Bizarre Tail Issue on 500 FBL TT Clone Build (Solved)

[AK65]

Hi

I just wanted to report on a truly strange tail issue which dogged my HK 500FBL TT build.

I built the kit without incident, and checked that everything worked. The tail pitch control worked smoothly, everything was OK.

I used a Tarot ZYX MK 1 gyro with 4.01 firmware. I should point out at this stage that I have 2 450's one with a ZYX and another with a TGZ380 and they are both fine, so I'm no stranger to setting up a flybarless system.

Time for the test flight. After balancing the main blades, and dialling down the headspeed because of vibe issues, I was ready for test flight. I attached a very wide training gear made up from some carbon fibre poles, and boy was I glad that I did.

I got it into a test hover, cyclic control seemed fine. I then tried tail control, and it was in a way ... loose. Scary loose. Unflyable loose. Dialling up the gyro gain (Heading hold mode) did nothing to help. I was happy just to get the bird on the ground at this stage.

I figured maybe the tail servo was weak or faulty and replaced it. Same thing, rudder input started the chopper slowly pirouetting without control being able to stop it. Training gear again saved the day.

At this stage I left it for a couple of months, but it was eating away at me that the otherwise nice 500 was not flying.

I removed the main blades and then ran the tail only on the bench at various speeds. At low speeds, the gyro seemed to work (fought to keep the tail at the desired orientation), but at higher speeds, it lost authority, eventually allowing me to rotate the tail to any position without fighting it. In HH mode, a properly working tail and gyro will fight tooth and nail to keep the heading as commanded. I figured that it was maybe the vibration due to higher head speed confusing the fbl unit, so I removed the fbl and held it in my hand (to eliminate vibration), but same result.

So clearly, it was not an electronic / vibration issue, and could not be a servo issue, since the same behaviour was evident with two servos.

I decided to check the tail pitch control by hand by adjusting it with the rod manually. Bingo! At slow speeds, the pitch control was smooth to operate, but at higher speeds, it was virtually impossible to move the rod to adjust the pitch, and when the resistance was overcome, it almost 'notched' to another pitch position and needed a lot of force to 'un-notch' it back to neutral. Hardly great for the constantly varying pitch adjustments that the servo is required to make......

I then changed the hub assembly and the pitch slider, and now it seems to have fixed the problem - the test flight had a tail that was controllable.

But my question really, after this very lengthy explanation is why? What happened?

Could it be that as the tail speed increased, that the centrifugal loading on the bearings in the blade grips exacerbated revealed notchiness to a point where the bearings were literally frozen in place? Would it be a good idea to replace the hub with a system that uses thrust bearings, like Tarot have fitted to their 450's? Has anyone else come across this problem before? I'd appreciate come comments / insight, because this one really stumped me.

Note that the heli is brand new, so no crash damage (even though it tried very hard to crash while the tail was misbehaving.....), and I don't have the skills to correct the kind of spin it got into. I'm still 2D flying.

[Abenn]

I haven't heard of the tail slider binding up so badly that it wouldn't move, but I did hear many times that the HK clones' tail sliders were always notchy. So when I bought my HK500TT I was all prepared with a complete Align tail assembly

Unfortunately, on my maiden flight the vibration was so bad that I aborted after a couple of seconds. Original HK blades had been balanced before assembly, so I reckon it was simply an accumulation of slop in various bearings and joints, alongwith out-of balance minor components. Replacing the main shaft, complete head, blades, flybar and main and auto gears with parts I had from when I upgraded my Align 500 to fbl cured the problem. After about 18 months, flying Sundays only, last Sunday I noticed the TT drive (original HK item) sounding a bit noisy -- I've yet to check whether it's bearings or gears or something else.

[AK65]

Lol, it seems as if you replaced everything! Luckily, when I ordered the 500, I ordered an extra set of blades and extra tail hub, because the blades could not be tracked and the tail hub was grindy. The QC on those birds is appalling. I really regretted not getting the RJX X-TRON 500.

So far, this one is behaving itself, but we're taking baby steps. Still trying to handle the adrenaline associated with the bigger craft (as compared to the 450).

This is my first TT, and I can really do without. I'd far prefer a belt, although disassembly is easier, and belft tension is a non issue. Also, I've got away with one tail dunking, but my luck can't last. I'll have to modify my landing techique.

[Gladius]

I would definitely use grips with thrust bearings. That's a lot of axial load to not have bearings.

[SumOFparts]

On HK's - at least older ones the tail slider tube is pressed into the control yoke - the align screw together at this same point - If that press fit is not correct or square - it will jack/bind up that whole assembly under fast/hard demanding moves....its best to replace with aligns pieces here - everything but the box....tail spins 4 times the head speed - so little things have big meaning....

As Gladius has suggested - the head, the other high demand, high speed critical component is another problem area - especially after a crash - I would monitor it carefully.......

Most end up sticking a used align head - there is nothing wrong with HK - but you'll end sticking align parts on it to make it viable....

HK uses Gartt made components - some are ok - however; some like their swashes - RUN!
One of the CPO's guest had one of those fail on its 600HK maiden.....
If you put the align one next to HK swash - difference is easily seen.

At these prices - its never been this affordable to fly this size - just have sort thru and replace what troubles or dont trust.....these make excellent beaters.....seen pilots progress faster on these than some fancy money pit.....
-

[AK65]

Really trying to keep the costs down, but am seriously considering replacing the tail box with Tarot or Align. The tail grips imho should have thrust bearings. I got a Tarot hub for my 450 with thrust bearings and I know at least I won't have trouble in that department.

The more experience I gain, the more I understand that 90% of the trouble I've had came from the tail. Resonance, binding, bad bearings...the tail really exposes bad engineering....seeing as its spinning at 12000 + rpm.

Anyhow, we're good for now...

I'm doing what every HK beater owner does: reduce headspeed until the problems aint so bad....

It's not an Align...

[AK65]

Latest update:

Got her flying beautifully, stable as only a bigger heli can be.

She was about ten feet above the ground when one of the swash plate servo arms came loose. Mechanic's error - must have not put enough loctite on the ball joint.

Long story short, less than half a second later it hit the ground under full power, no time to react.

Main shaft, main gears, feathering shaft, main blades, boom, sheared torque tube, canopy, skids. Surprisingly the umbrella gears survived unscathed...

Now awaiting spare torque tubes.

Just to be safe I'll order a tarot tail box and hub.

At least it wasn't pilot error....

[AK65]

All reassembled. I found some horrible issues with the replacement tail hub and grips, and decided to refit the original. Took the grips apart and replaced a notchy bearing. The pitch adjuster was binding, so I went back to the original, and made sure there was no binding. It's as smooth as melted butter now.

New torque tube arrived (pre-bent in the post) from China. Very badly packaged. I spent some time straightening it, and it seems to be OK. Spooled up the tail with main blades off, and tail really fights nicely to stay in its orientation in HH mode.

Fitted the new main blades (fibreglass - I'm still a novice pilot even after years of on/off flying) and set the pitch, and spooled up the heli holding it at arms length. This is a fairly risky thing to do - don't try this at home!. Heli feels nice in the hand, not too much vibration, good blade tracking and fbl gyro nice and stable at hover pitch. Could almost let it go....Ready for the test flight tomorrow morning...

Wish me luck.....