Propeller

One of the key differences between single and multi-engine aircraft is the propeller system. Single engine aircraft with variable pitch propellers typically maintain a high pitch (low rpm) setting with high pressure oil from the propeller governor. In the event oil pressure is lost the blades will revert to a low pitch (high rpm) setting to maximize available power. In multi-engine aircraft the reverse is true. High pressure oil maintains the propeller blades in a low pitch (high rpm) angle when this setting is selected by the pilot, but if oil pressure is lost, the blades will revert to a feathered position—high pitch, low rpm—in order to minimize drag for single engine operations. 

Multi-engine aircraft often have a propeller synchronizer installed to eliminate the annoying “drumming” sound produced when the propeller RPMs are not precisely aligned. The synchronizer may be automatic or require the pilot to make RPM adjustments while referring to a small gauge. Some aircraft may be equipped with a synchrophaser, which adjusts individual propeller blades within their arcs in addition to matching the RPMs.

Fuel

The fuel systems of multi-engine aircraft are designed to allow each engine to draw fuel from the tank in the opposite wing in the event of an engine failure. This cross-feeding feature helps maintain balance in addition to extending endurance and range.

Combustion Heater

Many twin-engine aircraft utilize combustion heaters that burn some of the avgas from the fuel system to provide cabin heating. Single engine aircraft typically duct outside air through a shroud around the engine exhaust to provide heat, but this arrangement would be inefficient with engines mounted on the wings. A thermostat mounted on the unit will automatically shut it off if overheating occurs.