how do you find cg on a plane

Stolen from the wise moderator Da Rock;

It really is easy. Measure the airplane with a yardstick. Plug the numbers into the online application. Choose a Static Margin of 15% and click the COMPUTE button:

http://www.geistware.com/rcmodeling/cg_super_calc.htm

What are the measurements? They're listed in the application's input box and shown on the diagram beside it.

You have to remove the heat sink from the top of the engine's cylinder head, and look at the top of the piston....... you'll find the CG there EVERY TIME, I PROMISE ON MY WIFE'S LIFE It's a tiny little golden nugget that speaks in an irish accent and it calls you a "Crack Gigolo"










(jk)


(look for the 1st wingspar and go about 3 fingers back from there, pick up the plane by the wings at this point CLOSE to the fuselage. If it tips forward or backwards, ALOT, it's a problem you've got to fix before you get the plane in the air! I know this because I have a plane that is TOTALLY BUTTHEAVY!!!!)

You asked how to FIND the CG of an airplane. That is completely different from checking to see if the plane is balanced properly.
To find the CG of an airplane, the easiest way to do it is to suspend the plane by the wings from a single point in the ceiling. Then, with the datum line level (I usually just level the tail, if it is a flat-top tail, and the incidence is set at zero), hang a plumb-bob from the point of suspension. The plumb bob will point exactly at the center of gravity for the airplane.
Now, if you want to check to see if your plane will fly right, then the CG of the plane must be within the acceptable range as set by the manufacturer (or as calculated as instructed above). If the CG of the plane is behind the recommended point, then the plane is "tail heavy." If it is in front of the recommended point, then the plane is "nose heavy." Remember, CofG is a point on the airframe itself, it is NOT a number that you calculate. You try to MAKE the CofG on the airplane the same (or close) as the number you calculate. CofG is also a 3 dimensional point, not just a 2 dimensional point that most people think of it as. However, when balancing the airframe for proper flying characteristics, using the 2-D model works just fine, and there is no reason to ever try and find the 3-D CofG on a model.

Semantics… CofG is used by most to describe both the suggested balance point (recommended CG which is calculated) and the measured balance point (measured CG), but due to the very simple OP, I can understand how it is difficult to really know what is being asked here unless the OP clarifies the post. Besides, if we really want to get technical, it should be called center of mass, not center of gravity.

Yes, center of mass. You are correct.
And you are right, symantics. I thought maybe the OP was wanting to know his current cg, and the cradle is the best method I've seen. But, since he hasn't seemed to revisit this post, then I guess it's mute anyway.....

Since we are talking semantics, it's "moot" not "mute".

Sorry, I couldn't resist. This one drives me nuts!

Jack

I don't suggest using the 'three fingers behind the first spar' method. That could work on some planes, but if you do that on all of your planes you'll miss more times than not. Many planes have the first spar on or very near the balance point and three fingers width behind that could be a disaster on certain setups.

The most surefire method is to calculate your mean aerodynamic chord, or MAC and balance as a percent of that. Most conventional planes (one lifting surface, one horizontal stabilizer behind the wing) will be happy around 28-30% MAC. This works well for straight wings, tapered wings, and wings with either forward or rearward sweep. Deltas like to be somewhere between 15 and 20% MAC, and most flying wings like it around 12-15%. Calculating bipes is slightly tougher, but you can use the same method, but calculate the MAC from the planview of both wings.

A few things to keep in mind:

-The lower the aspect ratio and the larger the horizontal stabilizer, the more room for error you'll have.
-Any conventional layout designs with a lifting surface at the location of the horizontal stabilizer will want to have the CG farther back, thus making the H-Stab a true lifting surface. These are rare, but you may run into a few of these types in old timers or free flight.
-Canards are a bit different, and I've yet to come across any good and simple method of balance point estimation that works every time.
-If in doubt about your calculations, build a smaller chuck glider of similar demensions to test your work.

sorry. one of those things I say right, but spelled wrong. informal grammar/spelling/usage on these message boards, unfortunately, is all too often misinterpreted as ignorance (in the truest sense of the word). But I digress, as I do the same thing at times (point out misspellings, etc.)
Oh, and I'll go ahead and correct this one for me.....SEMANTICS, NOT SYMANTICS.....
all in fun....no harm, no foul
Thanks for the catch...
Hopefully there is something in this thread that will benefit the OP.

To locate where an airplane balances, there are a lot of methods.

To figure out where the CG should be for your first flight, the easiest method is to measure and plug the measurements into the geistware link provided in the 2nd post above. The location is a function of the size and location of the horizontal tail of the airplane and the chord and area of the wing. All those matter. And it's easier to use the geistware.com online application to take all that into consideration. Can you get by with just figuring out where 30% is on the wing? Yeah, IF your model is very average in almost every way. And if it is, then balancing on the spar is even simpler. And probably will be as good a figuring just 30% of the MAC you took the time to find and figure out. And if simple is more important than safe, have at it. Because if your models layout is average enough that 30% MAC works, it's probably average enough that the spar is along that line. You're relying on probability, right? to save you what.... So save some more and just use the spar. And get about as "accurate" results.

To figure the mean aerodynamic chord itself requires you to measure your wing and do a fair amount of drawing of diagrams. Save yourself the time to find paper and pencil and drawing out your planform. Make the same measurements on the tail you're going to have to make on the wing, plus one, and plug those 9 measurements into the geistware application and click the button. You already had to find the yardstick, so use it for a half minute more.

Last time I figured out MAC on paper it took me longer to figure out what scale to use drawing the measurements and projections on paper than it does to boot up my pc. And finding pencil and paper took longer than that did.

Find a yardstick, measure 9 things, type that into your pc, and click a button. Found paper and pencil yet?