HK 550 Tail Wag

[PcChip]

Originally Posted by Revenant759

One thing you can try (carefully, I might seriously add) is to take the main blades off and leave the tail blades on, disconnect tail pushrod from the tail servo, spool it up some (with tail blades on) and test the tail system under load. It should be fairly smooth to push/pull on the tail control rod while spooled up. Keep in mind, the tail WILL want to move, if you do this seriously be careful or have someone help. You can additionally try with the tail blades off, but it may not showcase the issue.

This might not be the smartest thing to suggest, because someone might get hurt.

(That said, it's totally something I myself would do, but I'm not smart.)

What you *should* do is remove main blades and tail blades, then do this!

Pop off the tail rod at the tail servo, and try moving it by hand while it's spooled up - make sure it's smooth. (You might feel some force pushing back, due to tennis raquet effect. you can install chinese weights if it's too much)

Also, how are the plastic tail pitch links looking? they're not tightened too much?

I think at this point we'd all like to see a video of the hovering to help further diagnose... make sure to include sound as well! (maybe set your camera on a tripod and hover in front of it ?)

[Borderlord]

PcChip

Just done the test, see previous post.
Did with blades on, seemed to be the best way, but I was very careful, and did not feel in danger at any time.
The whole range was smooth, no binding anywhere.
As expected, resistance increased as the pitch did, but not excessively. Bear in mind also that the wag occurs at hover, tail in, so at the point where the slider has least to do.
FYI, my tail has metal links.
Not keen on tightening the bolts too much, seemed to restrict movement, and also not keen on expecting threadlock to hold everything if I left the bolts loose, so I got some longer bolts and used locknuts on the outside to hold everything in place. Meant I could adjust the bolts so they were free moving but no play, then use the locknut to 'fix' the adjustment.
Have to think outside the box on this one.

Borderlord

[PcChip]

how long were the bolts?

have those custom long tail pitch bolts been on the whole time?

any chance of a video?

this is very strange indeed, as most causes of this have already been suggested and apparently tried...

maybe it's some kind of feedback loop between the FBL and GOV , I know that one user (Atomic Skull) had this happen with his YEP/Robird combo

[Borderlord]

Hi PcChip.

The tail pitch bolts are only long enough to get a locknut on the other end. Maybe 2mm or so. Only did it because if I tightened the bolts enough they caused the pivots to bind.
Also, couldn't find a way to get loctite into it without risking getting some on the bearings.
Been on there since I switched to the Align 600 hub.

Trying to get someone to video it for me. Might not be high quality, but at least you can have a look.

Agreed, we have tried all the obvious things.
Thinking maybe to put the original HK hub back on. It has thrust bearings, where the Align one does not.
Might make a difference. Also, some carbon blades to go on, although I think the movement is too slow to be blade related.

Can't be a feedback loop because it's not fbl, and I do not run governor.

Thanks anyway, all good thoughts.

Borderlord

[PcChip]

Have you tried it at all yet without your custom long bolts?

- almost grasping at straws here -

[Borderlord]

Hi PcChip

Believe me, we're all grasping at straws. Here's a few more for the collection.
Re-fitted the original HK tail hub. Effect seemed reduced, progress, I thought.
Concluded that the thrust bearings in the HK hub were making the difference, till I noticed that the Align one had these as well.
No answer there, then.
Did start to think it might not be the tail but the esc not producing constant h/s, so re-fitted the original Plush esc, to no effect.
Anyway, what I have now is the problem pronounced in HH, less so in Rate, but the gain in both modes turned down to a ridiculous level of 25%.
Have to admit the tail is still perfectly controllable at that level. Would have thought I would have seen signs of the tail response starting to suffer at such low gain, but not so. Going to re-fit the YEP controller, but reluctant to lower the gain any more.
I believe there is a relation between the allowable gain and the distance out of the ball, but no idea how this works.
If I move the ball, currently at 16.5mm, so I can use higher gain, I would feel happier lowering it still more.
Does this make any sense?

Borderlord

[Borderlord]

Hi Guys

Been doing a little research. Seems the dimensions of the tail linkage all add up to something called mechanical advantage. Although I could not find a way of putting a number to this, the higher the mechanical advantage the lower the needed gain.
Can't change much of the linkage dimensions, but moving the ball in on the servo arm will help.
Anyway, decided to move the ball in to 13mm from 16.5mm and that seems to have helped.
Left the gain at 25% as before. Although I still have the wag in HH, almost gone in Rate, so that's good enough for me. Just have to fly in Rate, no biggy.
Spent so long, and so much trying to troubleshoot this one, near enough is good enough.
Thanks to everyone for their help, especially PcChip.
Still open to any inspirations others may have, but for now, just going to fly the d****d thing.

Cheers

Borderlord

[scrofule]

Hello Borderlord,

I think I've got a tip for you.
I'm owner of a HK550 TT for 3 weeks and met exactly the same type of trouble than you.
My gyro is a Futaba GY502 with S9253 servo. I've been happy with this gyro for 10 years with my Raptor 300.
1st fly test of the HK, tail servo arm lenght at 14 mm as written on Align user manual if I remember well.
Seemed to have no yaw control, it was not possible to fly and after 5 take off tries it lead to a crash because of a switch error in panic.
One main shaft + canopy exploded + 1 pair of main blades + landing skids. Bad day.
I machined a new main shaft (the genuine HK was Ø9.92 mm, too loose in bearings !!!) and mine is now Ø9.995 mm. It fits perfectly the bearings and does less vibrations.
The slack between motor pinion and main gear can now be set correctly.
2nd test : with no main blades, I noticed that at low speed, the tail pitch control slide could move normally, but at hovering speed, it could not travel anymore !! What is this ???
3rd test : just replaced the HK tail blades by the Raptor 30 ones. Much better, the slide moves at hovering speed but not full travel.
4th test : the same + servo arm lenght at 10 mm instead of 14. Same torque-> higher force. Full travel obtained on the slide.
Flight test : it was now possible to hoover but with a great insecurity feeling. The heading was not really locked, with some erratic yaw movements. Too bad to try flying around.

I do not like the slack in every part of the tail system :
- axial slack of the shaft himself
- radial slack of the ball in the lever
- axial slack of the bearings of the slide command (some mayonnaise was pouring out of the bearings after 5 min test !)
- and so on.
The aluminium rods between the slide and the blades grips are not interesting.
On all my other machines they are made of plastic and the slack is much better.

5th test : Tail rotor, pitch slide and lever were replaced by some picked on a Mikado Logo10 carbon (10 years old). The dimensions are pretty much the same but the command lever is a bit longer so needs less force from the servo.
The tail blades were the HK from the kit.
I set an "average" gain of 70%, without searching to improve.

Oh miracle, the heading lock was now perfectly efficient, like what I'm used.
And this al though my tail servo arm lenght is only 10 mm which does not sounds optimized.

Conclusion : you are probably not facing a lack but an excess of mechanical slack in your tail command.
I will order an Align genuine slide and it will probaly be over.

Good luck in your reseach of the truth !

[PcChip]

Hi scrofule,

Hopefully the OP has long since had this issue sorted out, but yes - any slop in the tail system makes you have less control and is less precise, especially in backwards flight

As for you having lots of resistance when you spun the tail up (test 2), that sounds like the Tennis Racquet Effect, and chinese weights would solve this.

When *just* the right amount of chinese weights are added, it becomes effortless to move the tail slider when spun up to full speed. (too little weight and it's difficult to move due to resistance, exact weight there's no resistance, too much weight and it slings out to either one side or the other by itself)

[hdewinter]

I did fly the HK 550 for about a year in the original parts configuration, except for:

tail shaft
front tail gear
tail control hub

I replaced them all by align parts

I also did experience a wag I could not get rid off (align 780 gyro), things changed for the better when I shimmed the tail shaft. The shaft had a small amount of axial (back and forth) play. After shimming the tail behaved A OK.

[Borderlord]

Hi Guys

Been a while since I last posted. Thanks for keeping this in mind.
Never really got to the heart of the problem. I did get it to the stage where it was perfectly acceptable in Rate mode, but not in HH.
This was the most puzzling of all, why the effect should only be in HH, not in Rate. Didn't make sense.
Anyway, quite happy flying around in Rate, so left it there.
Since switched to FBL, and effect is even more reduced. The tail still seems not fully 'locked in' and a little sluggish.
Flies fine, but I'd like it better.
No slop in the mechanical system, but I did notice some in the servo gears, so this might be due for replacement.

PcChip

Heard of the Chinese Weight mod for 450 size, but never found any info on same at 550. Do you have a source?
Not thinking this is a cure for the tail wag, but the tail has always seemed a little sluggish in action. Got to be worth a try.

Cheers

Borderlord

[Atomic Skull]

Originally Posted by PcChip

Hi scrofule,
Hopefully the OP has long since had this issue sorted out, but yes - any slop in the tail system makes you have less control and is less precise, especially in backwards flight

I'm not sure that a little lateral slop in the tail grips is such a bad thing anymore. Tail rotors experience dissymetry of lift and blade flap just like the main blades. Full scale helicopters actually have either a teeter hub or flap hinges on the tail rotor for this reason. A totally rigid tail rotor puts a lot of strain on the hub and bolts as well as causing increased tail vibrations. Several recent model helicopter designs actually have a feathering shaft and dampers on the tail rotor.

The Blade 550X 600X and 700X use chinese weights on the tail grips but instead of being built into the grips they use special tail blade bolts with weights on them. I put a set on my XTreme 600 and I'm fairly sure they'd fit the Align 550/600/700 grips as well.



Those are actually KDS Innova/Agile tail grips. Pretty much a drop in replacement, I only had to change out the 5mm KDS balls with 4.75mm ones. I used Trex 600 flybar head links that I had laying around (you need balls with an M3 thread)

[PcChip]

Originally Posted by Atomic Skull

I'm not sure that a little lateral slop in the tail grips is such a bad thing anymore. Tail rotors experience dissymetry of lift and blade flap just like the main blades. Full scale helicopters actually have either a teeter hub or flap hinges on the tail rotor for this reason. A totally rigid tail rotor puts a lot of strain on the hub and bolts as well as causing increased tail vibrations. Several recent model helicopter designs actually have a feathering shaft and dampers on the tail rotor.

Hi Atomic,

I can't argue about damping in the hub and grips, however I was actually referring to slop in the linkage (the entire pitch control system between the servo output shaft and the link on the tail grips)

[Borderlord]

Hi guys, got some interesting results.

Since you raised the topic, decided to revisit the 550 tail. Have to admit, the effect is barely noticeable with the FBL unit, but the tail has always seemed a bit sluggish, and the comment about 'chinese weights' got me thinking.
Anyway, got a set of those Blade 550 ones AS showed. Unfortunately, didn't fit. I have the Align 600 plastic grips on mine, and the Blade bolts are simply too short.
Undaunted, decided to invent my own using 3mm bolts, nuts and washers I had lying around.
Tried to make the weights as similar as possible to the Blade ones, but the results were spectacular, in the wrong direction.
Took off the main blades, disconnected the tail rod and ran the motor up moving the rod by hand. Did a trial run with the existing set up just to get a feel.
Then fitted my home made weights and did the same test.
Even at a modest h/s the pitch slider immediately shot to one extreme, was very hard to get back to centre, and immediately shot to the other extreme.
Classic signs of too much correction. What surprised me was the violent reaction with quite a modest weight addition.
So lesson learned.
Gradually reduced the weights, with reducing effects until I ended up back where I started.

Fun afternoon used up. Ho hum

Borderlord

[pdooley]

i know this thread is a little stale, but i just got an HK550 and was wondering if you ever solved this?