Helicopter Stability - www.spinningwing.com
Spiral Stability We ignored a complication in the example of roll stability above. When a helicopter rolls right wing down and moves right its directional (yaw) stability also comes into play. Directional stability yaws the nose of the helicopter right, countering the increased sideslip. This reduces the (leftward) lateral flow across the rotor (which is what had provided the roll-corrective flapping). With too much directional stability, there will not be enough lateral flow to stabilize roll. The roll angle and yaw rate can increase leading to a spiral dive, called spiral instability. High directional stability reduces spiral stability.




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