Stability is the seasoning salt of aviation. Every airplane needs some, but going overboard will ruin a good bird. In airplane terms, Stability is the tendency of the model to return to a normal flight attitude. Unless you’re lucky enough to have a huge a gym to fly in, you will have to deal with the elements. Wind has always been a Frenemy. Just when you think you can count on it, it shifts. Your model will have to cope with the ever changing gusts.
Before you send your model airborne make sure it is setup for positive stability. This will help it come back to a normal flight path. Stability is controlled by the Neutral Point (NP) and the center of gravity (CG). The NP is fixed, its based on the plane’s geometry i.e. the area of the horizontal, the distance between the horizontal and the wing, the area of the wing. For most airplanes the NP lies somewhere around the middle of the wing. For the Glider Kit the NP is 42mm from the leading edge; the root chord is 90mm therefore the NP is 47% back.
The other factor that affects stability is the CG. This can be adjusted based on where the weight is located within the model. Stability comes on a spectrum with positive stability on one side and negative stability on the other. The type of flying will determine how much stability is needed. For hand launched gliders, positive stability is the only way to go.
What does stability mean for flight characteristics?
The more stable a model is, the quicker it will return to its original flight path, but it will also overshoot the path and oscillate. A slightly less stable model will not correct as fast, but will not overshoot as much. A neutral model will not correct at all; whatever new flight path it got pushed to, it will hold that until the next gust comes along, or the ground. An unstable plane will start to diverge and continuously move away from the original path. So more stability is better, to a point. Too much stability will require strong control inputs to change pitch and make the model less maneuverable.
For small models like the Glider Kit, moving the CG two mm will have a noticeable effect on the flight characterizes. However on large 1/4 scale plane, a two mm change will be unnoticeable. To compare stability across models, a ratio called Static Margin (SM) is used.
For the Glider Kit, a good location for the CG is 36 mm back from the leading edge. This produces a Static Margin of 6%.
By adjusting the CG we can control the static margin and thus the stability and flight characteristics. SM gives us a way to compare the stability across models, because CG location is only important relative to the Neutral Point.
The amount of stability comes down to personal preference, but unlike salt, start with a little extra and take some out as you go.
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