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Old 04-21-2021, 09:01 PM   #1 (permalink)
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Default Altitude hold in hover strange behavior (SOLVED) or NOT : video added

I had the H1 FBL controller on my ALERC Devil Fast 505 but later decided to replace it with the FC6250HX. I flew okay with the H1 but I prefer the 6250 for ease of programming and I already have 4 units in my SAB Goblin fleet.

It is difficult to hold altitude in hover. It seems like the throttle/pitch stick has dead band! Once lift up, it will keep climbing and when I lower the pitch it won't stop climbing right away. It will later slowly drop, so I add more pitch slowly (pass the hover point) but it keeps coming down until I have to raise the pitch again. This up/down is about 20 to 30 feet in height. It is also very difficult to land even though I gently lower the pitch but once it drops, it will keep dropping even I raise the pitch back on. I never experience this behavior before on any of my fleet.

I tried with different head speeds and some other adjustment in both the TX (throttle curve, pitch curve, pitch expo, etc.) and Forward Programming.

There's no drifting fore and aft or side way eiher. Forward flight is not that bad though.

BTW, it has plenty of lift because if I go the pitch pump, it will climb very fast.

I also did the bench test by removing both the main and tail rotor then spooled her up. The 3 cyclic servos moved in unison when I moved the throttle stick and I didn't see any dead band in the mid stick at all. This indicated that the swash plate moved along when the throttle stick moved and never stopped mid way either.

Note also that I only had a few flights with the H1 controller. But since it did NOT have the behavior I mentioned above with the FC6250HX, I assume that it has nothing wrong with the mechanics.

The other four FC6250HX work fine. After the setup, very minor adjustment needed for nice steady hover with or with out SAFE.

Any suggestions are welcome.

Thanks,


P.S. My Tx is DX18 Gen 2.


ALERC Devil 505 Yoyo effect!!! (1 min 56 sec)

Last edited by tanasit; 05-03-2021 at 09:20 AM..
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Old 04-27-2021, 12:30 PM   #2 (permalink)
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Other than panic recovery the collective is simply passed through to the swash mixing. When panic is active the fc6250hx will reduce the amount of - collective permitted once the model is near upright level.

In my experience the problem you described can be caused by;

a) Bad cyclic servo (check accuracy by moving collective up and down slowly on the bench)

c) Swashplate binding on main shaft (pop servo links and make sure it is free)

d) Bent/bowed spindle

e) Bad thrust bearings

f) Main shaft vertical play
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Old 04-27-2021, 02:31 PM   #3 (permalink)
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had a bad CG that I've addressed yesterday; never had a chance to fly it yet due to bad weather here in north Italy this week
On the bench servos are fine, no binding, heli is almost new (8 packs at the moment) and have to tune it.
By the way I like how it fly
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Old 04-28-2021, 08:42 PM   #4 (permalink)
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Quote:
Originally Posted by brianbremer View Post
Other than panic recovery the collective is simply passed through to the swash mixing. When panic is active the fc6250hx will reduce the amount of - collective permitted once the model is near upright level.

In my experience the problem you described can be caused by;

a) Bad cyclic servo (check accuracy by moving collective up and down slowly on the bench)

Bench test: smooth movement throughout.

c) Swashplate binding on main shaft (pop servo links and make sure it is free)

All okay.

d) Bent/bowed spindle

Test with straight edge steel ruler on sevral spots, no gap. Roll on thick glass pane, no wobble BUT will replace since others checked out ok.

e) Bad thrust bearings

New built

f) Main shaft vertical play
None with proper shim(s)



Thanks Brian
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Old 04-29-2021, 08:23 PM   #5 (permalink)
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Default Problem SOLVED or so I thought !!

While I was checking the set up of the swashplate and others, at eye level I could see that all seemed to be in order. In particular the radius arms that looked level but when I looked from the bird eye view, I saw one of the arms was not parallel to the other!! Closer look confirmed that the pivot screw that held the arm wasn't perfectly straight. I replaced the screw and hoped that's the case but no luck.

Next I replaced the spindle and went out testing. No luck again.

However, I noticed that the blade tracking kept changing!!! I tried to hover at eye level and looked at the swash while I was moving the pitch stick but I couldn't tell anything. Then I pinpointed my eyes at the tips of the blades and here's what I saw.

While I was hovering at best at I could, the blades tracked perfectly but as soon as I raised the pitch one of the blade tip jumped out of track. I immediately landed and looked for the culprit. And sure enough one of the main blade arm was loose but since I thought it's a one piece unit so I didn't take a good look at first. The kit came with some parts preassembled but I took all of them apart to clean and add Loctite since they came with none. But I did NOT take the main blade arm off. The arm fit onto the knub and the screw head was "inside" the holder thus out of sight.

It's raining now, so tomorrow it has to me a good day for me.

My conclusion: at hover the blades track fine but as soon as I added the pitch, the holder arm moved upward slightly because the force of the positive pitch and the heli climbed up and up. Next when I lowered the pitch at first the loose holder arm still locked in the up position (thus climb) until the swash is lowered enough to the point where the loose blade almost get to the negative pitch and only then it would suddenly flipped in the "down" position which caused the heli to drop and kept on dropping as I observed earlier. And if I added the pitch it would cycle through this yoyo effect I've never seen before.

Lesson learnt for me for sure.

Thanks for all the help.
Edit: even with almost 30 mph wind I went out to test fly anyway. I filed a spot with three surrounding. Fired her up and hoped to see steady hover but no cigar!!!
The blade tracking was perfect but somehow it’s still exhibited that strange yo-yo behavior.
I took the clip this time and will put on YouTube later today.
Oh well
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Last edited by tanasit; 04-30-2021 at 08:58 AM..
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Old 04-30-2021, 04:27 PM   #6 (permalink)
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Check for a stripped servo arm / spline, this would produce the behavior you described as well. Place a light load on the servos by hand and raise/lower the collective slowly looking for any odd behavior.



I am thinking servo issue based on your 20-30ft comment initially, you can try lowering the setup->swashplate->outputs->frequency to 70hz and see if it helps. Maybe the servos just do not like 333hz.
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Old 04-30-2021, 06:59 PM   #7 (permalink)
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Quote:
Originally Posted by brianbremer View Post
Check for a stripped servo arm / spline, this would produce the behavior you described as well. Place a light load on the servos by hand and raise/lower the collective slowly looking for any odd behavior.



I am thinking servo issue based on your 20-30ft comment initially, you can try lowering the setup->swashplate->outputs->frequency to 70hz and see if it helps. Maybe the servos just do not like 333hz.
The servo splines are brass and the arms are aluminum but I checked as you suggested anyway and found no irregularity.

Next I plan to reset the controller to factory default and go through the set up steps again if this still doesn’t help I will exchange with my other FC6250HX.

As for setting to 70 Hz I will test fly tomorrow and report back.

Thanks

EDIT: As I was checking the servo arms and splines, I noticed the flexibility of the whole servo because of the rubber grommets (see photos below which was called “servo washer”). None of my other helicopter call for these grommet on the servos except my old nitro rc models which needed to isolate the vibration of the engine to the servos.

Now I believe this may be the answer to my yoyo problems.
Again, I already counted the chicken twice (spindle and rotor arm).
Hopefully this will be it.

Too bad, the wind speed around here is "hovering" about 30 mph but I am so excited to test fly.





Edit: I couldn’t help it so came out to the field with wind gust about 30 mph.
Yes this time it works but just need to fine tune with total curve, pitch curve among others.
However I adjust the servo setting down to 70 Hz from 333 as per Brian suggestion and it even works better.

Finally...
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Last edited by tanasit; 05-01-2021 at 08:56 AM..
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Old 05-03-2021, 09:14 AM   #8 (permalink)
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Here're the video:

ALERC Devil 505 servo grommets faulty 1 of 2 (0 min 52 sec)


ALERC Devil 505 servo grommets faulty 2 of 2 (0 min 35 sec)
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Old 05-30-2021, 08:22 AM   #9 (permalink)
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Thumbs up Interesting

Thanks, I learned a lot from your posts. Interesting to see the result!
Cheers!
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