Spiral Instability

[Ref Pilot’s Handbook of Aeronautical Knowledge, FAA-H-8083-25B Page 5-20]

Spiral Instability exists when the static directional stability of the aircraft is stronger than the lateral stability (Directional stability > Lateral stability)

The stronger directional stability tends to yaw the nose into resultant relative wind and force the nose to a lower pitch attitude somehow could reduce the Dutch roll tendencies. However, the yawing tendencies causes the wing on outside travelling faster than the inside wing and the lift becomes greater and greater. It could bring about the overbanking tendency – bank angle becoming steeper and steeper. If the pilot don’t make a correction, the aircraft could enter downward spiral as well as the steep spiral dive.

 

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Solution for “overbanking tendency”?

Improper recovery from spiral instability leading to inflight structural failures! Since the airspeed in the spiral condition builds up rapidly, the application of back elevator force to reduce this speed and to pull the nose up only “tightens the turn,” increasing the load factor. So, is that any solution to the problem?

Ans: wing leveler is developed to correct or eliminate this instability.

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