VTOL and STOL aircraft

Even before the dawn of jet aircraft, aeronautical engineers have wanted to reduce the amount of runway required by fast aircraft, preferably eliminating runways completely. They wanted an aircraft that takes off and lands like a helicopter but flies with the efficiency of an airplane. These types of aircraft are generally classified either as Vertical Takeoff and Landing (VTOL) or Short Takeoff and Landing (STOL), and the two categories are often grouped together as V/STOL. However, designing such aircraft has been difficult and there have been few successes.

The Lockheed XFV-1 is a "tail-sitter" aircraft. It points straight up, which permits the entire thrust of its propulsion system to be converted directly into vertical lift. However, this feature also makes it difficult to land since the pilot needs to ease down onto the ground.

From the 1940s until today, slightly more than 40 vertical/short takeoff and landing aircraft have been tested. However, only four have actually gone into production: the British Harrier "jump jet" and its derivatives, the Soviet An-72/74 transport aircraft, the Soviet Yak-38 naval fighter, and the Bell/Boeing V-22 Osprey. Most of the other aircraft were highly experimental or proof-of-concept types never intended to lead to an actual production aircraft.

The biggest problem with achieving V/STOL flight is that conventional wings provide a good amount of lift for a relatively low amount of forward thrust. Getting an aircraft off the ground with little or no forward motion requires that engine thrust—and not wing lift—support a significant portion of the aircraft's weight—or all of it. This usually requires big engines, lots of fuel, and complicated flight controls, all of which weigh more.

Another problem is that these aircraft are often hard for the pilot to control during transition from horizontal flight to vertical flight and back again. Computerized flight control systems and better cockpit displays have helped with this. Some experimental V/STOL aircraft also simply have had a hard time accelerating in forward flight after lifting off the ground.

One early V/STOL aircraft was the XVW-1, which Lockheed tested in 1954. This "tailsitter" aircraft had two large contra-rotating propellers on its nose. In horizontal flight, the aircraft looked like many piston-engine fighters, although with a large tail. When on the ground, it actually sat on its tail. The plane would rise straight up during takeoff and transition to forward flight. The Convair XFY-1 Pogo, developed at the same time, operated similarly. Both were extremely difficult to control in vertical flight (the pilots had to look over their shoulders to see the ground when landing), and both were grounded in 1956. The Ryan X-13 Vertijet tailsitter was more successful. It used a single large jet engine for thrust and even landed and took off in the Pentagon parking lot. But it too never progressed beyond the experimental stage.

In the late 1950s and early 1960s, several European experimental V/STOL jets were developed. The French firm Dassault developed two versions of the Mirage III-V with small lift jets in the fuselage for lifting the aircraft off the ground. Germany's EWR developed the VJ101C, with wingtip-mounted pairs of tilting jet engines and lift jets behind the cockpit.

In the early 1960s, the British firm Hawker developed the experimental P.1127 Kestrel. This aircraft had a single Rolls-Royce Pegasus jet engine in the fuselage that vented its exhaust through four ports in the fuselage—two on each side. The ports could be directed downward to lift the aircraft off the ground, and then rotated back to direct the thrust to the rear, thus changing the aircraft from vertical to forward flight.

The Kestrel was soon developed into the British Aerospace GR.3 Harrier single-seat attack aircraft, which entered service in 1969. The Harrier is often called a "jump jet" and can operate from relatively unprepared locations. The Harrier proved to be successful and was later modified into a naval fighter version and also adopted by the U.S. Marine Corps as well as by other navies. Despite its success, the Harrier rarely takes off and lands vertically because this severely limits its lifting capabilities and its range. It usually makes short takeoffs and landings, sometimes rolling forward to lift off a "ski jump" ramp, both on the ground and on small aircraft carriers.

The Soviet Yak-38 "Forger" single-seat naval fighter was similar to the Harrier. It operated from the USSR's four multipurpose cruiser/aircraft carriers during the 1970s and 1980s. Unlike the Harrier, the Forger had three jet engines, two of which provided vertical lift and the third that could be vectored to provide both vertical lift and horizontal thrust. The Forger could not make rolling takeoffs like the Harrier but had to rise straight up, using more fuel. It was severely limited in range and performance and was withdrawn from service in the 1990s.

Probably the most commonly tested type of V/STOL aircraft has been the various tilting propulsion designs, often collectively called "convertiplanes." These aircraft tilt propellers, rotors, ducted propellers, or even their entire wings from vertical to horizontal.

The XV-3 in hover at Ames Research Centre, July 1962.

The Bell XV-3 was begun in 1951. It made its first flight in August 1955 as a helicopter but crashed only two months later. A second craft began flying in December 1958 and was more successful. It operated until 1965 when it was damaged in a wind tunnel test.

After the successful flight test of an experimental VTOL aircraft called the X-100, Curtiss-Wright designed the M-200 (also known as the X-200), later designated the X-19. The U.S. Air Force was sufficiently impressed with the new craft that it ordered two prototypes of the X-19. On its first flight on November 20, 1963, the aircraft was damaged in a hard landing. It was repaired and made 49 other flights of very short duration. On its 50th and last flight, a transmission part failure caused an "asymmetric lift" (one engine produced more lift than the other, causing the aircraft to suddenly begin to roll over) and the pilots ejected. The aircraft was repairable, but it never flew again. Five months later the program was cancelled. Its sister craft never flew.

Other similar American aircraft, designated VZ-2, VZ-4, X-22, and X-18, were also developed during the late 1950s and 1960s. The VZ-2 and X-18 tilted their entire wings to the horizontal while hovering. But this made them highly susceptible to wind gusts (the large flat wing acted like a barn door swinging in the wind) and they were difficult to control. The X-22 proved to be one of the most successful early convertiplanes and flew in various tests for more than a decade. The French firm Nord built two Nord 500 Cadets, equipped with twin wing-mounted propellers. The first aircraft hovered while tethered to the ground in 1968 but never flew again.

The XV-15 began development in the late 1970s. It was the most capable of these experimental aircraft. Developed by NASA and the Department of Defense, the XV-15 had two large rotors mounted at the ends of wings connected to what looked like the fuselage of a small business jet.

The XV-15 was so successful that it ultimately led to the V-22 Osprey, which was intended to be a multipurpose aircraft for the U.S. Marine Corps, the U.S. Coast Guard, and some special purpose missions, such as the recovery of downed pilots deep behind enemy lines (called Combat Search and Rescue, or CSAR). The V-22, which looks like a more muscular version of the XV-15, offered greater range, speed and lifting capability than conventional large transport helicopters. By essentially operating as a conventional aircraft in forward flight, it avoided the limitations of helicopters, such as less speed and range. One of the key developments for the Osprey was a transmission shaft running through the wing and connecting both engines. If one engine failed, the other engine could keep both propellers rotating.

Some aircraft use large flaps or other devices on their wings to dramatically increase lift at low speeds or even to deflect engine thrust downwards. The American YC-14 and YC-15 aircraft, tested in the 1970s, evaluated these technologies, some of which eventually made their way to the successful Boeing C-17 cargo plane (which is not considered a STOL aircraft but which can operate from shorter runways). The Soviet An-72 cargo aircraft also operates in this manner.

CLICK TO SEE ANIMATION