The Fiat G.91 is a jet fighter aircraft designed and built by the Italian aircraft manufacturer Fiat Aviazione, which later merged into Aeritalia.
The G.91 has its origins in the NATO-organised NBMR-1 competition started in 1953, which sought a light fighter-bomber (officially, the competition was seeking a "Light Weight Strike Fighter") to be adopted as standard equipment across the air forces of the various NATO nations. The G.91 was specifically designed to fulfil the requirements of this competition, being relatively lightweight and capable of operating from austere airstrips while also being armoured and suitably armed while remaining relatively affordable in comparison to many frontline fighters. On 9 August 1956, the prototype conducted its maiden flight. After reviewing multiple submissions, the G.91 was picked as the winning design of the NBMR-1 competition.
During 1961, the G.91 entered into operational service with the Italian Air Force, and with the West German Luftwaffe in the following year.Angelucci and Matricardi 1980, p. 272. Various other nations adopted it, such as the Portuguese Air Force, who made extensive use of the type during the Portuguese Colonial War in Angola and Mozambique. The G.91 remained in production for 19 years, during which a total of 756 aircraft were completed, including the prototypes and pre-production models. The assembly lines were finally closed in 1977. The G.91 was also used as a basis for a twin-engined derivative: the Fiat/Aeritalia G.91Y. The G.91 had a relatively lengthy service life, outlasting the Cold War and being finally withdrawn in 1995. It was displaced by newer types such as the Dassault/Dornier Alpha Jet and the Aermacchi MB-326.
Development Origins
As a result of experiences from the Korean War alongside newly developed concepts of aerial cooperation, the members of NATO recognised a need to reequip their inventories with suitable jet-powered ground attack aircraft.Apostolo 1966, p. 3. In December 1953, NATO Supreme Command issued specifications for a new light tactical support aircraft.Angelucci and Matricardi 1980, p. 273. European manufacturers were invited to submit their designs for this requested Light Weight Strike Fighter role. The G.91 was designed to this specification by the Italian engineer Giuseppe Gabrielli, hence the "G" designation. The competition was intended to produce an aircraft that was light, small, expendable, equipped with basic weapons and avionics and capable of operating with minimal ground support. These specifications were developed for two reasons: the first was the nuclear threat to large air bases, many cheaper aircraft could be better dispersed, and the other was to counter the trend towards larger and more expensive aircraft.
The specified technical requirements included a 1,100 m (3,610 ft) takeoff distance over a 15 m (49 ft) obstacle, the capability to operate from semi-prepared grass airstrips and roads, a maximum speed of Mach 0.95, a range of 280 km (170 mi) with 10 minutes over the target while possessing a maximum of 2,200 kg (4,850 lb) empty weight and 4,700 kg (10,360 lb) max weight. Equipment requirements were the presence of armoured protection for the pilot and the fuel tanks along with alternative arrangements of 4 × 12.7 mm (.5 in) machine guns or 2 × 20 mm or 30 mm autocannon.Ferrari 1992, p. 83. These operational specifications were viewed as not being straightforward to fulfil at that time.
The challenge of providing an engine that matched the requirements of lightness and power, reliability and ease of maintenance was solved by using the Bristol Siddeley Orpheus turbojet then at the start of development in the UK. Development of this engine was aided by substantial contribution from the US Mutual Weapons Development Programme. Nine of the ten designs to be subsequently submitted for the competition were powered by the Orpheus engine."NATO Choice Highlights Orpheus Success." Flight, No. 2618 Volume 75, 27 March 1959. p. 22.
In order to evaluate the bids that various aircraft manufacturers submitted in response, a special Advisory Group for Aeronautical Research and Development (AGARD) committee conducted extensive evaluations. The designs were required within two months of the competition, in which time an assortment of submissions were made, mainly by European companies. Besides the G.91, these included the Northrop N-156, Dassault Mystère XXVI, Sud-Est Baroudeur, Aerfer Sagittario 2 and the Breguet Br.1001 Taon. On 18 March 1953, these submitted designs formally began to be assessed by the AGARD committee, which was under the chairmanship of Theodore von Kármán. The initial project selection process took 18 months to complete.
Fly-off and selection On 3 June 1955, it was announced that the Fiat G.91 had been selected as the design with the most promise. Out of the submissions, the two winning projects were, in order: the Breguet Br.1001 Taon and the Fiat G.91. A third aircraft design was subsequently added: the Dassault Mystère XXVI. As a result of the G.91's selection, an immediate order was received for three prototypes and a total of 27 pre-production aircraft. Fiat quickly set about developing the concept to the prototype phase; noticeable changes in the design occurred during this phase, such as increases in the aircraft's maximum weight due to the addition of supplementary equipment and structural strengthening measures.
On 9 August 1956, the prototype G.91 conducted its maiden flight at the Caselle airfield, Turin, Italy; it was flown by Chief Test Pilot Riccardo Bignamini. Accordingly, the G.91 had achieved a seven-month head start over the Dassault prototype and almost a year ahead of the Breguet Taon.Angelucci and Matricardi 1980, p. 274. An intensive series of test flights followed the type's maiden flight, these were not without setbacks. The most serious problem discovered during these was the presence of aeroelastic vibrations, leading to a series of investigative flights to resolve this condition. On 20 February 1957, during a test flight intended to explore the limits of the aircraft's speed-load envelope, difficulties led to the destruction of the first G.91 prototype. The cause of the prototype's loss was linked to problems encountered with the horizontal control system which had led to structural failure; this led to the tail assembly being subjected to rigorous testing to identify the cause of the failure.
The re-engineering work that had been performed to cure the vibration problem was very extensive and resulted in the second prototype being fitted with a larger tail, a six cm (two in) higher canopy, and the addition of a ventral fin. The second prototype had other modifications, including the installation of a full armament and a more powerful Orpheus engine capable of 4,850 lbf. In July 1957, the second prototype performed its first flight; it was not immediately sent to participate in the final evaluation process.Ferrari 1992, p. 85. Both the third and fourth G.91 prototypes were later sent to France to continue the evaluation flights.Niccoli 2002, p. 168.
The final selection of the competing designs was planned for late 1957. In September 1957, at the Centre d'Essais en Vol at Brétigny-sur-Orge, in France, the three rival aircraft types met for evaluation trials. During these trials, the Italian aircraft is claimed to have delivered an impressive performance. In January 1958, the Fiat G.91 was officially declared the winner of the competition.
In April 1958, following a meeting of NATO defence ministers, an agreement was formed that the G.91 would be the first NATO lightweight strike fighter, and that it was to be followed in 1961 by "a developed version" of the Breguet Taon. In May 1958, a production meeting was scheduled to discuss the production of the aircraft with financial support from the United States; the Americans would provide some of the finance for the French, German and Italian aircraft, in addition to paying for the Turkish aircraft. The defence ministers reached an agreement to order 50 aircraft for each country.
Given the large economic and commercial interests at stake, there was a certain amount of controversy surrounding this decision. After the loss of the first G.91 prototype, the French government preferred to pursue development of the locally designed Étendard instead.Crosby 2002, p. 183. The British government similarly ignored the competition to concentrate on Hawker Hunter production for the same role. The Italian government had elected to order the G.91 for the Italian Air Force prior to the results of the competition being known. These pre-production machines would later go on to serve for many years with the Italian aerobatic team, the Frecce Tricolori, designated as the G.91 PAN.Green 1964, p. 35.Apostolo 1966, p. 2.
Production By 1957, Fiat Aviazione was in the process of establishing the first production line for the G.91 at their facility at Turin-Aeritalia Airport, Piedmont, Italy.Lambert 1957, p. 111. In total, Fiat constructed 174 G.91s of various variants for Italy, along with an additional 144 G.91 R/3 variants for West Germany (including 50 that had been ordered and then cancelled by Greece and Turkey). The first order was for 50 aircraft from Aeritalia, then Dornier and other German firms had an order for 232 machines, which was later increased to 294.Apostolo 1966, p. 7. The Luftwaffe (German Air Force) also bought 44 G.91T/3 two-seat trainers and another 22 were produced in Germany, ending production in 1972.Doll and Dorner 1974, p. 19.
The German order involved a production run of 294 G.91s that were domestically constructed in Germany under a license production arrangement by Flugzeug-Union Süd, a consortium of former competitors Messerschmitt, Heinkel and Dornier; Messerschmitt produced the forward fuselage and tail assembly, Dornier manufactured the center fuselage along with final assembly and flight testing, while Heinkel fabricated the wing. The G.91 holds the distinction of being the first combat aircraft to be manufactured in Germany since the Second World War. On 20 July 1961, the first German-built G.91 conducted its first flight from Oberpfaffenhofen Airfield.
Aeritalia also negotiated a comprehensive license manufacturing agreement for the Orpheus engine that powered the type.Lambert 1957, p. 114. Messier-Bugatti-Dowty produced the aircraft's undercarriage.
The G.91 was also considered by a number of nations, including Austria, Norway, and Switzerland. During early 1961, four G.91s were shipped to the United States in order to participate in a formal evaluation program for the United States Army; these aircraft were subjected to several evaluation trials, the majority of which taking place within Alabama and New Mexico. The US Army studied the use of the type as a possible Forward Air Control aircraft.Apostolo 1966, pp. 8–9. However, the service subsequently relinquished all fixed-wing aircraft operations to the United States Air Force, and thus the prospective G.91 procurement was not pursued.
Design
The Fiat G.91 is a lightweight tactical ground attack aircraft. It is a subsonic aircraft that was designed for both simplicity and agility. A key function of the aircraft is its short-field capability, having been constructed with a rugged airframe to withstand the demands imposed by high-intensity operations and the use of semi-prepared airstrips; the undercarriage is furnished with low-pressure tyres for the same purpose.Apostolo 1966, pp. 3, 5. The standard mission of the G.91 was the attack of targets within a 170-mile radius of its base with a typical loiter time of ten minutes, travelling at maximum speed in the combat area and traversing at cruise speeds. The G.91 is typically powered by a single Bristol Siddeley Orpheus turbojet engine.