A little bit of engineering (562404), страница 18

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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) of an airplane 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.

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.

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.

Airplanes use either piston or turbine (rotating blades) engines to provide propulsion. In smaller airplanes, a conventional gas-powered piston engine turns a propeller, which either pulls or pushes an airplane through the air. In larger airplanes, a turbine engine either turns a propeller through a gearbox, or uses its jet thrust directly to move an airplane through the air. In either case, the engine must provide enough power to move the weight of the airplane forward through the airstream.

The earliest powered airplanes relied on crude steam or gas engines. These piston engines are examples of internal-combustion engines. Aircraft designers throughout the 20th century pushed their engineering colleagues constantly for engines with more power, lighter weight, and greater reliability. Piston engines, however, are still relatively complicated pieces of machinery, with many precision-machined parts moving through large ranges and in complex motions. Although enormously improved over the past 90 years of flight and still suitable for many smaller general aviation aircraft, they fall short of the higher performance possible with modern jet propulsion and required for commercial and military aviation.

The turbine or jet engine operates on the principle of Newton’s third law of motion, which states that for every action, there is an opposite but equal reaction. A jet sucks air into the front, squeezes the air by pulling it through a series of spinning compressors, mixes it with fuel and ignites the mixture, which then explodes with great force rearward through the exhaust nozzle. The rearward force is balanced with an equal force that pushes the jet engine, and the airplane attached to it, forward. A rocket engine operates on the same principle, except that, in order to operate in the airless vacuum of space, the rocket must carry along its own air, in the form of solid propellant or liquid oxidizer, for combustion.

There are several different types of jet engines. The simplest is the ramjet, which takes advantage of high speed to ram or force the air into the engine, eliminating the need for the spinning compressor section. This elegant simplicity is offset by the need to boost a ramjet to several hundred miles an hour before ram-air compression is sufficient to operate the engine.

The turbojet is based on the jet-propulsion system of the ramjet, but with the addition of a compressor section, a combustion chamber, a turbine to take some power out of the exhaust and spin the compressor, and an exhaust nozzle. In a turbojet, all of the air taken into the compressor at the front of the engine is sent through the core of the engine, burned, and released. Thrust from the engine is derived purely from the acceleration of the released exhaust gases out the rear.

A modern derivative known as the turbofan, or fan-jet, adds a large fan in front of the compressor section. This fan pulls an enormous amount of air into the engine case, only a relatively small fraction of which is sent through the core for combustion. The rest runs along the outside of the core case and inside the engine casing. This fan flow is mixed with the hot jet exhaust at the rear of the engine, where it cools and quiets the exhaust noise. In addition, this high-volume mass of air, accelerated rearward by the fan, produces a great deal of thrust by itself, even though it is never burned, acting much like a propeller.

In fact, some smaller jet engines are used to turn propellers. Known as turboprops, these engines produce most of their thrust through the propeller, which is usually driven by the jet engine through a set of gears. As a power source for a propeller, a turbine engine is extremely efficient, and many smaller airliners in the 19- to 70-passenger-capacity range use turboprops. They are particularly efficient at lower altitudes and medium speeds up to 640 km/h (400 mph).

There are a wide variety of types of airplanes. Land planes, carrier-based airplanes, seaplanes, amphibians, vertical takeoff and landing (VTOL), short takeoff and landing (STOL), and space shuttles all take advantage of the same basic technology, but their capabilities and uses make them seem only distantly related.

Land planes are designed to operate from a hard surface, typically a paved runway. Some land planes are specially equipped to operate from grass or other unfinished surfaces. A land plane usually has wheels to taxi, take off, and land, although some specialized aircraft operating in the Arctic or Antarctic regions have skis in place of wheels. The wheels are sometimes referred to as the undercarriage, although they are often called, together with the associated brakes, the landing gear. Landing gear may be fixed, as in some general-aviation airplanes, or retractable, usually into the fuselage or wings, as in more-sophisticated airplanes in general and commercial aviation.

Carrier-based airplanes are a specially modified type of land plane designed for takeoff from and landing aboard naval aircraft carriers. Carrier airplanes have a strengthened structure, including their landing gear, to handle the stresses of catapult-assisted takeoff, in which the craft is launched by a steam-driven catapult; and arrested landings, made by using a hook attached to the underside of the aircraft’s tail to catch one of four wires strung across the flight deck of the carrier.

Seaplanes, sometimes called floatplanes or pontoon planes, are often ordinary land planes modified with floats instead of wheels so they can operate from water. A number of seaplanes have been designed from scratch to operate only from water bases. Such seaplanes have fuselages that resemble and perform like ship hulls. Known as flying boats, they may have small floats attached to their outer wing panels to help steady them at low speeds on the water, but the weight of the airplane is borne by the floating hull.

Amphibians, like their animal namesakes, operate from both water and land bases. In many cases, an amphibian is a true seaplane, with a boat hull and the addition of specially designed landing gear that can be extended to allow the airplane to taxi right out of the water onto land. Historically, some flying boats were fitted with so-called beaching gear, a system of cradles on wheels positioned under the floating aircraft, which then allowed the aircraft to be rolled onto land.

Vertical Takeoff and Landing (VTOL) airplanes typically use the jet thrust from their engines, pointed down at the Earth, to take off and land straight up and down. After taking off, a VTOL airplane usually transitions to wing-borne flight in order to cover a longer distance or carry a significant load. A helicopter is a type of VTOL aircraft, but there are very few VTOL airplanes. One unique type of VTOL aircraft is the tilt-rotor, which has large, propeller-like rotating wings or rotors driven by jet engines at the wingtips. For takeoff and landing, the engines and rotors are positioned vertically, much like a helicopter. After takeoff, however, the engine/rotor combination tilts forward, and the wing takes on the load of the craft.

The most prominent example of a true VTOL airplane flying today is the AV-8B Harrier II, a military attack plane that uses rotating nozzles attached to its jet engine to direct the engine exhaust in the appropriate direction. Flown in the United States by the Marine Corps, as well as in Spain, Italy, India, and United Kingdom, where it was originally developed, the Harrier can take off vertically from smaller ships, or it can be flown to operating areas near the ground troops it supports in its ground-attack role.

Short Takeoff and Landing (STOL) airplanes are designed to be able to function on relatively short runways. Their designs usually employ wings and high-lift devices on the wings optimized for best performance during takeoff and landing, as distinguished from an airplane that has a wing optimized for high-speed cruise at high altitude. STOL airplanes are usually cargo airplanes, although some serve in a passenger-carrying capacity as well.

The space shuttle, flown by the National Aeronautics and Space Administration (NASA), is an aircraft unlike any other because it flies as a fixed-wing airplane within the atmosphere and as a spacecraft outside Earth’s atmosphere. When the space shuttle takes off, it flies like a rocket with wings, relying on the 3,175 metric tons of thrust generated by its solid-fuel rocket boosters and liquid-fueled main engines to power its way up, through, and out of the atmosphere. During landing, the shuttle becomes the world’s most sophisticated glider, landing without propulsion.

Airplanes can be grouped into a handful of major classes, such as commercial, military, and general-aviation airplanes, all of which fall under different government-mandated certification and operating rules.

Commercial aircraft are those used for profit making, usually by carrying cargo or passengers for hire (see Air Transport Industry). They are strictly regulated—in the United States, by the Federal Aviation Administration (FAA); in Canada, by Transport Canada; and in other countries, by other national aviation authorities.

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