how to add torque to correct bank?

I have a simple script that applies pitch (Z axis) and yaw (Y axis) to an object using relative torque. However, I have noticed that the object will generate a natural roll on combined inputs and I am trying to figure out how to correct this while preserving the new heading. (does that even make sense?, I'm fairly new to using physics)

here is an image:




the top image shows the original "world" (which is also starting local) vector (forward is aligned to X axis). The bottom image shows the rolling angle (red dotted line) that the arrow needs to be corrected. I found a simple example on stabilizing, but as my orientation has changed (white dotted line), I know I should apply a rotation to the negative X axis. Unfortunately, I'm not sure how to do this math. The code I have commented below does stabilize the entire arrow, but it acts more like a compass pointed to true North (or in this case, positive X axis).

so, how do I create a new vector3 that retains the rotations of Yaw and Pitch, but zeros the roll?

Code (csharp):

1.  
2.  
3.     public float gain = 3.0f;
4.     public float easeOut = 0.25f;
5.  
6.     private Vector3 controlDirection, stabilizeVector;
7.  
8.     public virtual float Horizontal
9.     {
10.         get
11.         {
12.             return Input.GetAxis("Horizontal");  //L,R, A,D *yaw*
13.         }
14.     }
15.     public virtual float Vertical
16.     {
17.         get
18.         {
19.             return Input.GetAxis("Vertical");  //U,D W,S *pitch*
20.         }
21.     }
22.  
23.     void Start()
24.     {
25.     //initialize
26.         controlDirection = Vector3.zero;
27.         stabilizeVector = Vector3.zero;
28.     }
29.  
30.     void FixedUpdate()
31.     {
32.         controlDirection.Set(0, Horizontal, (Vertical * -1));  //grab input
33.  
34.         //if (controlDirection == Vector3.zero) //no input
35.         //{
36.         //    //correct banking
37.         //    stabilizeVector = Vector3.Cross(transform.up, Vector3.up);
38.         //    rigidbody.AddRelativeTorque(stabilizeVector * easeOut, ForceMode.VelocityChange);
39.         //}
40.  
41.         rigidbody.AddRelativeTorque(controlDirection * gain, ForceMode.Acceleration);
42.     }

[edit] may have meant "roll" instead of bank [/edit]

 

well... this solution works to fix my problem: http://answers.unity3d.com/questions/10425/how-to-stabilize-angular-motion-alignment-of-hover.html

wish I understood Quaternions better...