There are few number of reasons. Redundancy is one of the reasons. The lower and upper rudder can be supplied by separate hydraulic systems, so that rudder authority is conserved if one or more hydraulic systems of the aircraft were to fail. In the A380, the rudders also have something called Electro hydrostatic backup actuators (EHBAs). These actuators are electrically powered with each actuator having its own small hydraulic reservoir. The upper rudder and lower rudder of the A380 has 4 EHBAs (two for each) and 3 of them are supplied by separate electrical busbars. This makes it possible to operate the rudder in the event of a full hydraulic failure with some major electrical failures.
The other reason to split the rudders into to two is to ensure the structural integrity of the vertical stabilizer. When you apply the rudder in an aircraft there is a load applied on the vertical stabilizer. If this load is too high, it can damage the surface and reduce its fatigue life. Again, looking at the A380 as an example, the vertical stabilizer of the aircraft stands as tall as 47 ft, which is nearly as long as a wing of an A320. If you were to place a single piece of rudder on the aircraft, and allow it deflect all by itself, it can put some incredible amounts of load on the tail. With this you have two options. Either you could reduce the deflection rate of the rudder or you could split the rudder (upper and lower) and give each of the rudders an individual deflection rate. The first option is not that desirable, because it takes away some of the control authority.
The second option is more desirable because if you design it in such a way that the lower rudder has higher deflection angles when compared to the upper rudder, it makes the system more structurally sound. Why because, the lower rudder is closer to the aircraft fuselage and that means, most of the load is now transferred to the fuselage rather than the vertical stabilizer. In the A380, its designed in such a way that both the rudders deflect to a maximum angle of 30 degrees when the speeds are low. However, as the speed increases by about 160 knots, the upper rudder deflection angles are reduced by the rudder travel limiter. At about 180 knots, the lower rudder angles are also reduced, but at a much slower rate when compared to upper rudder. This continues up to close to 250 knots, where the lower rudder deflection is lowered to 10 degrees either side and it remains so until the speed is again reduced. The upper rudder angle continues to decrease up to 5 degrees as the aircraft speeds up.
Have a look at the A380’s maximum rudder deflection angles in the graph below.
The picture below shows, what I have just said in action. Notice that the lower rudder has a higher deflection than the upper rudder as the A380 comes into land.
Some additional details about why the rudder on an airplane is split:
- Yaw Control: The primary function of the rudder is to control the yaw motion of the aircraft. Yaw refers to the rotation of the aircraft around its vertical axis, which is perpendicular to the wings. By deflecting the rudder, the pilot can create a side force that helps to counteract any unwanted yawing moments. This allows the pilot to maintain directional control of the aircraft, especially during maneuvers such as turns or crosswind landings.
- Differential Deflection: The split design of the rudder provides a concept called “differential deflection.” When the pilot applies rudder input, the downward deflection of one side of the rudder is greater than the upward deflection of the other side. This asymmetrical deflection creates a difference in drag forces between the two sides of the rudder, generating a yawing moment in the desired direction. The split rudder design enhances the effectiveness and precision of the yaw control.
- Aerodynamic Considerations: The split rudder design also has aerodynamic benefits. When the rudder is deflected, it creates a sideways force that generates a torque around the aircraft’s vertical axis. This torque can induce adverse effects such as increased drag and sideslip. The split rudder design helps to mitigate these effects by redirecting some of the airflow around the rudder, reducing drag and improving the overall aerodynamic efficiency of the aircraft.
- Stall Prevention: Another advantage of the split rudder is its ability to counteract adverse yaw during certain flight conditions, particularly during slow-speed or high-angle-of-attack situations that can lead to a stall. Adverse yaw is a phenomenon where a yawing moment is created in the opposite direction of the intended turn due to the increased drag on the outer wing during aileron deflection. By applying rudder input in the opposite direction of the desired turn, the split rudder can help minimize adverse yaw and maintain coordinated flight.
It’s important to note that while the split rudder is a common design feature in many aircraft, not all airplanes have this configuration. Some aircraft, such as small general aviation planes, may have a single-piece rudder that achieves similar functions through different mechanisms or control surfaces. The specific design and configuration of the rudder can vary depending on the aircraft type, size, and intended purpose.
By Anas Maaz (Airline Pilot) & Aeropeep Team
