What a gyro is, what it does, and a little background
The gyros (or gyroscopes) on R/C helis these days are single-axis rotational rate sensors. They effectively become the controller between the pilot input and the servo output. We use a gyro on our yaw axis because the dynamics in yaw would otherwise be far too fast to comfortably deal with on such small helicopters. In pitch and roll, the flybar effectively does the same job for us.
In years past we used to use mechanical gyros on our R/C helis. These typically consisted of a small motor with two flywheels (one on each end of the shaft). As the heli yawed, the motor tilted on a small gimbal inside the gyro case. This tilt was measured electronically, and taken as an indication of yaw rate.
These days the gyros we use don't have any moving parts (at least not in the sense we're used to). Current gyros have an extremely small vibrating prism that senses rotational rate through the coriolis effect. Basically, this triangular prism has a transducer on one side to make it vibrate, and sensors on the other two sides to measure that vibration. This prism tends to want to vibrate in one plane as the heli turns around it. But the transducer continues to vibrate it in the longitudinal axis of the heli. The result is that the plane of vibration lags the plane of induced vibration by an amount proportional to the yaw rate of the heli.
The two types of Gyros, and how they act
as part of the Control system
There are basically two “types” of gyros for R/C helis these days – “proportional” and “heading-hold” (or AVCS). In both cases the gyro acts as the controller in a closed loop system. The pilot commands a yaw input to the rudder servo, and the gyro passes that command on to the servo. The servo changes the pitch of the tail-rotor blades and this results in a yaw rate. The gyro senses that yaw rate and subtracts a command proportional to that rate from the command to the servo. This forms a negative-feedback, closed-loop control system. While it seems counter-intuitive to alter the command to the servo, just because the heli did what you told it to, consider this… Imagine for a moment the pilot issues NO command, but the wind blows the tail to the right. The gyro will sense this motion and act to either stop the motion (in the case of a proportional gyro), or counteract the motion and return the heli to the desired orientation in the case of a heading-hold gyro. This will be true whether the undesired motion comes from a wind gust, a change in torque to the main blades, or any other non-commanded source – just what we want!
So, the gyro seems to counter our commanded input, and also the unintended inputs (wind, torque, etc.). But we can increase the gain on our commanded input because we know what the gyro will do to it. What does this mean? It means we would drive our servo well past the desired mechanical limits when the heli doesn’t yaw in response to our commands – such as when it’s sitting still on the ground without the blades spinning. In the old days (just a few years ago) we used to have to make a choice - either set the system up to overdrive the servo on the ground (and be careful not to give full inputs), or drive the servo only to the limits, and live with reduced yaw rate when flying. These days, most gyros have “limit” settings. These allow us to overdrive the servos as far as we want, but tell the gyro where the physical limits are. Thus, when we perform a yaw command on the ground we see that the last 1/3 of the stick motion has no effect. The servo reaches the programmed limit by about 2/3rds stick, and goes no further. But this is not a bad thing. When the heli is flying the entire stick deflection will be used, because our command will be reduced significantly when the heli is yawing.
The big difference between a proportional gyro and a heading-hold gyro is actually a matter of firmware – mechanically, they’re identical. The proportional gyro simply tries to counter the sensed yaw rate. The heading hold gyro integrates the yaw rate over time (to give yaw angle), and thus seeks to maintain the commanded yaw angle.
My Servo doesn’t Center Properly in HH mode!!!
You can imagine now that with a heading-hold unit, the gyro seeks to put and keep the heli’s tail at the position commanded by the pilot. So, if we issue a command while the heli is on the ground and blades are not spinning, the gyro tells the servo to move. But the tail does not move. So the gyro just keeps trying to get the tail where it belongs. The servo will not center, because the tail hasn’t yet gone where we told it to go. The proportional gyro (or proportional mode on our HH gyro) doesn’t exhibit this behavior because it doesn’t know anything about yaw angle. It only knows about yaw rate. When we release the stick we are no longer commanding a yaw rate. The heli isn’t yawing, and all is right with the world. So the servo is commanded to return to center. As a result we normally set the centering and limits of heading-hold gyros while in proportional mode. The Finless video on gyro setup describes this in much greater detail.
So How do I setup my HH Gyro?
Your servo and linkages should be properly centered, and limits should be set in
non-HH mode (with 0 trim). Then put the gyro in HH mode and fly.
Increase gyro gain until tail begins to wag or “hunt”; then back off a bit. If you want greater piro rates, increase your ATV's. This won't affect the limits set on the gyro. But it will give you more throw in flight.
This extremely brief description of gyro setup is really intended for reference. I highly recommend the Finless video for a detailed description of how to set up your Futaba GY401 heading-hold gyro.
Useful gyro links: