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Dragging Dynamics

We have already seen that rotorblades lead and lag in the plane of rotation. We will refer to these movements by using the term 'dragging'. When a blade drags, it must also somehow return to its 'home' position (for sake of simplicity, we will assume that there is a rotating frame of reference, see the next figure). The origin of the force with this function depends upon the type of rotorhead. In a hingeless rotorsystem, the blade is more or less fixed to the rotorhead, and there is, thus, a particularly strong homing force because there is material stress when the rotorblade drags. In a rotorsystem with drag hinges, these, to a great extent, prevent restorative moments. In these circumstances, and due to a drag hinge offset, some restorative moment comes from the centrifugal force which produces it. This can be explained by looking at the next figure. When the blade drags, the CM moves somewhat closer to the shaft. However, centrifugal force wants to return the CM back to its original position. This generates a restoring force (blue arrow).

Dragging forces drive a system that consists of a blade (mass) with restoring moments (either due to material stress and /or due to centrifugal forces), which will resonate at a certain frequency. This system must be stable by providing an appropriate amount of damping. This damping cannot be generated aerodynamically, as is the case with blade flapping. Therefore, some mechanical damping device is usually used (see picture).

Helicopter rotorblade drag damper
Helicopter blade drag damper

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Cyclic & Collective

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