Devices Mounted on Wings: What are They?
The explanation of commonly-sighted devices that not everybody knows about.
Sometimes when we get on a commercial plane and sit at the window, we see the wings of the plane we are flying. The same is true when we view the aircraft digitally (photos or videos) or directly as in an airshow. The longer we look, the more we see in detail the shape. Then, suddenly we are focused on the strange devices attached to the wing that stands out from the “slick” wing design we usually see. What are these devices, and what are their functions? This article aims to provide an explanation of our questions.
The devices we see are there to improve the aerodynamics and flight performance of the aircraft in certain (if not all) flight conditions. The devices are seen in this article’s cover photo, however, are different from one another, especially their functions. Here, we take a look at each of the devices one by one.
Vortex Generator
A vortex generator is an aerodynamic device, consisting of a small vane usually attached to a lifting surface (in this case, an aircraft’s wing), that creates an air vortex that delays local flow separation and aerodynamic stalling, thereby improving the effectiveness of aircraft’s wings and control surfaces at very low speeds.
How vortex generator works
The aircraft wing has an aerodynamic cross-section called an airfoil. According to Bernoulli’s law, air travels through both above and below the airfoil with different velocities. The velocity difference caused pressure difference between the two sides of an airfoil, and the pressure difference created lift for the wing and eventually the aircraft. However, due to the viscous and frictional forces between air and airfoil surface, the velocity of air traveling through the upper side of airfoil alone is not the same in every location. The velocity depends on the position of the air relative to the surface of the airfoil. The velocity of air just above the surface of the airfoil will be zero and will gradually increase if the air is located further away from the surface to a point where the velocity of the air will be equal to the velocity of incoming air. The region between the two air velocities is called a boundary layer.
The formation of a boundary layer with high energy (high speed) keeps the airflow in contact with the wing surface. However, at slow speed, the airflow separates from the wing before reaching the aircraft control unit, such as ailerons. The separation made any effort by the pilot to control the aircraft by moving the ailerons useless. In the end, the pilot lost control of the aircraft, and the aircraft went into a stall.
To overcome the problem, a vortex generator is incorporated into the wing design. Vortex generators would generate vortices that will energize the boundary layer and keep the airflow as close as possible to the wing and the control surfaces.
Vortex generators are also used underwing of Airbus A320 family aircraft to reduce noise generated by airflow over circular pressure equalization vents for the fuel tanks. Lufthansa claims a noise reduction of up to 2 dB can thus be achieved.
Wing Fence
Wing fence, also known as boundary layer fence or potential fence, is a fixed aerodynamic device that looks like flat plates fixed to the upper or lower surfaces of the wing, parallel to the wing chord, and in line with the stream of incoming airflow. It is designed to prevent flow from moving away from a control surface of a trailing edge, thus preventing the entire wing from stalling at once or the wingtip from stalling before the wing root. Wing fence devices are often seen on swept-wing aircraft.
How wing fence works
As a swept-wing aircraft slows toward the stall speed of the wing, the angle of the leading edge forces some of the airflows sidewise toward the wingtip. This process is progressive: airflow near the middle of the wing is affected not only by the leading edge angle but also the span-wise airflow from the wing root. At the wingtip, the airflow can end up being almost all span-wise, as opposed to front-to-back over the wing, meaning that the effective airspeed drops well below the stall. Because the geometry of swept wings typically places the wingtips of an aircraft aft of its center of gravity, the lift generated at the wingtips tends to create a nose-down pitching moment. When the wingtips stall, both the lift and the associated nose-down pitching moment rapidly diminish.
The loss of the nose-down pitching moment leaves the previously balanced aircraft with a net nose-up pitching moment. This forces the nose of the aircraft up, increasing the angle of attack and leading to stalling over a greater portion of the wing. The result is a rapid and powerful pitch-up force followed by a complete stall. The “Sabre Dance” that plagued North American F-100 Super Sabre is one example of swept wing’s tendency to stall at the wingtip before the wing root.
Wing fences delay or eliminate these effects by preventing the span-wise flow from moving too far along the wing and gaining speed. When hitting the fence, the air is directed back over the wing surface.
Other devices that can produce the same result as wing fence are:
Leading edge slat
Slats act as a fence in the form of their actuators. They also reduce the effect by improving the aircraft’s angle of attack and creating a much lower stall speed.
Pylon
Even though pylon creates the same effect as wing fences, the pylon is originally designed for an aircraft to be able to carry an external load. Modern military aircraft have pylons that can carry multiple different loads without having to change the pylon itself, while commercial aircraft have pylons that are used to mount the aircraft’s engine.
Vortilon
Vortilon is a pylon that is reduced in size and created vortices, hence the name vortilon (VORtex-generaTing-pyLON). Vortilons consist of one or more flat plates attached to the underside of the wing near its leading-edge, aligned with the flight direction. When the speed is reduced and the aircraft approaches stall, the local flow at the leading edge is diverted outwards; this span-wise component of velocity around the vortilon creates a vortex streamed around the top surface, which energizes the boundary layer. A more turbulent boundary layer, in turn, delays the local flow separation.
Vortilons are often used to improve low-speed aileron performance, thereby increasing resistance to spin. Vortilons only stream vortices at a high angle of attack and produce less drag at higher speeds.
Wingtip fence: a hybrid between wing fence and winglet
Wingtip fence, although similar in its form to winglet, should not be confused with its counterpart. It has two surfaces, with each extending above and below the wingtip, but both surfaces are usually shorter in length than a winglet. A wingtip fence has the aerodynamics effect of both a wing fence and a winglet but with lower efficiency.
Anti-shock Body
An anti-shock body is a pod-like structure integrated into an aircraft wing that is usually placed on the upper-back surface of the wing. It was designed to reduce the drag on transonic speed.
How anti-shock body works
At high-subsonic flight speeds, the local speed of the airflow can reach the speed of sound, where the flow accelerates around the aircraft’s body and wings. The speed at which this development occurs varies from aircraft to aircraft and is known as the critical Mach number. The resulting shock waves formed at these zones of sonic flow cause a sudden increase in drag, called wave drag. To reduce the number and strength of these shock waves, an aerodynamic shape should change in cross-sectional area as smoothly as possible from front to rear. The design procedure used to design such an aerodynamic shape is called the Whitcomb area rule.
One of the resulting designs is the anti-shock body. An anti-shock body decreases the strength of the shock waves, thus reducing the aircraft’s drag at transonic speeds, which occur between Mach 0.75 and 1.2.
Flaps Fairings
Flap fairings are devices that are used to protect the flap retraction and extension mechanisms from elements and make the aircraft more aerodynamic by reducing the amount of interference drag acting upon the aircraft. This increases the performance and makes the aircraft efficient. They also help to reduce drag caused by the effects of compressibility as the aircraft nears the speed of sound by acting like anti-shock bodies.
Winglets
A winglet is a wingtip device shaped like a near-vertical extension of a wingtip. The winglet is intended to reduce the drag of an aircraft, thus making the aircraft fly more efficiently, reducing the operating cost, and increasing its range.
How winglets work
A traditional wing (wing without winglets) would produce a large vortex on its wingtips. These vortices are called the wingtip vortex and are caused by lift differences on the wing span-wise. Wingtip vortex originating at the leading edge of the wingtip and propagating backward and inboard, causing turbulence. This turbulence ‘delaminates’ the airflow over a small triangular section of the outboard wing, which destroys lift in that area. Additionally, the wingtip vortex also caused high vibration on an aircraft’s wing, causing instability and adding aerodynamic drag.
If there is a winglet installed at the wingtip, the wingtip vortex, which rotates around from below the wing, strikes the cambered surface of the winglet, generating a force that angles inward and slightly forward, greatly reducing the wingtip vortex’s size. The winglet also converts some of the otherwise-wasted energy in the wingtip vortex to an apparent thrust for the aircraft.
Another potential benefit of winglets is that they reduce the intensity of wake vortices (vortices that trails behind an aircraft) produced by the aircraft. Winglets also increase efficiency by reducing vortex interference with laminar airflow near the tips of the wing, by ‘moving’ the confluence of low-pressure (overwing) and high-pressure (underwing) air away from the surface of the wing.
Other than that, the winglet also creates fuel economy improvement that increases in conjunction with the flight time. One of the winglet variations, blended winglets, even allows a steeper angle of attack at takeoff, thus reducing takeoff distance.
Winglet Substitute
Not all aircraft benefited from winglets. That is why military combat aircraft (fighters) does not use winglet directly into their wings. This is due to a number of reasons, such as winglets only perform best at certain speeds (transonic) while most modern fighters could accelerate into supersonic speed. Additionally, winglets are only used on highly loaded, high-aspect-ratio wings when their L/D at a rather high lift coefficient needs to be pushed up even more. Highly maneuverable configurations are less concerned with single-percentage increases in L/D for a small part of the flight, and in order to improve turn performance, increasing the wingspan is much more effective. But that increases roll inertia and roll damping, so fighters use low-aspect-ratio wings and compensate their higher drag in turn with more powerful engines. (Note that the trend in fighter aspect ratios went down with the improving thrust-to-weight ratio of jet engines.)
In order to utilize the advantage and minimize the downside of the winglet, military aircraft use a substitute wingtip device. The device can come in multiple forms. In Lockheed F-104 Starfighter, the device came in the form of wingtip-mounted external fuel tanks that had small “wings” on them. On more modern aircraft such as General Dynamics F-16 Falcon, the substitute is missile mounted on the wingtip.
