Propulsion systems of the Savoia-Marchetti SM.81
| Industry | Aerospace |
|---|---|
| Products | Aircraft |
The Savoia-Marchetti SM.81 was an Italian tri-motor aircraft. Unlike the contemporary Savoia-Marchetti SM.79, it featured a variety of engine types from the outset.
Engines
To ensure operational efficiency, engine standardization was implemented across different deployment regions. Aircraft stationed in Italy and Spain employed Alfa 125/126 engines, while those in Libya were equipped with GR 14K engines, and units operating in East Africa utilized Piaggio powerplants. Additionally, a unique twin-engine prototype, designated S.M.81B, was tested using Isotta Fraschini engines.
Alfa Romeo 125 RC35
- Power output ranged from 432 kW (579 hp) to −507 kW (−680 hp).
- Combined engine power: between 1,298 and 1,521 kW (1,740 to 2,040 hp).
Gnome-Rhône 14Kc
- Individual engine power varied from 485 kW (650 hp) to −746 kW (−1,000 hp).
- Aggregate power delivered: 1,455 to 2,237 kW (1,950 to 3,000 hp).
Piaggio P.X. RC15
- Engine output between 501 kW (672 hp) and −522 kW (−700 hp).
- Total combined power measured at 1,499 to 1,567 kW (2,010 to 2,100 hp).
Alfa Romeo 126 RC34
- Power per engine ranged from 582 kW (780 hp) up to −671 kW (−900 hp).
- Overall power output spanned 1,746 to 2,014 kW (2,340 to 2,700 hp).
Piaggio P.IX RC40
- Engine rated at approximately 507 kW (680 hp).
- Combined power totals around 1,522 kW (2,041 hp).
Isotta Fraschini Asso IX RC
- Single engine produced roughly 626 kW.
- Total power delivered by two engines amounted to approximately 1,252 kW.
Propellers
The aircraft utilized three-blade metal propellers featuring duralumin blades paired with steel hubs. These propellers had diameters ranging between 3.4–3.5 m (11–11 ft).
Fuel Tanks
Consistent with Italian multi-engine aircraft practices of the period, the SM.81 was fitted with metal self-sealing fuel tanks developed by the specialist manufacturer SEMAPE. The configuration included eight tanks: six located within the central wing section (comprising four tanks of 150 L/40 US gallons each and two larger tanks of 1,140 L/301 US gallons), alongside two outer wing tanks holding either 370 L (100 US gallons) or, when equipped with Gnome-Rhône 14K engines, increased capacity of 780 L (210 US gallons). This arrangement provided a total fuel volume of either 3,620 or 4,400 liters (960 or 1,160 US gallons).
Performance
The SM.81 was equipped with engines producing between 1,305 and 1,752 kW (1,750 to 2,350 hp), which, despite each engine being somewhat underpowered on its own, collectively provided adequate power. When fitted with the AR.125 engine, the aircraft could reach a top speed of 340 km/h (210 mph). Other engine configurations allowed speeds ranging from 320–347 km/h (199–216 mph). Its optimal cruising velocity was approximately 260 km/h (160 mph), although some reports indicated it could achieve up to 330 km/h (210 mph). While more powerful engine combinations enhanced performance in hot and high-altitude environments, their increased diameter sometimes caused additional drag that slightly decreased maximum speed. Nevertheless, the SM.81 outpaced the Junkers Ju 52, which was considered its closest counterpart.
Despite carrying fuel loads equal to or exceeding those of the SM.79, the SM.81’s operational range was shorter. Under standard conditions, it could cover a maximum distance of about 2,000 km (1,200 mi). The ferry flight to Spain in 1936 highlighted this relatively limited range, likely influenced by aerodynamic drag, which also contributed to its modest speed.
With a useful load capacity of 3,799 kg (8,375 lb), and carrying up to 2,400 kg (5,300 lb) of fuel, the bomb payload was limited to 1,000 kg (2,200 lb), resulting in a range between roughly 600–700 km (370–430 mi), depending on the engine type installed. When fully loaded with bombs (2,000 kg (4,400 lb)), the ferry range dropped to just 460 km (290 mi), while its combat radius was approximately 640 km (400 mi).