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Between
10/38 and 4/39, Sergio Stefanutti designed the SAI.7 as a
high-performance touring plane for the civil market. Undoubtedly
one of the best looking aircraft ever designed, the SAI.7
possessed exceptionally clean lines, was of wooden construction
with plywood skin. The SAI.7 was the first Ambrosini airplane
with retractable landing gear, in this instance of the
fully-retractable tail wheel type with wide-track main units that
swung inward and upward into the roots of the cantilever low-set
wing. The first two aircraft were completed in 7/39 with a
windscreen design that extended right to the nose for a very
clean entry, and were entered in the Avio Raduno del Littorio
competition that started a few days after the machines made
their maiden flights. The aircraft were too under developed to
win the competition, but nonetheless put in a very credible
performance that included a maximum speed of 251 mph with a 280
hp air-cooled Hirth HM 508D inverted-Vee engine. One aircraft
took a class closed-circuit speed record over 61.2 miles with a
speed of 244 mph. The high
speed/power ratio of the SAI.7 caught the imagination of the Italian air
force which saw considerable possibilities in the concept of a
lightweight interceptor that could be built in large numbers without
drawing on the country's strategic stockpile of aluminum alloys. Little
was done about the military potential of the SAI.7 in the short-term,
however, and it was 1941 before Stefanutti began work on a fighter
trainer derived from the SAI.7 with the Hirth engine being replaced by a
280 hp Isotta-Fraschini Beta RC.10 inverted-Vee engine. The fuselage and
wingspan were increased and the faired racing windscreen was replaced by
a conventional stepped windscreen at the front of a more heavily framed
cockpit enclosure with two rearward sliding sections for access, and the
landing gear was modified with a fixed tailwheel. These changes
increased the maximum take-off weight, but the maximum speed was reduced
only slightly to 248.5 mph. By the time the first of an eventual 10
SAI.7 fighter trainers appeared, the increasingly difficult military
position in which Italy found itself was reflected in the emphasis on
combat aircraft rather than trainers; and so further work on the fighter
trainer variant was abandoned until after the war when it was revived
and led to the S.7 and Supersette trainers. Throughout
this process Stefanutti and the Italian air ministry had not lost sight
of the SAI.7's potential for development into a lightweight fighter. In
1942 Ambrosini evolved the SAI.107 as an experimental fighter for full
evaluation of this potential. The new type was essentially a single-seat
derivative of the fighter trainer was a considerably more powerful
engine in the form of a 540 hp Isotta-Frashini Gamma RC.35 IS inverted-Vee
engine driving a two bladed propeller. This prototype recorded a maximum
speed of 348 mph and confirmed that a viable interceptor was possible.
Stefanutti then proceeded to the design of a fighter with full
operational equipment including armament. This was the SAI.207 that
otherwise differed from the SAI.107 only in its more powerful engine
driving a three-bladed propeller. Flight trials began in 1942, and in
the course of these, the first SAI.207 recorded a dive speed of 596 mph
at 9,845 ft, corresponding to Mach .86.
The Italian
air ministry ordered Ambrosini to begin work on a pre-production batch
and placed an order for 2,000 production aircraft. In the event only 13
of the pre-production aircraft were completed, three of them being
allocated to the 3rd Stormo Caccia Terrestre during 7/43 for operational
trials. Italy secured an armistice with the Allies just two months
later. Production of the SAI.207 had meanwhile been cancelled in favor
of its SAI.403 Dardo (dart) derivative. This essentially was a more
sophisticated version of the SAI.207 with the same type of stressed-skin
wooden construction, but which had fully retractable landing gear rather
than the 207's combination of retractable main gear and fixed tailwheel.
In addition, changes included a revised tail unit with a
variable-incidence tailplane, greater ammunition capacities, and an
upgraded powerplant in the form of the 750 hp Isotta-Fraschini Delta
RC.21/60 Serie I-IV driving a three-bladed Piaggio constant-speed
propeller.
The three
variants of the SAI.403 planned were the Dardo-A lightweight interceptor
with armament restricted to two 12.7mm machine guns, the Dardo-B
general-purpose fighter with two 20mm cannon and two 12.7mm machine
guns, and the Dardo-C long-range fighter with 2 20mm cannon and
provision for two 39.6 US gallon drop tanks supplementing the internal
fuel capacity from the 79.25 US gallon of the Dardo A/B to 108.3 US
gallon. The first SAI.403 flew late in 1942 and revealed exceptional
performance, including a maximum speed of 404 mph at 23,620 ft. This
resulted in the termination of the SAI.207 in favor of 3,000 of the
SAI.403 to be produced. None of these fighters had been delivered before
the Italian armistice with the Allies and the closing of the program.
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