Dialling in 3G for the first time

[ollie2893]

Just launched my new FBL 600e for the first time. Haven't p***ed my pants like this for a long time. Actually got my rusty old X trainer gear out - just in case Needn't have worried. Despite the random settings of the unit, nothing exciting happened. Naturally, I observe a slight drift, which I would ordinarily correct through my sub-trims. I am little confused, though: the manual is adamant that I must not do this (unless I repeat the 3G setup). I am supposed to use the trim tabs only. Sorry to ask so dumb: what's the difference between sub-trim and trim? Put another way, how can the 3G unit tell which one I moved?

I would also invite your advice on these:
(1) At my slow HS setting (1500rpm), the FBL is very twitchy in a hover. Should I increase the gains?
(2) The bird is massively more responsive to cyclic input than on the FB. Should I counter this on the Tx (through my D/R) or through some parameters on the 3G? (I have not got the cable yet, but that purchase would seem unavoidable.)

Thanks.

EDIT:
A related question is this: Can any modern-day FBL unit at all be expected to stabilize a two-speed setup, where the two speeds are quite different (I fly 1500 and 2000)? There're enough problems on the tail with this. I effectively need different gains (which, depending on the radio gear, a Tx may be able to switch). But the 3G unit has no input channels at all to remotely adjust the ELV/AIL gain...

[npomeroy]

Originally Posted by ollie2893

... Naturally, I observe a slight drift, which I would ordinarily correct through my sub-trims. I am little confused, though: the manual is adamant that I must not do this (unless I repeat the 3G setup). I am supposed to use the trim tabs only. Sorry to ask so dumb: what's the difference between sub-trim and trim? Put another way, how can the 3G unit tell which one I moved? ...

The reason for not changing the subtrims is that the 3G needs to register the neutral setup position (servo arm geometry, horizontal swash etc) as its "zero" point. Any trim is read as a command. I think you can indeed use trim to correct a slight drift, but it is probably due to slightly off initial setting which should ideally be corrected by re-doing the initial setup.

Nelson P

[ollie2893]

Thanks for your response!! I have been musing some myself since no replies were forthcoming.

Sub-trims and trims are both commands to the Tx, but they merely shift the signal to the Rx. I am positive that the 3G cannot tell the difference between those trims. And I am also positive that both affect the neutral position in the same way. Which is baffling because, as you say, a gyro needs to know center-stick.

However, the way all gyros actually calibrate their center is at boot time. Which is why it is so important that you do not move the sticks on power-up. (Hold that thought: this actually implies that while we are free to use trim tabs, after doing so, we would really need to reboot the gyro - which isn't mentioned in any manual I have ever read.)

I conclude therefore that the difference between trims and sub-trims probably concerns the upper and lower limits. I think the input to output transformation performed by the Tx may be a formula like so (ignoring ATV here):

txOut= subTrim + max(-100,min(100, potIn + trimTab))

That is, the Tx perhaps generates a signal of the range [subTrim-100;subTrim+100] and given how incredibly paranoid the Align manual harps on about not changing the sub-trims, perhaps something terrible happens to their s/w if an input signal exceeds the valid signal range originally computed for AIL and ELV?

Meaning in practice, though, that I will use sub-trims as before to find the neutral center (which will happen w/o my banging the sticks to either limit anyway) and then afterwards quickly run through the setup again to reset the AIL and ELV limits.

[npomeroy]

I wish someone with deeper experience would respond to you.
The thing to remember with the 3G is the initial setup gives the microprocessor the neutrals and endpoints. Thereafter you are not commanding the servos: You are instructing the 3G, which does its internal computing and "decides" what to do with the servos.
I could stand corrected, but reading my Hitec Optic 6 manual (which is more thorough than my newer Spektrum DX6i manual) it seems the nature of the subtrim may be different from trim (e.g. what the "centre" is when you have selected some expo) which is a simple command for an input + or - the neutral point.

Nelson P

[ollie2893]

Originally Posted by npomeroy

reading my Hitec Optic 6 manual. it seems the nature of the subtrim may be different from trim

Yes, I conclude (from my transformation formula) that the sub-trim is different in so much as it shifts the available signal range on the channel. In my exmaple, I gave this range as simply +/- 100 around the sub trim (ignoring ATV). If the sub-trim sits at +40, say, then an available range of [-60;+140] results. There's nothing you can do to the signal after that with the trim tabs that would bust that range. (Which is interesting in its own right because when you shift your trim tab to the max, what the Tx will effectively then implement for that channel is some asymmetric exponential: above center towards the trimmed side, every mm of pot movement results in a greater signal deflection than on the opposite throw, which is slowed down.)

That said, there is no way the 3G can tell the difference between my moving the sub-trim from 0 to 40 or my moving the trim tab from 0 to 40 - either way, "neutral" is suddenly 40 (and the 3G latches on to that at power-up). Just the end points will be different.

Thanks again for your reply!

[npomeroy]

I guess what Align are getting at is that by separating the use of sub and "normal" trim you don't fiddle with the neutral point THAT THE 3G HAS STORED.

[markleyland]

If you are getting drifting turn up the aileron gain on the 3G.

[ollie2893]

You assume that my swash is level. I don't. Not for a completely rebuilt bird. I simply ignored the Align instructions and nudged the sub-trims into place a few ticks until the bird settled nicely. Then I re-ran the initialization to fix "neutral" in Step 1, as Nelson correctly pointed out. Flies like a charm now. Just pushed it through some horribly gusty winds. It wasn't much fun but it went straight like a whistle. Never was that stable with the FB. I am very happy.

[celm]

can you use subtrim to get the servos to 90 when doing the set up before setting up the swash?

[ollie2893]

Yes. That's really what you should do. And if you have a swash plate leveling tool, then perhaps you will even get there.

I have always been very relaxed about dialing in the swash. I eye-ball it to be level, then hover it and trim out the last few sub-clicks. You only need to be a little careful with this relaxed approach when flying inverted. It only works in this lazy fashion if your bird's 100% balanced.

If you are just starting out in this hobby, then don't worry about balance too much and simply sub-trim it out.

[celm]

i always use a swash leveling tool