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The wings are the primary source of lift for fixed-winged aircraft, and at the heart of its construction, is the airplane wing spar.
From the perspective of someone not very conversant with an airplane’s construction, the wing spar would be a new term, and they might also wonder why we’d only refer to it when focusing on fixed-winged aircraft. Well, this is because rotary-winged aircraft use a different mechanism.
An airplane wing spar is the primary structural member of the wing, designed to hold all loads experienced along its span either during flight or when on the ground. They run spanwise along the wing and, depending on the sweep, at right angles to the fuselage.
How an airplane can fly is a complex question. One that may be answered by looking at the features and components at play in its design. However, if the wings weren't considered, this scenario would be impossible.
This article focuses on the wing spar as the primary structural component of the wing. And it seeks to understand how the spar assists in achieving lift and distributing loads along the wing.
How Does an Airplane Wing Spar Work?
When studying the spar's operation, it is preferable, to begin with, an examination of the forces at play along the wings' surface to gain an understanding of how the spar contributes to overall performance.
Some of these forces are:
Tension and compression forces - At any given time, an aircraft will either experience forces pushing the wing upwards or pulling it down along its span. Depending on the direction of the force, one side of the wing will either be stretched or compressed while the other side stays the same.
Practically speaking, the lift produced by an aircraft's wings during flight is what keeps the entire aircraft airborne. This causes the wing to fold upward along its whole length.
Similarly, when on the ground, the wings will experience downward bending forces along their span as they are subjected to weight from fuel stored along the wing span or at the wing-tips as these are the areas where fuel tanks are located.
Some aircraft also have landing gear and engines mounted along the wings, which also contribute to downward forces.
Drag - Based on the laws of aerodynamics, when an airplane moves forward, it experiences drag forces that go in the opposite direction of the thrust. These forces act on the wing and tend to push it backward.
Twisting forces - Due to aerodynamic effects during flight, vortices are generated on the wing which tends to twist it. Also, when ailerons are used to achieve roll, the resulting change in control causes torsional forces on the wing.
As the center of all these forces, the wing experiences loads related to lift, coupled with the associated drag, use of ailerons, and all the relative weights placed on it. These loads have to be somehow absorbed otherwise the structural integrity of the wing will be compromised.
The basic role of spars is to support such loads.
But as much as spars are the most important structural components making up the wing, they are not the only components that form it. And an aircraft cannot fly without the assistance of these other members (though they may not all be found in every aircraft wing design).
The skin is the outermost material covering the wing and all its internal components. It is responsible for producing lift by giving the wing that smooth aerofoil shape. Stress on the skin during flight is transferred to the stringers and ribs.
Stringers are longitudinal structures running along the wing’s span and are attached to the ribs. They transfer the loads acting on the skin to the ribs.
Ribs run chordwise along the wing span (from the leading to the trailing edge) and are the ones now responsible for transferring loads from the skin and stingers to the airplane wing spar.
How Is the Spar Designed?
As has already been demonstrated, airplane wing spars play an important role in dispersing forces and loads applied to the wing span, both in flight and on the ground.
There are three major designs of the spar around which a wing is constructed. They include:
One main spar is used in this design. The wing's structure and aerofoil shape are provided by the ribs and bulkheads that attach to it. Although, this design is rarely adopted among designers.
This design has two main spars that are connected to the bulkheads and stringers forming a box shape to make the wing stronger.
This is the most common wing shape, which features two spars, one of which is positioned near the leading edge of the wing. And almost all of the wing loads are taken care of by this one.
The other spar, though smaller than the main one, is located at the rear part of the wing, a distance of approximately two-thirds away from the trailing edge. This is what keeps the ribs and skin aligned and helps prevent the wing from twisting.
The wing spars are connected to the fuselage with bolts as opposed to rivets and are connected to main attachment points on the fuselage, offering increased strength at these points.
The use of bolts is particularly important since overall loading is greater at the root section of the wing compared to the wing tips. Although one might be amazed at the few bolts used, they still provide the required strength.
Even though they're composed of different materials and have distinct designs, they nonetheless perform the same function.
What Materials Make up the Spar?
Spars can be built from a variety of materials, the most common of which are wood, metal (mostly aluminum), or composites.
Though mostly used in early aircraft models, wooden wing spars can still be found on a few more recent airplanes such as the all-wood Robin DR400.
The wooden spars are made from special tree species like the Sitka Spruce or Ash. The wood from these trees is known for its straight, uniform, and long grain, which is compressed and glued together to retain the wing’s shape and laminated to increase strength.
The initial design of these spars was made as solid rectangular blocks, referred to as slab spars. But it was later noted that this design could be improved.
The mid-section of these thick slabs was found to be taking a little share of the wing loads, with most of the stress and strain focused on the top and bottom ends of the spar. This extra thickness also meant increased weight.
The I-beam was later developed as an improvement to this design by shaving off a section of the mid-cross-section to form a shape resembling the letter "I". This new design was able to make the spar lighter, which is something that plane makers always try to do.
With the wing loads concentrated on the two edges (spar caps), the middle section (shear web) is left just as a support feature.
Making this component thinner does not affect the load-carrying capacity of the spar, but rather makes it lighter.
Despite its improved features, the wooden wing spar has several drawbacks that limit its use, such as being vulnerable to biological attacks from insects and fungi. Furthermore, both dry and damp weather conditions damage the material.
Aluminum is the most common material for making airplane wing spars, and it is better than wood in many ways. On top of it being light in weight, aluminum is not affected as much by weather and biological factors.
Although this doesn’t make it fail-proof, as they also exhibit structural failure and need regular inspections and maintenance checks to ascertain their integrity.
Some aviation accidents, especially with older plane models, have been linked to metal wing spar fatigue.
Such an incident involved a Cessna 210 while conducting geological surveys, crashed near Mount Isa, Queensland, fatally injuring the two pilots on board. Investigations confirmed that the aircraft’s wing spar experienced fatigue and fractured, separating its right-wing mid-flight.
The aircraft was manufactured in 1976 and had over 12,000 flight hours.
Some older spar designs were made with thick laminated metal sheets to avoid the need for heavy riveting. This made the spar stronger, but it also made it heavier and less effective.
Recent metal spar designs use separate large caps and thin webs, laminated with multiple aluminum sheets and joined together to make a single beam.
This is a fail-safe feature that helps keep the whole spar structure from failing. Its construction permits a single section to carry the whole load in case the other section fails.
Carbon fiber or Kevlar composite materials are commonly employed in the manufacturing of airplane wing spars. These materials are employed in many modern airplanes because they have the advantages of greater strength and reduced weight.
Although they share the same principles and mechanics as metallic spars, composite spars can look a bit different.
Of course, you can still find the I-beam structures in these wings, but the designer has more flexibility in construction due to the nature of the material.
One can integrate the spar into the wing with no obvious signs of the spar.
False spars may sound strange, and understandably so: they are extremely real. In many ways, they are similar to the main spars that run from one end of the wing to the other, however, they do not extend the complete length of the wing.
They are commonly used to carry loads of moving surfaces, particularly ailerons.