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I have currently have a path that wraps around a Sphere from one side to another - this is basically a 3D highway in the sky type object that is a textured road. I have an Aircraft model that I can get to follow this path, however I'm having trouble getting it to rotate correctly so that it is parallel to the surface of the globe

Using the following code:

this.transform.forward = Vector3.RotateTowards(this.transform.forward, this.transform.position - TargetWaypointPos, Airspeed * Time.deltaTime, 0.0f);

this.transform.position = Vector3.MoveTowards(this.transform.position, TargetWaypointPos, Airspeed * Time.deltaTime);

This gets the plane to correctly face the next waypoint and follow the path, however I cannot get it to rotate correctly in the Z-space to be parallel to the path/globe. I can compute the correct angle by:

Vector3 yAxis = this.transform.position;
yAxis.Normalize();
Vector3 zAxis = nextPoint - this.transform.position;
zAxis.Normalize();
xAxis = Vector3.Cross(yAxis, zAxis);
xAxis.Normalize();
float angleZ = Vector3.Angle(Vector3.right, xAxis);

However attempting to apply this to my model in any way appears to ruin the Vector3.RotateTowards logic. I must be missing something simple to combine these two.

enter image description here

Any ideas? I'm new to Unity, still figuring out all the available options.

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This is easier than you think!

// Get the vector toward the next waypoint from here.
Vector3 forward =  TargetWaypointPos - transform.position;

// Get the local "up" vector pointing out of the planet to here.
Vector3 up = transform.position - SphereCenterPos;

// Form an orientation that points the z+ axis along "forward",
// and the y+ axis as close as possible to "up".
Quaternion targetRotation = Quaternion.LookRotation(forward, up);

// Rotate toward this target at a controlled rate.
transform.rotation = Quaternion.RotateTowards(
    transform.rotation,
    targetRotation,
    Time.deltaTime * DegreesPerSecond
);
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