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. Both the third and fourth G.91 prototypes were later sent to France to continue the evaluation flights.