Savoia-Marchetti SM.81 propulsion

Infobox Aircraft
name=Savoia-Marchetti SM.81


caption=Savoia Marchetti SM.81 in action (escorted by Fiat CR.32 fighters)
type=Bomber and transport aircraft
manufacturer=Savoia-Marchetti
designer=
first flight=1934
introduced=1935
retired=1950
status=
primary user=Regia Aeronautica
more users=Spanish Air Force
Aeronautica Militare Italiana
produced=
number built=534
unit cost=
developed from=Savoia-Marchetti S.73
variants with their own articles=

The Savoia-Marchetti SM.81 was a three-engined aircraft, but unlike the contemporary Savoia-Marchetti SM.79, it used a wide range of engines from the start:

:Alfa Romeo 125 RC.35:: 580 hp (or 680 hp, depending on the source) at take-off:: 650 hp at 3,500 m:: Total power: 1,740-1,950 hp Fact|date=September 2007

:Gnome-Rhône 14KC:: 650 hp at take-off:: 740 hp at 3,000 m:: Total power: 1,950-2,220 hp

:Piaggio P.X R.C.15:: 670 hp at take-off:: 700 hp at 1,500 m:: Total power: 2,010-2,100 hp

:Alfa Romeo 126 RC.34:: 680 hp at take-off:: 750 hp at 3,400 m:: Total power: 2,040-2,250 hp

:(possibly) Piaggio P.XI RC40:: 780 hp at take-off:: Total power: 2,340-?

Three bladed metal propellers were used, with Duralumin blades and steel hubs, and with a diameter of 3.4 - 3.5 m Fact|date=September 2007.

The fuel tanks, as standard for Italian multi-engined aircraft, were metal self-sealing tanks using materials developed by SEMAPE, the specialized manufacturer. Eight tanks were fitted, with six in the centre wing (4 x 150 litre and 2 x 1,140 litre tanks), and two 370 litre tanks (or 780 litre with Gnome-Rhône 14K engines) in the outer wings Fact|date=September 2007. This gave an overall fuel capacity of 3,620 or 4,400 litres.

Standardizing the engines in individual theatres of operations was necessary. Aircraft based in Italy and Spain had Alfa 125/126 engines, those based in Libya had GR 14K s, and Eastern Africa based aircraft were powered by Piaggio engines.

Performance

With a total of 1,750-2,350 hp the SM.81 was well served, even if individually the engines were underpowered. The maximum speed with the AR.125 engine was 340 km/h, with others it was from 320-347 km/h. Cruise speed at its best was 260 km/h, but there are reported values of up to 330 km/h. The extra power of many engine-sets was valuable in high and hot conditions, but their larger diameter was sometimes enough to reduce maximum speed by adding more drag. Even so, the aircraft was faster than the Junkers Ju 52, its most direct equivalent.

Even with the same or even greater fuel load than the SM.79, the SM.81 had a shorter range. It had a maximum 2,000 km endurance in normal conditions. The ferry range to Spain in 1936, was an example of the relatively short range of the aircraft, perhaps caused (as was the low speed) by drag.

Given the 3,799 kg useful load, with the maximum 2,400 kg of fuel on board, the bomb load was reduced to 1,000 kg, with a range of around 600-700 km, depending on the type of engines. With a full 2,000 kg bomb load its range was only 460 km, while endurance was 640 km.