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The critical engine of a multi-engine fixed-wing aircraft is the engine that, in the event of failure, would most adversely affect the performance or handling abilities of an aircraft. On propeller aircraft, there is a difference in the remaining yawing moments after failure of the left or the right (outboard) engine when all propellers rotate in the same direction due to the P-factor. On turbojet and turbofan twin-engine aircraft, there usually is no difference between the yawing moments after failure of a left or right engine in no-wind condition.
Most aircraft that have counter-rotating propellers do not have a critical engine defined by the above mechanism because the two propellers are made to rotate inward from the top of the arc; both engines are critical. Some aircraft, such as the Lockheed P-38 Lightning, purposefully have propellers that rotate outward from the top of the arc, to reduce downward air turbulence, known as downwash, on the central horizontal stabilizer, which makes it easier to fire guns from the aircraft. These engines are both critical, but more critical than inward-rotating propellers.
Aircraft with propellers in a push-pull configuration, such as the Cessna 337, may have a critical engine, if failure of one engine has a greater negative effect on aircraft control or climb performance than failure of the other engine. The failure of a critical engine in an aircraft with propellers in a push-pull configuration typically will not generate large yawing or rolling moments.
The Airbus A400M has an atypical design, because it has counter-rotating propellers on both wings. The propellers on a wing rotate in opposite directions to each other: the propellers rotate from the top of the arc downward toward each other. If both engines on a wing are operative, the shift of the thrust vector with increasing angle of attack is always towards the other engine on the same wing. The effect is that the resultant thrust vector of both engines on the same wing does not shift as the angle of attack of the airplane increases as long as both engines are operating. There is no total P-factor, and failure of either outboard engine (i.e.: engines 1 or 4) will result in no difference in magnitude of the remaining thrust yawing moments with increasing angle of attack, only in the direction left or right. The minimum control speed during takeoff (V speeds) and during flight (V speeds) after failure of either one of the outboard engines will be the same unless boosting systems that may be required for controlling the airplane are installed on only one of the outboard engines. Both outboard engines would be critical.
If an outboard engine fails, such as engine 1 as shown in Figure 2, the moment arm of the vector of the remaining thrust on that wing moves from in between the engines to a bit outside of the remaining inboard engine. The vector itself is 50% of the opposite thrust vector. The resulting thrust yawing moment is much smaller in this case than for conventional propeller rotation. The maximum rudder yawing moment to counteract the asymmetrical thrust can be smaller and, consequently, the size of the vertical tail can be smaller. The feathering system of the large, 8-bladed, 17.5-foot (5.33 m) diameter drag propellers must be automatic, very rapid and failure-free, to ensure the lowest possible propeller drag following a propulsion-system malfunction. If not, a failure of the feathering system of an outboard engine will increase propeller drag, which in turn enhances the thrust yawing moment considerably, thus increasing actual VMC(A). The control power generated by the small vertical tail and rudder alone is low by the small design. Only rapid reduction of thrust of the opposite engine, or increased airspeed can restore the required control power to maintain straight flight following the failure of a feathering system. Designing and approving the feathering system for this airplane is challenging for design engineers and certification authorities.
On airplanes with very powerful engines, the problem of asymmetrical thrust is solved by applying automatic thrust asymmetry compensation, but this has consequences for takeoff performance.
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