GY401/TREX450 (not so) QUICKIE

[kitkarsons]

Hi volks

I've been chasing a number of niggly (possibly related) problems on my 450 Sport and in doing so I have realized that I have never actually performed a rate-mode setup on my GY401/9527 tail. So, having reminded myself of the way to achieve this, I began by hovering and flicking to Rate mode to see which way (if at all) the tail would move.

The result of this experiment leaves me a little confused and I am now wondering if this is an indication of some other problem.

BTW: My radio is a DX6i

The results were as follows...

having adopted a stable hover, I switched to rate mode (gain set at 25% on the DX6i which I believe is equivalent to 50%?). The tail immediately moved anti-clockwise at a rate that was neither alarming nor negligable, rather, a direct move to the right which, by the time I switched back to HH, had turned the heli through almost 90-degrees.

I interpreted this result as an indication that the rate-mode gain was too high (?), i.e. it was over-compensating and moving the tail rightwards. Is this correct?

So, I adjusted the rate-mode gain from 25% to 30% (equivalent to 45%, correct?) and tried again. Same result and, although it is hard to say for sure since I just flick the switch one way then the other quite quickly (I'm in a fairly confined space and cannot really afford to piro), there didn't seem to be much dirrerence in the angular acceleration/velocity of the tail once I hit rate mode.

I tried again at 35% == 40% and still, if there was any difference it was negligable. Given that by now I had dropped the gain by a total of 10%, I certainly expected to see a greater effect?

This is when I started to wonder if there was another problem? Hence... this post.

I should also point out that I am a (dirty) TAIL-FLYER, i.e. my rudder moves in the opposite direction to that of a normal person!

The way things currently stand:

i) the 401 is set for 'digital' servo and the sense is set to 'reverse'.

ii) the ball link on the servo wheel is at 90-degrees to servo shaft after initalization,

iii) the servo is positioned such that, (by virtue of (i) above) when gyro has initialized, the leading edges of tail rotor blades are in direct alignment (i.e. zero pitch)

iv) my HH gain is drift-free at around 68%

v) as of this morning, my throttle curve is a flat 85%. I dropped it from a flat 90% since this helps with the niggly problems I've been trying to solve. Whilst my pitch curve is a linear 50..100% above mid-sticks and drops to give a maximum of about -2 degrees pitch below midsticks.

SO MY MAIN QUESTION IS: What's up with my rate mode gain?


The niggly problem is as follows but it is the above question that I'm specifically interested in. I include the following in case it is of any use.

For the most part, the tail behaves as I want it to, but I am getting some niggly/intermittent rapid-wagging which comes and goes sometimes in relation to changes in throttle stick position, but at other times it happens without any change to stick input. Having dropped my throttle curve by 5% as described above, things are better but it does still happen and now my stick position for hovering is up around 75%-80% which doesn't leave much to play with.

I have checked and rechecked my collective-pitch settings and I definitely have zero-pitch at midsticks with about +/- 8 degrees either side. In any case, the heli as a wholoe is p[retty well-behaved, in fact, all of my subtrims are ZERO with the exception of just 4-clicks to the right on my aileron channel. The heli hovers very nicely.

This all started after a crash a few weeks ago hence I have recently replaced...

1. main shaft,
2. main & a/r gears,
3. tail drive gear,
4. tail rotor shaft,
5. tail boom,
6. drive belt
7. blade grip ball links
8. replaced my original s9257 with another metal-geared one I got from another member (Sacko, cheers mate) and I also tried an ACER-LAB D12-MG which worked OK although a little slower, but I still had this same wagging problem.

I cannot see any sign of wobble on the head as a whole, I've checked my feathering-shaft since the crash and it is straight.

MY SUBSIDUARY QUESTION IS: what gives? is there anything else I could try?

Thanks heaps for taking the time to read my screed.
Double heaps if you think you can help.

Cheers,
Rich.

[J-S-Q]

The rate mode gain is not the issue, it's the centre point of your tail pitch slider -you will need to physically adjust the length of the tail control rod (or move the servo along the boom) in order to get the correct 'neutral' amount of pitch so the heli does not rotate at centre stick.

[Vic.P]

Hi Rich,
Your tail mechanical set-up should be done in Rate mode. The reason your heli started an anti-clockwise turn is because there is currently no tail offset, ie, your tail servo when at neutral has zero pitch on the blades. Adjusting the Gain will make no difference to the rotation of the heli!!

What you need to do is move the servo brackets so that you have a tail pitch offset of roughly 8 degrees. The pitch should be to give thrust to the right as you look at the heli from behind to counter an anti-clockwise turn so the blades should point like this .... /..... with thust -->
Keep adjusting the servo brackets until you get no tendancy to pirouette while in RATE mode.
Now you should have the tail geometry set correctly so that when you use H/H mode the tail servo will not be overworked trying to compensate for an incorrect tail neutral position.

[kitkarsons]

Thanks to you both. You have helped correct an error in my thinking that I've been carrying around for months! Tail-neutral != zero-pitch on tail rotor. Now that I understand this I should be fine. I have been moving the servo around thinking that I was setting tail-neutral when all I was doing was setting zero-pitch.

The killer is that, since I posted I've made more adjustments so that my rate/hh gains are now at 12% / 73% and the tail is rock-solid, plus with my throttle back at a flat 90% my niggly wag is completely cured! I've been wandering around here whistling to myself thinking that I'd finally sussed it.

Oh well. It's much better to know the real cause and rid myself of faulty thinking. I'll follow your recipe Vic and have another go tomorrow.

Thanks again.

[J-S-Q]

Well, as ever opinions will differ but I think most people don't bother to set up for Rate mode these days. I just set the tail slider at the centre of travel at centre stick and always fly in HH mode -not had a problem so far (apart from with the dreaded TRex 250 tail!). Also, regarding the throttle curve, I've usually found upping the headspeed makes for a better tail so I've generally run my electric helis with a flat 100% throttle curve.

[J-S-Q]

The killer is that, since I posted I've made more adjustments so that my rate/hh gains are now at 12% / 73% and the tail is rock-solid, plus with my throttle back at a flat 90% my niggly wag is completely cured!

If it feels solid then personally I'd just leave it at that, unless of course you specifically want to fly in Rate mode.

[machasm]

The problem with setting up for a neutral tail in rate mode with a 401 is that once you achieve this then you have unequal throws to get the tail at both endstops.
This isn't a problem for gyros with separately adjustable endpoints but the 401s 'limit' adjust affects BOTH ends of the travel at the same time.
So once you have adjusted the limit to not bind in both directions then you will find that the tail will not travel to its full throw in one of the directions.
Personally, I set mine up for a zero pitch tail rather than a neutral tail.
Ade.

[GravityKills]

>> This all started after a crash a few weeks ago
there's your answer. Don't bother tweaking parameters when the hardware is broken.

A myriad parts could be bent, I can't list them all.
* For example, the tail rotor hub usually bends when I break a tail blade.
* Check the tail blade bearings, they may lock up under load.
* Spool up the heli without blades, unlink the rudder pushrod from the servo and operate it by hand. Do you notice anything?

[kitkarsons]

The problem with setting up for a neutral tail in rate mode with a 401 is that once you achieve this then you have unequal throws to get the tail at both endstops.
This isn't a problem for gyros with separately adjustable endpoints but the 401s 'limit' adjust affects BOTH ends of the travel at the same time.
So once you have adjusted the limit to not bind in both directions then you will find that the tail will not travel to its full throw in one of the directions.
Personally, I set mine up for a zero pitch tail rather than a neutral tail.
Ade.

Exactly as I have just discovered. I've actually lost about 1-2mm of slider travel from one end so that my full left-stick looks a lot less than full-right.

I am glad to finally properly understand this process, but on balance, I think I might take a while to try my own adjustments with the 12%/73% setup. Then perhaps later have a go at setting-up for rate-mode. It is true that (in the sim at least) I find some manouvers easier in rate mode? Don't know how PC it is to admit it though? But as a tail-flyer I'm probably already beyond the pale ...lol

A myriad parts could be bent, I can't list them all.
* For example, the tail rotor hub usually bends when I break a tail blade.
* Check the tail blade bearings, they may lock up under load.

In fact, the crash didn't damage my tail rotor assembly as far as I could see, i.e. blades still intact, just a very slight bend in the boom which I swapped-out. I did replace the tail rotor shaft later but that was a speculative move in an attempt to trace the wagging problem. Also, I've dismantelled, cleaned and reassembled the tail-rotor hub assembly since and I couldn't find any obvious sign of damage. However, it may well be getting near to the time for me to replace some bearings.

One thing I didn't mention before is that, up until the crash, if I spun my main rotors by hand they would continue to turn for a minimum of 6-seconds up to a mximum of 8-seconds, depending upon the belt tension. But since the crash, the very best I can get, regardless of belt tension is 4 seconds. This loss (power disipation) seems defintely to be in the tail drive somewhere; at first I thought it might be the main bearing blocks, but if I unlink the tail drive and spin the main rotors they go on forever... as it were. So it looks like the tail? But as I said before, I've replaced most of the obvious components.

* Spool up the heli without blades, unlink the rudder pushrod from the servo and operate it by hand. Do you notice anything?

What was it that you have in mind? How should/shouldn't it feel?

Thanks again to you all for your help & support.
Rich.

[J-S-Q]

I know you say the main blades spin freely without the tail connected but have you checked that the main shaft bearings are still nice and smooth? I find they nearly always get damaged in a crash.

[kitkarsons]

have you checked that the main shaft bearings are still nice and smooth?

I haven't had them out of the frame. Can't see anything from outside but I'll have to remove them for a closer look... the old eyesight is not what it was ...I need some spec's. Would you expect to see visible marks/scratches?

[J-S-Q]

I haven't had them out of the frame. Can't see anything from outside but I'll have to remove them for a closer look... the old eyesight is not what it was ...I need some spec's. Would you expect to see visible marks/scratches?

Nothing visible but they may feel a bit rough when you rotate them. It's also worth putting the main shaft in just one of the bearings at a time and applying a bit of side pressure to the shaft while you rotate it as this can reveal damage to the bearings that is not immediately apparent. They should be silky smooth otherwise they can create unwanted vibration and additional drag.

[J-S-Q]

In fact, it would also be a good idea to do a similar check on the blade grip bearings because I've found that these also get damaged in the majority of crashes.

[kitkarsons]

Nice one mate. I'll post back tomorrow when I've had a look.


Many thanks.
Rich.

[moyesboy]

Glad you've got it working better.
I found with my 401/9650(with 9257 gears to make it faster) when I had the gain up high enough to get a locked tail though rapid collective changes, I got a slight bounce on piroette stops and dialing in a little on the mysterious and not fully understood delay pot stopped that without affecting anything else adversely.

The way the 401 actually works is often misunderstood.
most people like to set rate mode so they get the tail still with no rudder trim - which means mechanically setting the linkage to get 4 degrees or so of right rudder tail pitch. This works very well, but you lose a little bit of tail pitch travel in one direction which will make it lose the tail slighlty earlier than necessary. Until you are doing fast backwards and sideways manoevres it doesn't matter though.

When the 401 is first switched on (you must be in HH), and the led is flashing before going steady on, its actually looking at the rudder channel. What it sees for the rudder signal then is what it considers to be no rudder for HH. It doesn't matter what the rudder stick or trim is it will see a bit of rudder trim and consider that to be zero. This is how you can set your 401 to have assymentric end points to its travel and use the full travel of the tail pich - yet still hover straight in rate mode.

Set the mechanics to give you maximum linkage travel, be sure to rotate the tail to chack nothing binds. you can use the limit pot on the 401 to get the full range. do this with the gyro in rate mode. It doesn't matter what the tail pitch comes out at at centre stick. You want the limit pot to be near to 100% - adjust the length of the tail servo arm if it is below 90 or above 110.

Now you can go fly the heli in HH mode and everything will be fine. you have all the 3D performance you can get - I'm not sure setting up rate mode will make it any better in fact, but you can set up rate mode as follows:

While in the hover switch to rate mode and back. Note which way it revolves (probably fairly fast left). Land and apply rudder sub trim to correct that turn, power down the heli, power up in HH and try again. Note that as long as you power down the heli and power up again it will learn the new rudder centre and treat it as rudder centre. Alternatively if you fly the heli with the rudder stick central and switch rapidly in and out of HH and rate it will teach HH the new rudder centre, but thats a bit hard until you are very close to having a heli that doesn't turn! (read the japlish instructions and now they will make a bit more sense!).

note you must use rudder sub trim, not the rudder stick trim, because then it stays the same for all flight modes. you can of course do it with the stick trim if you stick to NORM, and then set the sub trim and recentre the stick trim with your heli on the ground.

Probably I've confused you and everyone else, but this is the way to get the absolute maximum hold out of your 401, and the most consistent travelling piros.

By the way, I really like the K&BDD plastic tail blades on my 450 - much better than the curvy end plastic aligns. They are a little wider than the 450 pro tail blades which I've not yet tried.

You can also do the chinese weights mod if you are into fiddling - that improves things too.

But it the 401 will never be as good as a CSM720