The Piaggio P.108 Bombardiere was an Italian four-engine heavy bomber that saw service with the Regia Aeronautica during World War II.Matricardi 2006, p. 257. The prototype first flew on 24 November 1939 and it entered service in 1941. It was one of a handful of Italian combat aircraft that could match the best manufactured by the Allies.Angelucci and Matricardi 1978, p. 226. Four versions of the P.108 were designed, but only one, the P.108B bomber, was produced in any quantity before the armistice. The other variants included the P.108A anti-ship aircraft with a 102 mm (4 in) gun, the P.108C, an airliner with an extended wingspan and re-modelled fuselage capable of carrying 32 passengers, and the P.108T transport version designed specifically for military use. Only one P.108A and 24 P.108Bs were built. The combined total number of all versions (and prototypes) was at least 39, almost certainly more than 44. Most of the P.108Cs were subsequently modified for use as military transport aircraft and could accommodate up to sixty passengers.Sgarlato Nine P.108 Ts were used by Luftwaffe transport units until the end of the war.
P.108B
Design and development The P.108 was unique in the history of Italian aviation, as it was the only four-engine strategic bomber used by the Regia Aeronautica during World War II. It was a development of the earlier underpowered and wooden-structured P.50-II which was unable to take off at its designed maximum weight.
The designer of the aircraft was Giovanni Casiraghi, an experienced engineer who had worked in the US from 1927 to 1936. On the basis of his experience he designed a radically new aeroplane. The P.108 was an all-metal low wing bomber with a retractable under-carriage. During the Air Ministry official requirement of 1939, it won the Regia Aeronauticas contract for a new bomber over the CANT Z.1014 as it became obvious that the other competitors could not deliver useful numbers of aircraft to the Italian Royal Air Force before the mid-1940s.Bignozzi 1986, p. 304.
The first prototype P.108B flew on 24 November 1939. It performed extremely well in a series of tests and required refining in only a few minor areas, but it took some time for pilots to get used to the new aircraft. The P.108 was delivered to a single unit, the 274a Squadriglia (274th Squadron), in 1941. But there were several accidents, one of them involving the son of Italian dictator Benito Mussolini. On 7 August 1941, Bruno Mussolini, commander of the 274a Squadriglia, was piloting one of the prototypes of the "secret" bomber. He flew too low and crashed into a house.Bignozzi 1986, p. 305. The cockpit section separated from the rest of the aircraft and although the aircraft did not catch fire, it was nevertheless totally destroyed in the impact. Commander Bruno Mussolini died from his injuries.
By the end of 1941, the P.108B had demonstrated just 391 flying hours. Nonetheless, the new bomber showed much promise. The average Italian bomber cost around 2.1 million lire, the SM.79 cost 1.7 million, while the P.108 cost 5.2 million.Pedriali This would seem to favor the smaller bomber. But in a comparison to deliver a given weight of bombs at the same distance, the P.108B had the advantage. With a single squadron of nine P.108s capable of flying with , the estimated efficiency was comparable to a group of 26 SM.79s covering with . The total cost of each 'group' of aircraft required was about the same at 46.8 and 45.6 million lire respectively, but only 54 crew were required to man the P.108s compared to 130 required to man the SM.79s.
The P.108B's engines were designed to be more powerful than those propelling the B-17, and most of its defensive gun turrets were remote-controlled.
The second series, designated P.108B II, were a revised sub-category with the nose turret removed. While this reduced defence against head-on attacks, the aircraft was operated mainly at night. The speed gain was ten km/h (six mph), due to weight reduction and the more aerodynamic nose.Sgarlato
Technical details
The P.108 was an all-metal, four-engine bomber, with a crew of eight. It had a very robust modern structure (with a six g tolerance) designed by Giovanni Casiraghi, and built almost entirely of duralumin.
Provisions for the crew included a two-pilot cockpit with five to six crew members located in the mid-fuselage and nose; like the early B-17 Flying Fortresses, the P.108 had no tail turret. The most noticeable feature was the nose, having a separate structure for the bombardier/bomb-aimer, with the front turret above him, similar to the nose of the Armstrong Whitworth Whitley with the lower part protruding from the rest. The P.108's tail was even larger, because of the need to stabilise the heavy, powerful aircraft ( and up to at take-off, around 20% heavier than early B-17s).Pedriali
Engines and performance Piaggio was one of the few Italian aviation companies that had the capability to develop both aircraft and engines: their P.XII engines, although unreliable, were among the most powerful of their time. Fiat was a much larger company that also designed and manufactured airframes and powerplants, but limited its production to aircraft like the BR.20 Cicogna, a conventional medium bomber.
The P.108 was equipped with four air-cooled 18-cylinder P.XII radial engines, which suffered from reliability problems, but produced at , with at takeoff, and drove three-blade constant-speed Piaggio propellers. The P.XII was two Piaggio P.X. engines in tandem, which were versions of the French Gnome-Rhône 9K Mistral made under license, themselves being derived from the license-built Bristol Jupiters. Alfa Romeo also lacked a powerful, modern engine, and its AR.132s were two Bristol Mercurys in tandem.Pedriali
12 self-sealing fuel tanks could hold around 12,000 L (3,100 US gal), and eight oil tanks, two for each engine, held a total of 350 L (90 US gal). The combination of a heavy airframe and powerful engines resulted in high fuel consumption, and the P.108's performance at high altitude was mediocre, with a practical ceiling of about and a theoretical . By comparison, the B-17C was capable of attaining a higher ceiling.Pedriali The figure for the Italian bomber was comparable to contemporary bombers: the ceiling for the RAF's best bomber, the Avro Lancaster was similar, while the USAAF's B-17, B-24 and later B-29 were unusual in being able to gain such altitudes. This was due to their installed turbochargers, as well as doctrinal differences in high altitude precision bombing advocated by the United States Army Air Forces.
The P.108 could reach speeds of at , which was slower than several types of biplane. Whereas the B-17C was capable of at over , even if its range and bombload, with , were slightly inferior to those of the P.108's with .Pedriali The aircraft could fly with a bombload of , and with . Though much better than a typical Italian bomber ( carrying , this was not impressive compared to the Avro Lancaster, able to fly carrying , and the B-24, about with the same load. The SM.82, normally with around half of the P-108's fuel (around 5,500 L/1,500 US gal), was able to reach long-range targets, such as Gibraltar, when adapted to carry the necessary additional fuel.
Bomb load The P.108 had a large bomb bay which was capable of carrying of bombs.
Depending on the target - these were uploaded either as: 7 × bombs. 34 × (true weight: ). 38 × (true weight: ) bombs.
The bomb bay was located centrally in the fuselage, and divided longitudinally into three sections which prevented it from carrying heavier bombs like the types. This was a considerable limitation, whereas the SM.82 was capable of accommodating larger loads (being able to be used both in transport and the bomber role). In the torpedo-bomber configuration, three torpedoes could be carried under the belly and the wings.
Defensive armament The defensive armament of the first P.108 series consisted of eight Breda-SAFAT machine guns. One Breda "O" 12.7 mm (.5 in) with 450 rounds was fitted in the nose, and a Breda "G9" 12.7 mm (.5 in) in a retractable ventral turret, with 400-450 rounds per gun. In addition to these two fully hydraulically powered turrets, there were two 7.7 mm (.303 in) guns in the flanks, with 500 rounds per gun. The wing armament consisted of two remote-controlled, hydraulically powered Breda "Z" turrets with 600 rounds per gun in the outer-engine wing nacelles, linked to one of the two cupolas in the fuselage "hump", with an operator in each. The wing turrets represented the most innovative aspect of the P.108's technology.Pedriali
Although considered a very advanced design, the operational suitability of the wing nacelle turret installations was questionable:P.108s and B-17s had three turrets as main defensive armament, but the latter also had ventral, dorsal and tail gun positions. The P.108 had fewer machine guns (eight instead of 10 to 13), no dorsal or tail turret, while the ventral turret had only one machine gun. Two further turrets were placed in the wings, providing a better field of fire, coupled with the wide field of view from the cupolas placed in the dorsal fuselage, but this complex and innovative layout was not without shortcomings.
P.108A
In response to a request in November 1942, the P.108A Artigliere "gunship" was developed for anti-shipping duties to supplement torpedo bombers. It was armed with a modified high velocity Cannone da 90/53 gun mounted in a redesigned nose. This was considered the most effective artillery piece in service, and in several versions was used as an anti-aircraft and anti-tank gun by the Army and the Navy. To be more effective in its new role, the size of the gun was increased from 90 mm (3.5 in) to 102 mm (4 in), a non-standard Italian artillery calibre, and fired shells weighing 13 kg (29 lb) as opposed to the standard gun's 10 kg (22 lb), with a muzzle velocity of over 600 m/s (1,970 ft/s).The gun together with its recoil system weighed 1,500 kg (3,300 lb).Bignozzi 1987, p. 31. Due to it being a re-bored smaller gun, its weight was relatively low for its calibre.
The P.108A concept was in line with other contemporary developments, as medium bombers such as the North American B-25 Mitchell and even smaller attack-aircraft like the Henschel Hs 129 were being fitted with high-velocity 75 mm (3 in) guns.
The gun was mounted longitudinally in the fuselage centreline, at a depressed angle, and had a powerful recoil, which the 27 tonne (30 ton) airframe was able to absorb. The amount of ammunition that could be carried was around 50-60 rounds for the main gun, as well as up to three standard torpedoes or two radio-guided torpedoes (a secret weapon which was never used in combat), and for the defensive wing and fuselage turrets. The 102 mm (4 in) gun was intended to be fitted with a ballistic sight with an analogue computer, and a six or 12 round mechanical loader.Sgarlato
Initial modifications were made to MM.24318 which on 16 December 1942 flew to Savona. The modifications were completed in February 1943, and testing commenced on 3 March.