You don't need an inverted pendulum to make an aircraft unstable. See the Grumman X-29, the forward-swept wing made it aerodynamically unstable, and it piloting

You don't need an inverted pendulum to make an aircraft unstable. See the Grumman X-29, the forward-swept wing made it aerodynamically unstable, and it piloting it was only made possible by feedback control software. https://en.wikipedia.org/wiki/Grumman_X-29 https://en.wikipedia.org/wiki/Grumman_X-29