VTOL

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The Harrier Jump Jet, one of the most famous and successful fixed-wing single-engine VTOL aircraft.

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The Soviet Union’s VTOL aircraft, the Yakovlev Yak-38

It was a French engineer, Michel Wibault – whose company had built a range of commercial aircraft during the 1930s – who came up with a possible solution. Wibault envisaged a turbojet using vectored thrust, whereby rotating nozzles could be used to direct exhaust gases either vertically downwards or horizontally aft. Seeking funds to develop his theme, he approached the Paris office of the Mutual Weapons Development Team, which at that time was headed by Colonel Bill Chapman. This was in 1956, at the time when Bristol Siddeley was working on the Orpheus engine to power NATO’s lightweight fighter, so Chapman approached Dr Stanley Hooker, Bristol’s Technical Director, and sought his views on the Wibault project. Hooker was enthusiastic, and one of his project engineers, Gordon Lewis, was briefed to investigate the possibilities. After preliminary studies, in January 1957 Wibault and Lewis applied for a joint patent covering the design of a vectored-thrust engine known as the BE.52, this was further developed into the BE.53 Pegasus I, which was based on the Orpheus.

In the summer of 1957, details of the proposed engine were passed to Sir Sydney Camm at Hawker Aircraft, who made a preliminary design for an aircraft to go around it. The design was allocated the project number P.1127. It bore no resemblance, at this stage, to the amazing combat aircraft that was ultimately to be developed from it – the Harrier – but it was a firm beginning.

In June 1958, the Mutual Weapons Development Team agreed to pay 75 per cent of the development costs of the Pegasus engine. Funding the airframe, however, proved a tougher obstacle, for research funds had been eaten up by other projects. Hawker had no alternative but to proceed as a private venture while the Air Staff set about drafting an operational requirement to cover the concept. This emerged in April 1959 as GOR345, and Specification ER204D was issued to cover the P.1127, but it was not until October 1959 that Hawker received a preliminary contract for the building of two prototypes. It was fortunate that Hawker recognised potential when they saw it, or there might never have been a Harrier.

In Hawker’s opinion, the P.1127 was the ideal design to meet a new NATO requirement, NBMR-3 (NATO Basic Military Requirement No. 3, which was issued in 1961 after several revisions and called for a VTOL strike fighter with a sustained capability of Mach 0.92 at low level and supersonic speed at altitude). The P.1127 was not supersonic, but it had a vast amount of development potential ahead of it, and so Sir Sydney Camm proposed a modified version, the P.1150, which was to have a thin wing and an advanced Pegasus engine. Hawker could have progressed with the building of a prototype almost immediately, but yet another revision to NBMR-3, requiring greater range and load-carrying capacity, meant that the P.1150 would have been too small. Camm and his team therefore set about designing a scaled-up version, the P.1154, which was to have a BS.100 engine of 33,000 lb thrust.

However, the P.1154 had a formidable challenger, at least in theory, in the shape of the Dassault Mirage IIIV, whose forerunner, the Balzac VTOL research aircraft, was then under construction. The Balzac used the wings and tail surfaces of the Mirage III-01, married to a fuselage that was completely redesigned except for the main frames and the cockpit section. French research into VTOL, in fact, pre-dated both the Hawker P.1127 and NBMR-3, having been initiated in response to a French Air Force requirement. However, the French chose to pursue their experiments with a combination of lift jets and propulsion engines, rather than vectored thrust. Even then, the engines they chose to power the Balzac were British, consisting of eight lightweight Rolls-Royce RB.108 lift engines and a Bristol Siddeley Orpheus B.Or.3 turbojet for forward propulsion. Ironically, the use of British engines of proven design led to a strong lobby in both the Ministry of Aviation and the RAF that favoured concentrating on the development of the Balzac/Mirage IIIV as the standard NATO strike fighter, at the expense of the P.1154.

The Balzac made its first tethered flight on 12 October 1962 in the rig once used by the ill-fated Coléoptère at Melun-Villaroche, and initial tests were made with a non-retractable landing gear. The aircraft made its first free vertical take-off on 18 October 1962 and the first transition to horizontal flight on 18 March 1963. The test programme continued until 27 January 1964, when the aircraft suffered a critical divergent lateral oscillation during hovering descent. It dropped out of the sky like a falling leaf, crashing and killing its pilot. It was rebuilt, but crashed again on 8 September 1965, killing another pilot. This time it was beyond repair.

Meanwhile, the first flight of the Mirage IIIV had been delayed because of problems in selecting an appropriate propulsion engine. The prototype eventually flew on 12 February 1965, when hovering trials began; at that time the aircraft was fitted with a SNECMA TF104 turbofan, but this was subsequently replaced by a more powerful TF-106. The lift engines were eight Rolls-Royce RB.162-1 turbojets. During flight testing, the first prototype Mirage IIIV reached a speed at high altitude of Mach 1.35. The second prototype, which flew for the first time on 22 June 1966, was fitted with a Pratt & Whitney TF-30 turbofan rated at 11,330 lb thrust (18,520 lb with afterburning), and on 12 September the aircraft reached a speed of Mach 2.04. However, it was destroyed in an accident on 28 November, resulting in the cancellation of plans to build further prototypes and develop the aircraft to production standards. In fact, the Mirage IIIV programme had been under critical review for some time, not only on grounds of escalating costs but also because the programme had slipped badly. Originally, it had been expected that the prototype Mirage IIIV would fly late in 1963, and that the first squadron would form in 1966, if trials were successful. Ironically, another Dassault design, the Mirage F, which had been built solely to test the Mirage IIIV’s armament system and the TF-306 engine that was to have powered the operational version of the VTOL fighter, was found to have enormous potential in its own right as an operational strike fighter. It eventually entered service as the Mirage F-1, and did everything the Mirage IIIV was expected to do except take off vertically.

In Federal Germany, the design teams of Bölkow, Heinkel and Messerschmitt had joined forces in 1959 at the suggestion of the German Defence Ministry to develop a Mach 2 VTOL interceptor. The design they adopted involved an aircraft of conventional configuration, but with turbojet engines mounted in swivelling wing-tip pods to provide both lift and control in vertical and low-speed flight, together with fuselage-mounted lift engines. A bedstead-type test rig was built and had made 126 flights by April 1965, fitted with a single RB.108 lift engine. The consortium, known as the Entwicklungsring Süd Arbeitsgemeinschaft, produced two prototypes of an experimental single-seat VTOL aircraft, the VJ-101C, which were fitted with six RB.145 engines developed jointly by Rolls-Royce and MAN Turbomotoren. Tethered trials of the VJ-101CX-1 began in December 1962, the first free hover being made on 10 April 1963. The aircraft made its first horizontal take-off on 31 August 1963, and its first transition on 20 September 1963. During further trials the following spring the VJ-101CX-1 exceeded Mach 1 in level flight on several occasions, proving the viability of the concept; unfortunately, the aircraft crashed after a normal horizontal take-off on 14 September 1964, the pilot escaping thanks to his Martin-Baker Mk GA7 zero-zero-ejection seat.

Hovering trials of the second prototype, the VJ-101CX-2, began in the spring of 1965, and it made its first free flight on 12 June that year. By this time, Heinkel had dropped out of the consortium, and the resources of Bölkow and Messerschmitt were being channelled into other programmes. Plans to produce an operational version of the VTOL research aircraft, the VJ-101D, were therefore never implemented.

Nevertheless, the two VJ-101Cs had provided a wealth of knowledge about VTOL techniques, and it formed a sound basis for other German companies involved in the field. Foremost among them was the former Focke-Wulf company, which had produced a design study to meet a German Defence Ministry Requirement – VAK 191B – for a subsonic VTOL tactical fighter to replace the Fiat G.91. The initial design study was designated FW 1262, and in 1964 VFW and Fiat agreed to collaborate in development work under a Memorandum of Agreement signed by the German and Italian Defence Ministers. The Italians later dropped out of the programme, but VFW found another partner, Fokker of Holland, and Fiat agreed to carry on as sub-contractor.

Work proceeded with Federal German Government funding, and the first VAK 191B was rolled out in April 1970. It made its first conventional flight on 10 September 1971, and this was followed by a period of tethered hovering trials. By this time the other two prototypes had also joined the test programme, and on 26 October 1972 one of these made the type’s first vertical-to-horizontal transition. During this test the aircraft reached a speed of 276 mph, and its RB.126 lift jets were shut down and restarted in flight for the first time. At the end of 1972, however, German Government funding of the VAK 191B was terminated, and no further development was undertaken. By this time, RAF Harriers were being deployed to Germany, and their presence more than adequately filled the V/STOL (vertical/short take-off and landing) requirement in NATO’s front line. Hawker’s earlier faith had paid dividends, and the Harrier remained Western Europe’s only operational V/STOL combat aircraft.

Meanwhile, Soviet experiments with VTOL had resulted in an experimental prototype, the Yakovlev Yak-36, dubbed ‘Freehand’ by NATO. The Freehand was powered by two non-afterburning Soyuz Tumanskiy/Khatchaturov R-27-300 turbojet engines of 11,000 lb thrust each, mounted forward of and below the cockpit. They were fitted with louvred nozzles, which could be vectored through about 90 degrees and exhausted at the centre of gravity. Engine bleed air was used for reaction control nozzles at each wingtip fairing, on the tailcone, and at the tip of a ten foot long nose probe. The Yak-36 made its first untethered hover on 9 January 1963. From there, the flight envelope was slowly expanded, with a double transition from vertical take-off to forward flight and back to vertical landing performed on 16 September 1963. The Yak-36 was, in effect, a technology demonstrator that led to the development of the Yak-38 (NATO reporting name ‘Forger’), an operational strike aircraft that served on the Soviet Navy’s Kiev-class aircraft carriers.

Russia’s last venture into the V/STOL field was ambitious, involving the supersonic Yakovlev Yak-41 (NATO reporting name ‘Freestyle’). The Yak-41 programme was initiated in 1975, about the same time that the Yak-38 was first being deployed. The supersonic Freestyle was optimised for air defence with an attack capability as a secondary role. The first conventional flight was made on 9 March 1987 and the first hover on 29 December 1989. The first official details were not released by the Soviet Union until the 1991 Paris Air Show, by which time the two flying prototypes, now redesignated Yak-141, had accumulated about 210 hours’ flying time. A dozen FAI-recognised Class H. III records for V/STOL were set in April 1991, consisting of altitudes and times to altitudes with loads. During flight testing, the ‘Freestyle’ achieved a maximum speed of Mach 1.7. Flight testing was originally intended to continue until 1995, but development was stopped in August 1991 because of the shrinking Soviet military budget. Yakovlev funded the development from its own resources for a while, in the hopes of attracting a foreign investor. The second prototype was destroyed after a hard landing on the aircraft carrier Admiral Gorshkov on 5 October 1991. The following year, the surviving prototype was demonstrated at the Farnborough Air Show, but the design bureau was still unable to find a market for the design. The Yak-141 was claimed to be as manoeuvrable as the MiG-29, which is doubtful.

The operational use of V/STOL aircraft, by the very nature of the concept, was always going to be dangerous. Between 1961 and 2000 the RAF lost 100 Harriers, including P.1127 and Kestrel development aircraft, to non-combat causes. In addition, the US Marine Corps had lost 143 examples of its versions, the AV-8A and AV-8B, since V/STOL operations began in 1971 and up to 2004. This is a high price to pay for combat versatility. Because of its complexity, the Harrier remains one of aviation history’s most unforgiving aircraft, and it will ‘bite’ the unwary at the slightest pretext. It remains to be seen whether its supersonic successor, the Lockheed Martin F-35, will have gentler habits.

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