Consider it as a mass flow problem. You're ingesting a certain mass of air m at a velocity v and releasing it in the opposite direction. Assuming no losses in the U tube, the only thing that changes is the direction of v. What matters is the total momentum, mv.
If the tube were straight, mv would point out the back and the airplane would go forward. With a U shaped tube, you have instead -mv which is the same momentum, but a different direction, and the airplane moves backwards.
Now that I think of it, it might be easier to visualize as a conservation of energy problem.
Consider it as a mass flow problem. You're ingesting a certain mass of air m at a velocity v and releasing it in the opposite direction. Assuming no losses in the U tube, the only thing that changes is the direction of v. What matters is the total momentum, mv.
If the tube were straight, mv would point out the back and the airplane would go forward. With a U shaped tube, you have instead -mv which is the same momentum, but a different direction, and the airplane moves backwards.
Now that I think of it, it might be easier to visualize as a conservation of energy problem.