Helicopters Summary

  • Last updated on November 10, 2022

Any rotary-wing aircraft having powered, fixed rotors that provide lift and propulsion for the aircraft.

Configurations

The helicopter is the principal VTOL aircraft in service throughout the world. The name “helicopter” was coined by a Frenchman, Viscomte Gustave de Ponton d’Amecourt, circa 1863. Helicopters can be distinguished from other rotary-wing aircraft by the fact that their rotors are fixed in position on the aircraft fuselage and simultaneously provide lift and propulsion. The vast majority of modern helicopters have either one or two rotors that provide lift and propulsive force.

Although helicopters can take off and land vertically, their maximum forward speed is much lower than that of fixed-wing aircraft. This limitation is due to the fact that the rotor or rotors must provide both propulsion and lift. Under high-speed flight conditions, the vibratory forces on the rotor blades become very large, thereby limiting the top speed of the helicopter. In order to increase the top speed, some helicopters, known as compound helicopters, have been equipped with auxiliary means of propulsion, such as propellers or jet engines.

Helicopters are built in a variety of configurations, including the single-rotor, the tandem, the coaxial, and the side-by-side helicopters. The single-rotor helicopter is the most common configuration currently in use. It can be identified by the single main rotor that provides thrust and propulsion, as well as pitch and roll control. A smaller tail rotor usually provides antitorque directional yaw control. However, other devices may be used instead of a tail rotor.

Another common configuration is the tandem helicopter. The tandem helicopter has two large rotors, one at the forward end of the helicopter and the other at the aft end. The two rotors rotate in opposite directions, thus eliminating the need for an antitorque device, such as a tail rotor. This configuration is particularly well-suited for the transport of heavy cargo, because the two rotors can accommodate large changes in the aircraft center of gravity due to the cargo load.

Less common configurations include side-by-side and coaxial helicopters. Like the tandem helicopters, side-by-side helicopters have two main rotors, but one is located on the right side of the aircraft, and the other is located on the left side. The rotors rotate in opposite directions, again eliminating the need for a tail rotor.

A variant of the side-by-side helicopter is the synchropter, on which the two rotors are placed close together, so that the rotors intermesh. The synchropter has the advantage of being able to take off and land in more confined areas than either a side-by-side or tandem helicopter, because the close proximity of rotor masts reduces the area required for clearance around the rotors.

The coaxial helicopter has two counterrotating rotors that share a common mast. Because the rotors rotate in opposite directions, no tail rotor is needed for this configuration either. Coaxial helicopters also have the advantage of being able to land in more confined areas than any other configuration, because the swept area of the rotors is the smallest of all configurations.

Missions

Because helicopters are able to take off vertically, hover in midair, and land vertically, they are ideal vehicles for a wide variety of missions. They do not require prepared landing areas, so they can take off and land in forest clearings, on the tops of buildings, and on ships at sea. As a result, they can be used in civil and military applications for which fixed-wing aircraft are unsuitable.

The transportation of passengers is one of the primary missions of helicopters. The largest civilian user of helicopter transportation is the petroleum industry. Helicopters regularly transport petroleum workers to and from offshore oil platforms, because they are much faster and more cost effective than boats. Many large corporations use helicopters to ferry their executives between sites. Commercial helicopter operators in scenic locations, such as the Grand Canyon and Hawaii, regularly carry passengers on sight-seeing tours, although increasingly stringent noise regulations have somewhat curtailed their business.

Commercial helicopter airlines have not been economically viable, despite the obvious advantages of ferrying passengers between airports and between airports and inner-city heliports. The U.S. military services, particularly the Army and the Marines, make extensive use of helicopters for troop transport. Naval helicopters are often used for ship-to-shore and ship-to-ship transportation of personnel. In all services, helicopters are used for the insertion and extraction of special-operations forces at remote sites.

Cargo transportation is another important helicopter function. In the logging industry, helicopters are used to transport logs from remote areas either directly to a mill or to rivers in which the logs are floated to a mill. Construction projects often use helicopters to transport heavy equipment, such as heating, ventilation, and air conditioning units, to the tops of tall buildings. The Statue of Freedom, atop the U.S. Capitol Building, was removed by helicopter in 1993 for restoration and was later replaced in the same manner. Helicopters with large buckets slung beneath them are used to transport water from nearby lakes to the site of a forest fire. On the military side, the Army and Marines use helicopters to transport supplies and even small- and medium-sized vehicles from rear areas to troops in the field. The Navy uses helicopters to transport supplies from shore to ships at sea and between ships at sea.

Many police departments, particularly in large cities, use helicopters for airborne patrol and surveillance. Because they operate at altitude, helicopters have a wider field of view than ground patrols. In cases of pursuit, it is much easier for a helicopter to keep a fleeing suspect in view and safer for the ground units and the general public. In addition, when on patrol, a helicopter can often reach the crime scene more rapidly than can a ground unit. In a similar application, radio and television stations use helicopters for acquiring traffic reports and news gathering. News helicopters can often reach the scene of a news event more rapidly than can ground vehicles.

Another mission for which helicopters are particularly well-suited is search and rescue. Although this is primarily a military mission, police departments and the U.S. National Park Service will use helicopters to find and rescue hikers, campers, and others who find themselves in dangerous situations. The U.S. Coast Guard is very active in search and rescue, patrolling the waters off the coast of the United States. A typical Coast Guard rescue mission would be to extract passengers from foundering sailing vessels. Combat search-and-rescue missions are flown primarily by the Air Force and the Navy to locate and return aircrews of aircraft downed in combat. During the Vietnam War, the Jolly Green Giant (CH/HH-3E) helicopters were a welcome sight for many pilots who had been shot down while flying over North Vietnam.

Combat close air support and antiarmor are purely military missions. Close air support involves using helicopters to support friendly ground troops by directing fire on enemy troops in the near vicinity. Helicopters used in antiarmor missions are equipped with ordnance that is capable of disabling or destroying tanks and other armored vehicles. The Marines use the AH-1 and the Army uses the AH-64 for these missions.

Flight Control

One of the first problems of helicopter flight control that must be solved is the question of how to keep the fuselage from rotating opposite the rotor. In order to spin the rotor, torque is applied by the engine to the rotor driveshaft. Therefore, the rotor has an angular momentum, which must be counteracted in some manner. If the angular momentum of the rotor is not equalized, the fuselage will begin to rotate in the opposite direction to the rotor rotation. Single-rotor helicopters equalize the angular momentum with countertorque devices, such as a tail rotor or a NOTAR (no tail rotor) system. The tail rotor is a smaller rotor mounted vertically at the end of a tail boom that generates a lateral thrust. The NOTAR system also generates lateral thrust but does so using the slipstream of the rotor and air ejected from a slot in the tail boom to produce the Coanda effect. Helicopters with more than one rotor, such as the tandem, side-by-side, and coaxial types, equalize the angular momentum by employing equally sized rotors rotating in opposite directions.

In order to fly a helicopter, the pilot must be able to control the translation of the aircraft in the vertical, lateral (side-to-side), and longitudinal (forward-and-back) directions, as well as rotation in roll, pitch, and yaw. The pilot’s controls include a collective lever beside the pilot seat, a cyclic stick between the pilot’s knees, and foot pedals. It is interesting to note that in helicopters, the pilot sits in the right seat and the copilot sits in the left. In fixed-wing aircraft, the pilot sits in the left seat, and the copilot sits in the right. This seating arrangement is an artifact from one of Igor Sikorsky’s early helicopters, which had such “backward” seating.

To explain helicopter control, consider a single-rotor helicopter. The main rotor of a single-rotor helicopter produces a thrust, which acts in a direction roughly normal to the rotor disk. Therefore, in order to control the helicopter, the pilot must be able to control the magnitude and direction of this thrust. The magnitude of the thrust is controlled by the collective lever, which equally increases or decreases the pitch angle of all rotor blades, thereby increasing or decreasing the thrust. In order to control the direction of the thrust, the pilot must be able to control the orientation of the rotor disk. One way to change the orientation of the rotor disk is to physically tilt the rotor hub.

For very small helicopters, hub tilt is a practical control method. However, for larger helicopters, the rotor acts like a large gyroscope, which makes tilting the hub extremely difficult. The alternative is to increase the thrust on one half of the disk, while simultaneously decreasing the thrust on the other half. This cyclic change in thrust causes the rotor disk to tilt and does so with much less effort than hub tilt.

In all but a few modern helicopters, the pilot’s cyclic stick, acting through a swashplate, is used to change the cyclic pitch of the rotor blades. The swashplate consists of two parts: a nonrotating plate and a rotating plate. The nonrotating plate, which is connected to the pilot collective and cyclic pitch controls, slides up and down for collective-pitch changes and tilts for cyclic-pitch changes. The rotating plate sits on top of the nonrotating plate and spins with the rotor. Pitch links, attached to the rotating plate and the rotor blades, mechanically change the pitch angle of the blades. Yaw control is obtained through the foot pedals, which are connected to the collective pitch controls for the tail rotor.

History

Although the development of an operational helicopter is a relatively recent accomplishment, many of the concepts necessary for designing a practical helicopter have been known for a very long time. In fact, one could argue that a maple seed falling from a tree is nature’s model for the helicopter. The Chinese top, which predates the Roman Empire, is perhaps humankind’s first step toward modern helicopters. In addition, Leonardo da Vinci considered the possibility of vertical flight, and made a sketch of his concept for such a vehicle.

The development of a practical helicopter was made possible by overcoming three major technology barriers. The first barrier, and the easiest to overcome, was the design of a rotor system with rotor blades and a rotor hub that were strong but lightweight, with adequate aerodynamic efficiency. The second was to engineer a power plant with a sufficiently high ratio of power to weight, required in order to lift the aircraft off the ground. This barrier was overcome with the invention of the internal-combustion engine. The third technology barrier was to devise a method for controlling the helicopter in flight. The principles leading to controlled helicopter flight were developed gradually by helicopter pioneers.

Early helicopter pioneers tried a variety of power plants in their helicopter designs. During the latter half of the eighteenth century, Mikhail Vasilyevich Lomonosov in Russia, Launoy and Bienvenu in France, and Sir George Cayley in England provided power to their helicopters by using different spring mechanisms. While spring-driven power plants have a good ratio of power to weight, they cannot provide sufficient sustained power for long flights.

In the nineteenth century, steam-powered helicopters were designed by Horatio Frederick Phillips in England, d’Amecourt and Alphonse Pénaud in France, Enrico Forlanini in Italy, and Thomas Edison in the United States. In contrast to spring power, steam power could provide sufficient sustained power, but its ratio of power to weight was very low.

Like that of the airplane, the concept of the helicopter did not become truly feasible until the invention of the internal combustion engine. Developments leading to a practical helicopter began to be achieved not long after Orville and Wilbur Wright flew their first airplane, but the availability of an adequate power plant brought problems of control to the fore. Paul Cornu and Charles Renard in France, Emile and Henry Berliner in the United States, and Igor Sikorsky and Boris Yuriev in Russia made significant contributions prior to 1920.

Renard introduced the flapping hinge, which improved rotor control; and Yuriev introduced the antitorque tail rotor for yaw control. In 1907, Cornu made the first piloted, free-flight, vertical takeoff, but the aircraft had to be stabilized manually by a ground crew. In the 1920’s and early 1930’s, George de Bothezat in the United States, Etienne Oemichen and Louis-Charles Breguet in France, Raoul Pescara in Spain, Emile and Henry Berliner in the United States, Louis Brennan in England, A. G. von Baumhauer in Holland, and Corradino D’Ascanio in Italy, M. B. Bleeker in the United States, and Yuriev in Russia all built prototype helicopters. Unfortunately, all of these designs either had controllability problems or were too complex to be practical.

However, important contributions toward improved control were made by Bothezat, in differential collective pitch control; Pescara, in cyclic pitch control; von Baumhauer, in the area of the swashplate; and d’Ascanio, in servotab cyclic pitch control.

In 1936, German aircraft designer Heinrich Focke introduced the first practical helicopter, the Focke-Achgelis Fa-61, a side-by-side design in which all of the stability problems had been solved. In 1938, Hanna Reitsch flew the Fa-61 inside the Deutschland-Halle in Berlin, demonstrating its flying precision. In 1939, in the United States, Igor Sikorsky introduced the VS-300, a single-rotor helicopter, which may have been the world’s first useful helicopter. Germany continued its development of the helicopter during World War II, and Anton Flettner’s synchropter design, the FL-282 Kolibri, became the first production helicopter.

At about the same time, other individuals, including Arthur Young, Frank Piasecki, and Stanley Hiller in the United States, and Nikolai Kamov, Mikhail Mil, and Ivan Bratukhin in the Soviet Union were developing their own independent helicopter designs.

Modern Helicopters

The basics of helicopter design have not changed greatly since the early days of helicopters in the 1940’s. However, technological improvements have been incorporated that make the modern helicopter safer, easier, and more efficient to fly. One of the most significant advances in helicopter performance resulted from the introduction of the gas-turbine engine. The maximum power-to-weight ratio achievable with piston engines by the end of World War II was approximately 1 horsepower per pound. However, by the 1960, turbine engines had achieved power-to-weight ratios of 3 horsepower per pound, and by 2000 they had achieved weight ratios of up to 6 horsepower per pound.

Helicopter rotor systems have also undergone significant changes. In the early years, rotor blades were made exclusively of wood, one of the principal materials used for aircraft construction. In 1944, Hiller introduced metal rotor blades on the XH-44, but it was not until 1952 that metal blades were delivered on a production aircraft, the Sikorsky S-52.

The use of composite materials for rotor blade construction began in the early 1960’s, and, by the 1970’s, the Messerschmitt-Bölkow-Blohm company in Germany had built all-composite blades for the BO-105 helicopter. Virtually every modern helicopter is now equipped with composite blades. The rotor hub has also undergone changes in the way that the blades are attached. Many helicopter rotors are fully articulated. That is, each blade has physical hinges, which allow the blade to flap out of the plane of rotation and lag in the plane of rotation. A bearing also allows the blade to pitch. The concepts of a hingeless rotor that eliminates the flap and lag hinges and a bearingless rotor, which is basically a hingeless rotor without a pitch bearing, have found their way into the designs of many modern helicopters.

Technological improvements, such as vibration control devices in the rotor system and the fuselage, have improved the comfort level for passengers, as well as the performance of the flight crew due to reduced fatigue. Crash-worthy structural design, seats, and fuel systems have improved the safety of helicopters in emergency situations. Hydraulic control systems have replaced the mechanical control systems of early helicopters, and modern helicopters are often equipped with electronic flight control and stability augmentation systems to reduce pilot workload. Digital fly-by-wire and fly-by-light control systems, as well as glass cockpits, have begun to be introduced in advanced production helicopters.

Bibliography
  • Fay, John. The Helicopter, History, Piloting, and How It Flies. London: David & Charles, 1976. A description of the fundamentals of helicopter design and flight, using simple explanations of aeronautical theory.
  • Gablehouse, Charles. Helicopters and Autogiros: A Chronicle of Rotating-Wing Aircraft Since 1907. London: Scientific Book Club, 1967. A history of rotorcraft, including both helicopters and autogiros.
  • Hirschberg, M. J. The American Helicopter, An Overview of Helicopter Developments in America, 1907-1999. Arlington, Va.: ANSER, 2000. An historical account of twentieth century helicopter developments, with pictures and descriptions of many different designs.
  • Liberatore, Helicopters Before Helicopters. Malabar, Fla.: Krieger, 1998. A historical account of helicopter development from early concepts to practical models, updated with interpretations based on current knowledge.
  • Taylor, Michael J. History of Helicopters. London: Hamlyn, 1984. A chronicle of helicopter development.

Apache helicopter

Bell Aircraft

Sir George Cayley

Firefighting aircraft

Gyros

Leonardo da Vinci

Hanna Reitsch

Rescue aircraft

Rotorcraft

Igor Sikorsky

Vertical takeoff and landing

Helicopters use rotors rather than wings to achieve vertical takeoff and landing.

(NASA)
Categories: History Content