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How Airplane Wings Work: Ailerons to Flaps and Everything In Between

Feb. 06, 2019
12 min read
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You've probably stuck your hand out the window of a moving vehicle in the summertime — pivoted it up and down, changing the angle at which the oncoming wind bends around it. If you tilt it up ever so slightly, your hand goes up.

It turns out that your hand can be a pretty good approximation of an airplane wing. Out the window of a moving car, your hand is at once a wing, an aileron, a spoiler and a flap. (And if you stretch out your fingers, maybe even a slat.)

The primary and secondary control surfaces of a modern commercial jet. Image from the YouTube video below, modified by author.
The primary and secondary control surfaces on the wing of a modern commercial jet, an Airbus A320 (Image from the YouTube video below, modified by author.)

Airplane wings are a majestic and highly complex piece of engineering. Quite simply, they're almost alive. On the Boeing 787, computer systems control the wing's components to adjust to flight conditions — gusts, wind shear, turbulence, even being slightly too high for landing, and more — and all of this independent of the pilot inputs. You'll sometimes see those wing parts move quickly, sometimes with near-imperceptible adjustments — and on landing, those movements may happen very frequently.

This article explores some of the components of an airplane wing that you'll see when you look out the passenger window.

Two experts helped me demystify how the components work together. Dean Plumb is British Airways' Chief Technical Pilot, with more than 30 years of flying experience. He's got 10 years in the Royal Air Force flying C-130 Hercules transports, followed by 20 years at BA, including time on the Boeing 787 Dreamliner. Chip Kiehn, Director of Sales and Marketing for Aviation Partners Boeing, which makes and installs a part of the wing called a winglet, has spent close to 20 years at this Boeing joint-venture project. (Winglets are those curved ends jutting up from the end of the wing on many planes, including current models of the Boeing 737 and Airbus A320.)

Ailerons: The Little Wing

Ailerons — a commercial aircraft has two — control the movement of the aircraft on its longitudinal axis, causing it to roll left to right. Aileron is French for "little wing" — and that's exactly what they are. "Like the wing, the aileron is tear-shaped when viewed from the side and has the thinnest edge at the back," Plumb explained. "And...the aileron is surprisingly large when seen close-up."

Aileron deflected down on a Gulf Air Airbus A330-200. (Photo by: via Getty Images)
Aileron deflected down on a Gulf Air Airbus A330-200. (Photo by: via Getty Images)

The ailerons are located on the outside trailing edge of the wing. To spot the ailerons, you'll have to look closely. On a passenger aircraft, ailerons move ever so slightly from the passenger's perspective. Indeed, when your aircraft is banking in a turn, you may notice that the aileron returns to its flush-with-the-wing position, yet the aircraft continues to bank. It does this because of centripetal force holding it in a turn.

When the control column is shifted to the right by a pilot (or by the autopilot more often than not), the aileron on the right wing is then raised while the aileron on the opposite wing drops down. (They move opposite each other.) The act of raising the aileron on the right wing reduces the lift on the right wing — and when wings have a reduction in lift, they drop. Here, the right wing dips down in a controlled turn to the right.

A NASA animation showing aileron deflection and roll about the longitudinal axis of the aircraft.
A NASA animation showing aileron deflection and roll about the longitudinal axis of the aircraft.

"The ailerons are helped by spoilers on the wing which can reduce lift very slightly, aiding the turn," said Plumb.

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Spoilers and Airbrakes: Decrease Lift, Primarily

As the name suggest, spoilers spoil something. Here, they ruin the lift produced by the wing, much the same way an aileron does. So what's the point? Spoilers allow the plane to lose lift and descend in a controllable way.

"The spoilers act by making the wing less efficient, in a controlled way," said Plumb. "This is a great way of reducing unwanted airspeed as you slow down ready for the approach to land. It also allows the aircraft to descend at a quicker but comfortable rate if you have a lot of altitude to lose. You don't always need to use the spoilers in this way but it is very common and allows you to quickly match the speed of other aircraft ahead and behind you who are all in the same stream of aircraft coming into land."

There are often two sets of spoilers on airplane wings. The set close to the fuselage is called ground spoilers or airbrakes. "The ground spoilers are the exact same panels that are used as speed brakes in flight, except that on the ground they are allowed to deflect fully up and maximize the 'lift dumping' effect," Plumb said.

The Boeing 737 wing from a 1986 Boeing Maintenance Manual, modified by author.

Plumb explained that the pilots pre-arm the system during descent to automatically activate when the wheels touch down.

"A lot of people assume the spoilers work by acting as an airbrake, but in fact 80 percent of their contribution to stopping the aircraft is by stopping the wing from producing lift, and this forces the full weight of the aircraft onto the main wheels, thus making the wheel brakes much more efficient," Plumb explained.

Flaps: Increase Lift

That first machine-like whirring noise you hear as your aircraft descends for landing is the sound of the flaps deploying. Flaps are both lift and drag devices. Deploying flaps allows the pilot to descend and maintain lift at a much slower speed on approach. At the same time, deploying flaps provides drag, which slows the aircraft. On most jetliners today, there are inboard flaps and outboard flaps, with the inboard flaps being closest to the fuselage. They are deployed in degrees, as the aircraft descends for landing.

man washing the wing trailing-edge flaps of a Boeing 747-400 with a long handled brush. (Photo by: via Getty Images)
These components are huge. A man washing the wing trailing-edge flaps of a Boeing 747-400. (Photo by: via Getty Images).

The flaps are raised and lowered via aircraft hydraulics inside the torpedo-shaped bodies under the wing, called track fairings. These also serve a dual purpose of improving aerodynamic flow under the wing.

Flaperons: To Assist with Roll

As the name suggests, a flaperon is a device that is both an aileron and flap. They operate more like ailerons than flaps; they can adjust quickly up and down like an aileron, especially compared to flaps (which deploy ploddingly). Flaperons are found on larger twin jets such as the Boeing 777, Airbus A350 and Dreamliners. Flaperons will deploy during a turn when control inputs are performed. For a right turn, for example, the right wing's aileron will raise ever so slightly, decreasing lift on the wing, whereas the flaperon will extend ever so slightly to counteract a portion of that loss of lift in a controlled manner. This all done by the airplane's computers without additional input from the pilot.

Spoilerons: A Misnomer

A spoileron is a spoiler that also acts similar to an aileron — and these days they all do. It is not a separate component but a term used to describe the function of spoilers on many modern commercial aircraft. Spoilers automatically, and without pilot input, in tandem with the aileron, as Plumb described above, to aid in the roll along the longitudinal axis. You won't find "spoileron" in official documentation from Boeing or Airbus.

Slats: Like Flaps, but on the Other Side of the Wing

wing leading edge slats deployed showing extension mechanism of an Asiana Airlines Boeing 747-400. (Photo by: via Getty Images)
Wing leading edge slats deployed showing the extension mechanism on an Asiana Airlines Boeing 747-400. (Photo by: via Getty Images)

Slats extend outwards from the leading edge of the wing. Their main purpose is to allow the plane to fly at a higher angle compared to the relative wind; slats act to shift the oncoming air over the top surface of the wing. This allows the wing to maintain lift at a lower speed, such as when landing.

You don't get to see the slats in operation from underneath, as a passenger.

"The modern wing can pull the slats and flaps in so that it is the perfect shape for high speed, high altitude flying where the engines are most efficient," Plumb said. "However, when aircraft come in to land we want the wing to produce maximum lift at much slower speeds so, much like a soaring eagle coming in to land, the slats at the front of the wing and the flaps at the back extend to create a much bigger surface area and produce more lift."

"[Slats] are incredibly important on jet aircraft with swept-back wings, which are ideal for high-speed flight but need enhancement to provide enough lift for low-speed flight."

What About Up and Down? A Quick Word About Elevators

The angle of the aircraft relative to the wind (and generally the horizon) is controlled by the elevators, but those aren't found on the wing; they are part of the tail . "Pull back on the stick, the houses get smaller. Push forward, and the houses get bigger," pilots like to say. As a passenger, you can't see the elevators; they're placed on the trailing edge of the horizontal stabilizer, i.e. the tail of the aircraft.

Putting It All Together

"All of these flight controls work together to get the perfect wing shape for efficient flight. This is highly complex to coordinate but is made possible by modern flight control computers and means the wing is constantly adapting to conditions in a way that is not dissimilar to the way a bird flexes and adapts the shape of its wings," said Plumb.

You can see this in action with the video below, where an aircraft departs on its takeoff roll. Look for the flap-like flaperons moving quite a bit, as well as the spoilers acting in tandem to spoil lift and help stabilize the aircraft automatically.

Winglets: Increase Lift and Reduce Drag

Most modern jets have winglets — those upward-sloped pieces on the ends of wings, often sporting advertising for the airline (!). They serve an important purpose for efficiency. Chip Kiehn explained why.

A Boeing 737-800 with "scimitar" winglets
A Boeing 737-700 with "split scimitar" winglets. (Photo by Alberto Riva/TPG)

"Vortices of air are produced on wingtips due to the pressure differences between the upper and lower wing surfaces, which cause excess drag," said Kiehn. "Wingtip devices are generally designed to disrupt those vortices in such a way that reduce overall air drag and increase overall wing lift."

"On new wing designs, designers have clean-slates when considering wing tip shapes and can incorporate the necessary efficiency at the onset," Kiehn said. "As technology improves, so too does wingbox design. [Aviation Partners Boeing] winglets are designed as an improvement on older wingbox technology. Our split-scimitar design is a further improvement on our Blended Winglet technology, providing the 737-NG as much as 2.2% additional reduction in fuel consumption over the original blended winglets. "

To see this whole dance in action, watch this video of a Lufthansa Airbus A320 — featuring blended winglets — aircraft descending to landing; you'll see the ailerons move just a tiny bit, the flaps deploy, spoilers to slow the aircraft and ground spoilers deploy just after touchdown. This particularly aircraft does not have flaperons.

"The wing of a bird is a clue to future developments in wing design as some rather futuristic designs already imagine the whole wing morphing subtly, rather than just the flaps and slats moving," Plumb said. "So next time you see a soaring eagle, pay attention to how the wing changes shape depending on whether it needs high or low speed; you may well be looking at the inspiration for the next generation of aircraft wings."

Mike Arnot is the founder of Boarding Pass NYC, a New York-based travel brand, and a private pilot.

Featured image by JT Genter

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