Gyroscopes

[BEagle]

Ah - so they've got you, have they Tonks?

Herewith the words from BEagle's 'A2 for idiots':

Gyroscopic instruments. These are a right bug.ger. But you only need to know a few basics:

Gyros have 2 basic properties, rigidity and precession. By using these properties plus a few springs and widgets, one can equip one’s aeroplane with stable flight instruments for use in cloud - such as an artificial horizon, old-fashioned Directional Indicator, Turn Indicator and most of a gyro-compass (you also need the compass bit from a magnetic monitoring unit!).

The first property, rigidity, means that the axis about which your gyro is spinning will remain fixed in space unless acted upon by an external force. Torque, actually. It will oppose this torque if it rotates quickly and also if it has a high moment of inertia. If two gyros of the same mass have different shapes, the one with the most mass at the rim of the gyro will have the greater moment of inertia and the greater rigidity. A fast, high moment of inertia gyro will not want to be displaced!

Which brings me on to the second property, precession. Weird this and I never did really understand it, but just accept the fact that if a gyro is rotating about one axis and is acted upon by an external torque, then just to be bloody-minded it’ll deflect about a mutually perpendicular axis - as though the torque had been applied 90º further around in the direction of rotation!

Gyros, like most other things except A2 QFIs, have errors. These are Real and Apparent Wander:

Real wander is caused by frictional forces and imperfections in the gyro assembly which act like little mini-torquettes. Hence the gyro thinks it’s being given applied torques and moves slightly; if it tilts as a result, that’s called ‘topple’, if it moves in azimuth, that’s called ‘drift’.

Apparent wander is rather more spooky. As the gyro spins merrily about a fixed reference axis in space, any measurement of the angles between this axis and your chosen frame of reference can easily be measured. But then move the frame of reference and these angles will obviously change - so if you choose the surface of the earth as your reference, a freely mounted gyro will still point towards the planet Qakll or wherever. It didn’t move, the Earth did (darling) - so you’ll just notice a bit of apparent wander due to the Earth’s rotation known as ‘Earth drift’. Now if you move the gyro as well, you’ll get what’s known as ‘Transport wander’ since you shifted it across your frame of reference. But the good news is that you can kid the gyro slightly with a correction for latitude to compensate for ‘Earth drift’ and by telling it where your idea of North is with a north-seeking magnet you can keep it orientated about Earth’s North. Not the planet Qakll!

Moving on to instruments, an artificial horizon has a gyro spun about a vertical axis (i.e. normal to the Earth’s surface. Movement of the aeroplane causes forces to be applied to the gyro mountings, fairly obviously. Gyroscopic rigidity and precession thus cause deflexions of the gyro about its axes, these are displayed to the pilot as pitch and roll displacements relative to the local horizon. Whereas a Directional Indicator has a gyro whose spin axis is maintained in a horizontal plane. Changes in heading are sensed by the gyro’s bearings; these are displayed to the pilot as a change of heading. The DI will be nice and steady, but real wander will cause inaccuracy. However, keep telling it where North is with an external magnetic monitoring unit and it’ll be very accurate.

Then there’s the Turn and Slip. Or rather the Turn bit. Much more complicated to understand; once you’ve digested this bit I might reveal more.....