Centreline Thrust

Aircraft with centreline thrust have two piston-engines mounted in tandem on the aircraft’s centreline. The loss of one engine alleviates the asymmetric thrust condition associated with twin-engine aircraft with wing-mounted engines. Cessna used the push/pull configuration on their twin-engine Cessna 336/337 Skymaster, with an engine mounted at each end of the fuselage pod. It is more commonly known as a centreline thrust configuration. The first aircraft with centreline thrust was designed and patented by Claudius Honore Dornier (1884-1969) a German airplane designer and manufacturer. One of his more famous designs, in the 1940s was the Dornier Do 335 Pheil (Arrow) heavy fighter with two piston-engines mounted at each end of the fuselage in the centreline thrust configuration. The aircraft arrived too late in WW II to see active service.

Minimum Control Speed (Vmc)

The minimum control speed (VMC) is the speed at which a multi-engine aircraft can fly with a failed engine and still maintain directional control.

The VMC speed is determined by the force from the rudder required to maintain directional control to counteract the yaw force caused by an engine failure. Below the VMC speed, rudder authority is reduced and the aircraft will yaw and diverge from the required heading. It was mentioned above: a twin-engine aircraft with both propellers rotating in the same direction has the greatest yaw force with the critical engine failed. To be precise, there are two different air speeds at which the rudder fails to maintain directional control: there is a VMC for each engine. However, the higher of the two air speeds is taken as the operational VMC. Aircraft with counter-rotating propellers have the same amount of yaw force with either engine failed, therefore, the VMC is the same when either engine is failed.

The term VMCA applies to the minimum engine failure control speed when the aircraft is airborne. The VMCA should be no higher than 1.2 times the stalling speed. The term VMCG defines the minimum control speed on the ground, and it must be lower than the take-off decision speed (V1) to ensure directional control can be maintained following an engine failure.