A characteristic of a Semi-Rigid Rotor System is that it usually has how many blades?

A Semi-Rigid Rotor System typically incorporates two blades. This design allows for greater flexibility compared to other rotor systems while still providing adequate strength and stability during flight. The two-blade configuration is advantageous because it offers a balance between complexity and performance, allowing for easy construction and maintenance.

In a semi-rigid system, the blades are hinged in such a way that they can flap and teeter around a central point. This teetering motion is essential for accommodating changes in lift and maintaining balance as the helicopter maneuvers. The two-blade system also helps reduce overall weight and drag, contributing to the efficiency of the aircraft.

While other rotor systems may have different blade counts (like one for a single rotor or more for other configurations), the specific characteristics and operational requirements of a semi-rigid system lend themselves to the two-blade layout, making it the correct answer for this question.