I think you're having a hard time because you may not be clear what your end goal should look like. A well described question is halfway answered.
You say it's on rails. But then you want to account for angle of attack and wingspan while having a constant force driving you forwards. You seem to be creating a hodgepodge of rails and physics based behavior. I caution that such a hybrid may create strange edge-case behaviors and may tend to be unintuitive.
I was imagining something along the lines of Star Fox or Panzer Dragoon, where even though it was on rails you could still move a little bit within the level. The Fluzzard levels from Mario Galaxy 2 is probably closer to what I'm looking for though.
Let's break this down to the simplest forces you need to fly. Airplane, dragon, whatever.
Lift, thrust, drag, gravity.
You can use simple lift coefficient for lift force.
Its sounds like you want thrust to be constant forward.
You can use simple drag coefficients to get a drag force.
Gravity is gravity.
For your rotional control you have 3 axis.
Pitch, roll, yaw.
Here is where you need to better define your control scheme. Should the triggers roll you directly? Should they be mapped to angle of attack of left and right wings? Should they be mapped to give an aileron effect (left wing down, right wing up)? How do you pitch? You need a tail. Ditto for yaw.
Left analog stick will control pitch and horizontal movement. Whether to make it control yaw directly, I'll have to play around with it to see. And yes, I was thinking the triggers roll you directly as well as making you lose a little lift in the process.
Unity3D's engine already has variables built into it's physics system for drag and constant forward force, so that part is easy. I'm was using them to keep a constant forward velocity when I was playing around with the system.