IV.

Airplane Structure

Airplanes generally share the same basic configuration—each usually has a fuselage, wings, tail, landing gear, and a set of specialized control surfaces mounted on the wings and tail.

The materials that airplanes are made from have evolved as technology has advanced. The earliest airplanes were built mainly from wood and fabric with some metal parts. By the end of the 1920s many airplanes had metal frames and were covered with riveted metal sheets. Lightweight aluminum became the metal most commonly used in manufacturing airplanes for most of the 20th century. In the 1980s composite materials such as carbon fiber-reinforced plastic (CFRP) began to be incorporated into aircraft, helping to make them lighter and more fuel efficient.

A.

Fuselage

The fuselage is the main cabin, or body of the airplane. Generally the fuselage has a cockpit section at the front end, where the pilot controls the airplane, and a cabin section. The cabin section may be designed to carry passengers, cargo, or both. In a military fighter plane, the fuselage may house the engines, fuel, electronics, and some weapons. In some of the sleekest of gliders and ultralight airplanes, the fuselage may be nothing more than a minimal structure connecting the wings, tail, cockpit, and engines.

B.

Wings

All airplanes, by definition, have wings. Some are nearly all wing with a very small cockpit. Others have minimal wings, or wings that seem to be merely extensions of a blended, aerodynamic fuselage, such as the space shuttle.

Before the 20th century, wings were made of wooden ribs and spars (or beams), covered with fabric that was sewn tightly and varnished to be extremely stiff. A conventional wing has one or more spars that run from one end of the wing to the other. Perpendicular to the spar are a series of ribs, which run from the front, or leading edge, to the rear, or trailing edge, of the wing. These are carefully constructed to shape the wing in a manner that determines its lifting properties. Wood and fabric wings often used spruce for the structure, because of that material’s relatively light weight and high strength, and linen for the cloth covering.

Early airplanes were usually biplanes—craft with two wings on each side of the fuselage, usually one mounted about 1.5 m (about 5 to 6 ft) above the other. Aircraft pioneers found they could build such wings relatively easily and brace them together using wires to connect the upper and lower wing to create a strong structure with substantial lift. In pushing the many cables, wood, and fabric through the air, these designs created a great deal of drag, so aircraft engineers eventually pursued the monoplane, or single-wing airplane. A monoplane’s single wing gives it great advantages in speed, simplicity, and visibility for the pilot.

After World War I (1914-1918), designers began moving toward wings made of steel and aluminum, and, combined with new construction techniques, these materials enabled the development of modern all-metal wings capable not only of developing lift but of housing landing gear, weapons, and fuel.

Over the years, many airplane designers have postulated that the ideal airplane would, in fact, be nothing but wing. Flying wings, as they are called, were first developed in the 1930s and 1940s. American aerospace manufacturer Northrop Grumman Corporation’s flying wing, the B-2 bomber, or stealth bomber, developed in the 1980s, has been a great success as a flying machine, benefiting from modern computer-aided design (CAD), advanced materials, and computerized flight controls. Popular magazines routinely show artists’ concepts of flying-wing airliners, but airline and airport managers have been unable to integrate these unusual shapes into conventional airline and airport facilities.

C.

Tail Assembly

Most airplanes, except for flying wings, have a tail assembly attached to the rear of the fuselage, consisting of vertical and horizontal stabilizers, which look like small wings; a rudder; and elevators. The components of the tail assembly are collectively referred to as the empennage.

The stabilizers serve to help keep the airplane stable while in flight. The rudder is at the trailing edge of the vertical stabilizer and is used by the airplane to help control turns. An airplane actually turns by banking, or moving, its wings laterally, but the rudder helps keep the turn coordinated by serving much like a boat’s rudder to move the nose of the airplane left or right. Moving an airplane’s nose left or right is known as a yaw motion. Rudder motion is usually controlled by two pedals on the floor of the cockpit, which are pushed by the pilot.

Elevators are control surfaces at the trailing edge of horizontal stabilizers. The elevators control the up-and-down motion, or pitch, of the airplane’s nose. Moving the elevators up into the airstream will cause the tail to go down and the nose to pitch up. A pilot controls pitch by moving a control column or stick.

D.

Landing Gear

All airplanes must have some type of landing gear. Modern aircraft employ brakes, wheels, and tires designed specifically for the demands of flight. Tires must be capable of going from a standstill to nearly 322 km/h (200 mph) at landing, as well as carrying nearly 454 metric tons. Brakes, often incorporating special heat-resistant materials, must be able to handle emergencies, such as a 400-metric-ton airliner aborting a takeoff at the last possible moment. Antiskid braking systems, common on automobiles today, were originally developed for aircraft and are used to gain maximum possible braking power on wet or icy runways.

Larger and more complex aircraft typically have retractable landing gear—so called because they can be pulled up into the wing or fuselage after takeoff. Having retractable gear greatly reduces the drag generated by the wheel structures that would otherwise hang out in the airstream.

E.

Control Components

An airplane is capable of three types of motion that revolve around three separate axes. The plane may fly steadily in one direction and at one altitude—or it may turn, climb, or descend. An airplane may roll, banking its wings either left or right, about the longitudinal axis, which runs the length of the craft. The airplane may yaw its nose either left or right about the vertical axis, which runs straight down through the middle of the airplane. Finally, a plane may pitch its nose up or down, moving about its lateral axis, which may be thought of as a straight line running from wingtip to wingtip.

An airplane relies on the movement of air across its wings for lift, and it makes use of this same airflow to move in any way about the three axes. To do so, the pilot will manipulate controls in the cockpit that direct control surfaces on the wings and tail to move into the airstream. The airplane will yaw, pitch, or roll, depending on which control surfaces or combination of surfaces are moved, or deflected, by the pilot.

In order to bank and begin a turn, a conventional airplane will deflect control surfaces on the trailing edge of the wings known as ailerons. In order to bank left, the left aileron is lifted up into the airstream over the left wing, creating a small amount of drag and decreasing the lift produced by that wing. At the same time, the right aileron is pushed down into the airstream, thereby increasing slightly the lift produced by the right wing. The right wing then comes up, the left wing goes down, and the airplane banks to the left. To bank to the right, the ailerons are moved in exactly the opposite fashion.

In order to yaw, or turn the airplane’s nose left or right, the pilot must press upon rudder pedals on the floor of the cockpit. Push down on the left pedal, and the rudder at the trailing edge of the vertical stabilizer moves to the left. As in a boat, the left rudder moves the nose of the plane to the left. A push on the right pedal causes the airplane to yaw to the right.

In order to pitch the nose up or down, the pilot usually pulls or pushes on a control wheel or stick, thereby moving the elevators at the trailing edge of the horizontal stabilizer. Pulling back on the wheel deflects the elevators upward into the airstream, pushing the tail down and the nose up. Pushing forward on the wheel causes the elevators to drop down, lifting the tail and forcing the nose down.

Airplanes that are more complex also have a set of secondary control surfaces that may include devices such as flaps, slats, trim tabs, spoilers, and speed brakes. Flaps and slats are generally used during takeoff and landing to increase the amount of lift produced by the wing at low speeds. Flaps usually droop down from the trailing edge of the wing, although some jets have leading-edge flaps as well. On some airplanes, they also can be extended back beyond the normal trailing edge of the wing to increase the surface area of the wing as well as change its shape. Leading-edge slats usually extend from the front of the wing at low speeds to change the way the air flows over the wing, thereby increasing lift. Flaps also often serve to increase drag and slow the approach of a landing airplane.

Trim tabs are miniature control surfaces incorporated into larger control surfaces. For example, an aileron tab acts like a miniature aileron within the larger aileron. These kinds of controls are used to adjust more precisely the flight path of an airplane that may be slightly out of balance or alignment. Elevator trim tabs are usually used to help set the pitch attitude (the angle of the airplane in relation to the Earth) for a given speed through the air. On some airplanes, the entire horizontal stabilizer moves in small increments to serve the same function as a trim tab.

F.

Instruments

Airplane pilots rely on a set of instruments in the cockpit to monitor airplane systems, to control the flight of the aircraft, and to navigate. By the end of the 20th century traditional instrument displays using analog dials and indicators began to be replaced with computer-controlled electronic displays in new designs of aircraft.

Systems instruments will tell a pilot about the condition of the airplane’s engines and electrical, hydraulic, and fuel systems. Piston-engine instruments monitor engine and exhaust-gas temperatures, and oil pressures and temperatures. Jet-engine instruments measure the rotational speeds of the rotating blades in the turbines, as well as gas temperatures and fuel flow.

Flight instruments are those used to tell a pilot the course, speed, altitude, and attitude of the airplane. They may include an airspeed indicator, an artificial horizon, an altimeter, and a compass. These instruments have many variations, depending on the complexity and performance of the airplane. For example, high-speed jet aircraft have airspeed indicators that may indicate speeds both in nautical miles per hour (slightly faster than miles per hour used with ground vehicles) and in Mach number. The artificial horizon indicates whether the airplane is banking, climbing, or diving, in relation to the Earth. An airplane with its nose up may or may not be climbing, depending on its airspeed and momentum.

General-aviation (private aircraft), military, and commercial airplanes also have instruments that aid in navigation. The compass is the simplest of these, but many airplanes now employ satellite navigation systems and computers to navigate from any point on the globe to another without any help from the ground. The Global Positioning System (GPS), developed for the United States military but now used by many civilian pilots, provides an airplane with its position to within a few meters. Many airplanes still employ radio receivers that tune to a ground-based radio-beacon system in order to navigate cross-country. Specially equipped airplanes can use ultraprecise radio beacons and receivers, known as Instrument Landing Systems (ILS) and Microwave Landing Systems (MLS), combined with special cockpit displays, to land during conditions of poor visibility.