What is a delta-3 hinge?

Diagram explaining helicopter delta 3 hingeA delta-3 hinge is a hinge that is not at a 90-degree angle, it is offset to some degree.

When a delta-3 hinge is used in a rotor blade, the blade angle is changed as the blade flaps.  This change in angle is because the hinge is offset.  The delta 3-hinge is commonly used in tail rotors to reduce the degree of flapping that is required.  This practice allows for the tail rotor to be closer to the tailboom, ultimately reducing vibration.  This type of hinge is used in some main rotors to a lesser degree.  The primary deterrent for the use of the delta-3 hinge is the increased stress imposed on the hinge.

Reference(s):

Principles of Helicopter Flight, 2nd Edition, pg. 96

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