Any aircraft that uses a rotor, or rotating wings, to provide the craft’s lifting force.
All aircraft can be divided into two general categories: fixed-wing aircraft and rotary-wing aircraft, or rotorcraft. The principal difference between the two types of aircraft is the method used to provide the lifting force that allows the aircraft to fly. Fixed-wing aircraft use large, stationary wings to provide the required lift. The lift on the wings is generated by pulling the entire aircraft through the air with some type of propulsion system, such as propellers or jet engines. Rotorcraft, on the other hand, are equipped with at least one rotor, made up of a set of two or more rotating wings, that provides the lift that the aircraft needs to stay aloft. The lift on each of the rotating wings is generated as the rotor spins through the air, around the rotor shaft. As a result, the rotor can generate lift without the entire aircraft necessarily having to be in motion.
The two most common types of rotorcraft are the helicopter and the autogiro. Helicopters and autogiros are similar in that they both have large-diameter rotors that provide lift for the aircraft in all flight conditions. Helicopters are used in a large number of civilian and military applications. Civilian applications include airborne ambulance, police surveillance, news gathering, fire fighting, logging, heavy construction, intracity passenger transportation, tourism, cargo transportation, and search and rescue. Military applications include troop transport, logistical support, combat air support, and combat search and rescue. Autogiros, in contrast, have few commercial or military applications and are flown mainly by sport aviators.
Virtually all other rotorcraft can be broadly grouped into a category called tilting proprotor aircraft. These aircraft include the tilt-shaft/rotor, tilt-prop, tilt-wing, and tilt-rotor aircraft. All have two or more rotors, which provide either lift or propulsive force by tilting to a vertical position for vertical flight and to a horizontal position for forward flight. In vertical flight, the rotors provide all the lift necessary to keep the aircraft aloft in takeoff, landing, and hover. In forward flight, wings perform that function, and the rotors provide propulsive force. With the exception of the V-22 Osprey, a tilt-rotor aircraft, tilting proprotor aircraft have never progressed beyond the prototype stage.
Helicopters can be distinguished from other rotorcraft by the fact that their rotors have a fixed orientation relative to the aircraft fuselage and simultaneously provide lift and propulsion. On a helicopter, engines provide the power that drives the rotation of the rotor. Most modern helicopters have either one or two rotors that provide lift and propulsive force. The maximum forward speed of a helicopter is limited by 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 extreme, 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 propulsion, such as propellers or jet engines.
Helicopters have been built in a variety of configurations. The most common configuration is the single-rotor helicopter, which has a single main rotor for thrust and pitch and roll control and often has a smaller tail rotor that provides directional control. Another common configuration is the tandem helicopter, which has two large rotors, one near the forward end of the helicopter and the other near the aft end. This configuration is particularly well suited for the transport of heavy cargo, because the two rotors can accommodate large changes in the aircraft’s center of gravity.
Less common configurations include the coaxial and side-by-side helicopters. The coaxial helicopter has two counterrotating rotors that share a common mast. Side-by-side helicopters also have two rotors, but one is located on the right side of the aircraft and the other is located on the left side. A variant of the side-by-side helicopter is the synchropter, on which the two rotors are placed close together so that they intermesh.
The concept for the helicopter has been around since the Chinese top, which predates the Roman Empire. Leonardo da Vinci also considered the possibility of vertical flight. However, like the airplane, the helicopter did not become a practical concept until the invention of the internal combustion engine. Significant developments in the direction of a practical helicopter began to be achieved not long after Orville and Wilbur Wright flew their first airplane in 1903. Men such as Emile and Henry Berliner, Raoul Pescara, Louis-Charles Breguet, Heinrich Focke, and Anton Flettner made major contributions to the development of the helicopter. Although some give credit to Igor Sikorsky for building the first successful helicopter, the VS-300, others argue that the Focke-Wulf Fw-61 was the world’s first practical helicopter. At the same time, other individuals, including Arthur Young, Frank Piasecki, and Stanley Hiller, were developing their own designs.
After World War II ended, a number of companies began designing and building helicopters. The most successful were Sikorsky, Bell, Piasecki (later Boeing), Kaman, and Hiller. During the Korean War, the use of helicopters for medical evacuation showcased the usefulness of helicopters and spurred further developments. Among the most important developments was the introduction of the turboshaft engine. Later, during the Vietnam War, the role of helicopters was expanded to include troop and cargo movement and attack missions. The expanded uses for the helicopter led to more developments that resulted in modern helicopters.
Autogiros look very much like helicopters, except that they typically have only one rotor. The rotor on an autogiro also has a fixed orientation relative to the fuselage, but, in contrast to that of the helicopter, the autogiro’s rotor provides only lift. Propulsive force is provided by an auxiliary power source, such as a propeller. In addition, the rotation of the rotor is driven not by an engine, but by the air that passes through the rotor disk as it is dragged through the atmosphere by the aircraft. This behavior is similar to that of a maple seed, which spins as it falls from the tree. Because the rotor requires the aircraft’s forward motion in order to rotate, the autogiro can neither take off nor land vertically, nor can it hover. It is a short takeoff and landing (STOL) vehicle.
Autogiro development began in about 1920 and was considered a viable alternative to the helicopter until the development of Sikorsky’s helicopter. The father of the autogiro is Juan de la Cierva of Spain. After building two unsuccessful aircraft, Cierva flew his first successful autogiro in 1923. In the United States, the Pitcairn and Kellett Aircraft Companies were principally responsible for the development of the autogiro. Operating under a license from Cierva, they cooperated with Cierva in developing his original design. Cierva used conventional aircraft controls to fly his autogiro. By 1932, control was achieved by tilting the rotor with respect to the fuselage, eliminating the need for all aircraft controls except the rudder. Also, early autogiros started their rotors by taxiing around on the ground. Later models were equipped with a geared connection to the engine.
The principal reason for the development of tilting proprotor aircraft was to overcome the inability of helicopters to fly at high forward speeds, while retaining the ability to hover, take off, and land vertically. A comparison of the rotors of helicopters and autogiros to those of tilting proprotor aircraft shows that the latter have much smaller diameters. Tilting proprotor aircraft also have wings and vertical and horizontal tail surfaces like those of conventional fixed-wing aircraft.
Tilt-shaft/rotor aircraft were the predecessors of the modern tilt-rotor aircraft. This rotorcraft was able to take off and land vertically, as well as hover, by virtue of the fact that its rotors could pivot to provide lift for takeoff, landing, and hover, as well as propulsive force in forward flight. Stationary wings provided the required lift in forward flight. The first tilt-shaft/rotor aircraft was the Model 1-G, which was built by the Transcendental Aircraft Corporation and became the first such aircraft to successfully make the transition from hover to forward flight in December, 1954. The rotors on the Model 1-G were located at the ends of the wings, and the engine that provided power to them was located in the fuselage. The Model 2 followed the Model 1-G and was tested from 1956 to 1957. Bell Helicopter Company, which had been working on a similar concept since 1951, introduced the XV-3 in 1955. Two prototype aircraft were eventually built and flew many flight tests. The XV-3 was similar in design to the Model 1-G, which was not surprising because aircraft engineer Robert Lichten played a major role in the design of both aircraft.
The concept for the tilt-prop aircraft was similar to that of the tilt-shaft/rotor aircraft, except that the diameter of the tilt-prop rotors was smaller, like a large propeller. Only two aircraft of this type were ever built, both of which were built by Curtiss-Wright and called the X-100 and the X-19. The X-100 had two tilting props at the ends of a stationary wing. The X-19 had four tilting props, two at the ends of the main wing and two at the ends of a smaller wing at the tail of the aircraft.
The tilt-wing aircraft was another variant of the tilting proprotor aircraft. As with the others, the orientation of the rotors could be changed from horizontal to vertical, so that the aircraft could both hover and fly at high forward speeds. However, on a tilt-wing aircraft, the rotors were rigidly attached to the wing, and the entire wing pivoted. The first tilt-wing aircraft was the Vertol 76 (VZ-2), which first flew in 1958. Hiller Aircraft then built and flew the X-18 in 1959. The two most successful tilt-wing aircraft, the LTV-Hiller-Ryan XC-142 and the Canadair CL-84 Dynavert, appeared in the mid-1960’s. In 1964, the XC-142 became the largest vertical takeoff and landing aircraft to fly. Five prototypes were built, but all eventually crashed or were otherwise accidentally destroyed.
Similar in configuration to other tilting proprotor aircraft, the tilt-rotor appears to be a fixed-wing aircraft with large propellers attached to nacelles at the tips of the wings. The engines that provide the power to turn the rotors are located in the nacelles. In 1975, Bell Helicopter Company began the development of the XV-15 tilt-rotor research aircraft. Two prototype aircraft were built, and the first successful flight with conversion to forward flight took place in 1977. Since that time, the XV-15 has performed hundreds of hours of research flight testing and flight demonstrations. The unprecedented success of the XV-15 led directly to the development of the Bell-Boeing V-22 Osprey, the first operational tilt-rotor aircraft.
Campbell, J. P. Vertical Takeoff and Landing Aircraft. New York: Macmillan, 1962. Overview of aircraft developed with vertical takeoff and landing capabilities. Gabelhouse, C. Helicopters and Autogiros, A Chronicle of Rotating-Wing Aircraft. 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. A historical account of twentieth century helicopter developments, with pictures and descriptions of many different designs. Lindenbaum, B. L. V/STOL Concepts and Developed Aircraft: A Historical Review. Dayton, Ohio: University of Dayton, 1982. A historical review of concepts for vertical/short takeoff and landing aircraft, including descriptions of research aircraft. Muson, K. Helicopters and Other Rotorcraft Since 1907. London: Macmillan, 1968. A historical account of helicopter development.
Vertical takeoff and landing