The Challenge of Vertical Take-Off and Landing (VTOL) Aircraft
Military aircraft face significant challenges when operating from runways, which are often vulnerable due to their reliance on long stretches of concrete. This limitation restricts flexibility and increases the risk of damage during operations. Operating from an aircraft carrier’s pitching deck is even more complex, with limited space making it a dangerous endeavor. As a result, designers have long sought to develop vertical take-off-and-landing (VTOL) aircraft that can function like helicopters, offering greater operational freedom.
Creating a VTOL aircraft is no easy task, and many projects have struggled to achieve success. Despite these difficulties, some remarkable designs have emerged, showcasing the complexity and innovation involved in this field. Only three out of ten listed projects made it into service, highlighting how challenging it is to produce a viable VTOL system.
10: VFW VAK 191B
The VFW VAK 191B was an ambitious attempt by West Germany to develop a supersonic VTOL strike aircraft as part of a NATO competition. It featured a propulsion system using a Rolls-Royce/MAN Turbo RB.193 engine and two lift jets. However, after the NATO requirement was scrapped, the project continued for research purposes. Eventually, it was abandoned in 1972 due to lack of funding and relevance.
The VAK 191B had a simple propulsion system that used steerable jet thrust, similar to the Harrier. Its design was sleeker than the Harrier, with a higher fineness ratio, making it better suited for high-speed flight. However, it suffered from a low thrust-to-weight ratio and small wings, which would have made landing difficult. Despite its promising features, it never entered service.
9: Ryan X-13 Vertijet
The Ryan X-13 Vertijet was another attempt at a tail-sitter VTOL aircraft. It featured a Rolls-Royce Avon turbojet engine and was notable for its successful flight across the Potomac River to the Pentagon. However, the military ultimately decided it was not needed, and the project did not progress further.
The X-13 was more successful than other tail-sitters but failed to meet the military’s needs. Its approach was deemed outdated, and it never entered service.
8: Lockheed XV-4 Hummingbird
The Lockheed XV-4 Hummingbird was designed as a target-spotting aircraft for the US Army. It used multiple nozzles for vertical lift, but the thrust was insufficient, leading to a shift toward short take-off capabilities. It was the first demonstration of an ejector-augmentor VTOL system, which aimed to increase thrust through fluid dynamics.
The XV-4A had a top speed of 518 mph, but its thrust-to-weight ratio was inadequate. Both prototypes crashed, with one resulting in the death of a test pilot. The XV-4B had a more complex propulsion system, but it left little room for useful payloads.
7: Yakovlev Yak-38 ‘Forger’
The Yak-38 ‘Forger’ was a Soviet interim VTOL aircraft that served from 1976 to 1991. It was similar to the British Sea Harrier but with a higher maximum speed. It laid the foundation for the more advanced Yak-41. Despite its limitations, it was one of only three VTOL aircraft to enter service.
The Yak-38 featured two lift engines and a cruise engine with thrust-vectoring. It also had an automatic ejector seat, a feature later adopted by the F-35B. However, it was vulnerable to lift-engine failures and had poor range and load capacity.
6: Ryan XV-5A Vertifan
The Ryan XV-5A Vertifan was designed to support close-air operations. It used a fan-in-wing lift system, which provided more thrust than conventional nozzles. However, the complex system made it difficult to control, especially during transitions between flight modes.
Despite its innovative design, the XV-5A suffered from poor low-speed handling and acceleration. It also faced several fatal crashes, which ultimately led to the project’s cancellation.
5: EWR VJ 101
The EWR VJ 101 was a six-engined VTOL aircraft developed by Heinkel, Messerschmitt, and Bölkow. It had a unique propulsion system with integrated controls for pitch, roll, and yaw. Although it reached Mach 1.14, issues with ground erosion and hot-gas ingestion plagued the design.
The VJ 101 was a promising concept, but it was eventually replaced by more practical aircraft like the F-4 and Typhoon.
4: Dornier Do 31
The Dornier Do 31 was a German attempt at a supersonic VTOL fast-jet. It was designed to provide tactical support to forward bases. However, the development of the Fiat G.222 rendered it unnecessary, as it offered similar performance without the complexities of VTOL.
The Do 31 was an impressive engineering achievement, but it was ultimately too costly and complex for practical use.
3: Yakovlev Yak-141/41/43/201 Series
The Yak-141 was a Russian VTOL aircraft with a swivelling rear nozzle and a high speed of Mach 1.4. However, its use of afterburning for vertical flight caused problems with noise and engine intake issues. The program was halted in 1991 due to economic conditions.
The Yak-141 was a stepping stone to the more advanced Yak-43, which was never built. A stealthy version, the Yak-201, was also studied but never produced.
2: Lockheed Martin F-35B Lightning II
The F-35B is the STOVL variant of the F-35 family and is the first supersonic jump jet to enter service. It uses an integrated lift fan propulsion system, allowing it to perform vertical take-offs and landings. It has been adopted by several countries, including the US, UK, and Japan.
The F-35B offers advanced sensor systems and radar stealth, giving pilots superior situational awareness. However, its STOVL systems reduce its fuel capacity and range compared to the CTOL version.
1: Harrier
The Harrier is widely regarded as the most successful VTOL aircraft. It was the first operational STOVL strike fighter and introduced the term “jump jet.” Its simplicity and reliability set it apart from other VTOL designs.
The Harrier was initially funded by the US Army and later operated by the RAF and Indian Navy. It was eventually replaced by the Harrier II, which served with several other nations.
