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How do you figer out CG??

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How do you figer out CG??

Old 10-14-2007, 06:43 PM
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Default How do you figer out CG??

I have a question how do you figure out the cg for a tapered wing I'm wanting the formula because I bought a used plane and it didn't come with a set of instructions and I don't know what kind of plane manufacture made the plane to get it so that why I'm asking. This would rely help me out !!

Thanks ,In advance Josh
Old 10-14-2007, 06:48 PM
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Default RE: How do you figer out CG??

http://www.palosrc.com/instructors/cg.htm
Just fill in the blanks. Start with about 28-30% for MAC value.
Old 10-14-2007, 07:02 PM
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Default RE: How do you figer out CG??

Actually, MAC is the Mean Aerodynamic Cord and isn't something you input. It's figured out from the Tip Cord and the Root Cord, and is simply the average of the two.

And figuring the CG based solely on the wing is not the best way to figure out where an airplane's center of gravity should be. That needs to consider lots more things than just the wing. Like how large the tail is and how far back it is.

try this one:

http://www.geistware.com/rcmodeling/cg_super_calc.htm
Old 10-14-2007, 08:57 PM
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Default RE: How do you figer out CG??

Actually that calculator works perfect, and has on at least a dozen aircraft over the last few years. Including several large scale models, such as a 140" Constellation, 94" P-39, 110" B-24, and a 1/2 scale Spacewalker.
I like the "KISS" principle.
Old 10-15-2007, 04:38 AM
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Default RE: How do you figer out CG??

Very often in our hobby, there is more than one way to do something. And there are often two or more answers to a question. In some cases, one of the solutions works but leaves out some aspects that can be very important, but still works "good enough".

Why does the location of the CG matter? If we're keeping this simple, then it's simple what it does. It and the horizontal tail balance the airplane and stabilize the pitch of the airplane. Both the tail and the CG work together to do that job. And the size of the tail and how far back it is really is important to where the CG winds up.

So very simply, the tail and the CG make the airplane stable in pitch. When the CG is located properly for the airplane, the tail can keep that airplane flying level. When the CG is too far forward, the tail has too much poorly located weight to deal with and the elevator winds up doing most of it's work keeping the nose up and loses it's effectiveness. When the CG is too far forward, the tail simply can't keep that poorly located weight from making the airplane unstable no matter how big the elevator is or how much it can be deflected.

As you can see, the CG location and the tail have to work together to control the airplane. If your airplane has a tail, it and the CG work together. And both are equally important in balancing the airplane's pitch stability.

If you want to balance the airplane, and the airplane has a tail, you'd consider the size and location of the tail.



Old 10-15-2007, 10:55 AM
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Default RE: How do you figer out CG??

An existing plane such as you described, not something you're designing, will have the c.g. about 30% of the m.a.c.
The horizontal has already been sized and positioned for good flight.
If you're working up a design, using moments and area distributions of an existing design will in most instances also work with a 30% c.g. (or wherever it is on the shape being used as a pattern).
This "rule of thumb" works pretty good most of the time.
You'd be surprised how many models use this approach.
Old 10-16-2007, 07:09 AM
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Default RE: How do you figer out CG??

If you want the easiest to do and the shortest sound byte, then simply balance your model on the spar. Simple. Works lots of times. The spar is usually around where the cg can be to help most models probably fly ok. The spar is usually around the 30% chord location all along the wing. You won't have to know what a MAC is, or figure out where it is along the span, or compute 30% of anything, or project the 30% location on the MAC back to the root chord of the wing, or anything.

That plan has some drawbacks, but heck, it works. Lots of times.

In the picture, the wing with all the colored lines on and around it has an 11" chord. The red line at the tip shows that 11inches divided into inches. The broad blue line that runs spanwise actually follows the wings spar. It also is the manufacturer's suggest RANGE of CGs for the airplane. When that model was setup for it's maiden, that's where the CG was located. The airplane flew ok. It was a bit sluggish on the elevator but flew ok otherwise.

You could pick the airplane up with your fingertips under the spar anywhere along the wing and it'd balance. What more do you need. What more would you want.

I wanted a better flying airplane. So I took a couple of minutes with a yardstick and found a few locations. The location shown by the green arrow is the neutral point of the airplane. The whole airplane. It's the centerpoint of the balancing act the CG and the tail do. Any CG forward that point will give positive pitch stability, in different amounts obviously. The blue arrow shows where the CG wound up after finding the real CG range for the airplane and testing with the CG in that range. Right now the airplane flies great. I've also tuned the elevator throws to match the increased efficiency of the elevator and have less doing more back there too. The airplane is better in blustery conditions and a number of other good things. But all that is subjective. Was anything beyond fingertips at the spar necessary? I didn't know for sure until I did it, but no, not the way it worked out even with this simple, no taper, no sweep, everyday average airplane.

The good thing about our hobby is that simple stuff does work lots of times. And the really good thing about model airplanes is that you can do exactly how much you want to do from simple to complex. And sometimes the complex is as easy as measuring your airplane with a yardstick and plugging 9 numbers into an online application that will give you absolutely reliable information for almost any model's layout. Works for swept wings, tail less, big tailed, small, green, average, anything.

It's a kewl hobby. That can also teach us some things along the way.

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Old 10-16-2007, 07:14 AM
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Default RE: How do you figer out CG??

OH, BTW, balancing on the spar doesn't work if the wing is swept, but what the heck. It works otherwise............... and is simple to do.

And when it is swept, you can usually go halfway out the wing and put your fingers under the spar there and that works more often than not. And is simple to do.













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