Futaba GY401 piezoelectric gyro and Futaba S2954 high-speed digital servo
In R/C modeling, a gyroscope or gyro is a device used to stabilize aircraft along one or more axis. R/C helicopters typically employ a single-axis gyro to counter the effects of main rotor torque on the tail. The gyro counters the effects of main rotor torque on the tail's heading, so the pilot can concentrate on control input and does not have to counter-balance all the external forces affecting the angle of the tail boom.
The gyro measures the acceleration forces along its axis and directly conteracts them by sending according corrective control impulses to the servo, thereby modifying the tail rotor's pitch. If, for example, a sudden gust of wind makes the tail turn to the right, the gyro will add just enough left-steering to keep the tail in position - unless the pilot instructs the vehicle to do a right turn.
Gyros can be built mechanically, electronically (using piezo-elements) or optically. Mechanical gyros are mostly outdated and optical gyros are usually used for navigation in full-size aircraft, not R/C models. Modern electronic gyros have varying degrees of efficiency and precision and take anywhere from 40 seconds (for cheap models) to less than one second to initialize, during which the model should be located on a flat surface and not moved or shaken.
Most gyros have a sensitivity setting that can be either set manually or remotely. A too low sensitivity setting results in the tail not staying in place, while a too high setting causes the tail to swing left and right in quick succession. Finding the right setting is a major contributing factor to the performance of the gyro. A quick servo is also of vital importance, since any delay between corrective control input and it's execution adds instability to the system. Lastly, gyros are sensitive to vibration and should be mounted with a thick double-sided adhesive tape, preferably on a location that is both close to the rotor shaft AND as non-vibrating as possible.
Heading Lock[]
Heading lock gyros are more sophisticated than regular models and are able to effectively "glue" the tail of a helicopter into the desired position, even during wild 3D maneuvers. This is done by keeping a history of accelerations along the gyro's stabilization axis, which enables the gyro to sum up all the accelerations and apply just the right control output to stop the tail from rotating. A normal gyro does not sum up the accelerations, and simply applies a counter-force depending on the acceleration measured at that very moment in time. This causes the tail to drift or turn during maneuvers, which makes backward flight extremely difficult. On the other hand, a helicopter with a heading-lock gyro will always keep the tail set to the desired position, no matter how much wind is blowing - which also means that the pilot has to deliberately turn the tail when making a turn, whereas a non-heading-lock gyro would allow the tail to turn according to the flow of air, thus making turns a bit easier.
While regular gyros apply corrective maneuvers on top of the pilot's command input, the heading-lock gyro essentially control the tail servo on their own, with the pilot's commands being fed into the Gyro and added into the calculated control output.
SMM[]
Regular Piezo gyros are heavily affected by temperature drift and vibrations. State-of-the-art gyros employ SMM sensors like the ones manufactured by Silicon Sensing (http://www.siliconsensing.com), which are much less temperature- and vibration-sensitive. Until the dawn of SMM elements, high-performance piezo gyros had to be encased in temperature-isolated housings, and suspended from the outer casing to minimize the negative influence of vibrations.
Other Applications[]
Gyros are also used for fixed-wing models and have a tremendous stabilizing effect on whatever flight axis they are installed on. They do not just add comfort or security for less-experienced pilots, but also make the model fly with incredible stability, making even the behaviour of small models believably lifelike. Heading-lock is usually not employed for this type of gyro.
Compatibility[]
It should be noted that not every gyro is compatible with every receiver or servo. For example, the Robbe G300 does not work with Scan-Receivers like the DS8 from Graupner, simply failing to initialize. High-performance gyros like the GY601 are only to be operated together with the included S9251 digital Servo. The GY401 can be used with regular servos, but has to be switched to the appropriate operating mode to prevent the high-frequency control signals from damaging the servo. Not every digital servo can deal with this rapid succession of control input, while some analog servos actually can. Thus, extra care should be taken before enabling the "digital output" mode on a modern gyro.
As a rule of thumb, it is recommended to search various RC forums for reports on incompatibilities prior to purchase of a gyro.