problem with changing direction of relative force

Hello every one I have problem with my code and when I change the direction of my object the relative force don't change properly and my object get out of control. I need to know how can I change my force direction smoothly and quickly? I write this code in Update and next in Fixed update and test it with AddForce and AddRelativeForce but in none of them I can't find my answer.

     var UpForce : float;
     var MaxSpeed : float = 100;
     var MinSpeed : float;
     var Speed : float = 1;
     var Acceleration : float = 100;
 function Update () {
     UpForce = rigidbody.velocity.z/10;
     UpForce = Mathf.Clamp(UpForce,0,7);
     rigidbody.AddForce(0,UpForce,0);
     // speed control
     Speed = Mathf.Clamp(Speed,MinSpeed,MaxSpeed); 
     if(Speed <= MinSpeed){
     Speed = MinSpeed;
     }
     if(Speed >= MaxSpeed){
     Speed = MaxSpeed;
     }
     if(Input.GetKey(KeyCode.B)){
     Speed = Speed + 1;
     }
     if(Input.GetKey(KeyCode.N)){
     Speed = Speed - 1;
     }
     if(Input.GetKey(KeyCode.A)){
     transform.Rotate(0,0,Acceleration*Time.deltaTime);
     }
     if(Input.GetKey(KeyCode.D)){
     transform.Rotate(0,0,-Acceleration*Time.deltaTime);
     }
     if(Input.GetKey(KeyCode.S)){
     transform.Rotate(-Acceleration*Time.deltaTime,0,0);
     }
     if(Input.GetKey(KeyCode.W)){
     transform.Rotate(Acceleration*Time.deltaTime,0,0);
     }
     Debug.Log("force:" &&  rigidbody.AddRelativeForce.z);
 }
 function FixedUpdate(){
 rigidbody.AddRelativeForce(0,0,Speed);
 }