Comment by schiffern

  >Not sure if it counts as under-actuated

Generally these aircraft still have three+ actuators, they just replace the rudder with differential spoiler or split elevons. Three actuators, three axes. Same number, so it's not under-actuated.

You're referencing the fact that it's dynamically unstable, and thus needs to be stabilized with a computer. That's a different thing.

You can also have aircraft which are under-actuated yet are dynamically stable. One example is a parafoil, which achieves controlled flight using only two actuators (pull lines).