Savoia-Marchetti S.55

The Savoia-Marchetti S.55 was a double-hulled monoplane flying boat designed and produced by the Italian aircraft manufacturer Savoia-Marchetti. It was designed to perform both commercial and military applications.

The S.55 featured numerous innovative design features. While the passengers and cargo were housed within the twin hulls, the flight crew controlled the aircraft from a cockpit located in the thicker section of the wing between the two hulls. The S.55 had two inline contra-rotating propellers, mounted in tandem. The engines were canted sharply at an upward angle. Two wire-braced booms connected the triple-finned tail structure to the twin hulls and wing.

During August 1924, the S.55 performed its maiden flight. Shortly after its introduction, the S.55 set multiple records for speed, payload, altitude and range. Numerous flying boats saw service with the Regia Aeronautica and export operators such as the Spanish Air Force and the Royal Romanian Naval Aviation as reconnaissance bombers and maritime patrol aircraft. While some examples were active by the outbreak of the Second World War, the type only played a marginal role in the conflict due to their age.

Design and development
The Savoia-Marchetti S.55 was a double-hulled thick-section cantilever monoplane flying boat that was designed to perform both civilian and military operations, dependent upon equipment fitout. In general terms, its design was largely original and possessed numerous features that increased its practicality as a platform for naval military tasks such as minelaying and torpedo attacks. Various aspects of the flying boat eased aspects of its operation; it was relatively easy to dismantle for ground transport; the wings comprised three detachable sections while each of the hulls could be entirely detached from the centre section along with the tail unit. The engines could be readily inspected at all times, even during flight.

The twin hulls of the flying boat were spaced roughly 13 feet apart, ran for just over half of the total length of the flying boat, and had a single-step V-shaped bottom. They were constructed in a manner that permitted the internal space to be configured into a series of cabins, accommodating a maximum of 12 passengers, if so required. The flying boat's cockpit was positioned in the centre section of the leading edge, between the two hulls; a passageway permitted personnel movements between the cockpit and the cabin space. On militarised flying boats, the stern of each hull would accommodate a tail gunner from which a relatively favourable range of fire could be obtained. Underneath the middle of the flying boat's centre section was an intentionally clear space of roughly eight foot; this space was suitable for the mounting of either torpedoes or mine-laying apparatus.

The cuter wing sections had a pronounced dihedral and sharply tapered from root to tip. The leading edge swept back about 15 degrees while the trailing edge was only slightly inclined. The wing was furnished with balanced ailerons that had an inclined axis. The tail surfaces were carried by two pairs of V-shaped outriggers; the top beam of each outrigger was attached to the upper surface of the centre section, near the rear main spar, while the lower beam was attached to the sternpost of the hull. They were cross-braced and each had a vertical strut at its forward and rear ends. Mounted on top of the rectangular horizontal stabilizing surface were three triangular vertical fins, the trailing edges of which were outfitted with hinged balanced rudders.

It was powered by a pair of Isotta Fraschini Asso 500 V-12 piston engines, each capable of providing up to 400 hp. These engines were arranged in tandem within a streamlined nacelle that was mounted high above the centre section of the aircraft, supported by a series of struts. The forward engine drove a tractor propeller while the rear engine had a pusher propeller. Cooling of both engines was achieved via a single radiator that was mounted in the nose of the nacelle. The engine nacelle was inclined, placing the line of thrust from the engines at roughly eight degrees off from the horizontal, this exerting a constant but slight upwards push throughout flight. A key reason for this arrangement was to direct the generated slipstream on to the tail surfaces. Level flight could be maintained with only the one operational engine.

Operational history
During August 1924, the S.55P prototype conducted its maiden flight. Throughout 1926, this same prototype set 14 separate world records for speed, altitude and distance with a payload. The type's greatest successes, however, were its many flights between Europe and the Americas.

The First aerial crossing of the South Atlantic had been made in stages in 1922 using three different Fairey III machines, and a subsequent crossing was made by the Dornier Do J Plus Ultra in January 1926 with a single stop at the Cape Verde Islands. One year later, the first S.55 crossing was made by the Santa Maria under Francesco de Pinedo. After flying south to Bolama, at that time in Portuguese Guinea, they hoped to cross the ocean without another stop but were forced down at Cape Verde, reaching Brazil on 23 February 1927. After the crossing, the aircraft was traded to Brazil for coffee beans.

The Brazilian João Ribeiro de Barros and his crew of three made another crossing in S.55 "Jahú" on 24 April 1927. Departing from Santiago Island, he crossed the Atlantic in "Jahú" and landed at Fernando de Noronha Island, Brazil.

Pilots Francesco de Pinedo and Carlo del Prete took off from Sesto Calende, Italy, in an S-55 on 13 February 1927. Four months later, on 16 June 1927, they arrived back in Italy, having flown nearly 48,280 km in 193 flying hours and having made just over 50 stops, including Rio de Janeiro, Buenos Aires and New York City.

On 20 June 1928, S.55 I-SAAT Santa Maria, piloted by Ten. Col. Umberto Maddalena of the Italian air force, located survivors of Arctic explorer Umberto Nobile's crashed airship Italia on an ice floe about 120 km (75 mi) northeast of Nordaustlandet, Svalbard.

The Italian Air Marshal of the time, Italo Balbo, became famous for organizing a squadron of S.55s for Atlantic crossings, culminating in his 1933 flight with 24 aircraft to Chicago's Century of Progress International Exposition. On 1 July 1933, General Balbo commanded a flight of S-55s from Orbetello, Italy, completing the flight in just over 48 hours, maintaining a tight "V" formation. These large fleets of aircraft were sometimes called a "Balbo".

Numerous aircraft were introduced by the Regia Aeronautica and served as a long-range bomber and patrol aircraft. By the Second World War, the service's S.55 fleet was no longer considered to be serviceable and the type was only ever held in reserve.

Variants

 * S.55
 * Prototypes and original production model delivered from 1927 to 1930, 90 built, including two prototypes.


 * S.55C
 * Civil variant delivered from 1925 to 1926, eight built.


 * S.55P
 * Improved civil variant with enlarged hull for ten passengers and enclosed cockpits delivered from 1928 to 1932, 23 built.


 * S.55A
 * Military variant delivered with 418 kW (560 hp) Fiat A.22R engines, 16 built.


 * S.55M
 * Variant with some wood structures replaced by metal, seven built by Piaggio in 1930.


 * S.55 Scafo Allargato
 * Widened and deepened hull and enclosed cockpits, 16 built by Savoia-Marchetti and 16 built by CANT.


 * S.55 Scafo Allargatissimo
 * Variant with greatly enlarged hull, 20 built by Savoia-Marchetti, 16 built by Macchi and six built by CANT.


 * S.55X
 * Variant fitted with Isotta Fraschini Asso 750 engines for North Atlantic formation flights, later armed and used as a reconnaissance-bomber. 25 built.

Civil operators

 * Kingdom of Italy
 * Aero Espresso Italiana
 * Società Aerea Mediterranea


 * Aeroflot
 * Aeroflot


 * Aero Transport Company d.b.a. AirVia.
 * Marine Air Transport Co.
 * Alaska Airways
 * Alaska Airways

Military operators

 * Kingdom of Italy
 * Regia Aeronautica
 * Brazilian Navy (eight flying boats)
 * 🇪🇸 Spain (1937)
 * Spanish Air Force
 * 🇷🇴 Romania
 * Royal Romanian Naval Aviation (seven flying boats)
 * Royal Romanian Naval Aviation (seven flying boats)

Surviving aircraft
The last remaining example is preserved in Brazil, at the TAM "Asas de um sonho" museum, at São Carlos, São Paulo. The aircraft, registered I-BAUQ and named "Jahú", was the S.55 used by Commander João Ribeiro de Barros in his crossing of the South Atlantic in 1927.