AIRCRAFT Boeing 747-400.

This is a wide-bod­ied long-haul aircraft. An aircraft is any machine capable of atmospheric

Flight.

Aircraft fall into two broad categories:

Heavier than air

Heavier than air aerodynes, including autogyros, helicopters and variants, and conventional fixed-wing aircraft (airplanes or aeroplanes). Fixed-wing aircraft generally use an internal-combustion engine in the form of a piston engine (with a propeller) |or a turbine engine Get or turboprop), to provide thrust that moves the craft forward through the air. The movement of air over the airfoil produces lift that causes the aircraft to fly. Exceptions include gliders which have no engines and gain their thrust, initially, from winches or gliders and then from gravity and thermal currents. For a glider to maintain its forward speed it must descend in relation to the air (but not necessarily in relation to the ground). Helicopters and autogyros use a spinning rotor (a rotary wing) to provide lift helicopters also use the rotor to provide thrust. The abbreviation VTOL is applied to aircraft that can take off and land vertically. STOL stands for Short Take Off and Landing.

Lighter than air aerostats: hot air balloons and airships. Aerostats use buoyancy to float in the air in much the same manner as ships float on the water. In particular, these aircraft use a relatively low density gas such as helium, hydrogen or heated air. The distinction between a balloon and an airship is that an airship has some means of controlling both its forward motion and steering itself, while balloons are carried along with the wind.

In heavier-than-air aircraft, there are two ways to produce lift: aerodynamic lift and engine lift. In the case of aerodynamic lift, the aircraft is kept in the air by wings or rotors. With engine lift, the aircraft defeats gravity by use of vertical thrust. Examples of engine lift aircraft are rockets, and VTOL aircraft such as the Hawker-Siddeley (HS) Harrier.

In a "conventional" configuration, the lift surfaces are placed in front of a control surface or tailplane. The other configuration is the canard where small horizontal con­trol surfaces are placed forward of the wings, near the nose of the aircraft. Canards are becoming more common as supersonic aerodynamics grows more mature and because the forward surface contributes lift during straight-and-level flight.

A second category of aerodynamically lifted aircraft are the rotary-wing aircraft. Here, the lift is provided by rotating aerofoils or rotors. The best-known examples are the helicopter, the autogyro and the tiltrotor aircraft (such as the V-22 Osprey).

A further category might encompass the wing-in-ground-effect types, for example the Russian ekranoplan also nicknamed the "Caspian Sea Monster" and hovercraft; most of the latter employinga skirt and achieving limited ground or water clearance to reduce friction and achieve speeds above those achieved by boats of similar weight.

A recent innovation is a completely new class of aircraft, the fan wing. This uses a fixed wing with a forced airflow produced by cylindrical fans mounted above. It is (2005) in development in the United Kingdom.

Until World War II, the internal combustion piston engine was virtually the only type of propulsion used for powered aircraft. The piston engine is still used in the majority of aircraft produced, since it is efficient at the lower altitudes used by small aircraft but the radial engine (with the cylinders arranged in a circle around the crankshaft) has largely given way to the horizontally-opposed engine (with the cylinders lined up on two sides of the crankshaft). Water cooled V engines, as used in automobiles, were common in high speed aircraft, until they were replaced by jet and turbine power. Piston engine typically operate using avgas or regular gasoline, though some new ones are being designed to operate on diesel or jet fuel. Piston engines normally become less efficient above 7,000-8,000 ft (2100-2400 m) above sea level because there is less oxygen available for combustion; to solve that problem, some piston engines have mechanically powered compressors (blowers) or turbine-powered turbochargers or turbonormalizers that compress the air before feeding it into the engine; these piston engines can often operate efficiently at 20,000 ft (6100 m) above sea level or higher, altitudes that require the use of supplemental

oxygen or cabin pressurization. Ppressurised aircraft,however, are more likely to use the turbine

engine, since it is naturally efficient at higher altitudes and can operate above 40,000 ft. Helicopters also typically use turbine engines. In addition to turbine engines like the turboprop and turbojet, other types of high-altitude, high- performance engines have included the ramjet and the pulse jet. Rocket. aircraft have occasionally been experimented with. They are restricted to rather spe­cialised niches, such as spaceflight, Where no oxygen is available for combustion (rockets carry their own oxygen.