Wing spars are the principal structural members of the wing. These members carry the whole weight of the wing as well as the concentrated weight of the fuselage and landing gear. In flight, their main function is to carry the bending loads generated by the aerodynamic forces on the wing. In parallel, the spar web shoulders the vertical shear loads - the lift - that the aerodynamic loading of the wing creates. In a low-wing airplane, a stub spar, built as thick as the root section of the main spar, is made part of the fuselage structure, transmitting distributed load and completing the structural loop.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is a spar in aviation?
A spar is a structural member in an aircraft wing. Wing spars can be thought of as the skeleton or the frame of an aviation wing. The fuselage is fastened to the wing spars to be held in place. Airframe and fuselage inspections involve the inspection of the spars to determine the structural integrity of the wing.
In aviation, the spar is the skeleton of the wing: the principal spanwise structural member that carries every pound of lift and every concentrated load from fuselage, engine mounts and landing gear. Built as an I-type beam that is taller than it is wide, it extends from root to tip, clamped into the fuselage by wing fittings, plain beams or a truss so that the entire weight of the aircraft is transferred through the spar roots. The upper spar cap is loaded in compression and the lower in tension. The web between them carries vertical shear loads and keeps the caps a fixed distance apart, while the ribs that give the airfoil its shape are attached to the same caps. Whether the airplane is high- or low-wing, the main spar forms one face of the wing box and acts as the backbone of the wing, with auxiliary or false spars creating closed sections for undercarriage stowage. A stub spar is part of the fuselage itself.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
What is the primary purpose of the wing spar in an aircraft?
The primary purpose of the wing spar is to serve as the main backbone of the aircraft. It runs the width of the aircraft horizontally and gives each wing the strength to take the bending load for lift generated by the aerodynamic forces. Spars carry the bending loads in the wing generated as a result of aerodynamic forces on the wing. The main spar resists bending primarily in the direction of the lift vector up and down. Since wings will bend upwards, spars usually carry shear forces and bending moments, transmitting these loads into the fuselage. Spar caps carry the bending moment generated by the wing in flight, with the upper cap loaded in compression and the lower in tension for positive load factor. The spar web is responsible for carrying the vertical shear load lift which arises from aerodynamic loading of the wing. In larger aircraft, multiple spars - primary spars to carry bending loads and auxiliary spars to create closed sections - work together with ribs and struts to create a rigid, strong, yet light space frame.
