How would you rotate a plane? I'm trying to use rigidbody.AddTorque, but I want it to work for turning left and right.

var turnSpeed = 5.0;

function FixedUpdate() {
    if (Input.GetButtonDown("Horizontal")){
        rigidbody.AddTorque(transform.left * turnspeed);
    }

Is there a way to make it work both directions? Horizontal won't work with only "transform.left"; it has to go right as well when needed.