Ambrosini SS.4

infobox Aircraft
name = Ambrosini SS.4
type =Fighter
manufacturer = SAI-Ambrosini



caption = Three views of the SS.4
designer = Sergio Stefanutti
first flight = 1935
number built = 3(?)
status =prototype
primary user =Regia Aeronautica

The SAI-Ambrosini SS.4 was an Italian fighter prototype developed in the late 1930s but never mass produced. The radical design for its day featured a canard-style wing layout and a 'pusher' propeller. Although intended as an air-superiority interceptor, the SS.4 never entered service because the design suffered from numerous faultsFact|date=January 2008 which caused the prototype to crash-land, killing the test pilot. The design, although unique, barely compared favorably to contemporary fighters, and the Regia Aeronautica decided not to pursue the design because of these shortcomings.

Development

The many types of Italian aircraft developed in the 1920'-40's had widely varied characteristics. There were mediocre aircraft, often put into production but generally not in great numbers; aircraft that were an honest compromise of many design characteristics which were the most produced (such as Italian fighters of '0' series); machines very advanced in structure but still quite conventional in project design (such as Reggiane Re.2005), and then there were machines that were unconventional in structure and design like the Piaggio P.119. However, some Italian designs were not only advanced and unconventional, but also exhibited completely novel concepts, with few if any comparable aircraft in the world at the time of their appearance. the SS.4 is one example of the latter.

The SS.4's designation comes from the initials of Sergio Stefanutti, the designer of this plane. This curious-looking machine was one of the more obscure World War II-era designs, and also one of the most unusual and elaborate Italian aircraft of the period.

SAI (Società Aeronautica Italiana)-Ambrosini was an ancient but small manufacturing firm based in Passignano sul Trasimeno, Umbria. Sergio Stefanutti was a G.A.R.I. engineer who focused most of his attention on the aerodynamics of his aircraft in an effort to optimize it for the task needed.

Stefanutti did not invent the SS.4 from scratch. The canard concept was used since early in aviation history and the aft 'pusher' propeller was also well-known (The Wright Flyer is an example of this design). However by the outbreak of World War II they were believed to be impractical features for military purposes, particularly with the demanding requirements of modern fighter designs. During the 1930s there were many projects involving experimental or unconventional aircraft, notably redesigned engines and wings of new models (such as the flying wing of Jack Northrop). Stefanutti started small, with the SS.2, simply a light aircraft with a canard wing configuration, built in the construction center at Guidonia, the experimental 'city' built for aviation. It was a simple yet original canard light aircraft with a 2-cylinder Keller engine rated at 16 hp. This was not intended to be a combat model, but rather a concept testbed for the unconventional design. The SS.2 flew for the first time in 1935, and one of the two prototypes was transformed into a two-seat model with an enhanced CNA 38 hp engine. This new aircraft first took to the air on 2 October 1937 and was officially revealed in Milan at an international air exposition. The small fixed-undercarraige SS.2 was sent to Passignano for evaluations. Experience with this machine and the study of its aerodynamics sent Stefanutti to imagine a new 'version', this time with a front-line task: interception and air superiority fighter. The basic characteristics- propeller in aft fuselage, canard wing, and tricycle undercarriage were already tested with the SS.1/2 and after over a year of design work the new machine was built and dubbed the SS.4.

Description

In brief, the SS.4 was an all-metal single seat fighter with a canard wing configuration and tricycle undercarriage. It had a very small fuselage with a rear-mounted propeller and a clean, sleek design with silver paint.

The Ambrosini SS.4 was short because of its canard configuration, which needed an aerodynamic engine-free nose for an optimal layout (due to the air flow to the rear propeller being deflected by the movement of the elevons, making the flight commands very heavy and precision control difficult). To correct this, the engine was shifted into the aft fuselage. This made a conventional tail design impossible, so the vertical stabilizers were doubled and shifted to the wings, which themselves needed to be shifted to the aft fuselage in order to let the tail rudders work more effectively. The presence of the propeller in the rear forced the aircraft designers to include a tricycle undercarriage to avoid collisions between the runway and the blades as the plane pitched upwards during takeoff (other 'pusher' propeller designs such as the Dornier Do-335 had similar design complications). All these features totally defined the layout of this fighter, which was nothing conventional by any standards of the time.

By design, this aircraft had a low and short nose, which provided excellent forward visibility for the pilot, which was augmented by an extremely simple canopy with only one metallic part over his head. This was a huge difference compared to conventional fighters, with their big noses aimed to the sky because of the classical 'taildragger' undercarriage, necessary for the forward propeller, which limited visibility.

The visibility for the pilot was not hampered by canard controls, which were in a lower position, nor by the armament fitted in the nose. Behind him, the twin tail and high fuselage were more problematic for viewing the '6 o'clock' position. The air control surfaces were elevators in the canards, two rudders behind the wings, and two ailerons in the wings. The canards were quite large, starting at the nose and stretching backwards to the cockpit. They had large moving surfaces, and the right canard had a trim surface.

The wings were peculiar, because it was important to leave space for the propeller and also respect the necessity to be as far aft as possible, so they were practically designed with a sweeping configuration. This was not meant to reduce the drag at transonic speeds, but rather to place the rudders far enough behind the fuselage to be effective. The two rudders were also of prominent dimensions, with the mobile parts actually behind the wing. The wingspan was noticeable, stretching to almost twice the length of the fuselage. The wings were even wider than those of the Zero, but were four square meters smaller, so they were slim with pointed extremities. In contrast to the short fuselage, they gave the SS.4 a futuristic look, even though the design was simply a logical response to the requirements of canard-wing design. The wings also housed the two posterior landing gear. The undercarriage itself was totally retractable, with one nose and two wing housings of light construction.

The engine was a powerful Isotta Fraschini Asso IX RC.40, a liquid cooled engine capable of 960 hp which drove a 3 bladed metallic propeller. To efficiently cool the engines, given the difficulty with aft engines, there were two air intakes just behind the cockpit that very much resembled those used by jets, except that they were too small to provide enough air for turbines. Two fuel tanks were located in the mid-fuselage, near the oil tank.

For armament, fitted linearly in the nose, some early sources say the SS.4 was armed with a 30 mm cannon and a pair of 20 mm guns, however this is an unreasonably heavy configuration: no 30 mm cannon was available at the time and no space for such complex armament was provided in the small nose of the fuselage; the surviving photos of the SS.4 clearly display the barrels of two Breda machine guns (12.7 mm) in the flank positions, while there was apparently nothing mounted in the center, although the 20 mm gun may have been recessed farther back in the fuselage. The evaluations that state the SS.4 had a 20 mm gun and two Bredas are more reliable and more readily supported by photos. It is possible that even the 20 mm gun wasn't available in sufficient quantities in Italy at that time so the aircraft flew with only the dual Bredas fitted in the nose.

As for dimensions and performance, the aircraft was capable of 540 km/h max speed, and stalled around 110-120 km/h. It had length of 6.74m, a wingspan of 12.32m, and wing area of 17.5m.

The short existence

The prototype SS.4 was built in SAI Ambrosini, Passignano sul Trasimeno, but then the aircraft was sent to Eleuteri airport, Castiglione del Lago, also in the Trasimeno area. There the aircraft was successfully tested on 7 March 1939. The next day, the SS.4 prototype was ready to be moved to Aviano airbase (by rail, not by flying), but the chief test pilot, eng. Ambrogio Colombo, wanted to fly a second test flight. After 45 minutes of normal flight an aileron malfunctioned and the test pilot attempted to land his aircraft. Even though he was only two kilometers from Eleuteri he could not reach the runway and landed in Campagna. This proved to be a fatal accident when the plane, upon landing, careened into a tree. The pilot Colombo died because the engine came through the fuselage from the rear and squashed him against the front of the cockpit. A memorial commemorating Ambrogio Colombo's death is still visible near one street in the zone where it crashed.

The investigation concluded that the accident was due to imperfect construction which led to a faulty installation of the failed aileron. The study also pointed out the excessive vibrations trasmitted by the engine to the wing.

Even though this original fighter was much more advanced in design than any Regia had at this time, and although it was still praised for its unique characteristics, the development of this machine was interrupted and the production work was shifted to the SAI 207.

Assessment

Even though the canard concept theoretically allowed for superior agility and a propeller-free nose was available for weapons, the performance of the SS.4 was not outstanding. Regardless of the agility the aircraft displayed, its performance was not significantly different than what would be expected of a conventional airframe with a 1000 hp inline engine. In comparison, the Dewoitine D.520, a close contemporary, had a 930 hp engine, 1x20 mm and 4x7.5 mm armament in the wings. Apart from the inferior visibility on the ground, it was only slightly slower, but was also a bit less powerful (-10 km/h and -30 hp) and the weaponry was comparable and did not require a fire synchroniser either. The minimum speed of the SS.4 was low but not sufficiently slow to justify this radical redesign compared to other Italian fighters.

The canard wing concept proved in this case to be more of a burden than a benefit to the reliability and performance of the aircraft, and once again cast the feasibility of canard-wing aircraft into doubt for military purposes.

If the engine was damaged or broken, the pilot was in great trouble; if he attempted to bail out he risked being cut to pieces by the aft propeller, but if he tried to land he risked being crushed between the terrain and the engine, as happened with the prototype. The 600 kg engine did not protect the pilot in hard landings (or from A/A fire), but acted instead as if it were a hammer. Other rear-propeller aircraft which had the same weakness incorporated design elements to ensure the survivability of the pilot in light of the aft propeller; the Dornier Do-335, for example, was fitted with one of the first ejection seats.

The small improvements that were eventually made to SS.4 were compromised by some basic problems, and other more serious difficulties like the overheating of the engine (especially with a high angle of attack, which caused interference with the air intakes because of the canard forward surface controls), and the vibrations transmitted by the engine to the aft structure.

Even if the performance of the SS.4 was better than the other '0' fighters, this was only due to the powerful engine, and if a Macchi 200 was fitted with the engine it is likely that it could have had superior performance over the SS.4 (in fact, with 1,100 hp DB601's it was redesigned as C.202) because the SS.4 had a higher drag coefficient. The rearward visibility was also dismal. This could have allowed enemy fighters to 'splash' the SS.4 from behind, and even if the engine protected the pilot from machine-gun fire, then he would risk his life attempted to bail out or force-land the fighter.

Another problem would have been the modification possibilities; with the adoption of more powerful engines, like the DB series, the baricenter would shift and cause more serious vibrations and need improved cooling systems.

Another issue was handling at the extreme speeds of a real air combat. Only the newer generation of jet fighters such as the (1960s-era) Saab 37 Viggen and (modern)Eurofighter Typhoon have been fitted with adequate flight control systems for the optimal performance of the canard configuration. In fact, even though they improved the Viggen's agility, they were meant in this case mainly for short take-off and landing operations (STOL).

As a result of the SS.4's failures in 1939, the possibility of an Italian airforce (then called the Regia Aeronautica) having a canard fighter in service 60 years before the Eurofighter ended.

After this experience, Stefanutti changed to more conventional, exceptionally streamlined aircraft such as the SAI 207. But the misfortunes with these extreme (albeit more conventional) aircraft continued and the SS.4 was only the first of several Stefanutti fighters that killed the test pilot (the others were S.107, 207 and 403).

Other aircraft similar to the SS.4 were made with the Shinden (Japan) and XP-55 (USA). They were even more advanced, but still troublesome machines, bigger and more powerful than SS.4, and were similarly disappointing with performance results. It seems that propulsion and control surface technology had yet to catch up with canard-wing design; the ideal propulsion systems would have been jet engines (as were tested in the Shinden), while the adequate flight control system (mediated by computers) was still decades away.

However impressively or poorly the SS.4 would have performed had it entered service, effectively it was not until the Saab 37 Viggen in 1967 that canard fighters were born as a real valid alternative to the conventional wings-in-front aircraft design.

ources

Lembo, Daniele "i brutti anatroccoli della Regia", Aerei nella Storia n.15.