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Riporto da un articolo di un "non niubbo" (è in inglese, purtoppo):

I've seen a number of folks post regarding setting up the GY401 so I thought I'd repost my setup tips based on what I do with my 3 Trexes.... this is copied from a much earlier post but I hope it makes it easier for folks to find.

Here is how I set up... maybe right maybe wrong but it seems to work.
First do mechanical setup.
- CH 4 endpoints set to 100% each side. This does NOT affect throw but start with this as you will adjust later to get the desired piro rate.
- Trim and Subtrim centered.
- Power on in AVCS mode (using whatever gain setting in the TX / switch setting s are needed) and wait for the gyro to initialise. You may see a blinking LED (rather than steady) you can ignore this... it seems to indicate an 'uncentered' gyro for what I can tell and goes away once you set the neutral position.
- Set the gyro GAIN to Normal using your Tx (varies from Tx to Tx.. Futaba it's easy, JR etc. you need to select a gain less than 50% I think).
- Now see how much tail slider throw you have. You're actually going to do two things here - set the RIGHT AMOUNT of throw and set the THROW TO BE CENTERED.
- You may need to go around the next couple of steps a few times.
1) Set the LIMIT on the 401 so that you do not hear any binding when moving the rudder full left and right. Binding = servo making a straining / buzzing noise.
2) At this stage you will probably be getting full throw one way on the slider but not the other - we need to CENTER the slider relative to the servo.
3) Look at the tail slider travel... if it is not moving full in one direction then work out which way you need to move the servo on the boom (only a TINY amount) to get more throw in that direction. Move the servo - leave the boom clamp screws loose for now.
4) Check the throw, increase the LIMIT on the gyro to accomodate the additional 'room' made by moving the servo until you hear it bind one side or the other. Go to 2)
- When you get to a point where the throw is FULL in both directions and the LIMIT is set so that no binding occurs then tighten the servo clamps. The servo is now 'centered' on the tail slider and the LIMIT is set to obtain full throw.

Next you need to tell the 401 where 'neutral' is . In other words it needs to understand how much pitch to apply to get the heli to hover straight. This will be the gyro's reference point for determining what it needs to do to correct left or right - or where to return the rudder when you finish a turn or piro and want it to STOP the tail dead (i.e this is IMPORTANT to get snappy 'locked' stops).
EDIT: Added as folks may power off the heli between the last step and this one. You need to do the rudder neutral setup with the gyro set to a Normal gain - if you powered off then set the Tx so the gyro will be in HH / AVCS mode (ALWAYS power on in HH/AVCS) and power up the heli. Once the gyro is initialised you can switch back to Normal gain on the Tx.

- Set your TX to subtrim adjustment mode and select channel 4 - should be centered / 0 right now. You can also adjust using the rudder trim on your Tx is you do not have a subtrim option!
- You will need to take off but ideally you should do some basic adjustment on a smooth surface (on the ground or a lazy susan). Spool up to full HS, 0 pitch and apply a little + pitch. If the heli rotates then adjust subtrim / trim to correct.
- Once you have a manageable level of 'spin', take off and adjust subtrim / trim until the heli will hover straight with no rudder input. Depending on your TX and level of flying experience you may be able to adjust in the air (I can do this with my Fut 7CHP as the adjuster wheel is easy to reach whilst hovering) . If you are not confident or the TX is not easy to configure then note which way you need to adjust, land, adjust and go again. NOTE you need to be in a 'stable' hover.. if you're applying pitch / throttle then the tail WILL wander because YOU DO NOT HAVE REVO ENABLED (you SHOULD NOT have REVO enabled!)
- Once you have a stable hover in Normal (non AVCS) mode, so you have defined the subtrim / trim for rudder neutral, then you can set this for AVCS mode.
- Land the heli
- Easiest option is to power off the heli, Switch the gain setting in the TX to a HH / AVCS gain setting and then power the heli back on. The 401 will memorise the rudder subtrim / trim setting a this point as the rudder neutral. IMPORTANT.. from now on you must ALWAYS power on the heli with AVCS / HH gain selected so thatt he subtrim / trim settign that you have set is memorised as rudder neutral DO NOT adjust the subtrim / trim for the rudder in AVCS / HH mode.
- (OPTIONAL!) You can also 'set' the rudder neutral for AVCS / HH mode by switching between Normal mode and HH mode 3 or more times at intervals of less than one second ending in HH (AVCS) mode with the heli powered on (Futaba document this so that you COULD set the rudder neutral in flight if desired!). If your TX has a gyro gain switch (like a Fut 7CHP) then this is easy.. for some lower end JR models you will need to juggle the gain setting in the menu system up and down around 50% (e.g. <49 = Normal mode, >51 = AVCS so you toggle 49 to 51, set, 49, set , 51, set, 49, set, 51, set... you can now increase gain to just less than your Normal gain... work it out.. NORMAL = 30, HH - 69 or so, offset from 50).
- Fly and confirm your gain setting - reduce if it wags.
- Now you can set the endpoints on channel 4 in the TX... this will affect the piro rate. It does NOT affect the throw of the pitch slider (this is 'owned' by the LIMIT on the 401 - which you should NOT adjust).
- Usually your left turn endpoint can (may) be a little less than the right turn - the heli will piro (nose) left quicker than right because the main blade torque helps. Try 60 left and 70 right or somesuch.

Hope this helps some folks out...

UKG.