Know your CG

Model Airplane News, Apr 2004 by Smith, Jerry

Flight insurance for your model

Many airplanes are damaged or destroyed on the first flight because the builder failed to check the model's center of gravity (CG) before he took it to the flying field. Some assume that if you assemble the model as directed by the instruction manual, a successful first flight is assured. Not always true! A little nose-heaviness is tolerable (in fact, it's sometimes desirable), but if the model is too tail-heavy, you will be busy with the sticks.

Most kits and plans indicate that the model's CG should be located at a specific point on the wing's mean aerodynamic chord (MAC). This point is usually identified as a percentage of the MAC, but it's sometimes given as a measurement. When you check your model's CG, pay attention to what it says in the manual, and then determine exactly where the CG should be. Generally, it should fall somewhere in the MAC's 25- to 30-percent range. Measure your wing's chord and mark the CG location. For example, on a 12-inch constant-chord wing, the 28-percent MAC is 3 3/8 inches back from the leading edge (0.28 x 12 = 3 3/8).

On a constant-chord wing, the MAC can be measured anywhere along the wing. With a tapered-wing plan, the MAC will fall somewhere in the middle of the wing panel (see Figure 1).

WHERE IS THE MEAN AERODYNAMIC CHORD LOCATED?

By measuring a tapered wing panel, the MAC can be graphically determined. You can use the model's full-size plans or draw your own layout. If you have a CAD program, you can also easily draw an accurate scale drawing. At the wing root, draw a reference line that extends aft from the trailing edge that's parallel to the fuselage centerline. This line should be the same length as the tip chord. At the wingtip, draw another reference line forward of the tip that's the same length as the root chord. Now find the center of the root chord and tip chord, and connect the points with a centerline. Draw a diagonal line between the ends of the two extension reference lines at the tip and the root chord. The point at which the diagonal line and the centerline intersect is the spanwise location of the MAC. At this location, draw the MAC reference line parallel to the root and tip chord lines. To determine the CG location, measure the required percentage or distance from the leading edge back along the MAC.

You can balance your model at this point, or if it's more convenient, at any point along a line that extends from the CG and is perpendicular with the MAC. Figure 2 illustrates how the distance between the leading edge and the CG reference line changes because of the wing's taper. It is important to know where the CG should be measured relative to the MAC. Some assembly manuals in ARF kits don't give this information.

Balancing your model at the correct CG location is very important and can make the difference between a successful first flight and a complete disaster.

Great Planes Model Distributors (800) 682-8948; (217) 398-6300; greatplanes.com.