW, has a slightly nose down attitude, (to reduce the lift from the main wing to suit the aircraft required lift), in level flight is not surprising.

Have fun!
 

d8veh

Elite member
#31
Hmm! A lot of interesting theory. I too am an engineer rather than a scientist. I started designing and flying my own planes 56 years ago when there was no internet to tell you what to do and my 6 pence a week pocket money was all spent on wood, glue and tissue paper, so none left for books on aeroplane theory. I've built hundreds, and as far as I can remember, I never calculated or measured the CG/ balance point on any of them. I use my eyes to estimate where 1/4 to 1/3 of the wing chord is, then stick my fingers under the wings to see where it balances. As long as it balances somewhere in that range, the plane is going to be launched. I think my dad told me that rule because he built planes too. I never had one that didn't fly, but I have had to move some mass afterwards, more often because the plane wasn't lively enough for me, so mass got moved backwards.

I'm not suggesting that everybody should adopt this approach, but I do think that some people are a little too pedantic about it.
 

Hai-Lee

Old and Bold RC PILOT
#32
Hmm! A lot of interesting theory. I too am an engineer rather than a scientist. I started designing and flying my own planes 56 years ago when there was no internet to tell you what to do and my 6 pence a week pocket money was all spent on wood, glue and tissue paper, so none left for books on aeroplane theory. I've built hundreds, and as far as I can remember, I never calculated or measured the CG/ balance point on any of them. I use my eyes to estimate where 1/4 to 1/3 of the wing chord is, then stick my fingers under the wings to see where it balances. As long as it balances somewhere in that range, the plane is going to be launched. I think my dad told me that rule because he built planes too. I never had one that didn't fly, but I have had to move some mass afterwards, more often because the plane wasn't lively enough for me, so mass got moved backwards.

I'm not suggesting that everybody should adopt this approach, but I do think that some people are a little too pedantic about it.
You lucky B! You got pocket money?:eek:.

I started with just the paper and some glue around the same you started. I could make them fly freeflight no problems but give me the controller and it always ended in tears!

I agree with the balance approach you describe, (old school), but I have also been seriously involved in design and test piloting of late and being able to know the interaction between balance, flying surface incidences, Thrust lines, and the like has become vital to get designs sorted for distribution to the general flying public.

You would be amazed at the number of people complain that their Cg point is different to the published plans to get the plane to fly when in fact it is a wing incidence error that causes the actual issue. This same incidence issue combined with the chosen balance point that often gives rise to planes being too fast difficult to control. That is why I jumped in! CG is not the be all and end all when it comes to making your model fly properly.

Try increasing the wing incidence on the Simple cub or even the TT and see how much calmer and better the planes fly!

Not upset or angry just trying to set it straight!

Have fun!
 

jtuttle11

Junior Member
#33
Center of Gravity or Balance point should always be 25 to 30 percent of the 'Mean Average Chord' of the wing. This is straight forward for a Non-tapered-Straight wing however it gets considerably more involved for a Tapered or Swept wing. An example is the paper airplanes we all made in school. Those paper darts would balance much farther back.