A rate mode gyro will sense movement and drive your servo to attempt to stop it. As such, the movement will be damped or stopped, but the heli will not maintain its current heading (direction the nose is pointing). Wherever the nose happens to be pointing when movement stops, that's where it stays till the next upset.
When flying a rate-mode gyro, the gyro will actually fight you in turns as it will not only attempt to stop unwanted movement, it will attempt to stop the turn you've commanded (eventually, some rate gyros had a feature added called "stick priority" that at least made the gyro smart enough to know the turn was a commanded one, and not an accidental one).
Since the rate gyro isn't really too keen at doing much more than trying stop or dampen movement, most transmitters still retain a rarely used feature called "Revolution Mixing" or "Acceleration Mixing". This feature has two adjustable settings -- "UP" and "DOWN".
Typically, to set up a heli that will fly a rate mode gyro exclusively, you would set the tail rotor to have about six degrees of pitch at hover to keep the tail from yawing while hovering. With a rate gyro, you adjust it so that at hover, the heli doesn't want to yaw left or right. Other setup stuff includes setting the travel limits so that at full rudder you don't bind up the mechanics and making sure the gyro knows which way to compensate, so you need to get the direction set correctly.
But, if you then increase pitch/throttle (and add torque into the system) the rate gyro can't keep up, and the nose of the heli turns as it goes up. That is where you adjust the Revolution "UP" mix to stop that unwanted yaw.
Then, as you decrease pitch/throttle, the torque in the system is generally less, the nose goes the other direction. Here, you adjust the Revolution Mix "DOWN" setting to stop that yaw.
These three steps allow you to hover without unwanted yaw, to go up without unwanted yaw, and to descend without unwanted yaw. It kind of worked, but since there were so many variables, it wasn't perfect.
The higher end radios also had a "rudder offset" feature to use with your rate gyro. Everything seems to be set OK for hovering, for going up, and for going down, but when you transitioned to forward flight, the fact that your tail rotor gets additional lift from moving forward also causes the nose of the heli to change direction. The Rudder offset feature allowed you to tweak the rudder trim to be different at hovering than in forward flight.
Rate gyros took a lot of tinkering with to get set up correctly, but gave a more "scale-like" flight performance.
With a rate gyro, the rudder servo is controlled primarily by the transmitter, and the servo follows the stick "normally" and returns to center on its own. The gyro only aids the rudder.
Enter the Heading Hold (tail-lock, AVCS, head-lock...) mode.
With the ability to have better sensors and microcomputers built-in to the gyro, the ability to sense the rate at which the heli is yawing (as opposed to just knowing it's moving) added the possibility of having the gyro not only stop the movement, but to figure out how much compensation to add to the rudder servo to move the nose back to where it was and maintain that position was introduced. This became the basis for the Heading Hold gyro. If you know how fast you're turning and for how long, you know how to maintain heading.
The heading hold mode is a whole different mode of operation. The transmitter really no longer controls the servo, the gyro does. That is why on the ground, if you move the rudder stick, the servo moves, but stops in weird places relative to stick motion, and generally doesn't return to center on its own. You have given up direct control of the rudder in favor of letting the gyro do its thing.
The stick essentially gets relegated to telling the gyro which direction to let the nose turn to a new heading, and how fast it is allowed to get there (heading and yaw rate are about all you get to control in this mode, the gyro does the rest).
With a heading hold gyro, setup is quite different than that for a rate gyro. It is now typical to simply center the tail pitch slider at neutral, and tell the gyro where the mechanical left and right travel limits are (as the stick no longer tells the gyro how far to move the pitch slider). The rudder endpoint settings no longer control travel of the tail rotor, they control the maximum rate at which you will allow the heli to spin. Typically, the endpoints start at 100% and if you want a faster yaw ("piro" ) rate, increase the endpoint number; if you want a slower yaw rate, decrease the endpoint number. In most present-day gyros, the left and right yaw rates can be fine tuned to give you equal yaw rates for left and right, in spite of the system torque differences. This was not really possible with the GY401.
Of course, there are now a thousand additional settings to mess with in the higher end gyros, and USB adapters, Bluetooth adapters, and even Iphone/Android Apps are available so you can really fine tune (or screw up royally) your gyro.
Heading hold gyros typically have a start-up, initialization sequence that last from about three to five seconds. During this time, you need to let the heli sit and not move it. Among other things, the gyro is looking at the rudder channel neutral pulse width, and learning how wide it is.
Once it's learned where neutral IS, it expects it to remain the same until the next time the power is cycled and it gets a chance to learn again.
Therefore, once you initialize a HH gyro in HH mode, the rudder trim in all flight modes must be identical. If you have different rudder trims in different flight modes, when you switch flight modes, the trim changes, and the gyro no longer "knows" where neutral is. It will be angry with you and may drift, or do all sorts of weird, unwanted things.
For the same reason, once you have initialized the gyro in HH mode, NEVER NEVER NEVER use the rudder trim tab on your TX. You'll confuse the gyro and it won't work too well until you re-establish the neutral point.
The GY401 MUST be initialized in Heading Hold mode, or it just won't work well at all. Most other gyros these days aren't that picky. Just don't mess with their neutral settings after you let them initialize.
Gain setting these days in most gyros is accomplished by an additional plug that goes to an unused channel in your receiver. Tradition has somewhat settled on using channel 5 (the "landing gear" channel" in most systems), or in the case of JR/Spektrum, AUX 2 in a receiver having more than six channels.
The gain control is set up such that it looks at the width of the pulse coming down the gain lead from your receiver so the gyro knows which mode in which to operate (rate/Heading Hold), and what the actual gain should be.
If you were to plug the gain lead into your aileron channel for instance, you could essentially move the aileron stick full left and be in rate mode at 100% gain. As you move to neutral, you stay in rate mode, but the gain decreases till you hit neutral and are at zero gain. The gyro is essentially out of the picture at this point. Now continue to move the stick to the other extreme, and the gyro enters Heading Hold mode, and the gain increases from 0 at neutral up to 100% at full stick.
Most transmitters today have a Gyro Sense/Control menu that requires the gain lead be plugged into a specific receiver channel, and the numbers that you enter into that menu select the mode and gain for that mode (simply by adjusting the width of the pulse coming out of the RX channel controlling the gain). These menus typically allow the gain to be controlled by the flight mode (or other) switch, and you can have different gains (and the same or different modes) based on the switch setting.
In the gyro sense menus, the gain numbers may go from 0 to 100, or -100 to +100.
If they go from 0 to 100, then 0 usually sets 100% gain in Rate mode, 50 sets zero gain and no mode, 100 sets 100% gain in Heading hold mode. 25 would set 50% gain in rate mode, 75 would set 50% gain in HH mode.
If they go -100 to +100, then at -100, you would be in rate mode at 100% gain, and as you move to 0, the gain decreases to 0 at 0. As you start going from 0 to +100, the gyro enters HH mode at near zero gain, and the HH gain increases to 100% when you hit 100%. (This is just like using the Gear Channel and its two endpoints).
If you don't use that built-in Gyro Sense/Control menu, you can opt to plug the gain lead into the Gear channel (usually channel 5) of the RX, and use the Gear switch on your TX and its two endpoint settings to control gain. Doing so usually gets you one Rate mode gain, and one Heading Hold mode gain.
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