Sorry folks, there seem to be many mis-conceptions here.
The reason multi-engined propeller driven aircraft have a critical engine is due to the impact the rotating vortex trailing from each propeller makes on the wing and vertical surfaces behind, it while producing thrust: i.e. the angle of attack of the vortex on the wing and the vertical surfaces is different, depending upon the direction of rotation of the prop, the thrust being produced and the indicated airspeed of the aircraft.
The critical engine will be determined during flight testing for Vmca, which is defined as the minimum speed at which directional control can be maintained under defined circumstances (i.e. sudden power cut, maximum bank angle of 5 degrees, etc.).
On aircraft with propellers rotating in a clockwise direction, as viewed from behind, the critical engine will usually (but, not always) be the left engine. In other words, the failure of the left engine will cause loss of directional control at a higher speed than a failure of the right and therefore is more critical. On some aircraft, an engine failure at Vmca will result in a significant difference in performance between a left engine failure and a right engine failure.
Last edited by twochai; 16th January 2013 at 15:24.