Just going to put my anorak on.
The reactive torque produced in response to rotor movement is constantly changing. Changes in engine speed or pitch of the main rotor blades results in a corresponding change in torque, and these can be sudden and violent. These changes are happening constantly and are always trying to spin the helicopter. The wind is simultaneously interfering with the yaw control and if not adjusted for, will whip the tail of the helicopter around.
The gyro is a small electronic device which detects unwanted yaw and automatically controls the tail rotor servo to correct and control the yaw movement. It's an essential bit of kit which basically allows RC helis to fly. It plugs into the tail rotor channel of the receiver and the tail rotor servo then plugs into the gyro. The gyro is in full control of the tail rotor and interprets tail rotor commands from the receiver. Only when the pilot sends a tail rotor command to the helicopter does the gyro allow the helicopter to turn.
Gyros are based on piezo crystal technology, and they come in two main flavours - heading hold and yaw rate (non-heading hold).
A heading hold gyro works by keeping the helicopter pointed in the direction the pilot last pointed it no matter what's going on. The wind can blow all it wants but the helicopter will stay pointing in the chosen direction. On the ground this can cause a problem, for example if you pick up the tail of the heli and rotate it 180 degrees (with no stick input), upon take-off the gyro will correct for the "unwanted" 180 degree course change by giving full rudder input until the heli faces the right direction.
A yaw rate gyro only slows or dampens the amount of yaw movement from any source to a controllable level. These sources include torque variation as the main rotor speed, pitch, and cyclic are adjusted, a gust of wind blowing the tail around (weathervane effect) or a command from the pilot via the transmitter. The problem with yaw rate gyros is that the gyro-produced counter movement may be insufficient to completely correct for the unwanted yaw change.
I bought a yaw rate gyro because it was cheaper. It will clearly do the job, but now I'm begining to understand all the advantages associated with heading hold gyros I realise I may have made the wrong choice. Another mistake to add to the list.
The sensitivity of response/correction of the gyro can be adjusted (gyro gain) and it's very much a case of testing the heli and making adjustments. Too little gain and the yaw control becomes sluggish, too much and the gyro can over-control things and making the heli very sensitive with the tail oscillating from side to side ("hunting"). Although one gain value can be used for all aspects of flight, different gain settings can be used in flight to cope with different tail rotor speeds. Bear in mind that the tail rotor rpm is linked to the main rotor rpm, thus, the sensitivity of the tail rotor is lower at a lower rpm (for example during a hover where the main rotor rpm is lower), and higher at a higher rpm (for example during fast forward flight where the main rotor is at its highest rpm). With remote gain, a channel on the transmitter can be used to switch between two gain/sensitivity settings, i.e. more gain during lower rotor rpm, and less gain during high speed rotor rpm. This way you will still have good RC helicopter gyro performance during a hover at a lower rpm and can simply switch to less gain when flying with high head speeds.
The reactive torque produced in response to rotor movement is constantly changing. Changes in engine speed or pitch of the main rotor blades results in a corresponding change in torque, and these can be sudden and violent. These changes are happening constantly and are always trying to spin the helicopter. The wind is simultaneously interfering with the yaw control and if not adjusted for, will whip the tail of the helicopter around.
The gyro is a small electronic device which detects unwanted yaw and automatically controls the tail rotor servo to correct and control the yaw movement. It's an essential bit of kit which basically allows RC helis to fly. It plugs into the tail rotor channel of the receiver and the tail rotor servo then plugs into the gyro. The gyro is in full control of the tail rotor and interprets tail rotor commands from the receiver. Only when the pilot sends a tail rotor command to the helicopter does the gyro allow the helicopter to turn.
Gyros are based on piezo crystal technology, and they come in two main flavours - heading hold and yaw rate (non-heading hold).
A heading hold gyro works by keeping the helicopter pointed in the direction the pilot last pointed it no matter what's going on. The wind can blow all it wants but the helicopter will stay pointing in the chosen direction. On the ground this can cause a problem, for example if you pick up the tail of the heli and rotate it 180 degrees (with no stick input), upon take-off the gyro will correct for the "unwanted" 180 degree course change by giving full rudder input until the heli faces the right direction.
A yaw rate gyro only slows or dampens the amount of yaw movement from any source to a controllable level. These sources include torque variation as the main rotor speed, pitch, and cyclic are adjusted, a gust of wind blowing the tail around (weathervane effect) or a command from the pilot via the transmitter. The problem with yaw rate gyros is that the gyro-produced counter movement may be insufficient to completely correct for the unwanted yaw change.
I bought a yaw rate gyro because it was cheaper. It will clearly do the job, but now I'm begining to understand all the advantages associated with heading hold gyros I realise I may have made the wrong choice. Another mistake to add to the list.
The sensitivity of response/correction of the gyro can be adjusted (gyro gain) and it's very much a case of testing the heli and making adjustments. Too little gain and the yaw control becomes sluggish, too much and the gyro can over-control things and making the heli very sensitive with the tail oscillating from side to side ("hunting"). Although one gain value can be used for all aspects of flight, different gain settings can be used in flight to cope with different tail rotor speeds. Bear in mind that the tail rotor rpm is linked to the main rotor rpm, thus, the sensitivity of the tail rotor is lower at a lower rpm (for example during a hover where the main rotor rpm is lower), and higher at a higher rpm (for example during fast forward flight where the main rotor is at its highest rpm). With remote gain, a channel on the transmitter can be used to switch between two gain/sensitivity settings, i.e. more gain during lower rotor rpm, and less gain during high speed rotor rpm. This way you will still have good RC helicopter gyro performance during a hover at a lower rpm and can simply switch to less gain when flying with high head speeds.
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