rigidbodies with flight physics and autopilot - Approach?

Hey, I guess my problem is not that simple but I hope someone knows how I can approach this:

I am trying to design a RTS-like game using Unity's rigidbody-physics and I have succeeded so far but now I want to implement unit movement, especially of planes/hovering units but all this seems to be very complex. This is what I got so far:

• I am using a force to compansate gravity, the force points at the local down direction of the object causing it to accelerate and losing altitude when rotated as expected with constant lift. Another force accelerates the objects forwards.

• I managed to rotate the object by using torque and a PID controller (based on this thread http://answers.unity3d.com/questions/199055/addtorque-to-rotate-rigidbody-to-look-at-a-point.html) which works semi okay because its somehow shaky.

The results are okay; the object is rotated in the correct direction and the constant force is accelerating it but it looks like it moves on rails, there is no 'leaning into the corner' and if I rotate it manually the PID controller is not able to compansate for that and it flys sideways and crahes. When it arrives at the destination point it starts to fly circles which is okay but it does not stay in the plane between the starting point and the destination point and crashes after a while, in general it does not look very realistic.

Does anyone know how I can approach that better? I did a lot of research on this topic, like flight dynamics, aerodyamic but its not really helping because it expains more how planes fly but I am looking more for an autoplilot.

Thank you guys :)