Airplane

I

Introduction

Airplane, engine-driven vehicle that can fly through the air supported by the action of air against its wings. Airplanes are heavier than air, in contrast to vehicles such as balloons and airships, which are lighter than air. Airplanes also differ from other heavier-than-air craft, such as helicopters, because they have rigid wings; control surfaces, such as movable parts of the wings and tail, which make it possible to guide their flight; and power plants, or special engines that permit level or climbing flight.

Modern airplanes range from ultralight aircraft weighing no more than 46 kg (100 lb) and meant to carry a single pilot, to great jumbo jets, capable of carrying several hundred people or several hundred tons of cargo. The largest commercial passenger airplanes weigh nearly 560 metric tons and the largest cargo jets up to 640 metric tons.

Airplanes are adapted to specialized uses. Today there are land planes (aircraft that take off from and land on the ground), seaplanes (aircraft that take off from and land on water), amphibians (aircraft that can operate on both land and sea), and airplanes that can leave the ground using the jet thrust of their engines or rotors (rotating wings) and then switch to wing-borne flight.

II

How an Airplane Flies

An airplane flies because its wings create lift, the upward force on the plane, as they interact with the flow of air around them. The wings alter the direction of the flow of air as it passes. The exact shape of the surface of a wing is critical to its ability to generate lift. The speed of the airflow and the angle at which the wing meets the oncoming airstream also contribute to the amount of lift generated.

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An airplane’s wings push down on the air flowing past them, and in reaction, the air pushes up on the wings. When an airplane is level or rising, the front edges of its wings ride higher than the rear edges. The angle the wings make with the horizontal is called the angle of attack. As the wings move through the air, this angle causes them to push air flowing under them downward. Air flowing over the top of the wing is also deflected downward as it follows the specially designed shape of the wing. A steeper angle of attack will cause the wings to push more air downward. The third law of motion formulated by English physicist Isaac Newton states that every action produces an equal and opposite reaction (see Mechanics: The Third Law). In this case, the wings pushing air downward is the action, and the air pushing the wings upward is the reaction. This causes lift, the upward force on the plane.

Lift is also often explained using Bernoulli’s principle, which states that, under certain circumstances, a faster moving fluid (such as air) will have a lower pressure than a slower moving fluid. The air on the top of an airplane wing moves faster and is at a lower pressure than the air underneath the wing, and the lift generated by the wing can be modeled using equations derived from Bernoulli’s principle.

Lift is one of the four primary forces acting upon an airplane. The others are weight, thrust, and drag. Weight is the force that offsets lift, because it acts in the opposite direction. The weight of the airplane must be overcome by the lift produced by the wings. If an airplane weighs 4.5 metric tons, then the lift produced by its wings must be greater than 4.5 metric tons in order for the airplane to leave the ground. Designing a wing that is powerful enough to lift an airplane off the ground, and yet efficient enough to fly at high speeds over extremely long distances, is one of the marvels of modern aircraft technology.

Thrust is the force that propels an airplane forward through the air. Thrust is provided by the airplane’s propulsion system: either a propeller or jet engine or combination of the two.

A fourth force acting on all airplanes is drag. Drag is created because any object moving through a fluid, such as an airplane through air, produces friction as it interacts with that fluid and because it must move the fluid out of its way to do its work. A high-lift wing surface, for example, may create a great deal of lift for an airplane, but because of its large size, it is also creating a significant amount of drag. That is why high-speed fighters and missiles have such thin wings—they need to minimize drag created by lift. Conversely, a crop duster, which flies at relatively slow speeds, may have a big, thick wing because high lift is more important than the amount of drag associated with it. Drag is also minimized by designing sleek, aerodynamic airplanes, with shapes that slip easily through the air.

Managing the balance between these four forces is the challenge of flight. When thrust is greater than drag, an airplane will accelerate. When lift is greater than weight, it will climb. Using various control surfaces and propulsion systems, a pilot can manipulate the balance of the four forces to change the direction or speed. A pilot can reduce thrust in order to slow down or descend. The pilot can lower the landing gear into the airstream and deploy the landing flaps on the wings to increase drag, which has the same effect as reducing thrust. The pilot can add thrust either to speed up or climb. Or, by retracting the landing gear and flaps, and thereby reducing drag, the pilot can accelerate or climb.

III

Supersonic Flight

In addition to balancing lift, weight, thrust, and drag, modern airplanes have to contend with another phenomenon. The sound barrier is not a physical barrier but a speed at which the behavior of the airflow around an airplane changes dramatically. Fighter pilots in World War II (1939-1945) first ran up against this so-called barrier in high-speed dives during air combat. In some cases, pilots lost control of the aircraft as shock waves built up on control surfaces, effectively locking the controls and leaving the crews helpless. After World War II, designers tackled the realm of supersonic flight, primarily for military airplanes, but with commercial applications as well.

Supersonic flight is defined as flight at a speed greater than that of the local speed of sound. At sea level, sound travels through air at approximately 1,220 km/h (760 mph). At the speed of sound, a shock wave consisting of highly compressed air forms at the nose of the plane. This shock wave moves back at a sharp angle as the speed increases.

Supersonic flight was achieved in 1947 for the first time by the Bell X-1 rocket plane, flown by Air Force test pilot Chuck Yeager. Speeds at or near supersonic flight are measured in units called Mach numbers, which represent the ratio of the speed of the airplane to the speed of sound as it moves air. An airplane traveling at less than Mach 1 is traveling below the speed of sound (subsonic); at Mach 1, an airplane is traveling at the speed of sound (transonic); at Mach 2, an airplane is traveling at twice the speed of sound (supersonic flight). Speeds of Mach 1 to 5 are referred to as supersonic; speeds of Mach 5 and above are called hypersonic. Designers in Europe and the United States developed succeeding generations of military aircraft, culminating in the 1960s and 1970s with Mach 3+ speedsters such as the Soviet MiG-25 Foxbat interceptor, the XB-70 Valkyrie bomber, and the SR-71 spy plane. In 2004 the experimental X-43 plane smashed previous airplane speed records by flying at nearly Mach 10. The unpiloted craft was constructed by the National Aeronautics and Space Administration (NASA).

The shock wave created by an airplane moving at supersonic and hypersonic speeds represents a rather abrupt change in air pressure and is perceived on the ground as a sonic boom, the exact nature of which varies depending upon how far away the aircraft is and the distance of the observer from the flight path. Sonic booms at low altitudes over populated areas are generally considered a significant problem and have prevented most supersonic airplanes from efficiently utilizing overland routes. For example, the Anglo-French Concorde, a commercial supersonic aircraft, was generally limited to over-water routes, or to those over sparsely populated regions of the world. This limitation impacted the commercial viability of the Concorde, which ended its regular passenger service in October 2003. Designers today believe they can help lessen the impact of sonic booms created by supersonic airliners but probably cannot eliminate them.

One of the most difficult practical barriers to supersonic flight is the fact that high-speed flight produces heat through friction. At such high speeds, enormous temperatures are reached at the surface of the craft. For example, the Concorde was forced to fly a flight profile dictated by temperature requirements; if the aircraft moved too fast, then the temperature rose above safe limits for the aluminum structure of the airplane. Titanium and other relatively exotic, and expensive, metals are more heat-resistant, but harder to manufacture and maintain. Airplane designers have concluded that a speed of Mach 2.7 is about the limit for conventional, relatively inexpensive materials and fuels. Above that speed, an airplane needs to be constructed of more temperature-resistant materials.

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.