I have an airplane with my script, the main problem is that I use those 2 formulas :
L = (p*Mathf.Pow(transform.InverseTransformDirection(rigidbody.velocity).z*3.6,2)*A*Cl)/2;
for the lift
and
Dx = (p*Mathf.Pow(transform.InverseTransformDirection(rigidbody.velocity).x*3.6,2)*A*Cd)/2;
rigidbody.AddRelativeForce(-Vector3.right*Dx);
Dy = (p*Mathf.Pow(transform.InverseTransformDirection(rigidbody.velocity).y*3.6,2)*A*Cd)/2;
rigidbody.AddRelativeForce(-Vector3.up*Dy);
Dz = (p*Mathf.Pow(transform.InverseTransformDirection(rigidbody.velocity).z*3.6, 2)*A*Cd)/2;
rigidbody.AddRelativeForce(-Vector3.forward*Dz);
for the drag. I set the drag of my rigidbody to 0.1 and try to use those formulas, but airplane continues moving with momentum/inertia
No idea? 
Hmmm, i have anti-grav ship which had this annoying momentum too, so i did something like this:
var MaxSpeed : float = 1700000;
var ActSpeed : float = 0;
function FixedUpdate() {
verMovement = Input.GetAxis("Vertical");
rigidbody.AddForce(transform.forward * ActSpeed * Time.deltaTime);
if (verMovement && ActSpeed < MaxSpeed) {
ActSpeed += 2500;
}
else if (!verMovement && ActSpeed >= 1500){
ActSpeed -= 1500;
}
if (ActSpeed < 0){
ActSpeed = 0;
}
}
function OnCollisionEnter (StrHit : Collision){
rVelocity = rigidbody.velocity.magnitude * 3.6;
if (StrHit && ActSpeed > 0 && StrHit !== gameObject.tag == "Enemy"){
ActSpeed -= 900 * rVelocity;
}
}
function OnCollisionStay (WeakHit : Collision){
rVelocity2 = rigidbody.velocity.magnitude * 1.6;
if (WeakHit && ActSpeed > 0){
ActSpeed -= 200 * rVelocity2;
}
}
Hope it helps… somehow ^^d
(rigidbody is 60u heavy, and is set to drag = 3)