In Flight Structural Loads. Sizing of Wing Box

For more than half a century, the structures of transport aircraft have been mainly skinned and even self-supporting. This naturally applies to the main wing box, which is the wing's load-bearing element. Schematically, this box is made up of a front spar – on which the aerodynamic surfaces of the leading edge are supported, which are usually mobile –, a rear spar – supporting the flaps and ailerons –, connected by the upper and lower panels. These last two panels –– constitute both the upper and lower soles of the wing box, and the aerodynamic surfaces that generate most of the wing's lift.

In addition to its major structural and aerodynamic role, the volume of the box also acts as the main fuel tank. This volumetric characteristic, derived from the structural and aerodynamic thickness of the wing, holds great promise for future architectures such as the flying wing, where the available space can be used for the payload.

When it comes to structural dimensioning of the main wing box, there are a number of regulatory specifications to consider. This article focuses on hulls in terms of both skin thickness (structural strength criteria) and stability (stiffening technology). Internal ancillary elements (connecting rods, ribs, crossmembers) are not explicitly considered, as they are added solely to enable the load-bearing hull to perform its structural function.

The aim of this article is therefore to summarize the methods for pre-dimensioning a wing box, based on a typical example of a civil transport aircraft. Starting with the in-flight loads –, the reference being the limit load factor (CL) –, the panels are discussed one after the other, in order to arrive at a root dimensioning (critical section) very close to the optimum.