##  [Spar](/spar-0) 

 Definition

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

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

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

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

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

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

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

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

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.