+ /* Compute pitch angle from vertical by taking the pad
+ * orientation vector and rotating it by the current total
+ * rotation value. That will be a unit vector pointing along
+ * the airframe axis. The Z value will be the cosine of the
+ * angle from vertical.
+ *
+ * rot = ao_rotation * vertical * ao_rotation°
+ * rot = ao_rotation * (0,0,0,1) * ao_rotation°
+ * = ((a.z, a.y, -a.x, a.r) * (a.r, -a.x, -a.y, -a.z)) .z
+ *
+ * = (-a.z * -a.z) + (a.y * -a.y) - (-a.x * -a.x) + (a.r * a.r)
+ * = a.z² - a.y² - a.x² + a.r²
+ *
+ * rot = ao_rotation * (0, 0, 0, -1) * ao_rotation°
+ * = ((-a.z, -a.y, a.x, -a.r) * (a.r, -a.x, -a.y, -a.z)) .z
+ *
+ * = (a.z * -a.z) + (-a.y * -a.y) - (a.x * -a.x) + (-a.r * a.r)
+ * = -a.z² + a.y² + a.x² - a.r²
+ *
+ * tilt = acos(rot) (in radians)
+ */
+