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I saw that some aircraft's rudder is composed of an upper rudder and a lower rudder (B727, A380, ...) as illustrated:

Flight control

From https://en.wikipedia.org/wiki/File:Boeing_727_flight_control_surfaces.svg

I wonder how does it work compare to a normal rudder and the pro/con of the two-parts rudder over the one-part rudder.

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3 Answers 3

up vote 22 down vote accepted

This is called Split Rudder and it provides redundancy. They run on different systems so if one fails, the other one can be used.

Split rudders also provide a finer high speed control, in that only the lower one moves at high speed, reducing the exposed surface area and therefore the control effect.

Very importantly, there is a structural benefit. When we use the rudder it imposes a twisting load on the vertical fin. By only using the lower rudder, when the aircraft is at high speed, we reduce the twisting moment and transfer that load to a bigger, stronger, part of the airframe. The principle is the same as why the outboard ailerons are disabled at high speed.

The patent information is here.

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It allows one part to jam while the pilot retains control over the other part.

Also the higher portion of the rudder induces roll when deflected, at high speeds (where control surface deflection is more effective) only the lower portion can be deflected to minimize this effect.

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As pointed out by Farhan, it adds redundancy to the aircraft.

A 747 once experienced a rudder hard over (http://en.wikipedia.org/wiki/Northwest_Airlines_Flight_85). Even with the split rudder design, the crew had to rely on asymmetrical thrust to control the aircraft. If it had one very large rudder surface, control would be much more difficult, or may even be impossible.

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