There probably isn't any one "good" place for additional information on your radio and gyro setting, but understanding what makes the gyro work in normal mode, or rate mode, and what controls the gain, as well as a bit of basic knowledge about your radio in general will take you a long way.
First of all, the Gyro gain control plugs into an available channel for its control setting/input. Coming out of the receiver socket is a variable width control pulse, a pulse whose width is set by the position of a control stick, a switch, or perhaps some mixing you have set up in your transmitter.
For this discussion, assume that the pulse coming out of your receiver channel to control your gyro gain can vary in width from as short as 1 millisecond up to as long in duration as 2 milliseconds. Let's also assume that when the control stick associate with that pulse is centered ("neutral" ), the pulse coming out of the receiver channel is 1.5 milliseconds wide.
These pulse widths are not made up, they very closely represent the signal coming out of each of your receiver's channel plugs for full stick deflection in each direction (1 and 2 millisecond wide pulses) , and neutral (1.5 milliseconds wide).
Plug a servo into that receiver output, center the stick, the servo becomes centered and is considered to be at neutral. The pulse going into the servo is 1.5 milliseconds long.
Move the stick to its full left extreme, the servo may rotate from its neutral point, Counter Clock Wise, 60 degrees of rotation and stop. The width of the pulse going into the servo is 1 millisecond.
Return the stick to neutral, then move the stick to its full right extreme, the servo will rotate from neutral, Clock Wise, 60 degrees of rotation and stop. The width of the pulse going into the servo is 2.0 milliseconds.
Since your radio is a digital PROPORTIONAL radio, the servo movement is proportional to stick movement. The width of the output pulse from the receiver is linearly proportional to the amount of stick movement (assuming no EXPO is used).
If the receiver output pulse width is 1.5 milliseconds wide at neutral, and goes to 2.0 milliseconds for full right stick, that pulse will be 1.75 milliseconds wide for "half" right stick. The servo would turn 30 degrees Clockwise from neutral for "half" right stick.
If the pulse is 1.5 milliseconds wide at neutral, and 1 millisecond wide for full left stick, the pulse will be 1.25 milliseconds wide for "half" left stick. The servo would move 30 degree Counter Clockwise for "half" left stick.
No matter which channel you use, or how you control it, the pulse widths coming out of the receiver channel behave the same way. 1 millisecond wide for full control position in one direction, 1.5 milliseconds wide for neutral control position, and 2 milliseconds wide for full control position in the opposite direction.
Now let's take a look at your GY401, and just about every other gyro on the market.
The gain lead is usually a single wire plug dangling off the end of the gyro wire harness. The gain lead gets plugged into a receiver output channel that will be used to control gyro gain.
That same harness also includes a standard 3-wire plug, this gets plugged into the rudder channel output of the receiver, and is what the gyro uses to make the rudder servo respond to the rudder control stick. Full left rudder -- output signal is 1.0 milliseconds wide. Neutral rudder, the output signal is 1.5 milliseconds wide. Full right rudder, the output signal is 2.0 milliseconds wide.
The Gyro gain input uses the incoming pulse width of its control channel to select operating mode -- Normal/Rate mode, or AVCS/Heading Hold mode -- and the gain for that operating mode. It's all based on the width of the pulse coming out of the receiver for a given gain setting in your TX.
If the pulse width is between 1.0 and 1.5 milliseconds in width, the gyro will operate in Normal/Rate mode.
When the pulse is 1.0 milliseconds wide, the gyro is operating at 100% gain, in Normal/Rate mode.
When the pulse width is at 1.5 milliseconds, the gyro is operating at ZERO gain, essentially not operating as a gyro at all.
If the pulse width is between 1.5 milliseconds and 2.0 milliseconds, the gyro will operate in AVCS/Heading Hold mode.
A pulse width of 2.0 milliseconds will select 100% gain in AVCS/Heading Hold mode.
A pulse width of 1.5 milliseconds will have the gyro operating at ZERO gain, essentially not acting as a gyro at all.
If the pulse is 1.25 milliseconds wide, the gyro will be in Normal/Rate mode, at 50% gain.
If the pulse is 1.75 milliseconds wide, the gyro will be in AVCS/Heading Hold mode, at 50% gain.
What has all this to do with your transmitter and its Gyro Sense menu?
The Gyro Sense menu is a simple way of letting you control the gain and operating mode of your gyro. To simplify matters, the Futaba radio you have insists that you use receiver channel 5 to control gyro gain if you choose to do so using the Gyro Sense menu. You have plugged the gain lead into the correct receiver channel, so the gyro is receiving a pulse width on its gain control lead set by the Gyro Sense menu for a given flight mode.
I looked at the Futaba 9C user's manual and found this:
The picture shows what the output of the receiver gain control channel does when the Gyro Sense menu is set to STD or GY.
In either case, you can see by the picture that when the setting is "0" for gain, the receiver output pulse will be at neutral, or 1.5 milliseconds wide.
In GY mode, if you have AVC of 100%, the pulse width out is 2.0 milliseconds, setting the gyro to 100% gain in AVCS/Heading Hold mode.
In GY mode, a value of 0 outputs a pulse 1.5 milliseconds wide, and the gyro is at zero gain, essentially with the gyro function disabled.
In GY mode, if you have NOR of 100%, the pulse width out is 1.0 milliseconds wide, setting the gyro to 100% gain in Normal/Rate mode.
Operating in STD mode, a value of -100 causes the output pulse to be 1 millisecond wide, the gyro is operating at 100% gain, in Normal/Rate mode.
Operating in STD mode, a value of 0 sets the output pulse width at 1.5 milliseconds (neutral), and the gyro is at 0 gain, essentially not operating as a gyro.
Operating in STD mode, a value of +100 has the gyro operating at 100%
gain in AVCS/Heading Hold mode.
Now, a couple of pictures from the GY401 manual:
In this picture --
the gyro is being controlled using the Gear switch (or the STD mode of your transmitter).
Gain is controlled by the endpoint settings for the gear switch (and there is one endpoint value for each of the two switch positions).
Gear Down would allow you to set a value between 0 (no gain at all) and -100 (100% gain in Normal/Rate mode).
Gear UP would allow you to set a value between 0 (no gain at all) and +100 (100% gain in AVCS/Heading Hold mode.
This corresponds to the -100, 0, and +100 values allowed in the STD mode of your transmitter.
You can also envision this control of the gyro with your TX set to GY mode, where when operating in the NOR side of the settings, the value ranges from -100 to 0 for Normal/Rate mode operation; when operating in the AVC side of the settings, the values range from 0 to +100 for AVCS/Heading Hold operation. 0 sets NO gain, no gyro function at all.
In this picture:
the Gryo Sense menu acts more like that found in a JR transmitter Gyro Sense menu.
Here, the setting menu in the transmitter allows you to set a number between 0 and 100.
Zero in this case has the gyro operating at 100% gain in Normal/Rate mode (1.0 millisecond wide output pulse).
A value of 50 has the gyro set to ZERO gain (not really Normal/Rate mode OR AVCS/Heading Hold mode) and a 1.5 millisecond wide output pulse.
A value of 100 sets the gyro to 100% gain in AVCS/Heading Hold mode with an output pulse width of 2.0 milliseconds.
When you DISABLE your transmitter's Gyro Sense output, control of channel 5 is most likely relegated back to the landing Gear UP/Down switch, one position of the switch will put the gyro in AVCS/Heading Hold mode while the opposite position puts the gyro in Normal/Rate mode. This would explain why when you turn off your Gyro Sense menu, things mysteriously begin to work.
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