Gliders, hang-gliders, and ultralights

MODERN GLIDERS ARE AMONG the most graceful and aerodynamically efficient of all aircraft. Unpowered but with a large wingspan (up to about 82 ft, or 25 m), gliders use currents of hot, rising air (thermals) to stay aloft, and a rudder, elevators, and ailerons for control. Modern gliders have achieved flights of more than 900 miles (1,450 km) and altitudes above 49,000 ft (15,000 m). Hang-gliders consist of a simple frame across which rigid or flexible material is stretched to form the wings. The pilot is suspended below the wings in a harness or body bag and, gripping a triangular
A-frame, steers by shifting weight from side to side. Like gliders, hang-gliders rely on thermals for lift. Ultralights are basically powered hang-gliders. A small engine and an open fiberglass car (trike),
which can hold a crew of two, are suspended beneath a stronger version of a hang-glider frame; the frame may have rigid or flexible wings. Ultralight pilots, like hang-glider pilots, steer by shifting their weight against an A-frame. Ultralights can reach speeds of up to 100 mph (160 kph).

Light aircraft

LIGHT AIRCRAFT, SUCH AS THE ARV SUPER 2 shown here, are small, lightweight, and of simple construction. More than a million have been built since World War I, mainly for recreational use by private owners. Virtually all light aircraft have piston engines, most of which are air-cooled, although some are liquid-cooled. Open cockpits, almost universal in the 1920s, have today been replaced by enclosed cabins. The cabins of high-wing aircraft have one or two doors, whereas those of low-wing aircraft usually have a sliding or hinged canopy. Most modern light aircraft are made of aluminum alloy, although some are made of wood or of fiberreinforced materials. Light aircraft today also usually have navigational instruments, an electrical system, cabin heating, wheel brakes, and a two-way radio.

Helicopters

HELICOPTERS USE ROTATING BLADES for lift, propulsion, and steering. The first machine to achieve sustained, controlled flight using rotating blades was the autogiro built in the 1920s by the Spaniard Juan de la Cierva. His machine had unpowered blades above the fuselage that relied on the flow of air to rotate them and provide lift as the autogiro was driven forward by a conventional propeller. Then, in 1939, the Russian-born American Igor Sikorsky produced his VS-300, the forerunner of modern helicopters. Its engine-driven blades provided lift, propulsion, and steering. It could take off vertically, hover, and fly in any direction, and had a tail rotor to prevent the helicopter
body from spinning. The introduction of gas turbine jet engines to helicopters in 1955 produced quieter, safer, and more powerful machines. Because of their versatility in flight, helicopters are today used for many purposes, including crop spraying, traffic surveillance, and transporting crews to deep-sea oil rigs, as well as acting as gunships, air ambulances, and air taxis.

Modern military aircraft

MODERN MILITARY AIRCRAFT ARE AMONG THE MOST SOPHISTICATED and expensive
products of the 21st century. Fighters need computer-operated controls for maneuverability, powerful engines, and effective air-to-air weapons. Most modern fighters also have guided missiles, radar, and passive, infrared sensors. These developments enable today’s fighters to engage in combat with adversaries that are outside visual range. Bombers carry a large weapon load and enough fuel for long-range flights. A few military aircraft, such as the Tornado and the F-14 Tomcat, have variable-sweep (“swing”) wings. During takeoff and landing their wings are fully extended, but for high-speed
flight and low-level attacks the wings are pivoted fully back. A recent development is the “stealth” bomber, which is designed to absorb or deflect enemy radar in order to remain undetected. Earlier
bombers, such as the Tornado, use terrain-following radars to fly so close to the ground that they avoid enemy radar detection.

Jet engines

JET ENGINES ARE USED BY MOST MILITARY and heavy aircraft, and by many helicopters. The simplest type of jet engine, or gas turbine, is the turbojet. It works by continuously burning a mixture of fuel and air in a combustion chamber to produce a jet of hot exhaust gas that is expelled through a nozzle to produce thrust. The hot gas also spins turbine blades, which, in turn, spin the blades of an air compressor; the compressor forces air into the combustion chamber. Many of the fastest aircraft use turbojets, with additional booster units called afterburners, but their use is restricted by their high noise emission. Most jetliners use turbofan jet engines, which are quieter. An enormous fan, driven by a lowpressure turbine, feeds some air into the compressor but feeds most of it through bypass ducts to join the exhaust jetstream in the tail cone. The bypass stream produces
most of the thrust. Many smaller, propellerdriven aircraft use turboprop jet engines, in which the engine powers a propeller.

Supersonic jetliners

SUPERSONIC AIRCRAFT FLY FASTER than the speed of sound (Mach 1). There are many supersonic military aircraft, but only two supersonic passenger-carrying aircraft (also called SSTs, or
supersonic transports) have been produced: the Russian Tu-144, and the Concorde, produced jointly by Britain and France. The Tu-144 was withdrawn in 1978, after only seven months in service. The
Concorde remained in service from 1976 until 2003, with a break for modifications from July 2000 until October 2001. Its features included a droop nose, which lowered during takeoff and landing to aid visibility from the cockpit; the pumping of fuel between forward and aft trim tanks helped stabilize the aircraft. The Concorde had a narrow fuselage and short span wings to reduce drag during supersonic flight. Its noisy turbojet engines with afterburners enabled it to carry 100 passengers at a cruising speed of Mach 2 at 50,000-60,000 ft (15,000-18,000 m). Once an aircraft is flying faster than Mach 1, it produces a continuous air-pressure wave, which is heard as a “sonic boom.”