It is surely not hard to imagine that airplanes are very heavy structures. But just how much do you know about those huge wings that are attached to them? Did you know, for example, that a single wing of a Boeing 747-400 weighs nothing less than 43,000kg? To make things even more interesting, this same wing measures approximately 525 square meters — enough room to house 40 medium-sized automobiles.
The enormous weight and size of aircraft wings, therefore, make it necessary for them to be attached to the fuselage properly. If you were thinking that they are glued on, then you may need to rethink it. In general, there are quite many ways aircraft wings are attached to the frame of the plane. The most common of these is the wing-fuselage lug attachment.
Lugs are the most widely used connecting elements in aircraft. They are used to join different components of an airplane’s frame. The attachment bracket, also known as a wing box, consists of a lug and a portion of spar connected with several bolts. A lug is made up of 2 pinholes along with the integrated top flange and bottom flange which will be connected to the spar.
Wing to fuselage joints are mostly overdesigned. This is highly necessary due to the kind of forces that act on wings when in use. Therefore, the most important factor considered in the construction of lug attachment brackets is structural strength.
Before they are put into use on an aircraft, wings are tested to prove their strength and discover their threshold limit to carry loads.
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Design of Wing-Fuselage Lug Attachments
Every aircraft wing is designed to carry its weight. Also, its structural strength is such that it can support the weight of the engine, the fuel it carries and all the aerodynamic forces acting on it.
Wings are made up of three assemblies namely: the left, right, and center wing sections. All these three sections are joined together to form a single piece wing.
The most important structural part of an aircraft wing is the spar. The spar runs spanwise through the length of the wing at right angles to the fuselage. A spar carries the flight loads and the weight of the wing while the plane is on the ground. A typical wing relies on its main spar which is like an H or I shaped beam to which the nose and the main ribs are also attached. The end of the main ribs is then mounted on the rear spar.
The wing ribs form the frame of the wing and therefore decide the aerodynamic form of the wing. In other words, they also determine the wing’s performance as regards lift and drag. They are made from aluminum alloy.
The connection from wing to the fuselage must be highly durable to withstand all the loads attached to it. This connection (lug structure) is also responsible for transferring the bending moment and shear loads acting on the wing to the fuselage. It often contains four lugs, two at the front (main) spar and two at the rear spar.
Forces Acting on an Aircraft Wing
There are 3 primary loads acting on the wing of an aircraft. These forces are critical to the design of any aircraft wing. They include
- aerodynamic lift
- load due to the wing structure weight
- load due to the weight of the fuel in the wing
All these forces act perpendicularly to the wing’s surface and their magnitude varies along the length of the wing. The action of the two weight forces gives rise to a bending moment.
Forces on ground
As expected, the forces acting on a plane’s wing are different when it’s on the ground from when it’s flying in the air. When on the ground, the only two forces acting on the wings are the weights of the wing itself and that of the fuel in it. In cases where the engine is attached to the wing — which are often — the weight of the engine also acts on the wing when it is on the ground.
Forces in flight
When in flight, the shape of the wing (commonly called airfoil) allows it to generate lift. The choice of airfoil largely determines the amount of lift it can generate. In most cases, airplane wings can produce enough lift that is equal to the wing’s weight. This is sufficient to keep the aircraft in level flight.
So, in essence, the fuselage supports the wings when the plane is on the ground while the wings support the fuselage during flight.
Thanks to the lug attachment, a good portion of the three wing loads are distributed to the frame of the plane (fuselage) too. Most of the load distribution can be observed in detail at the root of the wing. This load distribution also backs the theory of wing loading in aerodynamics.
Wing loading is the ratio of the total weight of an aircraft to the area of its wing. In other words, the size of a plane is directly dependent on the size of its wings.
Strut-Braced Wing Aircraft: Extra Wing Support
Some light airplanes use strong supports between the wings and the landing gear or fuselage to give the wing structure strength. These supports are called wing struts. This kind of structure is limited to lower performance planes since the struts generate a large amount of drag while the structure that uses a spar has no drag at all.
In aeronautics, this type of method is called bracing. In the past, the majority of airplanes — monoplanes and biplanes inclusive — made use of bracing as an integral feature. However, in modern aviation, the use of bracing has become rather limited. It is now mostly found in the form of lift struts in light commercial aircraft where weight conservation and aerodynamics are prioritized over sheer performance.
Bracing works by creating a triangulated truss structure that resists bending or twisting.
By comparison, an unbraced cantilever structure (structure discussed earlier) bends easily unless it carries a lot of heavy reinforcement (spars and ribs). One simple way to make the wing lighter and stiffer is to have a deeper structure. While trying to conserve weight and minimize air resistance, the structure can be made hollow. Bracing is then used to connect the essential components of the airplane’s frame.
A good example is a high-wing monoplane which has a diagonally placed lifting strut that runs through from the rear of the fuselage to the wingtip’s extreme. This type of structural configuration effectively increases the depth of the root of the wing to the fuselage height without any significant increase in weight.
In most cases, the tips of bracing struts are linked to the major structural element inside that part of the plane. This can either be a spar in the case of a wing or a fuselage bulkhead.
Bracing is an efficient method of resistance against the different types of forces acting on an airframe such as weight, drag, lift, and torsion or twisting. In essence, a strut is the most essential bracing component which is strong enough to withstand the acting forces be it compression or tension.
Can Wings Break Off From An Airplane in the Air?
It is practically impossible for airplane wings to snap off during flight. It is not uncommon to see them shake in a rather scary manner especially during flight turbulence. But airplane wings and their lug attachments are built in a way that allows them to maintain their structural strength, even in turbulent weather conditions.
To cope with different flight conditions, aircraft wings are made to be able to flex up and down naturally. This is known as wing flex. Most airplane wings can normally flex 0-7 degrees up and down during flight. In highly turbulent conditions, this flex angle can even be higher. The heavier the turbulence, the more flex a wing tends to experience.
How Much Flex Can a Wing Accommodate?
These days, the type of testing that aircraft undergo is always very thorough and elaborate. A plane typically gets to make it out of the manufacturer’s factory only after passing a compressive list of airworthiness and certification tests. One of these tests involves bending the plane’s wings to extreme angles. A typical modern commercial plane is tested in a rig where its wings may be flexed up to 90 degrees in either direction. Manufacturers perform a series of tests that try to imitate ordinary and extreme real-life load conditions to observe the reaction of the wings and fuselage. These tests are often called “static tests”.
In December 2013, for example, Airbus took the specially designed airframe of its A350 XWB on an extreme load test. Loads measuring up to one and a half times more than what is ever possible in real life were applied to the plane’s wings. When the load was at the maximum, the tip of the plane’s wing deflected more than five meters. In other words, the wing flex was almost at an angle of 90 degrees.
In a final test, the wings of the aircraft are tested until they snap. This allows the manufacturers to know the breaking point of these wings and ensure that it is satisfactorily above the predicted load level maximum.
Summary
Apart from the engineering of jet engines, the design, and aerodynamics of airplane wings were the second most instrumental contributors to the development of present-day aviation. After several innovations, the technology of attaching wings to a plane’s fuselage has now become safer and more reliable than ever before. So, the next time you fly, you can very well rest assured that the turbulent weather can’t yank off your airplane’s wing.
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