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This question (which is the first result for a Google search for "Split rudder aircraft", btw), talks about some of the advantages of the split rudder, including the fact that the lower rudder is smaller for finer adjustments while flying.

If the rudders were equal size, or if there were three pieces, two of them being equal to the third, could they be used as an air brake?

Would there be advantages to doing so vs building in a dedicated air brake?

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    $\begingroup$ it'll induce a roll moment and it's less efftective than actual spoilers $\endgroup$
    – ratchet freak
    Commented Dec 22, 2014 at 19:08
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    $\begingroup$ The primary purpose of a spoiler is to reduce lift, whereas the primary purpose of a speed brake is to slow the airframe down. The functions do overlap but a speed brake is not a spoiler. $\endgroup$
    – Skip Miller
    Commented Dec 22, 2014 at 19:18
  • $\begingroup$ @ratchetfreak: A three-piece rudder could be used as an air brake without inducing a roll moment. Whether that would be of any practical use, I cannot say. $\endgroup$
    – Ilmari Karonen
    Commented Dec 22, 2014 at 22:01
  • $\begingroup$ Split rudder like this: commons.wikimedia.org/wiki/File:STS-116_landing_port_behind.jpg $\endgroup$
    – Nick T
    Commented Dec 23, 2014 at 5:17

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It is possible but there are side effects.

If both rudders would be deflected to opposite sides to cancel out each others yaw moments, their drag will act as an air brake. But since the top rudder is further from the center of gravity than the bottom rudder, the resulting roll moments of the rudders don't cancel each other out. As a result the aircraft will start to roll. This can then be counteracted with the ailerons.

Since the rudders don't deflect as far as air brakes would, they would be less effective. If they would be designed to deflect more, the increased forces upon them and on the control system would require a heavier construction and thus add weight to the aircraft.

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Yes, a split rudder can be used as a speed brake. The early versions of the Beech Staggerwing and the space shuttle both have split rudders utilized for speed brakes.

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    $\begingroup$ Given the big, blunt fuselage, the split rudder on the Shuttle did not add much drag. It reassured the crew that they have something to change drag, but changing the flight path (both horizontally and vertically) is immensely more effective for glide path control of a delta configuration. $\endgroup$
    – Peter Kämpf
    Commented Dec 22, 2014 at 19:20
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    $\begingroup$ "The shuttle was designed with a low L/D ratio (~ 1) because during the descent the spacecraft must be slowed from about 17,300 mph to about 250 mph at landing." grc.nasa.gov/WWW/k-12/airplane/glidshuttle.html $\endgroup$
    – rbp
    Commented Dec 22, 2014 at 20:59

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