Inappropriate Stability Augmentation

Yaw damping stability augmentation is required for high-altitude airplanes by inescapable physical facts. Dutch roll damping ratio is directly proportional to air density. No airplane can be expected to have satisfactory natural yaw damping at altitudes above about 35,000 feet. So it is also for directional stability at high supersonic speeds, and to a lesser extent for high-altitude pitch damping. Airplanes with stabilizing surfaces compromised or even eliminated for stealth also must have artificial stability or augmentation.

At the other extreme, one sees stability augmentation applied inappropriately, that is, to correct flying qualities deficiencies caused by poor basic aerodynamic design. For example, there is really no reason for static longitudinal stability augmenters to be used in the general – aviation field. This is particularly so when the augmenters are complex, going beyond simple downsprings and adding maintenance and failure problems to be solved.

A case in point is the stability augmentation system used in Piper PA-31T Series Cheyennes. This system is basically a downspring, but with operating speed range and variable spring tension controlled by an angle of attack vane and a computer. A bobweight completes the installation. All this is needed because the basic Cheyenne airframe was derived from a lower powered Piper model, the Navajo. The Cheyenne’s engines are more powerful, but also lighter, leading to aft center of gravity problems. Rebalancing the air­plane would have been a better solution than what we consider to be an inappropriate use of stability augmentation.

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