A pilot must enter and maintain a zero-sideslip during an engine failure in a multi-engine aircraft to achieve the greatest climb performance possible while still maintaining control of the aircraft.
A zero sideslip is a state of minimum sideslip. During a zero sideslip, an aircraft will maintain a direction of travel directly forward, rather than have a direction of travel towards one side or the other.
While performing a zero-sideslip, the pilot will input rudder and aileron to counteract the aircraft's tendency to enter a sideslip. Therefore, reducing the aircraft's drag and allowing the greatest performance.
It is theoretically possible to maintain directional control of a multi-engine aircraft during an engine failure using solely rudder or bank to compensate for the aircraft's yaw towards the dead engine. However, only using rudder or bank to maintain directional control will drastically reduce aircraft performance.
During an engine failure, a pilot that only uses the rudder to maintain directional control of the aircraft will enter into a sideslip to the left or the right, depending on the engine that has failed.
In the example shown above, the aircraft has a right engine failure. In this case, the pilot is applying full left rudder to compensate for the resulting right yaw.
In this example, the aircraft will enter into a sideslip towards the right. While in the right sideslip, the aircraft will have a direction of travel towards the right resulting in the relative wind to be directed slightly right of the nose.
This results in the relative wind striking the aircraft's fuselage creating an area of turbulent flow on the other side of the fuselage directly opposite the relative wind. This area of turbulent flow results in a dramatic increase in parasitic drag and ultimately eliminates any remaining climb performance the aircraft may have been capable of.
The aircraft's direction of travel is skewed left or right due to the sum of forces acting on the aircraft.
In this case, the combination of the forward thrust produced by the still-functioning left engine and the left rudder force created by the rudder creates a resultant force towards the right.
This right resultant force ultimately determines the aircraft's direction of travel and subsequently the relative wind's path across the aircraft surface.
Using the horizontal component of lift produced by an aircraft's wing in a bank to counteract the yawing motion created by the remaining engine is theoretically possible. However, the amount of bank required to produce the horizontal force required is dependent on multiple external factors and would most likely occur at higher bank angles.
With that being said, if the pilot only inputted bank to counteract the uncommanded yaw created by the remaining engine, the aircraft would enter a sideslip in the other direction. In this example, the sideslip would be towards the left with the right engine failure.
This sideslip would cause the aircraft to have a direction of travel towards the left, causing the relative wind to strike the aircraft from the left. With the relative wind to the left, an area of turbulent flow is generated opposite the relative wind on the other side of the aircraft.
Similar to only using rudder, this area of turbulent flow results in an increase of parasitic drag, eliminating any potential climb performance.
While only using bank, the combination of forward thrust produced by the remaining engine and the horizontal component of lift creates a resultant force towards the left.
Like the resultant right force created while only using rudder, the left resultant force results in the aircraft's direction of travel to be towards the left and the relative wind to also come from the left.
The only way to maintain directional control of a multi-engine aircraft during an engine failure and get the greatest amount of performance is through the zero-sideslip.
To enter a zero-sideslip requires the pilot to input both rudder and bank simultaneously. This combination of rudder and bank results in the aircraft's direction of travel being directly forward and forces the relative wind striking the aircraft to come head-on to the nose of the aircraft. Pilots will know they are in a zero-sideslip when the aircraft is banked approximately 2-3 degrees towards the operating engine, and the ball is about half out towards the operating engine.
The end result is the ability to maintain control of the aircraft during an engine failure and the elimination of parasitic drag - allowing the greatest possible climb performance.
The reason bank and rudder need to be used to enter a zero-sideslip is that the rudder and horizontal component of lift forces cancel out, leaving only the forward thrust produced by the remaining engine to have any effect on the aircraft.
As shown above, during a right engine failure, the pilot will initially apply left rudder to maintain control of the aircraft. This will result in a force being applied to the left of the rudder, pushing the tail to the right. At this point, the pilot is in a sideslip towards the right while only using rudder.
The pilot will then enter a zero-sideslip by inputting approximately 2-3 degrees of bank towards the operating engine, effectively canceling the right rudder force.
This leaves the pilot in a zero sideslip, with only the forward thrust produced by the remaining engine having any actual effect on the aircraft's direction of travel.