The Forth Rail Bridge, a cantilever bridge with three balanced (double) cantilevers

Cantilever is an engineering term referring to a construction technique in which loads are carried by a beam to a strong mounting point. The beam forms a lever, which carries the load by being held in position by the mount, turning the loads into torque on the mount. Cantilever construction allows for long structures without external bracing.

In bridges, towers, and buildings

Cantilevers are widely found in construction, notably in cantilever bridges and balconies. Frank Lloyd Wright's Fallingwater used a cantilever to project the large balcony out into "free space". A less obvious example are the common radio towers, which resist being blown over by the wind through a cantilever at their base; the ground below must be "twisted" for the tower to fall.

In aircraft

Another common use of the cantilever is in aircraft design. Early aircraft wings bore their loads by building two (or more) wings, and bracing them with wires. They were similar to truss bridges in some aspects, the wings on each side of the plane were braced with crossed wires both along their length, so they would stay parallel, as well as front-to-back to resist twisting. The cables generated considerable drag however, and there was constant experimentation on ways to eliminate them.

It was also desirable to build a monoplane aircraft, as additional drag is formed by having a stack of wings. Early monoplanes used either struts (as do some modern personal aircraft), or cables (as do some modern home-built aircraft). The advantage in using struts or cables is a reduction in weight for a given strength, but with the penalty of additional drag, which reduces maximum speed (for a given power) and increases fuel consumption (for a given speed).

The most successful wing design was the cantilever. A single large beam, referred to as the spar, runs through the wing, and often right through the aircraft. Looking at a plane from the front, the wings are both trying to rotate up at the tips, a force that is resisted either by mounting the two spars to each other (each one is twisting in the opposite direction) or to a strong box-like structure in the middle, or by a shell like structure forward of the spar that forms the aerodynamic shape and resists twisting (this is called a D tube).

Cantilever wings require a much heavier spar than would otherwise be needed in cable-stayed designs. However as the size of aircraft grew, this additional weight dropped in comparison to the overall weight, as well as the growing weight of the cables needed to brace larger wings. Eventually a line was crossed in the 1920s, and designs increasingly turned to the cantilever design. By the 1940s almost all larger aircraft used the cantilever exclusively, even on smaller surfaces such as the horizontal stabilizer.

See also: engineering mechanics, statics, and Moment (physics)