The development of the airplane and other heavier-than-air craft has had the most far-reaching effects of any 20th-century invention. Although many scientific disciplines are involved in the rapid advances in aviation technology, none is as important as the aircraft itself. In this article the anatomy of an airplane is examined along with recent developments in such unconventional aircraft as helicopters. Basic principles of flight can be found in \Taerodynamics\t; the history of airplane development and a description of contemporary air transportation are the subjects of \Taviation\t and \Tairport\t. Lighter-than-air craft are discussed in \Tairship\t. The first powered, controllable aircraft, Orville and Wilbur \TWright\t's flying machine, demonstrated in its structure the same basic principles of flight as do today's high-flying jets. The wings, or airfoils, of the original 1903 Wright Flyer resembled a box kite. A small pair of wings, called a canard, was located forward of the main wings and provided control about the pitch axis, allowing the aircraft to climb or descend. The canard performed the same function as the elevators that are attached to the horizontal stabilizers on most modern aircraft (and the device is increasingly used again today on small, experimental aircraft). Controlled, coordinated turns in the air were achieved through a method called "wing warping," which deflected the rear, or trailing, edges of the wing and rudder. With no cockpit, the pilot lay prone over the wing in a cradle arrangement and moved his body from side to side to actuate the controls that effected wing warping and changed the plane's direction. The Wright Flyer was an extremely difficult aircraft to fly because it was statically unstable: it could not "fly by itself" but had to be constantly controlled by the pilot. European inventors believed that an aircraft should be inherently stable, and they soon improved upon the Wright brothers' design by developing dynamically stable and controllable aircraft that were safer and easier to fly. The concept of static stability has carried over to virtually all aircraft designs--although recent developments in fighter aircraft, such as the General Dynamics F-16, show that aerodynamically unstable aircraft have some advantages in maneuverability. The success of any one of the thousands of different craft made since the Wright brothers' first flying machine depended on the quality of research, design, engineering, and manufacturing used to produce it. By the time World War II began, the aviation industry had accumulated enough experience in aerodynamics, materials, and structures to ensure uniformity in aircraft development. As a result, most modern aircraft exhibit many structural similarities. They are almost always monoplanes--single- rather than double-winged. They are made of metal, are powered by one to four jet or reciprocating engines, and are supported on the ground by retractable landing gear. ANATOMY OF THE AIRPLANE The main structural components of modern aircraft are the fuselage; wings; empennage, or tail surfaces; power plant; and landing gear, or undercarriage. Fuselage The fuselage is the main body structure to which the wings, tail, landing gear, and power plants are attached. It contains the cockpit or flight deck, passenger compartment, cargo compartment, and--in the case of fighter aircraft--the engines and fuel tanks. Wings The wing is the most important lift-producing element of an aircraft. Wing designs vary, depending on the aircraft type and purpose. Propeller-driven aircraft normally have an all-metal straight wing with a thick camber, or curvature. Jet transports have swept-back wings of medium camber that lower aerodynamic drag and improve performance at high airspeeds. Both straight and swept-wing aircraft normally have ailerons attached to the outermost trailing edges of the wing. These ailerons raise and lower in opposition to one another, to increase or decrease lift on their respective wing in order to facilitate turning the aircraft. The wing also has flaps along the trailing edge, inboard of the ailerons. Flaps increase aerodynamic lift and drag and are used during takeoff and landing to increase lift at low speeds. Modern swept-wing transport aircraft also have high lift devices called leading-edge slats, which extend in conjunction with the flaps to further increase the lifting capability of the wing. An aircraft flies when the lift, or upward force generated by the wing, increases to a value larger than the aircraft's total weight. The most critical element in a wing's ability to produce lift is its cross-sectional shape. Early aerodynamic research on kites and gliders indicated that a flat plate would produce lift, but even more lift could be produced if the plate was inclined slightly into the wind. If the leading edge of the flat plate was rounded and the trailing edge tapered to streamline the wing, drag could be reduced. By increasing the camber of the top surface of the wing, while flattening the lower surface, lift could be dramatically improved. Wind tunnels, which are used extensively in airfoil research, have facilitated the compilation of a large amount of data on airfoil types and design. Tail Surfaces The tail, or empennage, provides stability and control for the aircraft and is mounted on the aft portion of the fuselage. It consists of two main parts: the vertical stabilizer, or fin, to which the rudder is attached; and the horizontal stabilizer, to which the elevator is connected. The rudder is used in conjunction with the ailerons to make coordinated turns, while the elevator is used to climb or descend. The horizontal stabilizer is sometimes mounted high on the vertical stabilizer, as is the case with the DC-9 and Boeing 727. The new Boeing 757 and 767 have the horizontal stabilizer attached to the rear section of the fuselage. Propulsion Systems The many aircraft propulsion systems include those which drive a \Tpropeller\t, primarily reciprocating and turbine (turboprop) engines; and propellerless systems that use the energy of rapidly expanding gases as a propulsive force (see \Tjet propulsion\t). The turbojet and the turbofan--a turbojet modification--are the most widely used commercial jet engines, and the reciprocating engine is still used extensively in light general aviation aircraft. Jet engines are normally attached to the wing or aft fuselage on pylons, but occasionally they are imbedded in the wing root next to the fuselage. On many fighter-bomber aircraft they are mounted in the fuselage in order to reduce aerodynamic drag and improve performance. Engine-propeller combinations on single-engine aircraft are usually located in the nose, or forward-most, section of the fuselage and pull the aircraft through the air. When two or more engine-propeller combinations are used, they are mounted on the wing, but forward of the leading edge. "Pusher"-type aircraft have the engine mounted in the rear section of the fuselage. Several aircraft designs utilize two engines, one pushing and one pulling. With respect to performance, turbojet engines operate most efficiently at high altitudes (above 7,600 m/25,000 ft); turboprops at mid altitudes (4,500-7,600 m/15-25,000 ft); and reciprocating engines at low altitudes (sea level to 4,500 m/15,000 ft). Landing Gear Fixed gear consists of a simple design of struts, wheels, and brakes that is not retractable into the wings or fuselage. It is usually found on light aircraft of simple design. The static nature of fixed gear reduces the probability of landing gear problems, but it creates increased drag on a plane in flight. Retractable gear is used on more complex aircraft. Since it reduces drag, it increases range significantly. Flying Controls The relatively simple controls on a light, general aviation airplane govern the speed of the craft and its direction, both on the ground and in the air. The control wheel at which the pilot sits may be pushed forward or pulled back to move the tail elevators; pushing forward causes the plane to nose down. The control wheel also alters the position of the ailerons. The movements of the rudder are controlled from foot pedals in front of the pilot's seat; the pedals activate the wheel brakes when the plane is on the ground. Wing flap controls are usually powered, either hydraulically or by electric motors. Engine thrust, and thus airplane speed, is controlled by a throttle. UNCONVENTIONAL AIRCRAFT Many of today's new craft are heralded as innovations, although some are old, reliable concepts wrapped in new packaging. Hang gliders, for example, operate on the same principles as the early gliders developed by such 19th-century aerodynamicists as Sir George Caley and Otto \TLilienthal\t. The wood frames and heavy fabric coverings of the original gliders have been replaced by lightweight aluminum, fiberglass, and synthetic fibers. Design of the modern hang glider was evolved from research at the National Aeronautics Space Administration (\TNASA\t) into spacecraft reentry. The \Tglider\t, or sailplane, has the same basic structure as other "heavier than air" machines but does not use a power plant. Instead, it is towed into the air by another aircraft and released at an altitude that permits it to soar along the "thermals," the columns of warm air that help keep it aloft. Gliders normally have a thin fuselage and very long wings with a narrow width, or chord. Many of the aerodynamic characteristics of the glider have been incorporated into high altitude reconnaissance aircraft, such as the U-2 and the RB-57 operated by the U.S. Air Force. Research into glider aerodynamics continues today at \TNASA\t and military test-flying programs. \TSTOL\t Aircraft Short takeoff and landing aircraft (see \TSTOL\t) have gained popularity in recent years as air transportation has changed its emphasis from speed to efficiency. De Havilland Aircraft in Britain has developed the DHC-7, a four-engine \TSTOL\t aircraft that requires only a very short runway for takeoff and landing. It utilizes advanced light-weight structural materials, new improved engines, and high lift devices to give it its \TSTOL\t capability. An economical airplane to operate, it is used by many small commuter and local service airlines around the world. \TVTOL\t and V-STOL Aircraft Vertical takeoff and landing aircraft (see \TVTOL\t), which include \Lhelicopter\ls, are still being developed for commercial and military use. For many years the military has utilized helicopters with great success on a variety of missions: medical evacuation, supply, troop transport, reconnaissance, attack. Industrial helicopters transport materials and personnel to and from areas where no landing strip is available to accommodate a fixed-wing aircraft. Some aircraft combine vertical and short takeoff and landing (V/STOL) capabilities. The propellers, the craft's power plants, or the wings themselves can tilt upward for takeoff and landing, but reverse to a horizontal position for flight. The Harrier, a British fighter plane, achieves vertical takeoff by rotating the exhaust nozzles of its jet engines downward in what is called a deflected thrust. Airships, or Blimps The high price of aircraft fuel has inspired a new interest in lighter-than-air \Lairship\ls, which use relatively little fuel and offer the promise of great economy. Some analysts feel that 10 to 15 small, effective airships could be built for the price of one jumbo jet. One of these craft might be capable of airlifting 1,000 tons of cargo or a passenger load greater than that of a jumbo jet, at speeds of up to 200 miles per hour. It could operate at one-third the cost of a passenger liner and would not require a large airport for takeoff and landings. In addition, the airship is a quiet craft, and--unlike the jet--its exhausts are comparatively nonpolluting. Supersonic and Hypersonic Planes Supersonic transports, or SSTs, aircraft that can fly faster than the speed of sound, have been used by the military for many years. Commercial supersonic flight has been limited to the \TConcorde\t, built by the British and French in 1976, which has proved to be a commercial failure in large part because of the vast amounts of fuel it consumes. The Russians have also built an \TSST\t, the TU-144, but it was not a successful design and was withdrawn from service after several disastrous crashes. Occasionally, aviation futurists speak of developing a hypersonic transport that would travel beyond the stratosphere at speeds in excess of 8,000 km/h (5,000 mph); given the price of fuel, however, it is doubtful that this type of aircraft will be developed in the near future. Ultralight Aircraft Small aircraft made from ultralight, superstrong materials are increasing the range of present-day aeronautical technology. The current interest in ultralight planes began with the surge in popularity of the hang glider; a hang glider equipped with a small golf-cart engine and a propeller (1976) was the first member of this new generation of aircraft. The enthusiasm for ultralights came originally from a few engineers who manufactured make-it-yourself kits for home builders, and from the kit buyers who found that they could make their own planes for a cost as low as a few thousand dollars. Some of the new ultralights resemble the earliest experimental aircraft. Others incorporate the most advanced aerodynamic shapes--achieved by computer modeling--and the newest materials: carbon fiber, kevlar, fiberglass-and-epoxy laminates, plastic foams. Design innovations include the rebirth of the Wright brothers' canard, which now takes the form of a second wing and acts to maintain lift; "winglets," vertical fins on the wing ends that increase wing efficiency; and rear-mounted, or tail, propellers that provide greater speed and stability. Ultralights range in cruising speed from 50 to upward of 320 km/h (30 to 200 mph). Some can reach altitudes of 4,300 m (14,000 feet). World records achieved by ultralights include the 1986 triumph of the Voyager, whose hollow plastic body holds four tons of fuel. With two pilots, the Voyager flew around the world nonstop in nine days. Ultralight airplanes also figure in experiments with human powered-flight (see \Tflight, human-powered\t), first successfully achieved in 1977 by the 35-kg (77-lb) Gossamer Condor. AIRCRAFT NAVIGATION Map reading and "dead reckoning"--manual speed and distance calculations--were the principal methods of navigation during the pioneer days of aviation, when a sudden change in weather combined with inadequate charts or maps could result in a forced landing or crash. One of the first reliable radio navigation aids was the Non-Directional Beacon, a radio signal that could be picked up by a cockpit device and used as a checkpoint to verify that the aircraft was on course. VOR (Very High Frequency Omni Directional Range) and TACAN (Tactical Air Navigation) were developed by the military and combined into the present-day VORTAC system that provides the majority of checkpoints that mark today's airway system, although increased airway congestion will necessitate a modernization of these facilities. Among the new navigation technologies that are or will soon be used by commercial aviation, Area-Navigation, or R-Nav, is an on-plane system using computers, dopplers, and inertial navigation to produce a self-contained navigational system that needs no ground-based signals as reference points. Omega, developed originally for ships, uses Very Low Frequency radio beam sent out by eight ground-based stations located across the globe. The signals can be picked up by any plane equipped with Omega receivers. The Global Positioning Satellite system uses a number of earth-orbiting satellites to provide two- or three-dimensional fixes (including altitude). Although confined to military planes at present, GPS will be operational for civilian use in the near future and will allow pinpoint navigational accuracy at any location in the sky. All of these new systems permit the pilot to do his or her own navigation, so that navigation officers are no longer needed for most commercial and military flights. Fifty percent of all fatal air accidents occur during the approach and landing phase of flight, and it is imperative that the most precise navigational aids be utilized during this critical period. The ILS, or Instrument Landing System, is the best navigational aid available. An airport ILS transmits several electronic signals to the pilot of an approaching plane. These define the approach course and glide slope--the angle of descent that must be used. The MLS, or Microwave Landing System now being installed at many airports, is similar to the ILS but allows curved approaches to be flown to the runway, alleviating the noise and congestion problems that plague the modern jet airport. \Tradar\t is used by an airport's Air Traffic Control to separate aircraft and vector them to the airport for landing. Aircraft may utilize radar as a navigation aid, and new radars now being used on some commercial aircraft also have the capability of detecting turbulence. FLIGHT HAZARDS Turbulence is an atmospheric problem with which aircraft have to deal at one time or another. Mild turbulence is common and usually produces little more than the discomfort of a bumpy flight. Violent turbulence, however, is the cause of a large percentage of airplane crashes. It presents the further hazard of sometimes occurring in the invisible form known as clear air turbulence. It is this form of turbulence that some newer radar systems are designed to identify, so that an aircraft can maneuver to avoid it if warned in time. Clear air turbulence, which involves violent updrafts and downdrafts of air, may be produced at lower atmospheric levels in a number of ways from variations in atmospheric temperature. It also takes place high in the atmosphere in the neighborhood of jet streams. \Twind\t shear, one form of turbulence, is a rapid change in wind speed or direction or both. The change in direction is more or less at right angles to the direction of the prevailing wind, so that an aircraft caught in such motions experiences a tearing effect. Other kinds of turbulence are observed in the vicinity of high mountains, where powerful rising or falling currents of air are the rule. One particular form such turbulence may take downwind from high ranges is that of an eddy called a rotor. Likened to an invisible tornado lying on its side, it has caused planes caught in the eddy to flip sideways and crash. Under ordinary circumstances pilots are usually able to avoid such obvious visible sites of violent weather as massive thunderclouds, and they may also fly above other storms from which rain or snow is being precipitated. Clouds and heavy precipitation are zones of violent turbulence, of course, and in addition they greatly reduce visibility. Icing is another problem of flight through precipitation at low temperatures, particularly for propeller-driven aircraft. Ice increases drag and can cause an engine to shut down. Commercial jet aircraft usually can climb quickly enough to avoid severe icing, but their engines are otherwise also prone to such effects. AIRCRAFT INSTRUMENTATION Power, performance, and navigational instruments are used by the pilot to evaluate the well-being of the aircraft and to check its course. The power instruments check engine performance, power output, and airspeed. The \Ttachometer\t is the basic power-indicating instrument. Jet aircraft use engine-pressure ratio gauges to determine thrust output. On piston-powered aircraft manifold pressure gauges measure the pressure under which the fuel-air mixture is supplied to the engine. Turboprop aircraft measure power output on a torque gauge that monitors the power available at the prop shaft. All aircraft have instruments that check oil temperature and pressure and fuel flow. The electrical and hydraulic systems are also monitored with gauges and caution lights. Performance instruments show how well and at what altitude the aircraft is flying. They include the artificial horizon, a gyroscope-mounted device that shows the pilot the plane's relation to the real horizon; the altimeter and vertical velocity gauges, which indicate height above mean sea level and the rate of climb or descent; and the airspeed indicator. A turn-and-bank indicator and an accelerometer, or G meter, keep the pilot informed as to the direction of turn and the loading, or strain, on the aircraft. (On some military aircraft, accelerometers are also used in inertial guidance systems to detect flight-course deviations.) The location of all these instruments in the cockpit is vital to the pilot's ability to assess craft conditions quickly and accurately. Instrument design and location is a science in itself and is constantly undergoing study. The "electronic cockpit" or EFIS (Electronic Flight Information System), a computerized instrumentation array that includes the presentation of flight information via television screens, is revolutionizing the way flight instruments are used. Its introduction has caused some difficulty because it requires a change in habit patterns; in the long run, however, it will simplify cockpit design and enable the pilot to make better decisions in critical situations involving mechanical failure, weather avoidance, or navigational problems. The new Boeing 757, the 767, and the European Airbus A-310 are all being equipped with the EFIS. J. Michael Jobanek Bibliography: Anderson, John D., Jr., Introduction to Flight: Its Engineering and History (1978); Bowers, Peter M., Unconventional Aircraft (1984); Coombs, Charles, Ultralights: The Flying Featherweights (1984); Jerram, M., Classic Aircraft (1981); Markowski, Michael A., Ultralight Aircraft (1981); Montgomery, M. R., A Field Guide To Airplanes of North America (1984); Nayler, J. L., Aviation: Its Technical Development; (1965); Stinton, Darrol, The Anatomy of the Aeroplane (1980); Swanborough, Gordon, Civil Aircraft of the World, rev. ed. (1980); Taylor, J. W. R., ed., Jane's All the World's Aircraft (annual).