I'm often perplexed by the number of moving parts on a plane's exterior. There are panels that flap up and down, some that fold backward, others that swing forward, etc. What do all these things do?

When a bird needs to maneuver or slow down, it does so by twisting or bending its wings and tail, something pioneer aviators emulated by incorporating wing bending in the first biplanes. But airplanes today are built from aluminum, titanium, and high-strength composites, not wood, fabric, or feathers. Thus, they are augmented by various contrivances that help a plane climb, descend, turn and decelerate. Passengers often speak collectively of these devices as "flaps," but technically they consist of flaps, slats, ailerons, elevators, rudders and spoilers.

Let's take a typical airliner, from back to front:

Atop the rear fuselage of every plane is the tail, aka its vertical stabilizer, which functions exactly as its presence suggests -- by keeping the plane straight. Hinged to the tail's back edge is the rudder. Unlike a boat rudder, it complements but does not control turns; its function is chiefly one of stability, tempering a plane's side-to-side swerve, or "yaw." Pilots move the rudder by means of foot pedals in the cockpit. I'll admit that sounds quaint, and in reality the autopilot systems and an apparatus called a "yaw damper" actually do most of the work. Flying a four-seater requires some literal legwork, but pretty infrequently do you wrassle with the rudder pedals of an airliner. The rudder can be divided into different sections, each with particular limitations pertaining to airspeed and angles of travel. Such restrictions are taken care of electronically or mechanically by the rudder system itself.

Beneath the tail, or occasionally attached to it, are two small wings. These are the horizontal stabilizers, the hinged rear portions of which are known as elevators. The elevators command a plane's nose-up/nose-down pitch. This is something of a simplification, as aerodynamics 101 teaches us that climbs and descents are also affected by adjusting power. But for practical purposes, elevators point the nose up or down, as directed by the forward or aft motion of the cockpit wheel or joystick.

Which brings us to the wings -- the heart and soul of every airplane. A plane is built around its wings the way a car is built around a chassis or a bicycle is built around a frame. To get aerodynamically correct, we're supposed to think of the wing in singular -- there are not two wings as much as one, bisected by the fuselage. But let's not do that; the aesthetic nature of airplanes makes it difficult not to behold separate, left and right entities, and for the purposes of this discussion we'll assume two wings.

As most passengers know, wings are fitted with an array of supplemental components -- namely flaps, slats, ailerons and spoilers. I remember, as a kid in a window seat on a Boeing 727 just aft of the wing, how the entire structure seemed to disassemble itself during descent. Big, triple-segmented flaps would come barreling down, the spoilers and ailerons would flutter and wave, while up front the slats and leading edge flaps would drop into position. Magically, almost, you could see right through the very center of the wing, with houses and trees visible in the spaces where the sections had slid apart.

The faster a plane flies, the more lift its wings generate, but when it slows they need help. Flaps, which trail backward and downward, enhance the wing's camber, allowing safe, stable flight at lesser speeds. Airliners take off and land with their flaps extended, though the exact positioning varies with weight and other factors. There are normally inboard and outboard subsets of flaps, which themselves can be segmented horizontally.

Slats, which roll forward from the front of a wing, perform a similar function. There can be several slats along the leading edge, deployed as needed depending on speed.

Ailerons, located along the trailing edge, are responsible for turns. Pilots steer via the control wheel or joystick, which in turn directs the ailerons up or down. They are interconnected and apply opposite forces: When the aileron on the left wing goes up, the one on the right wing goes down. (An up aileron reduces lift on that side, dropping its wing, while a down aileron raises its wing.) The smallest twitch of an aileron will provide a good deal of turn, especially when going fast, so you can't always spot them moving. If you're seated over the wing, it might seem a plane is banking without anything having budged, but in fact the ailerons have done their thing, if ever so slightly. On most large planes there are two ailerons on each wing -- an inboard and an outboard -- which work either together or independently as speed dictates. Thus there are actually four ailerons, not two, working in pairs.

Spoilers are the rectangular planks that spring from the wing's upper surface. Their use is most obvious on touchdown, when they pop to full deflection to assist in deceleration. A raised spoiler greatly disturbs airflow across the wing, reducing lift while simultaneously adding drag. During flight, spoilers are used to increase rates of descent and to decrease airspeed. Additionally, they are often linked with the ailerons to aid in turning.

So, yes, this gets complicated: A simple turn is often a choreography of rudder, aileron and spoiler, all directed from the cockpit along with the flaps, spoilers, and everything else described above. But before you picture a hapless pilot kicking his feet and grasping madly for levers, keep in mind that many of these individual surfaces are linked to single controls, so that a simple turn of the wheel or lowering of the flap lever will cause any combo of movements outside. And during much of a flight, it's the autopilot that's coordinating the physics of turns, climbs and descents (under guidance and instructions from the crew, of course).

Adding to the confusion, rudders, elevators and ailerons are frequently equipped with smaller tabs that operate independently from the main surfaces. These "trim" tabs help fine-tune the motions of pitch, roll and yaw. And remember those horizontal stabilizers out back? They too are commonly trimmable, lightening the forces of pitch.

If you're still with me, you'll be thrilled to hear that various aircraft also incorporate their own idiosyncratic versions of all these things. One plane I used to fly had some spoilers that were used only after landing, others that assisted with turning, and still others employed strictly for inflight deceleration. The cargo jet I flew had no slats at all, but "slots" that opened at the front of each wing. And certain Boeing models are equipped not only with conventional flaps like those we've discussed but also ones that lower from beneath the leading edge as well, in addition to the slats. The Concorde has no horizontal stabilizers, so it has no elevators. But it does have "elevons." Meanwhile, we'll save "flaperons" for another time.

And no, by the way, to answer a question I'm bound to be asked, a plane cannot fly with one wing. With safety in mind, aircraft components -- everything from navigational systems to engines -- often exist in duplicate or triplicate, but alas, this spirit of redundancy cannot extend to something so wholly integral to flight itself.

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