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 Aerodynamic Center Stability Definition  Static StabilityStatic stability refers to the aircraft's initial response when disturbed from a given angle of attack, slip or bank. Static Directional stability is probably the easiest to visualize. In the movie below you can see that the aircraft tends to return to the original slip angle (zero) if the relative wind changes. |
In the above movie we see that the initial tendency for the aircraft once the slip angle changes is to return to the original slip angle. This is called Positive Static Stability. In this case it is specifically positive static directional stability. Dynamic StabilityDynamic stability refers to the aircraft response over time when disturbed from a given angle of attack, slip or bank. In the above movie the aircraft displays strong positive static directional stability. It also displays strong dynamic directional stability. Specifically the yawing motion damps out very quickly, with only a couple of small oscillations before returning to zero slip. On the other hand a pitch disturbance also displays positive static and dynamic stability. But, the dynamic oscillations usually take much longer to damp out. The movie below displays positive dynamic longitudinal stability.
An aircraft can have positive static longitudinal stability but negative dynamic stability. Positive static longitudinal stability means that initially the aircraft tends to return toward the original angle of attack once disturbed. Negative dynamic longitudinal stability means that over time the oscillations get larger. (Eventually the aircraft would stall, if left uncontrolled by the pilot.) The movie below shows negative dynamic longitudinal stability.
IN the above movie each oscillation is larger than the one before. If not corrected by the pilot the aircraft will eventually stall. Aircraft are required to display both static and dynamic longitudinal stability in order to be certified. However, an aircraft can be flow successfully if it has static longitudinal stability even if the dynamic stability is negative. But, negative static stability would be an impossible situation for the pilot to contend with.
Some new aircraft, particularly those built by Airbus, are designed to "artificially" display neutral longitudinal pitch stability. This means that if the pilot pitches to a new attitude and releases the controls the aircraft holds the new attitude. However, you should keep in mind that this is not "real" stability, the onboard computers are creating this effect. The aircraft is actually stable and would return to the original speed and angle of attack if the computer didn't interfere. Positive, Negative, and Neutral StabilityStability can be categorized as:
The above classifications can be applied to both the static and dynamic stability. They can also be applied to all three axis. This page has not explicitly covered all possible stability variation (there are 18.) You should however be able to understand the nine possible stabilities and describe the meaning of each.
Of the above several are strictly theoretical. For example it is almost impossible to imagine an airplane without positive static directional stability. But, does that mean it would automatically have positive dynamic directional stability? (No) Clear as mud right???!!! <LOL> |
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Updated January 1, 2003