Also known as: VTOL
Definition: Aircraft that are capable of taking off and landing without the benefit of a ground run.
Significance: Because VTOL aircraft do not require runways or airfield infrastructure to take off or land, they are flexibile aircraft that can be used in a variety of situations.
During World War II, the airplane proved its value for both commercial and military applications. The helicopter also began to prove valuable, but it remained the only type of vertical takeoff and landing (VTOL) aircraft that had been flown successfully. However, the helicopter’s relatively slow cruise speed motivated aircraft designers to create new and innovative designs. The main objective of these designs was to create an aircraft with a cruise speed near that of fixed-wing aircraft, but that retained the ability to take off and land vertically.
VTOL aircraft are usually categorized by the means they use to generate power for hover and forward flight. On this basis, VTOL aircraft can be separated into four classes. The first class uses a single propulsion system that alters the direction of the thrust or alters the attitude of the aircraft itself for hover and cruise flight. The second class has a separate group of power plants for hover and cruise. The third class uses its primary power plant for both hover and cruise but also has a separate propulsion system for additional hover lift. The fourth and final class of VTOL aircraft uses its primary propulsion system to drive auxiliary devices for additional vertical thrust in hover.
Single Propulsion System
Aircraft in this first class use the same propulsion system for both hover and cruise functions. They include tilt-shaft/rotor, tilt-prop, tilt-duct, tilt-wing, tilt-rotor, tilt-jet, deflected-slipstream, vectored-thrust, and tail-sitter aircraft types. The first two types use rotating blades that function like rotors in hover and propellers in cruise. Their power plants remain stationary, while the rotor shafts tilt from vertical to horizontal. The basic difference between these two types is that the rotors on the tilt-shaft/rotor aircraft are similar in size to helicopter rotors, and the rotors on the tilt-prop aircraft are the size of propellers.
Examples of tilt-shaft/rotor aircraft include the Transcendental Model 1G (1954-1957) and the Bell XV-3 (1958-1965). Tilt-duct aircraft were proposed to take advantage of the phenomenon of increased thrust that occurs when a propeller is enclosed in a duct. The Doak VZ-6 first flew in 1958 and completed more than fifty hours of flight-testing. Between its first flight in 1966 and 1980, the Bell X-22 completed more than two hundred hours of flight and set a record by hovering at more than 8,000 feet.
In tilt-wing aircraft, the entire wing is tilted to minimize loss of lift in hover; as a result, these aircraft have control problems. The Vertol VZ-2 was quite successful, making more than 450 flights between 1957 and 1965. However, the Hiller X-18, which first flew in 1958, only completed twenty flights before being grounded. The five LTV-Hiller-Ryan XC-142 aircraft that were built accrued more than 450 flight hours beginning in 1964. The smaller Canadair CL-84 made its first flight in 1965, but did not generate sufficient interest from either the United States or Canada.
One of the two types of tilt-shaft/rotor aircraft that resulted in a production aircraft is the tilt-rotor. Tilt-rotor aircraft tilt their rotors from vertical to horizontal and, like the tilt-wing aircraft, tilt the engines as well. In 1977, the Bell XV-15 made its first hovering flight. As of 1986, the two prototype aircraft had accumulated more than 530 flight hours and 1,500 conversions. The National Aeronautics and Space Administration (NASA) is still using it for flight research and development. The success of the XV-15 led directly to the development of the Bell-Boeing V-22 Osprey, which became operational in 2000.
Deflected-slipstream aircraft use flaps to change the direction of the propeller slipstream and create vertical thrust. The first of these aircraft was the Robertson VTOL, which made only tethered flights in 1957. Other, largely unsuccessful, designs included the Ryan VZ-3 Vertiplane (1959-1961) and the Fairchild VZ-5 Fledgling (1959).
A similar, and vastly more successful, type of single-propulsion-system VTOL is the vectored-thrust aircraft, which changes the direction of jet exhaust for hover or cruise flight. The first of these aircraft was the Bell X-14, which first flew in 1957 and remained in use as aflyingtest bed until 1981. In England, the Hawker P.1127 Kestrel made its first flight in 1960 and became the forerunner of the McDonnell Douglas/British Aerospace Harrier. The first flight of the Harrier took place in 1966, and it entered service with the Royal Air Force (RAF) and the U.S. Marine Corps in 1969, as the first production vertical-or-short-takeoff-and-landing (V/STOL) aircraft. In the Soviet Union, the Yakolev Yak-36 made its first untethered flight in 1963. Although it was never put into production, it led directly to the operational Yak-38 Forger.
Tail sitters are aircraft that produce vertical thrust by changing the aircraft attitude. Therefore, they take off and land on their tails. Although taking off in this manner is routine, landings are much more problematic. Aircraft developed in the 1950′s using this concept include the Lockheed XFV-1, the Convair XFY-1 Pogo, the Ryan X-13 Vertijet, and the SNECMA C-450 Coleoptere. None were particularly successful, and the concept was not pursued.
Separate Propulsion Systems
VTOL aircraft with separate propulsion systems use lift engines for hover and separate engines for cruising flight. The lift engine is mounted vertically, and optimized to produce a large amount of thrust for takeoff and landing. In 1954, work began in England on the Short Brothers SC-1, which made its first untethered vertical flight in 1958. Despite a crash of the second prototype, which was repaired, both aircraft flew though 1963.
During the same time period, a French company built the Dassault Balzac V using the airframe of a Mirage III. The craft made its first conventional flight in 1963, but two fatal crashes in 1964 and 1965 effectively ended this program. However, Dassault built a new aircraft, the Mirage III-V, which achieved its first hover in 1965. Both prototype aircraft were eventually lost, but not before setting the speed record of Mach 2.04 for V/STOL aircraft in 1966.
Combined Propulsion System
Aircraft in the combined propulsion system class of V/STOL vehicles use one set of engines for lift only and another set of engines for lift and cruise. One such aircraft, the EWR VJ-101C (1963-1964), became the first supersonic V/STOL aircraft. Others, such as the Dornier Do-31 (1967-1970), the Lockheed XV-4B Hummingbird II (1964-1969), and the VFW VAK 191B (1971-1972), achieved only limited success because of the complexity of their designs. However, the Yakolev Yak-38 Forger, which first flew in 1971, eventually became operational in the Soviet Union. By 1988, when production ended, 231 total aircraft had been built. The Russians also developed the Yakolev Yak-141 Freestyle, which was flight-tested between 1991 and 1995, but never reached production.
Another type of aircraft in this class are tip-jet rotor-craft, which are essentially compound autogyros that transmit full power to the rotor in hover, then transfer power to auxiliary engines for cruise. Powered by jets on the tips of the rotor blades, only two of these aircraft, the McDonnell XV-1 (1954-1957) and the Fairey Rotodyne (1953-1962), were developed. Neither led to operational aircraft.
Augmented Propulsion Systems
Augmented propulsion system aircraft use their power plants to drive auxiliary devices to achieve additional lift in hover. One auxiliary device is the ejector, which was employed on the Lockheed XV-4A Hummingbird (19621964) and the Rock well XFV-12A(1978-1981). The ejector channeled hot exhaust gases into an augmentor, which accelerated cooler air for additional thrust. Another device used was the lift fan, a propeller mounted horizontally in the aircraft fuselage or wing. The Vanguard Omniplane (1959-1962) and the GE-Ryan XV-5A Vertifan (19641968) used a lift fan. Aircraft that use rotors for hover flight and auxiliary propulsion for cruise flight are often referred to as compound helicopters. Such aircraft include the Kamov Ka-22 Vintokryl (1960-1964), the Piasecki 16H-1 Pathfinder (1962-1966), and the Lockheed AH-54 Cheyenne (1967-1972).