In a fixed-wing aircraft, the spar is often the main structural member of the wing, running spanwise at right angles to the fuselage. The spar carries flight loads and the weight of the wings while on the ground. Other structural and forming members such as ribs may be attached to the spar or spars, with stressed skin construction also sharing the loads where it is used. There may be more than one spar in a wing or none at all. Where a single spar carries most of the force, it is known as the main spar.
Main spar of a de Havilland DH.60 Moth
Basic metal-sparred wing using a honeycomb 'D' box leading edge
A cantilever is a rigid structural element that extends horizontally and is unsupported at one end. Typically it extends from a flat vertical surface such as a wall, to which it must be firmly attached. Like other structural elements, a cantilever can be formed as a beam, plate, truss, or slab.
The Forth Bridge, a cantilever truss bridge
This concrete bridge temporarily functions as a set of two balanced cantilevers during construction – with further cantilevers jutting out to support formwork.
Howrah Bridge in India, a cantilever bridge
A cantilevered balcony of the Fallingwater house, by Frank Lloyd Wright