How to roll a Jet (GameObject with a RigidBody)

So I want to rotate it smoothly around the x-axis. When the X-velocity is positive, I want the rotation to be zero, and when it is negative, 180 degrees. I got this far below, but I'm not sure how to finish it.

 public var rotationSpeed:float = 5;
 public var targetAngle:float = 0;
 public var mod:Transform;
 
 function Update () {
     var rig:Rigidbody = this.rigidbody;
     if(rig.velocity.x > 0){
         targetAngle = 0;
     }else{
         targetAngle = 180;
     }