By William Pearce
The Piaggio P.23M was a prototype commercial transport aircraft intended for northern transatlantic flights. If flight testing was successful, the possibility existed to develop the unique aircraft for regular passenger service. Designed in 1934 by Giovanni Pegna, the P.23M was partly inspired by the Piaggio P.16 bomber prototype (also designed by Pegna). Two examples of the P.23M were ordered on 31 May 1934 and given the serial numbers MM 263 and MM 264.
The P.23M was a four-engine, all-metal aircraft with twin tail fins and rudders. The underside of the aerodynamically clean fuselage had a keel, much like a flying boat, and was watertight. While the aircraft could not operate from the water, the keel fuselage design was incorporated to facilitate emergency water landings. This feature was reflected in the P.23M’s name, the “M” representing Marino (Marine) .
The P.23M’s wing was a semi-cantilever, inverted-gull design (similar to the P.16’s) and was supported by three struts on each side. Each wing carried a long nacelle that housed two Isotta-Fraschini Asso XI R engines mounted in tandem. The foreword engine drove a tractor propeller, and the rear engine drove a pusher propeller. Each nacelle also housed the radiators for the engine pair and the retractable main landing gear.
The Asso XI R engine was a 12-cylinder Vee with a 5.75 in (146 mm) bore and a 6.30 in (160 mm) stroke. Total displacement was 1,962 cu in (32.1 L), and the engine produced 900 hp (671 kW). The Piaggio P.23M had a wingspan of 88 ft 7 in (27.0 m) and was 54 ft 6 in (16.61 m) long. The aircraft’s empty weight was 16,290 lb (7,387 kg), and maximum weight was 40,651 lb (18,439 kg), resulting in an impressive useful load of 24,365 lb (11,052 kg). Calculated speeds were 249 mph (400 km/h) maximum and 186 mph (300 km/h) cruise. Cruising range was estimated at 3,167 miles (5,100 km). Climbing calculations indicated the P.23M could reach 13,125 ft (4,000 m) in 14 minutes.
The Piaggio P.23M first took to the air on 25 October 1935 at Villanova d’Albenga, Italy. Tragically, something during the flight went horribly wrong. The aircraft crashed, and both test pilots, Ciacci and Risso, were killed.
While the exact cause of the crash is not known, it could have had something to do with the aerodynamic effect of the tandem engines and propeller pulses on the twin tails. The design for the second P.23M (MM 264), which had not been built, was converted to a trimotor configuration and redesignated P.23T. As a further development of the P.23M, the Piaggio P.50 I heavy bomber retained the tandem engines in each nacelle, but the twin tails were replaced with a single tail. Subsequently, the P.50 II had a conventional layout for its four engines—each in a separate nacelle.
Ultimately, the P.23T trimotor transport proposal was abandoned, and an entirely new aircraft was designed and constructed as the P.23R. Despite the similar designation, the P.23R had nearly nothing in common with the P.23M. Neither the P.23 nor P.50 series of aircraft proved successful. However, they did provide much experience to facilitate development of the Piaggio P.108 heavy bomber of World War II.
– Italian Civil and Military Aircraft 1930-1945 by Jonathan W. Thompson (1963)