Helicopter.

Helicopter. I INTRODUCTION Helicopter, aircraft that can take off and land vertically and can also hover motionless in the air. Helicopters are known as rotary-wing aircraft, as opposed to fixed-wing aircraft such as airplanes. A helicopter produces thrust by means of the blades of a main rotor as they rotate above the fuselage, or body, of the aircraft. As the blades rotate, an airflow is created over them, resulting in lift, which raises the helicopter skyward. The same rotor blades can be controlled to make the helicopter travel forward, backward, or sideways. Some helicopters can achieve forward flight at speeds of over 320 km/h (200 mph). Helicopters vary in design, but all must provide a means of counteracting the torque, or rotational force, produced on the shaft that turns the main rotor. Otherwise, the main rotor would rotate in one direction while the fuselage would rotate in the other. One common solution is to use a tail rotor. A small tail rotor is mounted vertically at the rear of the helicopter and produces a side thrust, preventing the fuselage from rotating. By increasing or decreasing the thrust produced by the tail rotor, the pilot can steer the helicopter to the left or right. Another solution is to use tandem rotors, that is, two main rotors turning in opposite directions. Since each rotor cancels the torque produced by the other, a tandem-rotor helicopter does not need a tail rotor. Helicopters have both advantages and disadvantages compared to fixed-wing aircraft. The helicopter's ability to maneuver in and out of hard-to-reach areas and to hover efficiently for long periods of time makes it valuable for operating in places where airplanes cannot land. Helicopters can perform important military tasks such as ferrying troops directly into combat areas or quickly transporting wounded soldiers to hospitals. However, helicopters use more fuel than airplanes and cannot fly as fast. This is because the helicopter rotor must produce both lift, which raises the craft into the sky, and thrust, which enables it to move about. In an airplane, the wings create lift and the engine produces thrust. Despite its poor cruising performance, the helicopter is the obvious choice for tasks where vertical flight is necessary. II MANEUVERING A HELICOPTER A pilot maneuvers a helicopter by changing the pitch, or angle, of the rotor blades as they rotate through the air. As the blades rotate, they create lift. When the pitch of a blade is increased, more lift is produced. By directing the lift, the helicopter can be propelled in different directions. Pilots use three different controls to maneuver helicopters: anti-torque pedals, a cyclic pitch stick, and a collective pitch stick. The pilot's feet control two anti-torque pedals, which are used to turn the helicopter to the left or right. The pedals control the pitch of the tail rotor blades, increasing or decreasing the thrust produced by that rotor. The tail rotor provides the sideways thrust needed to counteract the torque produced by the main rotor. When the thrust from the tail rotor balances the torque on the main rotor's shaft, the helicopter points forward. However, when the right pedal is pushed, the pitch of the tail rotor blades decreases and the thrust is reduced. The torque from the main rotor shaft then turns the nose of the helicopter to the right. When the left pedal is pushed, the tail rotor thrust increases, and the nose turns to the left. Tandem-rotor helicopters, which use two main rotors instead of a main rotor and a tail rotor, turn by tilting the rotors in different directions. The cyclic pitch stick moves a helicopter in a chosen direction by controlling the direction of the main rotor's thrust. This stick affects the pitch of the rotor blades as they cycle through a rotation. Increasing the pitch of a blade at a particular point during its rotation increases the amount of lift at that point. By selecting where along the rotor's path lift is increased, the pilot can tilt the helicopter forward, backward, or to either side. The cyclic pitch stick changes rotor pitch through a device called a swashplate. This device consists of two circular plates that surround the rotor shaft. The upper plate rotates with the shaft and the rotor blades and rests on the lower plate, which is controlled by the cyclic pitch stick. Moving the cyclic pitch stick forward, for example, tilts the lower plate, which in turn tilts the upper plate controlling the rotor blades. The swashplate lowers the pitch of the blades as they pass the right side of the helicopter, momentarily decreasing lift and causing the blades to flap downward. The swashplate at the same time increases the pitch of the blades as they pass the left side of the helicopter, increasing lift and causing the blades to flap upward. The front of the helicopter then points lower than the rear, and so the helicopter moves forward. Pushing the cyclic pitch stick in any direction will tip the rotor blades accordingly, allowing the helicopter to travel in any direction. When the stick is centered, the helicopter hovers in midair. The collective pitch stick is a lever that allows the helicopter to climb and descend vertically. It changes the pitch of all the main rotor blades equally, and performs much the same function as the pedals perform on the tail rotor. Pulling or pushing on the lever increases or decreases the thrust produced, varying the lift. Most collective pitch sticks also have a twist grip that changes the speed of the engine, in much the same way as the throttle of a motorcycle. Increasing rotor speed is another way to increase lift, but this is not normally done. The engine of a helicopter powers a transmission system that turns the shaft of the main rotor blades. The tail rotor is driven by a gearbox powered by the main rotor as it spins. Piston engines, similar to those in small fixed-wing airplanes, power most small helicopters. Large commercial helicopters, and almost all military helicopters, use turboshaft engines. A turboshaft engine is similar to a turbojet engine used to power a jet aircraft (see Jet Propulsion: Turbojet Engines). A turbojet engine is essentially a large cylindrical chamber open on both ends, with a rotating shaft inside. Fan blades on the rotating shaft draw in air from one end of the jet. Additional blades compress the air. Fuel is injected into the compressed air and then ignited, producing hot expanding gas that exits the other end of the jet. In a turbojet, the thrust from the exhaust gases propels the aircraft forward. In a helicopter turboshaft, the thrust powers a second shaft that turns the main rotor blades. Unlike jet airplanes, which use incoming air in forward flight to cool the engine, helicopters use cooling fans driven by the engine. In the event of a power failure, a helicopter can land safely by going into autorotation, or unpowered rotation of the rotor blades. The rotors will continue to turn because the helicopter's descent through the air produces an airflow over the blades and rotates them. When the rotor blades turn as the helicopter falls, they produce enough lift to allow the pilot to control the landing. Since the tail rotor gets power from the spinning of the main rotor, rather than directly from the engine itself, the tail rotor will continue to provide directional control. This safety feature allows the pilot to maintain a limited degree of control during an emergency landing. III HELICOPTER AERODYNAMICS Helicopters experience unique forces when in flight, and the design of the rotor blades helps overcome problems created by these forces. When a helicopter is hovering, the speed at which the tips of the rotor blades move is constant. However, as soon as the helicopter starts moving forward, the tip speed begins to change as the blades rotate around the fuselage. The tip speed increases as the blade advances toward the nose in the direction of flight. This is because the speed of the helicopter is added to the speed of the tip. As the tip passes the nose of the helicopter and begins retreating around, the tip speed decreases, because the speed of the helicopter is subtracted from the tip speed. Since lift increases with airflow speed, the advancing blade will produce more lift than the retreating blade. Unless adjustments are made for this difference in lift, the helicopter will roll over. To compensate for the unbalanced lift, Spanish aeronautical engineer Juan de la Cierva conceived the idea of the flexible, or articulated, rotor blade, in the 1920s. The articulated rotor blade is used today on all helicopters. Each main rotor blade is connected to the shaft by a flexible hinge. The hinges allow the rotor blades to rise and fall slightly as they rotate. This is called flapping, and it allows the advancing blade to rise slightly to avoid creating too much lift. The retreating blade, creating less lift, naturally flaps down so as to increase lift. Flapping allows the differences in lift caused by uneven rotor tip speed to cancel out, producing a stable ride. Many helicopters use mechanical hinges with lubricated bearings, but some use flexible straps made of a composite material in order to reduce the required maintenance. Helicopters require different amounts of lift and thrust at different times during flight, because the aerodynamic forces acting on them change during hovering and acceleration. The power needed to overcome the aerodynamic drag, or wind resistance, of a helicopter increases as speed increases. There is also drag on the blades themselves as they pass through the air. And there is the power needed to produce lift, which decreases as the helicopter moves faster. These separate forces combine to require more power for flight as a helicopter takes off and hovers, but less power as it flies forward. However, as speed increases, eventually more power is needed. For example, a helicopter may require 1,100 horsepower to hover. But at a forward speed of 110 km/h (70 mph) the required power may drop to approximately 600 horsepower, since the helicopter is moving rather than hovering. But as speed increases, so does drag, and so at a speed of around 240 km/h (150 mph), as much as 1,200 horsepower may be required. Helicopter speed in forward flight is also limited because of physical stresses on the blades at high speeds. As forward speeds approach 320 km/h (200 mph), the tip speed of the advancing rotor blade approaches the speed of sound, increasing vibration levels and required power. The Westland Lynx, a British military helicopter, holds the speed record for a helicopter. The Lynx achieved a speed of 401 km/h (249 mph) in 1986. IV USES FOR HELICOPTERS Helicopters come in many sizes and are designed for a variety of roles. One of the smallest and least expensive helicopters available is the two-seat Robinson R22, popular for flight training and aerial observation. The R22 has a gross weight of 620 kg (1,370 lb). Its two-bladed rotor has a diameter of 7.6 m (25.2 ft). The maximum forward speed of the R-22 is 180 km/h (112 mph), with a cruising speed of 153 km/h (95 mph). The largest helicopter is the 80-seat Russian Mi-26 military helicopter. Its eight-bladed rotor has a diameter of 32 m (105 ft) and supports a gross weight of 56,000 kg (123,450 lb). Its maximum speed is 295 km/h (183 mph) with a cruising speed of 254 km/h (158 mph). Because of its ability to hover and to take off and land vertically, the helicopter performs many functions that a fixed-wing aircraft cannot. For civilian use, these include emergency medical services, search and rescue missions, police services, support of offshore oil operations, news and traffic reporting, and business travel. Thousands of lives have been saved by helicopters, which have rescued people from the tops of burning buildings, plucked them from trees surrounded by ravaging flood waters, or lifted them from the decks of sinking ships. Helicopters also play an important role as military aircraft. Helicopters were first used in significant numbers during the Korean War (1950-1953), evacuating wounded soldiers from the battlefield to field hospitals. During the Vietnam War (1959-1975) helicopters also participated in combat missions. An attack helicopter can provide fire support to ground troops or serve as an antitank vehicle, capable of firing wire-guided or laser-guided missiles. Helicopters are frequently used to move troops quickly into and out of combat zones. Navies use helicopters equipped with sonar buoys to listen for enemy submarines. Other naval helicopters can rescue downed pilots from the sea or tow sleds that sweep for underwater mines. V HISTORY OF THE HELICOPTER Inventors and engineers perfected the design of the helicopter gradually, over many years. Original inspiration came from objects like an ancient Chinese top, which rose upward when spun rapidly. One of the earliest inventors to design a helicopter was Leonardo da Vinci. In one of his notebooks from 1480, he illustrated a model helicopter driven by a clockwork motor. His notes imply that the model flew, but, from his sketch, an antitorque device is not apparent. One of the first mechanical devices actually to hover was the Bréguet-Richet Gyroplane No. 1, designed by French aircraft pioneer Louis Charles Bréguet. It first flew in France on September 29, 1907, piloted by one of Bréguet's engineers. Lifted by four rotors 8 m (26 ft) in diameter, this fragile craft hovered about 0.6m (2 ft) off the ground for a minute while being restrained by four men holding on to the frame. Without a control system, it was far from being a practical helicopter, but it demonstrated that it was possible to lift a person vertically by means of powered rotors. On November 13, 1907, Frenchman Paul Cornu became the first person in history to rise vertically in powered flight, completely unrestrained from any support. The Cornu helicopter used two rotors attached to each end of a skeletal frame and was powered by a 24-horsepower engine. Although Cornu achieved a historic first, the controls of his machine were completely inadequate, and the craft never developed into a practical helicopter. Spanish engineer Juan de la Cierva paved the way for the development of a successful helicopter, but never built a helicopter himself. Cierva developed the autogiro, which resembles the helicopter but which uses an unpowered rotor. The rotor autorotates, or autogyrates, as the autogiro is pulled through the air by a separate propeller. The turning rotor provides lift much like an aircraft wing. In January 1923, Cierva successfully flew his C.4 autogiro, which incorporated articulated rotor blades. This allowed the blades to flap freely up and down in response to the unsteady aerodynamic forces that arise in forward flight. The articulated rotor was the technical breakthrough that led others to develop the successful helicopter. Cierva might have eventually done so himself, but he died in an airplane crash in December 1936. Germany made rapid strides in helicopter development in the 1930s and 1940s. The FA-61, designed by Heinrich Focke, flew for the first time on June 26, 1936. The FA-61 was the first practical design for a maneuverable helicopter. In 1937, as a propaganda stunt for the Nazi regime, the renowned female pilot Hanna Reitsch flew the FA-61 inside the city of Berlin's Deutschlandhalle sports arena. Another German helicopter, the FL-282 Kolibri, was used by the German navy during World War II (1939-1945). It could fly at 140 km/h (90 mph) and reach an altitude of 4,000 m (13,000 ft) with a payload of 360 kg (800 lb). It was the first helicopter design produced in quantity, but only a few became operational before the war ended. Igor Sikorsky, a Russian-born American aeronautical engineer, flew the first successful single main rotor helicopter, the VS-300, in 1939. He flew the final version of his VS-300 helicopter in 1941. Unlike previous helicopter designs, the VS-300 was the first helicopter to use a tail rotor to counteract the torque of the main rotor. This represented a major accomplishment that has been copied by the majority of helicopter designs built since. Sikorsky's research and development of the VS-300 led to the R4, the first American helicopter built in large quantities. During the 1990s, aeronautical engineers applied radar-evading stealth technology to the design of certain military helicopters (see Stealth Aircraft). The first helicopter to incorporate this technology was the U.S. Army's RAH-66 Comanche, developed jointly by the Boeing Company and Sikorsky Aircraft Corporation. The fuselage is shaped to reduce the helicopter's visibility to enemy radar, and weapons are carried internally to further reduce the helicopter's detection by radar. The Comanche is also designed to radiate less heat than other helicopters in order to evade infrared (heat-seeing) detectors. See also Aviation; Military Aviation. Contributed By: Barnes W. McCormick Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.