The following article is taken from the AMA National Newsletter.
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This article is by Michael Heer from The Eagle's Nest.

THE IMPORTANCE OF THE CENTER OF GRAVITY

The center of gravity (CG) is the point where the plane is balanced evenly from front to rear. This point is important, especially in thermal flying, because it is the point where your airplane can obtain maximize duration in lift, sink, or still air. If you balance your airplane in front of the real CG, it is nose heavy and will sink faster than it should. For the beginner, the airplane is more easily controlled. If you balance it behind the CG, the airplane will again sink slightly faster, but it may more readily respond to lift or sink. The skilled pilot may do this to help spot lift. If the CG is moved too far back, the airplane will be uncontrollable.

The beginner pilot often thinks his/her airplane is balanced on the proper CG because he/she has balanced the airplane right on the spot where the CG is shown on the plans. Unfortunately, when it comes to the CG, most plans lie. This is not for mean or vicious reasons, but rather it is done to help the new pilot by making the airplane easier to control. As mentioned above, a noseheavy airplane responds more quickly to your direct control input and a tail-heavy airplane may not respond at all. Well-written instructions often give you a point to start for your CG location and instruct you to work your way back slowly to find the best point of balance. This is critical. Work your way back slowly and seek help from experienced pilots when searching for the CG location of your airplane.

Some of you may be wondering what is the big deal? If you slope fly it may not be a big deal if you have balanced in front of the CG. It may be easily countered by an extra mile or two of wind for minimum lift for your airplane and you may never even notice that problem. But in thermal flying it may make a large difference. DAW makes a nice trainer known as the TG-3. Built per the plans, it is a great trainer at the slope, easy to fly and responsive and able to take a lot of damage. However at the thermal field, that same airplane drops out of the sky like a power airplane with a dead engine. The plans are overly cautious. I moved the balance point back approximately one full inch and got about double the length of flight for beginners who just cruised the sky.

The Sagitta and other airplanes in the 1980s used a popular airfoil known as the Eppler 205. The plans show the CG to be on the front of the spar in some airplanes to the middle of the spar in some others. The actual CG point is 42% of the wing root rib, which is well behind the spar. The difference in average flight times in using the proper CG is really tremendous. Not only does your airplane fly more efficiently with the CG in the proper location, but it also more readily points out lift, by reacting to lift and sink in a more demonstrative manner. Light lift that you may have flown right through with your airplane nose heavy now lifts your airplane's tail and makes you aware of it.

As I close, I again warn you about going too far back in searching for the CG of your airplane, but encourage you to talk to experts to find the proper place. You may be amazed at how well it flies with the proper balance.

from The Eagle's Nest, by Michael Heer
Sacramento Valley Soaring Society
PO Box 2086
Elk Grove CA 95759