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Old 09-27-2010, 07:43 PM   #1
ReadyToCrash
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Default tail setup - why not to use subtrim?

hi,

i wonder if someone could explain why using the subtrim to center the tail servo arm is not a good idea, please.

as far as i know, the servo arm must be at 90'. so far, none of my servos arms have had this position naturally. i always need to use a liitle subtrim to make them get into position. if i can't use the subtrim, how to make them be at 90'?

thanks a lot.
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Old 09-27-2010, 07:49 PM   #2
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I myself use subtrim to get my servo arms at 90. And from what I have seen from the Finless Bob videos that is what he says to do also. I haven't had no problems with using subtrim yet and I suggest to keep using subtrim, but I am sure some more experienced will reply and give you a more definite answer.
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Old 09-27-2010, 08:01 PM   #3
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You use sub-trim on cyclic as there are no gyros involved. The tail is controlled
from the gyro. Any sub-trim MUST be made in the gyro itself. To utilize radio induced sub-trim on a tail is to screw up the gyro sensor.
DO NOT use sub-trim from your radio on a tail ever. Ensure your mechanical set up is spot on. It is normal for the servo arm to not be a true 90. Get it as close as mechanically possible, that's all you can do.
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Old 09-27-2010, 08:07 PM   #4
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thank you guys a lot.

i'm stupid and i still have a few questions, please.

can't i use the subtrim on the rudder even when the mechanical set up is made in rate mode and then swithed to hh?

if the subtrim is not to be used on the rudder, why is there such a feature on the tx?

thanks.
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Old 09-27-2010, 09:08 PM   #5
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I guess I should get some sleep because I was thinking of cyclic and didn't even pay attention to the title of this thread, so for my tail servo I don't use subtrim.
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Old 09-27-2010, 09:54 PM   #6
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Default Subtrim on AVCS Gyro

Hey Ready,
(Also see my PM)

On any of the AVCS gyros in Head Hold mode, the sub trim will be interpreted as rudder input, so the gyro will move the servo in that direction. Not good. The sub trim will work in rate mode, just not HH mode. Also, at Tom said, it is really not critical to have the servo arm at exactly 90 degrees, just as close as you can get mechanically. No trim or sub trim. Getting no drift in rate is more important. That way the servo will not be overworking making a correction you could have done by positioning the servo on the boom. Take a digital picture of where the slider is with your subtrim set with do drift in rate mode. Then zero out the sub trim and move the servo on the boom to get it right there. if the gyro is working and not bad, you should have it. BTW, if you want to reset the Zero point, with the heli on but not moving, switch the gyro switch on and off 3 times in one second and the LED will flash and reset just like a power up.

The thing to remember is that in HH mode, the AVCS software takes a fixed heading at power up and adds and subtracts all of the stick, trim, and sub trim inputs and compares all of that against the actual heading of the heli and tries to put the nose where it thinks it is supposed to be. In rate mode, it just passes rudder through and reacts in the opposite way to things like wind, etc. It does not keep a heading, or adding up all of the inputs.

BTW, on a Telebee and the old Align gyros, you do use sub trim to stop the tail servo from creeping, per the Finless video, This does not apply to the 401 and clones.

Terry
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Old 09-28-2010, 05:11 PM   #7
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If you cant get 90' mechanically buy extra servo horns one will fit at 90'

eg a set ds510's in a 500 kit will usually give one servo at 90' the others need trim but buy 3 sets of horns and 1 from each set will fit 1 servo at exactly 90'
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Old 09-28-2010, 06:40 PM   #8
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Quote:
Getting no drift in rate is more important. That way the servo will not be overworking making a correction you could have done by positioning the servo on the boom. Take a digital picture of where the slider is with your subtrim set with do drift in rate mode.
hey blade,

pls bear with me because i'm really stupid.

is this the way to do what you suggested above?

01) set to rate mode
02) set subtrim to 0
03) throttle the heli up on the turn table
04) compensate the natural spin with some subtrim so that the tail is always still
05) throttle up up to 75% always making sure the tail is held
06) cut off the throttle
07) mark the position of the slider on the tail shaft
08) set subtrim to 0
09) slide the servo on the boom so that the slider in on the right position
10) set to hh


Quote:
BTW, if you want to reset the Zero point, with the heli on but not moving, switch the gyro switch on and off 3 times in one second and the LED will flash and reset just like a power up.
i tried but nothing happened.:o

thanks.
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Old 09-28-2010, 07:57 PM   #9
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OK lets dispell some myths,

Your tail servo is working ALL the time, it can NOT be over worked making normal corrections (can be over driven by binding or over loaded by say using a 450 servo on a 700 )
It does this in both RATE and HH Mode.

Rate mode is pretty much irrelivant unless you intend to fly in rate mode
or
You dont have a brillinat gyro so having a good rate set up can sometimes "help" in HH mode although its very touch and go if this is in reality whsts happening.

That is because in HH your servo "neautral" position is completely irrelivant because the gyro puts the servo where ever it needs to be in order to stop drift and hold its heading. Which will vary depending on head speed, pitch of the main blades, attitude of the heli etc

Problem is with the stock gyro or cheap alternative drift is something you either have to live with or say screw the theory and trim it out while you learn the basics,
After that there is only one way to get a good tail, and thats sink some $$ into a decent gyro and tail servo and set it up properly.
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Old 09-29-2010, 06:47 AM   #10
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Quote:
Originally Posted by ReadyToCrash View Post

01) set to rate mode
02) set subtrim to 0
03) throttle the heli up on the turn table
04) compensate the natural spin with some subtrim so that the tail is always still
05) throttle up up to 75% always making sure the tail is held
06) cut off the throttle
07) mark the position of the slider on the tail shaft
08) set subtrim to 0
09) slide the servo on the boom so that the slider in on the right position
10) set to hh


i tried but nothing happened.:o

thanks.
Theoretically that pretty much sums it up, although the point 09 looks suspicious to me. On one of my Belts (v2 CF) for whatever reason similar procedure was resulting in the slider being way too much off to the right. The Belt should be neutral at hovering (60-70% of the throttle) with the tail slider with about 1/3 of the movement available to the left (towards the boom) and 2/3 to the right, that's because going to the left you're letting the main rotor torque to turn the tail, while going to the right you're working against the main rotor torque. So in the case of my v2 CF (it has all Esky aluminum head and tail components) I did the following:

01) set to rate mode
02) set subtrim to 0
03) energize the Tx
04) energize the heli (with motor leads disconnected)
05) slide the servo on the boom so that the slider has 1/3 of the movement available to the left and 2/3 to the right.
06) set to hh mode

This resulted in totally crappy rate mode but very decent hh mode performance using stock v2 gyro.
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Old 09-29-2010, 08:24 AM   #11
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With regard to the original question, sub trim or not. My opinion is, use it if you need it and don't if you don't. It is no use saying move the servo up and down the boom, if no matter how hard you try you cannot get it to hold, without it drifting. It is also no use saying, never use sub-trim, when often it can work with with no ill effects.

Imagine if the problem was that both the radio and the gyro, unless they are real top of the range, and recently calibrated, were not tied to be either sending or expecting the same zero signal, or whatever the true figure for zero is. It is almost certain that neither will be right. If however, the gyro wants a slightly off centre signal, for what it thinks is zero, and the Tx can be told to send that, even if this is an off centre signal, achieved by using sub trim, then what can be the problem with that. If you cannot manage it on the boom, and you can manage it with subtrim, and it is working, I see no reason not to use it.

As Alex said, in HH, especially with a good gyro servo combo, it really makes no difference. As long as the ends are set, to prevent binding, and the middle was roughly 90, the gyro does completely its own thing throughout the available range to keep things in track with what you have requested by your stick movements, or lack of.

Another quick note, don't misinterpret a creeping servo for a drifting heli. Just because your servo moves to one end of its travel or the other, when it is on the ground, or not actually spinning on the lazy susan, this is not idicative that the heli will drift off heading in the air. The only way to know this for sure, as was also mentioned is to get it up to flight speed, maybe 60% throttle or so, on the lazy susan, or if you are confident actually fly it and observe. Never think it is wrong just from the behaviour of the servo. in HH mode they nearly always creep to one end or the other, especially on the lower end models but this has no effect at all with regard to how the gyro will operate in the air.


Cheers


Sut
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Old 09-30-2010, 04:19 AM   #12
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I'm not a fan of subtrim on the tail.... as mentioned you can get a servo arm to 90° or near as dammit mechanically with the correct servo horn.

If you must use subtrim, apply it to get to 90, then do a re-bind.... the rudder neutral signal you just shifted with subtrim will then be seen as neutral after the bind.
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Old 10-01-2010, 07:41 PM   #13
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Quote:
Originally Posted by ReadyToCrash View Post
hey blade,

is this the way to do what you suggested above?

01) set to rate mode
02) set subtrim to 0
03) throttle the heli up on the turn table
04) compensate the natural spin with some subtrim so that the tail is always still
05) throttle up up to 75% always making sure the tail is held
06) cut off the throttle
07) mark the position of the slider on the tail shaft
08) set subtrim to 0
09) slide the servo on the boom so that the slider in on the right position
10) set to hh


i tried but nothing happened.:o

thanks.
All I was suggesting was a way to take the setup you said had worked in Rate mode and translate it to a mechanical setup. My experience with the low cost 401 style gyros is that they fly better in HH with a good rate setup. The Futaba GY401 instructions say it can be setup with trim and it is supposed to take that as the zero position. I have not tried to get that to work. So you might get it to work with sub trim, hard to tell what chip yours has in it. As mentioned prior, a good rate setup on a Belt CP turns out to be about 1/3 of the shaft showing on the boom side. I did have one of those gyros that had some drift and was able to fix it by gluing the wires inside the case with hot glue and adding a piece of foam to keep anything from vibrating inside the case.

I hope you get it working.

Terry
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Old 10-09-2010, 05:55 PM   #14
Tom Burgess
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I use sub trim on the tail all the time. Nothing wrong with it.

I can't speak for Futaba as I am a Spektrum user.

When you first power on your model the rx (on Spektrum remember) sees the mid stick pulse width. If you have added subtrim and REBOUND, the mid stick pulse width is stored. If you have not rebound it will se it as an input, and as soon as your gyro initialises, it will creep.

Setup your geometry in rate mode, use subtrim to get 90, rebind, go fly
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