Helicopters are classified as rotary wing aircraft, and their rotary wing is commonly referred to as the main rotor or simply the rotor. Unlike the more common fixed wing aircraft such as a sport biplane or an airliner, the helicopter is capable of direct vertical take-off and landing; it can also hover in a fixed position. These features render it ideal for use where space is limited or where the ability to hover over a precise area is necessary. Currently, helicopters are used to dust crops, apply pesticide, access remote areas for environmental work, deliver supplies to workers on remote maritime oil rigs, take photographs, film movies, rescue people trapped in inaccessible spots, transport accident victims, and put out fires. Moreover, they have numerous intelligence and military applications.
Numerous individuals have contributed to the conception and development of the helicopter. The idea appears to have been bionic in origin, meaning that it derived from an attempt to adapt a natural phenomena—in this case, the whirling, bifurcated fruit of the maple tree—to a mechanical design. Early efforts to imitate maple pods produced the whirligig, a children's toy popular in China as well as in medieval Europe. During the fifteenth century, Leonardo da Vinci, the renowned Italian painter, sculptor, architect, and engineer, sketched a flying machine that may have been based on the whirligig. The next surviving sketch of a helicopter dates from the early nineteenth century, when British scientist Sir George Cayley drew a twin-rotor aircraft in his notebook. During the early twentieth century, Frenchman Paul Cornu managed to lift himself off the ground for a few seconds in an early helicopter. However, Cornu was constrained by the same problems that would continue to plague all early designers for several decades: no one had yet devised an engine that could generate enough vertical thrust to lift both the helicopter and any significant load (including passengers) off the ground.
Igor Sikorsky, a Russian engineer, built his first helicopter in 1909. When neither this prototype nor its 1910 successor succeeded, Sikorsky decided that he could not build a helicopter without more sophisticated materials and money, so he transferred his attention to aircraft. During World War I, Hungarian engineer Theodore von Karman constructed a helicopter that, when tethered, was able to hover for extended periods. Several years later, Spaniard Juan de la Cierva developed a machine he called an autogiro in response to the tendency of conventional airplanes to lose engine power and crash while landing. If he could design an aircraft in which lift and thrust (forward speed) were separate functions, Cierva speculated, he could circumvent this problem. The autogiro he subsequently invented incorporated features of both the helicopter and the airplane, although it resembled the latter more. The autogiro had a rotor that functioned something like a windmill. Once set in motion by taxiing on the ground, the rotor could generate supplemental lift; however, the autogiro was powered primarily by a conventional airplane engine. To avoid landing problems, the engine could be disconnected and the autogiro brought gently to rest by the rotor, which would gradually cease spinning as the machine reached the ground. Popular during the 1920s and 1930s, autogiros ceased to be produced after the refinement of the conventional helicopter.
The helicopter was eventually perfected by Igor Sikorsky. Advances in aerodynamic theory and building materials had been made since Sikorsky's initial endeavor, and, in 1939, he lifted off the ground in his first operational helicopter. Two years later, an improved design enabled him to remain aloft for an hour and a half, setting a world record for sustained helicopter flight.
The helicopter was put to military use almost immediately after its introduction. While it was not utilized extensively during World War II, the jungle terrain of both Korea and Vietnam prompted the helicopter's widespread use during both of those wars, and technological refinements made it a valuable tool during the Persian Gulf War as well. In recent years, however, private industry has probably accounted for the greatest increase in helicopter use, as many companies have begun to transport their executives via helicopter. In addition, helicopter shuttle services have proliferated, particularly along the urban corridor of the American Northeast. Still, among civilians the helicopter remains best known for its medical, rescue, and relief uses.
A helicopter's power comes from either a piston engine or a gas turbine (recently, the latter has predominated), which moves the rotor shaft, causing the rotor to turn. While a standard plane generates thrust by pushing air behind its wing as it moves forward, the helicopter's rotor achieves lift by pushing the air beneath it downward as it spins. Lift is proportional to the change in the air's momentum (its mass times its velocity): the greater the momentum, the greater the lift.
Helicopter rotor systems consist of between two and six blades attached to a central hub. Usually long and narrow, the blades turn relatively slowly, because this minimizes the amount of power necessary to achieve and maintain lift, and also because it makes controlling the vehicle easier. While light-weight, general-purpose helicopters often have a two-bladed main rotor, heavier craft may use a four-blade design or two separate main rotors to accommodate heavy loads.
To steer a helicopter, the pilot must adjust the pitch of the blades, which can be set three ways. In the collective system, the pitch of all the blades attached to the rotor is identical; in the cyclic system, the pitch of each blade is designed to fluctuate as the rotor revolves, and the third system uses a combination of the first two. To move the helicopter in any direction, the pilot moves the lever that adjusts collective pitch and/or the stick that adjusts cyclic pitch; it may also be necessary to increase or reduce speed.
Unlike airplanes, which are designed to minimize bulk and protuberances that would weigh the craft down and impede airflow around it, helicopters have unavoidably high drag. Thus, designers have not utilized the sort of retractable landing gear familiar to people who have watched planes taking off or landing—the aerodynamic gains of such a system would be proportionally insignificant for a helicopter. In general, helicopter landing gear is much simpler than that of airplanes. Whereas the latter require long runways on which to reduce forward velocity, helicopters have to reduce only vertical lift, which they can do by hovering prior to landing. Thus, they don't even require shock absorbers: their landing gear usually comprises only wheels or skids, or both.
One problem associated with helicopter rotor blades occurs because airflow along the length of each blade differs widely. This means that lift and drag fluctuate for each blade throughout the rotational cycle, thereby exerting an unsteadying influence upon the helicopter. A related problem occurs because, as the helicopter moves forward, the lift beneath the blades that enter the airstream first is high, but that beneath the blades on the opposite side of the rotor is low. The net effect of these problems is to destabilize the helicopter. Typically, the means of compensating for these unpredictable variations in lift and drag is to manufacture flexible blades connected to the rotor by a hinge. This design allows each blade to shift up or down, adjusting to changes in lift and drag.
Torque, another problem associated with the physics of a rotating wing, causes the helicopter fuselage (cabin) to rotate in the opposite direction from the rotor, especially when the helicopter is moving at low speeds or hovering. To offset this reaction, many helicopters use a tail rotor, an exposed blade or ducted fan mounted on the end of the tail boom typically seen on these craft. Another means of counteracting torque entails installing two rotors, attached to the same engine but rotating in opposite directions, while a third, more space-efficient design features twin rotors that are enmeshed, something like an egg beater. Additional alternatives have been researched, and at least one NOTAR (no tail rotor) design has been introduced.
The airframe, or fundamental structure, of a helicopter can be made of either metal or organic composite materials, or some combination of the two. Higher performance requirements will incline the designer to favor composites with higher strength-to-weight ratio, often epoxy (a resin) reinforced with glass, aramid (a strong, flexible nylon fiber), or carbon fiber. Typically, a composite component consists of many layers of fiber-impregnated resins, bonded to form a smooth panel. Tubular and sheet metal substructures are usually made of aluminum, though stainless steel or titanium are sometimes used in areas subject to higher stress or heat. To facilitate bending during the manufacturing process, the structural tubing is often filled with molten sodium silicate. A helicopter's rotary wing blades are usually made of fiber-reinforced resin, which may be adhesively bonded with an external sheet metal layer to protect edges. The helicopter's windscreen and windows are formed of polycarbonate sheeting.
In 1939, a Russian emigre to the United States tested what was to become a prominent prototype for later helicopters. Already a prosperous aircraft manufacturer in his native land, Igor Sikorsky fled the 1917 revolution, drawn to the United States by stories of Thomas Edison and Henry Ford.
Sikorsky soon became a successful aircraft manufacturer in his adopted homeland. But his dream was vertical take-off, rotary wing flight. He experimented for more than twenty years and finally, in 1939, flew his first flight in a craft dubbed the VS 300. Tethered to the ground with long ropes, his craft flew no higher than 50 feet off the ground on its first several flights. Even then, there were problems: the craft flew up, down, and sideways, but not forward. However, helicopter technology developed so rapidly that some were actually put into use by U.S. troops during World War II.
The helicopter contributed directly to at least one revolutionary production technology. As helicopters grew larger and more powerful, the precision calculations needed for engineering the blades, which had exacting requirements, increased exponentially. In 1947, John C. Parsons of Traverse City, Michigan, began looking for ways to speed the engineering of blades produced by his company. Parsons contacted the International Business Machine Corp. and asked to try one of their new main frame office computers. By 1951, Parsons was experimenting with having the computer's calculations actually guide the machine tool. His ideas were ultimately developed into the computer-numerical-control (CNC) machine tool industry that has revolutionized modern production methods.
William S. Pretzer
Upon completion of the structure, the propulsion components are added, and wiring and hydraulics are installed and tested. Canopy, windows, doors, instruments, and interior elements are then added to complete the vehicle. Finish-painting and trimming are completed at appropriate points during this process.
Once tubular components have been formed, they are inspected for cracks. To find defects, workers treat the tubes with a fluorescent liquid penetrant that seeps into cracks and other surface flaws. After wiping off the excess fluid, they dust the coated tube with a fine powder that interacts with the penetrant to render defects visible. After the tubular components have been welded, they are inspected using X-ray and/or fluorescent penetrant methods to discover flaws. Upon completion, the contours of sheet metal details are checked against form templates and hand-worked as required to fit. After they have been autoclaved and trimmed, composite panels are ultrasonically inspected to identify any possible breaks in laminations or gas-filled voids that could lead to structural failure. Prior to installation, both the engine and the transmission subassemblies are carefully inspected, and special test equipment, custom-designed for each application, is used to examine the wiring systems. All of the other components are also tested before assembly, and the completed aircraft is flight-tested in addition to receiving an overall inspection.
Manufacturing processes and techniques will continue to change in response to the need to reduce costs and the introduction of new materials. Automation may further improve quality (and lower labor costs). Computers will become more important in improving designs, implementing design changes, and reducing the amount of paperwork created, used, and stored for each helicopter built. Furthermore, the use of robots to wind filament, wrap tape, and place fiber will permit fuselage structures to be made of fewer, more integrated pieces. In terms of materials, advanced, high-strength thermoplastic resins promise greater impact resistance and repairability than current thennosets such as epoxy and polyimide. Metallic composites such as aluminum reinforced with boron fiber, or magnesium reinforced with silicon carbide particles, also promise higher strength-to-weight ratios for critical components such as transmission cases while retaining the heat resistance advantage of metal over organic materials.
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Smith, Bruce A. "Helicoptor Manufacturers Divided on Development of New Aircraft." Aviation Week & Space Technology. February 29, 1988, p. 58.
— Phillip S. Waldrop