For all the planes I built i used the manuals recomended points for the CG. I recently built a 33% Yak54 without a manual. I've heard that the rule of thumb in finding the CG is 1/3rd the measurement of the widest part of the cord of the wing. I'd like to know if this is true for all planes. Thank you.

Have a look here
http://www.geistware.com/rcmodeling/cg_super_calc.htm

No, it is not true at all. The balance point is situated along the Mean Aerodynamic Chord (MAC)

Use the attached diagram exactly as shown.

Ed s

Since the initial CG is to give the model good pitch stability and solid elevator performance, no the CG isn't figured from just the wing dimensions.

Pitch stability has at least 4 major contributors. The size of the horizontal tail and how far back it is from the wing contribute tremendously as most would understand. The other two details that affect pitch are the wing area and chord. And then there are a number of other details. Considering only the wing planform doesn't get it by a long shot.

What is true for most planes (and that considers all the full-size flying right now) is to consider all the details that matter. The easiest way for us modelers to do that is to use the application that proptop linked to in his post above. It is easy and quick. Take 9 measurements and plug 'em in. Run the application with 5% as the StaticMargin and record the CG location. Then run it with 18% SM and record that CG location. The two locations give you the range your model can balance in for a safe and flyable 1st flight.

Another good result from having geistware compute a CG range for you, is a CG range gives you a very useful "tool" when trying to balance your model.

Knowing what an accurate and reliable range is will help greatly when moving parts around, or just adding weight to balance. You'll see the area where the CG can be, and how much more needs to be moved or weight to be added. You'll know too just how far out the balance is, if it is.

There are lots of planes that are carrying wasteful amounts of balance weight because of not knowing an accurate CG range location. The modelers thought they had to nail the CG to a point and lot's then want some nose down to be sure. They wind up not actually knowing anything. And carrying more weight than needed that also has reduced the elevator's effectiveness.

 Thanks guys,  I dont understand how to read that. Guess im just another dumby.lol. I'll try to get someone to help me with it. I got through the measurments of  first part then I started to scratch my head. I've depended on manuals to much to actually learn how to do it. Im mostly a Helicopter guy and im still learning that planes need more attention to weight and balance. Well thanks anyway. Im sure I'll have more questions down the road.

Aerodynamics can get very complex so I just do what I've done for last 50 years, locate CG at 25% to 30% of chord at root. Of course that may not work with every possible wing layout, such as swept wings but I think you are just looking for basic info. sometimes simple things can be made complex.

Go look at the sticky in the kit building forum about the CG calculator, life doesn't get much easier then that.

The object of the exercise is to provide a flyable airplane for the first few flights. From the simple diagram I posted to the complicated computer programs there is something missing. They provide positions for STATIC balance only. They fail to consider engine incidence and offset, if any. Wing and tail incidence, if any. The lifting component of wing and tail airfoils. Airspeed. So simple is best.

An airplane is not a static object. After the first few flights the REAL balancing and trimming of the model starts. And that is a whole different subject. I know where my Pylon Racers balance. It still takes a dozen flights or so to get a specific racer just right. And all I am doing is flying left hand circles.

Does anybody think that by sticking numbers in boxes one can balance and trim a full house Precision Aerobatic airplane?

Ed S

Lot's of people have those same opinions. Let's look at a closer comparison between the application and trying to draw up a 25% MAC location.

In actual fact, the geistware program is simple to use. It's quite a bit easier than getting out pencil and paper and drawing a scale drawing of a wing. I can take my yardstick off the wall in a lot less time than it takes to find pencil and paper and a place to draw. Furthermore, drawing a scale wing requires you make measurements. And then you have to figure out the scale to use on your paper. Heck, take the 9 measurements and plug 'em into the application and let it worry about figuring everything out. Nah, in fact figuring out the scale of your drawing is way harder than typing in 9 measurements.

It is also designed to compute the CG for flying the airplane. That's why it considers the effect the horizontal tail has on the stability of the airplane in flight.

And yes, sticking numbers in boxes will give an absolutely reliable and accurate CG range for establishing a safe starting point for your model's first flight. The object of the exercise is to provide a reliable CG for the first few flights, and it does that with ease. Anyone who can measure a model airplane with a yardstick can get a quick and reliable range. It's not sold as magic at all. It's purpose is to provide a safe starting point. As for Precision Aerobatic models, it'll do far better than considering less than half the things that affect pitch stability. The formula it uses is good enough for fullscale users, so there's almost a 100% probability it works as well for our models.

Aircraft designers like Lockheed use it's formulas. Airlines use the formulas in their operations 24/7. Of course, those don't use Geistware because they don't need to. Their computer systems are accessed directly. And they don't measure with a yardstick.

What is magic about the geistware application is that it does all the computation and we don't have to do any of it. All we need is a yardstick and the time to make 9 measurements. Do that and you get a reliable CG location for flying that considers wing sweep, taper, area, tail size, moment, and a bunch of other things that do affect CG location, instead of a rough guess that won't really be very good unless everything about the airplane is average, no wing sweep, tail size average and not farther back or closer than average, tail planform average like wing planform has to be average. And you wind up with a number for the CG that isn't any more reliable than if you simply placed the CG at the wing spar. After all, spar locations are as good as anything if you're just betting on the averages. There are a number of good reasons to use the formulas the aircraft industry and airline industry developed. And it's easier to do.

It's really easy. Let's look at it as a whole.

Take 9 measurements and type them in. Type a value of 5 into the Static Margin box. Click the button just below the SM box. Look down the page and find the CG location in the Calculated Results box.

Now let's take that a step at a time.
1. I use a yardstick to measure with. Half span, root chord, tip chord, and leading edge sweep are done for both the wing and horizontal tail. That's eight. The ninth is from the LE of the wing to the LE of the tail. The picture of the airplane planform that's on the geistware page shows those 9 measurements if you have any doubt where to measure.
2. To get a CG range, you'd run the application twice, once witha 5 in the SM box, and once with a 15. You can use other numbers, but if you're looking for a safe, reliable range, those are good for anyone unfamiliar with the values.


The good thing about having a range is it shows you when you're trying to adjust your CG on the bench, just where you need to get to and how far the actual CG is away from being in a safe range of locations. It makes it quite a bit easier to deal with moving a CG since you're not trying to hit an exact point that isn't "exactly" important. Yes, of course it's important, but the exactness isn't. We've developed a lot of slightly right, slightly wrong beliefs about the CG and using a range to balance clearly shows a developing modeler a lot more about the process. You will discover that some planes have narrower ranges than others. When you fly them, you'll often get a lesson on why. Some will have a wider range etc etc.......... But you'll certainly be coming up with an easier to discover and more reliable range.

A lot of designs with the swoopy lines, rounded periphery, strakes, etc. aren't easy to measure and make me more skeptical about using an on-line computer instead of going with chalk lines on the workshop floor.
To retrieve valid dimensions to plug into Geistware, I've had to do extensive layout work anyway. Still can't bring myself to not using my T squares and triangles to get a ball park idea.

Good idea those chalk lines.

Good idea to measure them with your yardstick too, if your projection of them was accurate at all, that is. Good thing about the task is that you're looking for a starting location for a safe CG for the first flight. You don't need to follow the swoopy lines with the yardstick any closer than any other method needs to figure out what to draw for the swoopy lines and perifery on a piece of paper for the less comprehensive method.

You do bring up an excellent question about the "simpler" method of drawing a scale wing planform. How would figuring the location of the tip chord for that scale drawing differ from using a yardstick? Can you give us your method of guestimation used to come up with the lines to draw? I'm guessing it just might be about the same estimations anyone would make while trying to figure where to hold the yardstick. So there's less problem guessing what lines to chalk than where to hold the yardstick? Do the swoopy lines become straight when chalked?

The chalk lines are like having an extra set of hands instead of floating a ruler is space while you consider how to interpret a complex shape. A lot of what I build is design as you go and never gets formally drawn up until the design has proven itself.
The more tapered and swept and curved the outline, the more critical the interpretation of that outline becomes. From what I've heard there is a CAD program that can convert the swoopy designs into a corresponding simpler form to do the calcs with?
So far, all I've ever done is my own TLAR version of straightening out some of those lines and plan on the first flight being possibly like breaking a horse that's never been rode.

The ball park idea works like gangbusters in Geistware.

To retrieve valid dimensions off a chalk outline would take you how long?

I'd bet with your experience, you'd have measured that wing the with the same estimations done in the attached picture. The attached is how that wing would be measured for Geistware. It took me longer to draw the picture than it'd taken me to measure the same way shown with a yardstick.

The orange lines are the root chord and tip chord. Yup, they're estimates. The red is the half-span and the green is the sweep. And yes, the sweep has to be done "in the air". Good of you to mention T-squares. The sweep measurement is more accurate if you have some reference line to work with. Figuring a means to do that accurately takes no less effort than working up lines to draw on paper, wouldn't you say? Of course, if someone is about to suggest that often the lines are already drawn on the plans................ measuring the plans is easier than blinking your eyes.

So measure the chalk lines if you feel they're accurate.
So you crush the prototype model flat on the floor to get accurate lines on the garage floor? We both know you don't.
So how do you get accurate measurements of your "not on paper yet" model? Same way anyone would work out the measurements for anything, and that works just as good for the application.

The simple method requires a drawing of the wing, and requires interpretations of tip chord location and dimensions. Where would you get the dimensions for that simple drawing method?

The simple method requires a root chord, tip chord, LE sweep and half span. Same with geistware. What about the simple method makes it easier for you to come up with those? How does the use of a measurement make it more or less accurately taken beforehand?

I've done what you show, but if any slight error in judgement is used for interpretting the shape, Geistware will still GIGO you an unworkable CG. The CG range of this .061 powered plane is the thickness of my pinkies.
I think my main point is that before we are allowed to use short cuts and calculators, we should learn how to "drive a stick shift" first. If a guy knows how to do it manually, then he'll have a better chance of knowing if he came away from Geistware with a workable solution.

...quite often the chalk line of the completed plane is my first draft. Similar to if you pull an ARF out of the box with no drawing or CG instructions.

...to extend key points from a model to the drawing board, block the model up squarely and use a T square or triangle to trace the outline of the object to points on the floor or table below...then connect all the dots. I like to have a datum line established first that you effectively plumb bob the plane to before you start working the outline. It really helps to weight the plane so it doesn't get nudged.

How did you figure that range?

The simple method only gives one point.

To get a range would require something that uses the Static Margin desired? The simple method really doesn't consider anything more than the wing shape.

You know, I think this thread is about people wanting a method to find a CG of an existing model. It's probably true nowadays their model is an ARF that they just bought almost-RTF.

I'm betting a lot of them actually don't have a yardstick, or don't remember where it is. Truth is, asking them to draw an accurate scale layout of the wing might be pressing them too. Figuring out how to scale the drawing isn't simple. And they'd have to measure then, and do it accurately. And not make mistakes drawing.

Since they're already measuring, they can save a bunch of time by plugging those numbers into the application. Any more effort than measuring certainly can't be simpler and can't possibly be easier.

I got the CG range from flight testing...the flyable range is actually much broader than the best "glide home deadstick" range...but those landings can't be avoided. For the first flights there are wraps of lead solder around the engine that get progressively peeled off as the flight testing progresses, with the ultimate goal of being totally lead free... but a 1/4-1/2 oz of the "metal of shame" placed where nobody can see it or knows it's there but me has been known to happen.

Im gonna go with TedMo, I went to the field today and asked around and I got the same answer 1/3rd,25%,30% of the wing cord is a good starting pointandafter the first flight i can make more adjustments to even everything out, Sorry guys you just make the MAC thing way too complicated. Im not an engineer or math wiz. I just wanna have fun not fry my brain. LOL. But thank you all for your input it was much appriciated. Thanks!

Maybe, but at which chord length on a tapered wing???

Ed S