Fiat G.91 - Design and Development

Design and Development

In December 1953, NATO Supreme Command issued specifications for a new light tactical support aircraft. 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 technical requirements were:

• 1,100 m (3,610 ft) takeoff distance over a 15 m (49 ft) obstacle
• Capability to operate from grass strips and streets
• Maximum speed of Mach 0.95
• Range of 280 km (170 mi) with 10 minutes over the target
• Armoured protection for the pilot and the fuel tanks
• 4 × 12.7 mm (.5 in) or 2 × 20 mm or 30 mm guns
• A maximum of 2,200 kg (4,850 lb) empty weight and 4,700 kg (10,360 lb) max weight

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.

The designs were required within two months of the competition. The special committee had to choose from eight projects. Besides the G.91, these included the Northrop N-156, Dassault Mystère XXVI (future Dassault Étendard IV), Sud-Est Baroudeur, Aerfer Sagittario II and the Breguet Taon Br.1001. These designs were assessed starting on 18 March 1953 by AGARD (Advisory Group for Aeronautical Research and Development) under the leadership of Von Karman.

Project selections took 18 months to complete. The first result was announced on 30 June 1955: the winning projects were, in order: the Breguet Br. 1001 Taon and the Fiat G.91. A third was added to these two aircraft: the Mystère XXVI, and three prototypes of this design were also ordered. The G.91 first flew on 9 August 1956 at the Caselle airfield, Turin, in the hands of Chief Test Pilot Riccardo Bignamini, with a seven-month lead over the Dassault prototype and almost a year ahead of the Breguet Taon.

The intensive series of tests that followed the maiden flight was not without its setbacks. The most serious problem was the elimination of aeroelastic vibrations which, on 20 February 1957, led to the destruction of the first G.91 prototype. The re-engineering work to cure the problem was very extensive and resulted in the second prototype being fitted with a larger tail, a 6 cm (2 in) higher canopy, a ventral fin and some other modifications. This aircraft flew in July 1957, but it was not sent to the final evaluation. The third and fourth G.91 prototypes were sent to France, for the competition.

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 the trials the Italian aircraft performed impressively and, in January 1958, the Fiat G.91 was officially declared the winner.

Following a meeting of NATO Defence Ministers in April 1958 it was agreed that the G91 would be the first NATO lightweight strike fighter, it would be followed in 1961 by the Breguet Taon. A production meeting was planned for May 1958 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 and pay for the Turkish aircraft. The defence ministers reached 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 G.91 prototype, the French government preferred to pursue development of the locally-designed Étendard. The British government similarly ignored the competition to concentrate on Hawker Hunter production for the same role. In fairness, it should also be pointed out that the Italian government also ordered the G.91 for the Italian Air Force before the results of the competition were known. These pre-production machines would later go on to serve for many years with the Italian aerobatic team, the Frecce Tricolori as the G.91 PAN. The type was also considered by Austria, Norway, Switzerland, and even the United States Army, which briefly evaluated the type as a possible Forward Air Control aircraft before relinquishing all fixed-wing aircraft operations to the Air Force.