A vertical and/or short take-off and landing (V/STOL) aircraft is an airplane able to take off or land vertically or on short runways. Vertical takeoff and landing (VTOL) aircraft are a subset of V/STOL craft that do not require runways at all. Generally, a V/STOL aircraft needs to be able to hover. Helicopters are not considered under the V/STOL classification as the classification is only used for aeroplanes, aircraft that achieve lift in forward flight by planing the air, thereby achieving speed and fuel efficiency that is typically greater than the capability of helicopters.
The main advantage of V/STOL aircraft is in their military performance, such as closer basing to the enemy, which reduces response time and tanker support requirements. In the case of the Falklands War, it also permitted high-performance fighter air cover and ground attack without a large aircraft carrier equipped with aircraft catapult. V/STOL was developed to allow fast jets to be operated from clearings in forests, from very short runways, and from small aircraft carriers that would previously only have been able to carry helicopters.
A rolling takeoff, sometimes with a ramp (ski-jump), reduces the amount of thrust required to lift an aircraft from the ground (compared with vertical takeoff), and hence increases the payload and range that can be achieved for a given thrust. For instance, the Harrier is incapable of taking off vertically with full weapons and fuel load. Hence V/STOL aircraft generally use a runway if it is available. In other words, short takeoff and vertical landing (STOVL) or conventional takeoff and landing (CTOL) operation is preferred to vertical takeoff and landing (VTOL) operation.On aircraft carriers, non-catapult-assisted fixed-wing short takeoffs are accomplished with the use of thrust vectoring, which may also be used in conjunction with a runway "ski-jump". There are 16 aircraft carriers that operate these STOVL aircraft: United States (9), Japan (2), United Kingdom (2), Italy (2), and Spain (1). Use of STOVL tends to allow aircraft to carry a larger payload compared to vertical take-off and landing (VTOL), while still only requiring a short runway. The most famous examples are the Hawker Siddeley Harrier and the BAE Sea Harrier. Although technically a V/STOL aircraft, they are operationally STOVL aircraft due to the extra weight carried at take-off for fuel and armaments. The same is true of the B variant of the Lockheed Martin F-35 Lightning II, which demonstrated VTOL capability in test flights but is operationally a STOVL.V-22 Osprey Pocket Guide . Bell Boeing, 2007. Retrieved 17 April 2010.
VTOL Vertical take-off and landing (VTOL) is a subset of V/STOL. This classification can include a variety of types of aircraft as well as thrust-vectoring fixed-wing aircraft and other hybrid aircraft with powered rotors such as cyclogyros and gyrodynes.
Some VTOL aircraft can operate in other modes as well, such as CTOL (conventional take-off and landing), STOL (short take-off and landing), or STOVL (short take-off and vertical landing). Others, such as some helicopters, can only operate as VTOL, due to the aircraft's lack of landing gear that can handle taxiing. Some lighter-than-air aircraft also qualify as VTOL aircraft, as they can hover, take off and land with vertical approach/departure profiles.
Electric vertical takeoff and landing aircraft, or eVTOLs, are being developed along with more autonomous flight control technologies and mobility-as-a-service (MaaS) to enable advanced air mobility (AAM), that could include on-demand air taxi services, regional air mobility, freight delivery, and personal air vehicles (PAVs).Le Bris, G. et al. (2022). "ACRP Research Report 236: Preparing Your Airport for Electric Aircraft and Hydrogen Technologies". Transportation Research Board, Washington, DC: https://nap.nationalacademies.org/catalog/26512/preparing-your-airport-for-electric-aircraft-and-hydrogen-technologies
Besides the ubiquitous helicopters, there are currently two types of VTOL aircraft in military service: tiltrotor aircraft, such as the Bell Boeing V-22 Osprey, and thrust-vectoring airplanes, such as the Harrier family and new F-35B Lightning II Joint Strike Fighter (JSF). In the civilian sector, currently only helicopters are in general use (some other types of commercial VTOL aircraft have been proposed and are under development ). Generally speaking, VTOL aircraft capable of STOVL use the latter wherever possible, since it typically significantly increases takeoff weight, range, or payload compared to pure VTOL.
History
Of dozens of V/STOL designs tried from the 1950s to 1980s, only the subsonic Hawker Siddeley Harrier and Yak-38 Forger reached operational status, with the Forger being withdrawn after the fall of the Soviet Union.
Props, proprotors, and advanced rotorcraft
The idea of vertical flight has been around for thousands of years, and sketches for a VTOL (helicopter) show up in Leonardo da Vinci's sketch book. Manned VTOL aircraft, in the form of primitive helicopters, first flew in 1907, but would take until after World War Two to be perfected.Yefim Gordon, The History of VTOL, page 28John Whiteclay Chambers, The Oxford Companion to American Military History, Oxford University Press, USA, 1999, page 748
In addition to helicopter development, many approaches have been tried to develop practical aircraft with vertical take-off and landing capabilities, including Henry Berliner's 1922–1925 experimental horizontal-rotor fixed-wing aircraft, and Nikola Tesla's 1928 patent, and George Lehberger's 1930 patent for relatively impractical VTOL fixed wing airplanes with tilting engines. In the late 1930s, British aircraft designer Leslie Everett Baynes was issued a patent for the Baynes Heliplane, another tiltrotor aircraft. In 1941, German designer Heinrich Focke's began work on the Focke-Achgelis Fa 269, which had two rotors that tilted downward for vertical takeoff, but wartime bombing halted development.
In May 1951, both Lockheed and Convair were awarded contracts in the attempt to design, construct, and test two experimental VTOL fighters. Lockheed produced the XFV, and Convair producing the Convair XFY Pogo. Both experimental programs proceeded to flight status and completed test flights 1954–1955, when the contracts were cancelled.Allen 2007, pp. 13–20. Similarly, the Ryan X-13 Vertijet flew a series of test flights between 1955 and 1957, but also suffered the same fate.
In 1962, Lockheed built the XV-4 Hummingbird for the U.S. Army. It sought to "augment" available thrust by injecting the engine exhaust into an ejector pump in the fuselage. First flying vertically in 1963, it suffered a fatal crash in 1964. It was converted into the XV-4B Hummingbird for the U.S. Air Force as a testbed for separate, vertically mounted lift engines, similar to those used in the Yakovlev Yak-38 'Forger'. That plane flew and later crashed in 1969.Jim Winchester, X-Planes and Prototypes, Barnes and Noble Books The Ryan XV-5 Vertifan, which was also built for the U.S. Army at the same time as the Hummingbird, experimented with gas-driven lift fans. That plane used fans in the nose and each wing, covered by doors which resembled half garbage can lids when raised. However, it crashed twice, and proved to generate a disappointing amount of lift, and was difficult to transition to horizontal flight.
Rockwell International built, and then abandoned, the Rockwell XFV-12 supersonic fighter which had an unusual wing which opened up like window blinds to create an ejector pump for vertical flight. It never generated enough lift to get off the ground despite developing 20,000 lbf of thrust. The French had a nominally Mach 2 Dassault Mirage IIIV fitted with no less than 8 lift engines that flew (and crashed), but did not have enough space for fuel or payload for combat missions. The German EWR VJ 101 used swiveling engines mounted on the wingtips with fuselage mounted lift engines, and the VJ 101C X1 reached supersonic flight (Mach 1.08) on 29 July 1964. The supersonic Hawker Siddeley P.1154, which competed with the Mirage IIIV for use in NATO, was cancelled even as the aircraft were being built. NASA has flown other VTOL craft such as the Bell XV-15 research craft (1977), as have the Soviet Navy and Luftwaffe. Sikorsky tested an aircraft dubbed the X-Wing, which took off in the manner of a helicopter. The rotors would become stationary in mid-flight, and function as wings, providing lift in addition to the static wings. Boeing X-50 is a Canard Rotor/Wing prototype that utilizes a similar concept.
A different British VTOL project was the gyrodyne, where a rotor is powered during take-off and landing but which then freewheels during flight, with separate propulsion engines providing forward thrust. Starting with the Fairey Gyrodyne, this type of aircraft later evolved into the much larger twin-engined Fairey Rotodyne, that used tipjets to power the rotor on take-off and landing but which then used two Napier Eland turboprops driving conventional propellers mounted on substantial wings to provide propulsion, the wings serving to unload the rotor during horizontal flight. The Rotodyne was developed to combine the efficiency of a fixed-wing aircraft at cruise with the VTOL capability of a helicopter to provide short-haul airliner service from city centres to airports.
The CL-84 Dynavert was a Canadian V/STOL turbine tilt-wing monoplane designed and manufactured by Canadair between 1964 and 1972. The Canadian government ordered three updated CL-84s for military evaluation in 1968, designated the CL-84-1. From 1972 to 1974, this version was demonstrated and evaluated in the United States aboard the aircraft carriers USS Guam and USS Guadalcanal, and at various other centres. These trials involved military pilots from the United States, the United Kingdom, and Canada. During testing, two of the CL-84s crashed due to mechanical failures, but no loss of life occurred as a result of these accidents. No production contracts resulted.Boniface 2000, p. 74.
Although tiltrotors such as the Focke-Achgelis Fa 269 of the mid-1940s and the Centro Técnico Aeroespacial "Convertiplano" of the 1950s reached testing or mock-up stages, the Bell-Boeing V-22 Osprey is considered the world's first production tiltrotor aircraft. It has one three-bladed proprotor, turboprop engine, and transmission nacelle mounted on each wingtip. The Osprey is a multi-mission aircraft with both a vertical takeoff and landing (VTOL) and short takeoff and landing capability (STOL). It is designed to perform missions like a conventional helicopter with the long-range, high-speed cruise performance of a turboprop aircraft. The FAA classifies the Osprey as a model of powered lift aircraft.Norton 2004, pp. 6–9, 95–96.
Attempts were made in the 1960s to develop a commercial passenger aircraft with VTOL capability. The Hawker Siddeley Inter-City Vertical-Lift proposal had two rows of lifting fans on either side. However, none of these aircraft made it to production after they were dismissed as too heavy and expensive to operate.