What defines a monocoque structure in aircraft design?

A monocoque structure in aircraft design is defined primarily by the fact that the outer skin bears the load. In this design approach, the structure relies on the external surface to provide the necessary strength and rigidity, allowing it to withstand various stresses during operation without the need for a traditional framework of internal ribs or supports. The skin itself is often engineered to handle tension, compression, and torsion, making it integral to the overall structural integrity.

This method can lead to a lighter overall structure, as it minimizes the need for additional materials that would typically be used for internal framing. Consequently, a well-designed monocoque structure can effectively distribute loads across the entire surface, contributing to both strength and aerodynamic efficiency.

The other options, while related to aircraft design, do not accurately describe the defining feature of a monocoque structure. For instance, structures that utilize large internal frames or that are primarily composed of hollow tubes would not fit the monocoque definition, as they rely on internal supports rather than the skin for load-bearing capacity. Composite materials may be used in monocoque designs but are not a defining feature; similarly, the presence of hollow tubes suggests a different structural approach.