Rigidity in space is defined as

• A. The four fundamental risk elements in aviation.
• B. The photosensitive pigments that initiate the visual response in the rods of the eye.
• C. The final adjustment and alignment of an aircraft and its flight control system that provides the proper aerodynamic characteristics.
• D. The principle that a wheel with a heavily weighted rim spinning rapidly will remain in a fixed position in the plane in which it is spinning.

Answer: (D)

Rigidity in space means a rapidly spinning rotor tends to keep its orientation fixed in space. Because of angular momentum, the axis of rotation resists changes in direction, so the spinning wheel or gyroscope stays pointing in roughly the same direction unless an external torque acts on it. If you try to tilt or tilt it, the reaction shows up as a precession, turning the rotation about a perpendicular axis rather than simply tipping where you push.

That’s why a wheel with a heavy, fast-spinning rim will seem to “hold” its orientation in the plane it’s spinning in; the spin axis remains fixed in space. In aircraft instruments, this rigidity provides a stable reference for attitude and heading.

The other statements describe unrelated concepts: one about risk elements, another about the eye’s visual pigments, and another about adjusting a flight control system. Those do not describe the gyroscopic behavior of maintaining orientation.