Proper balance

We all know that the cg must be in the right place and one wing shouldn't be heavier than the other for a plane to fly properly. The problem is that what is proper for one pilot may not be proper for another. We all have individual tastes and this extends to our planes as well. On nearly all of the plans we see now days, we see a symbol like that in figure 1 that designates the cg or balance point of the plane. Without exception, the first time builder will add weight here and there to balance the plane precisely on this point. Thats ok because the design has been tested and will perform good at this cg location. The problem is who tested it? Was he a beginner or expert? Is this cg location for optimum pitch control or minimum. Many years ago the plans showed a cg range such as is depicted in figure 2. In the instructions it would advise a beginner to start off with the forward cg range and as his flying skills improved, add a little tail weight to move the cg rearward. In order for your plane to perform to your individual taste or skill then you must flight balance it to do so.

The cg affects pitch control. Pitch is when you apply up or down elevator and point the nose of the plane up or down respectively. Here is another rule of thumb. The farther aft the cg the more sensitive the elevator. The farther forward the cg the less sensitive theelevator. In any case, with a conventional aircraft, the cg must be in front of the center of lift or cl. The cl of a wing is roughly 50% of the wing chord and the cg is roughly 25% to 30% of the wing chord as measured from the front as shown in figure 3. There is a good way to visualize the effect of cg on pitch stability. In figure 4 we have two wing panels each resting on a block of wood positioned between the cg and cl. If you grabbed each wing panel by the trailing edge and started moving them up and down, the top wing panel would offer more resistance than the bottom panel. This is true because the block of wood under the top panel is wider than the block under the bottom panel. This effect holds true during flight. The more distance between the center of lift and center of gravity the more stable the plane will be in pitch movement. As with anything else about an airplane the cg could be moved too far forward making the elevator ineffective but for a beginning pilot a forward cg is preferable to a rearward cg. By knowing and understanding how pitch control is adjusted a pilot can then start tailoring the flight charecteristics of his plane to suit his tastes in flying. <---Back to contents