SAI 403 - details

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Jan 18, 2005
I thought I knew most of the aircraft of WW2, but as usual with 'know-it-alls' I found myself tripped up when I came across a referece to the Ambrosini SAI 403 (apparently a single-engined, single-seat fighter). Does anyone know whether it actually got into service with the Regia Aeronautica and any other details?
The SAI 403 is a great plane! 8) I think it proves just how capable the Italians were at building great airframes.

At the beginning of the war, the Regia Aeronautica (italian air force) had a lot of obsolete planes,
most fighters were biplanes. They also had a few monoplane fighter types of excellent maneuverability,
but their radial powerplants of ca. 1.000PS didn´t allow at least 530km/h. In 1941,
german DB 601 and later also DB 605 engines arrived and were built under licence for the existing
monoplane fighters (some guys [that I insulted here in earlier versions] even tried to use a DB 601 with
a CR.42 biplane), and the results were good, but the speed didn´t reach allied standards. Further, italian fighters
were mostly too weak armed with only two 12,7mm guns. Because the italian industry wasn´t as fitted for war
and capable as the ones of its enemies, they tried to solve the problem with a lightweight fighter:
The Ambrosini S.A.I.207.
This one followed after the S.A.I.107, which had less than 540 PS, but reached 560km/h.
The S.A.I.207 were mostly built of nonstrategic materials, had excellent handling and maneuverability.
Its service ceiling was 12.000m and its armament was typical italian: two 12,7mm Breda MG.
It reached 625km/h with a a 750PS Isotta-Fraschini Delta RC40 V-engine and 2.000 orders came in.
They were cancelled after 15, when the advanced S.A.I.403 was in prototype stadium. 3.000 of them were ordered,
but only the prototype was finished. The S.A.I.403 had improved aerodynamics and armament.
But the development of these attractive planes ended in october, 1943 due to the armistice with the allies.

My comment:
Appearing in great number since spring/summer 1944, they would have been a problem for the allies.
With intact fuel supply and a anti-bomber-weapon like the later german R4M "Orkan" (Hurricane)
rocket, they would have blown the allied daylight bomber offensive over italy.
Imagine: 650km/h with only 750PS! Compare this with the biplanes of the beginning 1930´s,
which had almost the same power, but less than 400km/h!
Even the excellent Heinkel He 100 out of 1938 needed 1.1750PS for only 20km/h more!


Ambrosini S.A.I. 403 italian fighter
Wing span: 9,80m
Length: 8,20m
Height: 2,90m
Empty: 1.983kg
Maximum Take-Off: 2.640kg
Maximum Speed: 650km/h
Service Ceiling: 10.000m
Normal Range: 937km
One 750PS Isotta-Fraschini R.C.21/60 V-Engine
four 12,7mm Breda-SAFAT MG or each two
12,7mm Breda-SAFAT MG and 20mm MG151/20 cannon

The main reason I asked about it is that I've just found a card model of it, with all the instructions and gen in Polish...!

By the way, I think those performance figures you gave are probably before it was fitted with war equipment, which would have a pretty drastic effect, as it always does. Still impressive, though and rather reminiscent of the Saab J32 in terms of performance and power available.
That looks a bit SAAb-ish, too. Incidentally, thst J32 should read J21 - finger trouble!
Forgive me for resurrecting this 5-year old thread but couldn't find a more recent one and didn't want to start a new topic.

I was doing some more thinking about the SAI.403 and its possibilities.

But I would first like to add some information found at the Great Planes forum:

First to say, the SAI 403 was not a developement of the 207, but a parallel project.
The 207 was intended to be the "natural evolution" of the SAI 7 and 107, while the 403 was intended to solve some of the 207 expected problems. First the loss in handling due to the heavier wingload (using a different airfoil and tail section), second, the complexity of the construction (that was simplified) and third, the usage of several foreign types of wood (the 403 used only types of wood widely avaliable in Italy).
However, during the evolution of the project of the 403 (that was widely made by Ing. Ambrosini, the owner of SAI, and an entusiast aeronautical engineer himself) Ambrosini and Stefanutti came in discord, cause, it seems, Stefanutti thought Ambrosini used too extreme solutions for the 403, especially for the wings, so Stefanutti didn't want his name to be associated with SAI 403.
However, the discord was soonly over, and Stefanutti accepted to design one of the two projected developements of the 403, the SAI 503 (with a 1260 hp Isotta Fraschini Zeta engine), while one of his collaboretors designed the all metal SAI 404 (with the same Zeta engine).
After the war, Stefanutti stated that Ambrosini's solutions were effectively able to dramaticaly improve the handling of the 403.

The main differences between the three aircrafts (S7, 207 and 403) seems to be the wings' airfoil and structure.
the 207 had a 5 spars wing, with an airfoil of the NACA 230xx series. The S7 had a 2 spars wing, with a NACA 2212 airfoil. It can be interesting to find the airfoil used by Ing. Ambrosini for the 4 spars wing of the 403.

The SAI.207 was operationally tested but washed out. It really wasn't suited as a fighter because of structural deficiencies and limited climb rate.
These problems were supposed to be overcome with the SAI.403 but there exists little verifiable info on that type. To me it does seem however that the SAI.403 got an upgraded Delta engine of more than 800 hp. A next version would get the stronger Zeta engine but this powerplant proved even more troublesome than the Delta.

My own personal opinion on the possibilities of the SAI.403 has become more negative in recent months. By 1943/1944 the main threat came from the American bombers. The SAI.403 would have been horribly vulnerable against them: it had virtually no armour, a light armament of 2 MGs, no supercharger for higher altitudes and a speed unable to escape enemy escort fighters. There was a version planned to have 2 20mm cannons in wingpods (given that the wing would be able to hold the recoil forces) but this would have made the SAI.403 a great deal slower.
So it's clear that the only use of the SAI.403 would have been against enemy fighters at low or medium altitude. But when we compare with the average allied fighter aircraft of 1944 we see the Spitfire IX, the P-47 or P-38. All of these are faster and have more powerful armament.

So then one would say, yes, but the SAI.403 is a light and cheap fighter which can be built in larger numbers. Well, I have my doubts about that. First of all. the SAI.403 is NOT a light fighter. It weight over 2 tons, which makes it close to the Yak-1 or -3 which was also partially made out of wood. The IF Delta engine was lighter than other ww2 fighter engines but did not give the same amount of power either. It was smaller and lighter than other fighters and built out of wood. But nothing indicates that this type could have been built in twice the numbers of the old CR.42 or C.200 fighters.

And still .. it could have been a nice supplement to the Italian G.55 and Re.2005 which excelled at higher altitudes and had a powerful armament. The SAI.403 could have been used for CAP at lower altitude and as an unarmed photo-recon aircraft.

One more thing on the armament. The SAI's carried Italian Breda-SAFAT MGs which were heavier than the German MG 131. In fact it almost weight as much as the 20mm MG 151 cannon. As such it might have been possible to either install 2 MG 151s in the wings (with a bulge for the ammo) or to have 4 MG 131s in the cowling and the wings. The latter would give it the same armament as the P-51B/C at that time.


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