The engine that causes the most adverse control ( reduced control) when it fails. ( aerodynamically)
In some A/C it was the engine that drove the U/C Hyd pump.
Generally speaking now days it is the left engine when rotating clockwise from the cockpit, the one whos down going blade is closest to the fuselarge, (Al-beit fuselarge centerline) non counter rotating props.
What this means is that with the critical engine out, the right hand engine's down going blade is further from the fuselarge centerline, and strikes the air at a different angle, and thus you require more rudder to keep straight for a given speed & power setting, lever arm effect.
With the opposite, less rudder for the same speed and power, there is alot more to it than this, but this is the simplist that I can come up with.
Chr's H/Snort
