A principal spanwise structural member in a lifting surface (wing or rotor blade) that resists bending moments and shear forces, provides attachment points for ribs and skin/covering, and forms a principal element of the aerodynamic load path to the airframe.

Principle

A spar carries the majority of spanwise bending moment and shear arising from aerodynamic loads; its section and shear web design control deflection, strength and local load transfer to ribs and fittings.

Demonstration

Illustrative scenario — Situation: during maneuvering flight the wing experiences increased lift and bending moment. Recognition: the main spar (or multiple spars) running spanwise is identified as the element sized for bending and shear. Action: the spar resists the bending moment, transmits shear through its web to the wing box and attaches to the fuselage fittings. Consequence: the spar's stiffness and strength limit wing deflection and prevent structural collapse under the applied aerodynamic loads.

Misapplication

Referring to any longitudinal member in the wing as a spar (for example calling a rib or cap strip a spar) misidentifies the load-carrying role and can produce incorrect load distributions and unsafe sizing assumptions.

Consequence

A properly designed spar provides predictable wing stiffness and strength and a clear path for aerodynamic loads; failure, under‑design or damage to a spar can cause large deflections, flutter risk, loss of control surface effectiveness or catastrophic structural failure of the lifting surface.

Reversal

In modern integrated composite wing designs the load may be carried by a wing box or the skin‑stringer system rather than a single discrete spar; in those concepts the term 'spar' may refer to an equivalent load‑carrying element rather than a discrete beam.

Boundary

Clearly within: a primary front or main spar sized to carry spanwise bending and shear in a wing. Boundary case: a secondary spar or false spar that supports control surfaces or local loads but is not sized for primary bending. Clearly outside: a rib, which is transverse and shapes the airfoil rather than carrying primary spanwise bending.

Semantic Tension

Stiffness and strength ↔ Mass and spanwise continuity — increasing spar stiffness or cross‑section improves strength and reduces deflection but increases mass and may complicate weight distribution and manufacturability.

Synthesis

A spar is the primary spanwise bending element of a lifting surface; while its physical form varies by architecture (single spar, multi‑spar, wing box), its defining function is to take spanwise bending and shear and transfer those loads into the airframe.