S-800 oscillations/vibrations

[Mike Toy]

A couple of months ago I experienced a very close call with my S-800. It suddenly began to swing violently from side to side and I thought it was going to flip. It was going through almost 90 degrees and I consider myself lucky that it did not. FYI it uses and A2 controller was in GPS mode and carries a DSLR on a 3-axis gimbal. At 30ft I gave it full power and it stabilised itself and then landed without issue.

I took it home, double-checked everything, recalibrated the compass and did a couple of test flights without incident. I then went back to the same location and exactly the same thing happened, i.e violent oscillations. It could be local interference or it could have been coincidence. No cell towers, radio masts or similar in the vicinity as far as I can see.

Any ideas?

Next thing. The gimbal appears to shake from side to side which initially I thought was a gimbal issue. However looking at it closely it’s not, the whole bird is shuddering and that is simply being transferred to the gimbal and camera.

I’ve uploaded a couple of short videos to illustrate the point. The camera view clearly shows the side to side movement and the ground video shows the oscillations/vibrations, most notably at 0:27, 0:29, and 0:31. Look at the ends of the arms, not the gimbal.

https://www.youtube.com/watch?v=A3HyuvdoAGM

https://www.youtube.com/watch?v=sf1GhdtDs_0&list=UUiCnqWzx70ca-HEcPh5EnnQ


I’ve tried adjusting the gains, which helps, but by the time they are reduced enough to remove the shudder you might as well be flying a wet blanket, it is far too slow to react. We are blessed (not) with quite gusty winds here so you need to keep the gains at a reasonably high level to cope with this.

I’ve tried several combinations of vibration dampers but I think the real problem is the movement in the bird itself. If this could be removed then it wouldn’t be an issue, as it wouldn’t be transferred to the camera.

So, all you techies, does anyone have any suggestions as to what might be causing the problem and what I can do to rectify it? If you do please let me know, and the next time you are in Barbados the drinks are one me!

Thanks.

 

[Benjamin Kenobi]

Hey mate.

Looks like a blade or motor is unbalanced to me. How have you checked the balance on the blades? Are the motor bearings smooth to the touch?

High gains don't really help with strong winds. I use 135% on my WKM and it is rock solid in 25mph gusts (high gains aren't the problem here though).

 

[Mike Toy]

Benjamin,

The blades were all balanced just before we did the flight in the garden and as far as I can see the motors are OK. If you grab the propellor boss and pull up and away from the winding there is a little movement in a couple of them. Not much, probably 0.5 mm or so and only in the vertical plane, there's no side to side movement. Do you think that could have anything to do within it?

 

[Hexacrafter]

I have seen this before.
The gimbal is going CRAZY! AlexMOS? Makes the aircraft shake like a very bad balanced blade.
Turn off the gimbal and fasten in place with zip ties... I bet it all goes way.
The gimbal is going into a self feeding occillation. This could be due to a bad gimbal tune or worse// a gimbal design flaw.
I am VERY suspicous of the gimbal......
also...IHMO... that load is beyong the capabilities of a S800.

 

[Mike Toy]

That's what I thought in the first place but this occurs wether the gimbal is switched on or off. In fact in the ground video it is off.

Sorry, IHMO? As for the load I'll admit it's borderline but it is just about within the take-off weight specified by DJI. Certainly there is no issue with power, it goes up fast and we can get about 10 minutes out of a pair or 5000 mAh batteries.

 

[Hexacrafter]

This is more information....
So you are "shaking" the aircraft in the ground video? It is not the gimbal shaking on it's own?
If this is the case, then look at the isolation being too soft and/or the Yaw shaft/ bearings being loose or undersized.
On weight.....
I really do not care what DJI specifies....they truly bend the truth. Do a "real" motor/prop calc.....
With a stock S800 and that gimbal and camera, you are probably at 75-80% throttle at hover. Will it fly? Yes.....
Will it perform optimally... no....
WKM and A2 adjust the throttle to 50% stick for GPS....
If you went to manual mode... it would drop like a rock......
Many fly this way... it has difficulty holding altitude in GPS mode, does not behave well in gusts or wind and will lead to premature motor failure.
But... If you are happy then that is all that matters.

 

[Mike Toy]

Andrew,

Thanks for your information. Yes, you are correct, we were shaking the whole machine - I thought it was little 'loose' although of course there some of that will be the vibration dampers.

On weight…bear with me if you can - you build the things, I only take the pictures! I don't understand what you mean when you say WKM and A2 adjust the throttle to 50% stick for GPS and that it would fall if put into manual?? My pilot tells me that it hovers at 50% throttle. Anything above and it starts to ascend, anything below and it starters to descend.

FYI the gimbal and camera weight about 3.5 Kg and the two batteries add a further 1Kg each. Add the weight of the centre-frame and arms - another 2.2 Kg - and we're up to a total weight of 7.7 KG. I now have a feeling you're going to tell me that is way too heavy…...

 

[Hexacrafter]

I have watched the video a few times....
Who's Gimbal is this?
Is it rated for that large a camera (Especially the isolation)? I would ask them what weight the isolation is designed for and test to....
It really looks like the gimbal is far too heavy for the isolation system. Are the balls under compression or tension? "ball" style isolators are engineered to be under compression..... many use them in tension, but on heavier gimbals this can lead to a very bad shake.
To clarify what is going on, I would take zip ties and run them though at least one ball on each mount and compress the balls as tight as you can....
If the wiggle goes away.... then look heavily at the vibation isolation system..
I cannot go into too much in written form, but the WKM/ A2 use a 50% throttle position on the TX to "tell" the FC to hold altitude.....but this is NOT the actual throttle that is being applied to the motors by the FC to maintain that hover.
The "old" test is to switch to manual mode while at hover in GPS.......have enough altitude to recover and do this at your own risk as she is going to drop like a rock. Be prepared to switch back to GPS very quickly and recover from the free fall if you are not familiar with flying manual mode.
If the aircraft suddenly drops then you are hovering way over 50%
If the aircraft suddenly climbs, then you are hovering way under 50%
If comfortable with manual mode, then you would raise/lower the throttle to maintain altitude and the would be the "actual" throttle at hover. As stated before my guess is that you are pushing 75-80%.
The ideal goal is 50%. Too light and they get wobbly and balloonish..... too heavy and they do not have the required power reserves to deal with wind gusts, wind in flight, and maintaining altitude.
I hope this clarifies my thoughts.