Once a turn rate has been established and the rotational plane of the gyro has been displaced away from the vertical, there is a component of pitch rate that can be resolved perpendicular to the plane of rotation of the gyro. This increases the precessional moment generated by the yaw rate, and therefore the angular rate measured is greater than the pure yaw rate of the aircraft. I think, therefore, that it must measure turn rate with respect to the vertical earth axis but I have yet to work through the maths to prove it. It certainly indicates greater than the yaw rate.
The axis of a turn rate gyro is athwartships i.e. parallel to the lateral axis of the aircraft, with the gimbal giving freedom in roll (yz plane). If the axis were fore and aft, the gimbal would have to have freedom in pitch (xz plane). Therefore, it would only indicate yaw rate.
As for relevance, whatever happened to old skills such as limited panel IF? It may not be taught anymore but, if you do have a turn needle, one day this technique might just save your neck! And if you can fly limited panel, your full panel IF will doubtless benefit.
On a lighter note, I was told when in training that the turn gyro rotated up and away from you so that if it fell of its bearings it would roll away from you and not hit you in the nether regions!
Last edited by LOMCEVAK; 27th Feb 2003 at 07:22.