Just to clearify a few things!
Firstly both the turn & slip indicator and the Turn-coordinator is gyroscopic instruments.
Secondly Instruments of the such mentioned above is not designed for use in aerobatic aeroplanes, however they may well be used, but one should then be very aware of their limitations as with all mechanical gyros.
The turn and slip and turn-coordinator are mainly used in instrument flying as they indicate the rate of heading change. When flying IFR there are procedure turns, holdings, reversal procedures that are designed according to given rules. These being PANS-OPS (Old, New or even Newer), or TERPS; there are a bunch of these. Point being that all of these procedures are designed within a given airspace therefore also a given turn rate. these are referred to as rate one turns. 3 degrees / second. The istruments has dots on them to indicate when wether the aircraft wings of the instrument or the bat aligns with these dots they indicate you have a rate one turn, and therefore you are adhering to the desing of the procedure you are flying.
Regarding the question on how gyros are driven. There are four ways that is used today: They are Electrical, Vacuum, Laser or Piezo-elements. Electrical and Vacuum driven gyros are the old fashion gyros and are seldomly used in modern airliners, they are also called mecanical gyros as they have a mass spinning around an axis. The piezo-gyro is a piezo-electrical element welded in place with a silicon mass and sensors around it therby inducing electrical impulses that are sensed by the sensors around it and therby interpreted by a computer. This type of gyro is mostly used in model aeroplanes and helicopters and as far as I know not used in real-size aviation at all. The laser gyro is a ring or infact a triangle of two lasers. the lasers are directed opposite of one another and sends a beam of laser light hitting mirrors, reflecting the light in a triangle back to two sensors. The sensors registers the time from emmision to the time of reception for both lasers. Diffrence in the two emissions indicate that the laser platform has moved and therefore it is able to determine what way it has moved. These signals are then sent to the VDU by a computer (Visual display unit) telling the pilot the information he needs. This is generally used in modern glass cockpit designs.
Pretty technical, but explanatory I hope..
PS: And as M14P writes further down here, which is quite right, The time measured is not really what is measured. It is the Phase change of the laser beam. The phase change gives one the indication of TOT (Time of travel) for the beam which is necessary to calculate the amount of movment of the ring or (gyro platform).
Last edited by Fokker-Jock; 19th Jun 2002 at 12:01.