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.

Thanks. Always good to have easy to understand info nearby...........M

This has gotta be one for the sticky section.

Thanks UKG. Just what I needed. I'll let you, and all the others who have contributed, when I get the spin stopped.

Roll

Great write up Groucho.

Thanks Groucho. Im soon will venture into the separates stuff and this should help.

I'll toss my thanks onto the growing pile, UKG.

My tail was doing well enough, and didn't need any particular fixing. I remember reading conflicting advice some time back, that said secure the servo for 0 subtrim causing stable hover, and made that work for the last year+.

I felt a need to tinker, though, and followed your steps. Now, my T-Rex holds just as well, behaves mostly the same, yet I am happier because I have more travel available to the servo (even if it doesn't much get used on the one side) and no risk of binding.

Thanks for the input!

-Trev

Hi UKG

Good easy guide for the 401.

The tail on my SE is stable (401 and 9650) however when I give a quick Rudder input I get quite a "bounce", hunting - a wag which settles. There is less wag when I lower the gain from 70 to 50.

Gyro in HH, Delay = 0, DS = On.

I don't get this with my L10.
What to check?

Jeff

Yes, it does sound like hunting.

Gain can be set best in FFF - fly FFF and increase (or reduce) gain to just below the point where the tail wags as the heli flies forwards. This is the maximum gain - you do not need to use this much if you do not want to.

Hunting is usually caused by a servo being too slow. Given that you have a 9650 my guess would be that this is a binding issue - i.e. some binding is making the tail slider slow to react, so this 'looks' like a delay for the gyro.

You do not mention which hole in the Servo 'wheel' or control arm you are using... or what your limit is set to.

As a starting point make sure that you have gone through the setup above - and have the tail mechanically centered, with no binding and the rudder neutral set.

Check that the tail slider is very free to move (pop off the connectors and move it by hand). I've had tail shafts which very quickly became 'sticky' (maybe there was a bad batch, I had a couple of them). It was almost as if teh surface wore on teh shafts....

If you are using teh clip on the tail fin mount then make sure that it is NOT binding - I've had lots of issues with the clip. If you are using the clip then try to fly without it - you may need to reset rudder Neutral if you do this.

Maybe you have flex in the tail pushrod. Some folks upgrade to 3mm carbon... worth a try if nothing else helps.

Toshi:

Can we get this preserved somehow in the EHBG?

mw

Originally Posted by Mark Wood

Toshi:

Can we get this preserved somehow in the EHBG?

mw

You should look at the existing EHBG sections on tail rotor/gyro setup. It already has more setup information than the above description in a less verbose format:

http://www.swashplate.co.uk/ehbg-v16/ch12s16.html
http://www.swashplate.co.uk/ehbg-v16/ch12s18.html
http://www.swashplate.co.uk/ehbg-v16/ch12s17.html

The first part of the post by ukgroucho is covered by section 12.17.2.4 Setting Limit in the EHBG.

The second part of the post by ukgroucho is incorrect as far as I know. It assumes you can set the tail servo neutral position by using the "switch 3 times between HH/yaw rate mode" feature. This is not the function of this feature.

The "switch 3 times" option actually changes the gyro's stored center stick position. Basically, if you center the rudder stick and change the subtrim, the gyro will now think you are trying to turn the heli because the signal has changed. If you change the subtrim, you can use the "switch 3 times" option to tell the gyro to update its center stick position. This has no effect on the neutral tail servo position used by the gyro.

Also, if you are a newbie and try to hover in yaw rate mode as the second section suggests, then it's likely you will lose orientation and crash.

Toshi

Originally Posted by TMorita

You should look at the existing EHBG sections on tail rotor/gyro setup. It already has more setup information than the above description in a less verbose format:

http://www.swashplate.co.uk/ehbg-v16/ch12s16.html
http://www.swashplate.co.uk/ehbg-v16/ch12s18.html
http://www.swashplate.co.uk/ehbg-v16/ch12s17.html

The first part of the post by ukgroucho is covered by section 12.17.2.4 Setting Limit in the EHBG.

Maybe I'm missing something but the EHBG does not seem to include discussion of setting the rudder Neutral position. This is important to get the best performance out of the 401.

Originally Posted by TMorita

The second part of the post by ukgroucho is incorrect as far as I know. It assumes you can set the tail servo neutral position by using the "switch 3 times between HH/yaw rate mode" feature. This is not the function of this feature.

The "switch 3 times" option actually changes the gyro's stored center stick position. Basically, if you center the rudder stick and change the subtrim, the gyro will now think you are trying to turn the heli because the signal has changed. If you change the subtrim, you can use the "switch 3 times" option to tell the gyro to update its center stick position. This has no effect on the neutral tail servo position used by the gyro.

errr.. this has EVERY effect on the Neutral tail position used by the gyro. You're telling it that the rudder Neutral (pitch for normal hovering) that you just determined and set up in sub-trim is center stick... So when you return the rudder to center the tail slider goes back to a neutral position...

Originally Posted by TMorita

Also, if you are a newbie and try to hover in yaw rate mode as the second section suggests, then it's likely you will lose orientation and crash.

Agreed... which is why I suggest doing basic adjustment on the ground or a lazy Susan. However, I will make the point that the Trex flies 'OK' (as in pretty stable) with the gyro in yaw rate (Normal mode) and no REVO mix enabled for the purposes of this activity. i.e. you should ONLY be hovering not trying to fly circuits.

Originally Posted by ukgroucho

,..
errr.. this has EVERY effect on the Neutral tail position used by the gyro. You're telling it that the rudder Neutral (pitch for normal hovering) that you just determined and set up in sub-trim is center stick... So when you return the rudder to center the tail slider goes back to a neutral position...
...

I'm not sure whether you're referencing HH mode or yaw rate mode, so I'll cover both.

In HH gyro mode, the "switch 3 times" feature resets the stored center stick position, but this does not affect the tail servo neutral position, because there is no fixed mapping from stick position to tail servo position. The tail servo position is whatever the microprocessor in the gyro determines to be correct to hold the correct tail position.

In yaw rate gyro mode (single rate), the "switch 3 times" feature doesn't affect anything, because a yaw rate gyro has no concept of a center stick position. It only applies a positive or negative delta to the current servo signal depending on whether the tail is currently moving left or right.

Toshi

I think we're saying the same thing here Toshi...

The setup process for rudder Neutral is trying to determine the optimum amount of pitch to apply to allow the 401 to move from turning the heli to 0 turn. This optimises the ability to stop the tail dead after a piro or turn.

You determine the rudder neutral pitch in Normal mode by hovering and adjusting sub-trim. Adjusting sub-trim in Normal is fine, but it's NOT good to have any subtrim in AVCS mode as the 401 considers this to be a constant stick input.

This is why you do the triple transition from Normal to AVCS gain. You are "re-centering" the gyro in AVCS mode based on the sub-trim you determined for rudder neutral. When you do this you are telling the 401 the optimum position to return the slider to (rudder neutral) when you center the rudder lever. You are setting a rudder neutral position for AVCS mode.

As you state this has no effect in Normal mode.

Originally Posted by ukgroucho

I think we're saying the same thing here Toshi...
...

No, we're asserting completely different things.

I'm saying the "switch 3 times" calibrates the CENTER STICK POSITION. When the gyro sees the transmitter stick in this position, it assumes you don't want to turn the heli. This affects the relationship between the rudder transmitter stick and the gyro.

You're saying the "switch 3 times" sets the gyro's NEUTRAL TAIL SERVO position, which is something totally different. You're stating this changes the relationship between the gyro and the tail servo.

In heading hold mode, there isn't a direct relationship between the rudder stick position and the tail servo position. In fact, by definition, there is no neutral tail servo position, because the heading hold gyro applies however much throw is required to hold the tail position. If you have crosswind blowing on the left or right side of your heli, the tail thrust will need to be less or more. So there is no "neutral tail servo position".

Toshi