Turn Gyro Axis

[greengage22]

This might seem a pretty pointless and academic question with not much relationship to being a practical pilot, but the JAA ask it as an ATPL exam question anyway and I’d like to know if anyone can find an answer (or even a reference book).

Using a Turn and Slip indicator in a balanced turn, are you measuring angular rate with reference to the vertical or with reference to the aircraft’s yawing axis?

You see, in a balanced turn, the precession will cause the rotor of the rate gyro to go to the vertical. I think you’re measuring with respect to the vertical, ie, the rotor’s orientation at the time. If this is correct, then your Rate One turn will be measured with respect to the vertical axis and should then correspond to your stopwatch and the amount of heading change. If measured about the yawing axis, there will be a small difference (admittedy, not very much in a gentle turn, but it would build up over several continuous turns).

I can’t find any British pilot textbook that gives the answer. A university textbook on gyrodynamics will probably have the answer and maybe John or Ghengis know it anyway. Anyone know?

The thought does occur to me that if British pilots have not needed to answer this question for the many years we have had national exams, why does this suddenly become essential information when the French set the questions? I think that they must have different pilot textbooks. I wouldn’t mind if they’d only give us a clue as to what they are.