S800 minor crash (hard landing really) on first test flight with Zenmuse

[steveq]

Hey Gurus,

Up until today, my DJI-S800 was flying beautifully, just a dream. Easy to control and I never felt like it was ever going to drop out of the sky. It was the standard kit that I built myself.

Today was the first flight since adding the Zenmuse Z-15 + NEX-5R + iOSD + retractable legs. It felt completely different and went a little pear shaped. Here is a video of the crash:

Immediately after take off, the motors started revving (you can hear it in the video if you listen carefully), not heavily, but enough for me to notice things didn't seem right, it was struggling. It seemed to have trouble maintaining stable altitude and position (yes it had GPS and home lock). I was able to correct it but shortly after takeoff, it just dropped and I couldn't correct it in time. Fortunately there was no serious damage (as I didn't take it up too high, and the grass was soft), the Zenmuse popped out of its rubber dampeners, I popped them back in quite easily and bench tested it, all seems ok. I'm not flying it again until I have some answers. My motor gains are set as per the manual:

Basic Gain
Pitch 240
Roll 240
Yaw 200
Vertical 200

Altitude Gain
Pitch 220
Roll 220

The only thing I haven't done is move the IMU onto the new position (on the plate near the battery). It is still in the central hub (in case I want to fly it on the old legs again without the Z15). But I'm tempted to move it now in case it has contributed to today's problem?
Perhaps I need the new (metal) IMU, as I read somewhere that altitude loss can be an issue with a full payload, and the new one fixes this?

Here is a snapshot of my settings for the Wookong:

View attachment 12830

I'm using the newest firmware 5.24. I run 2 x 6s 5500mAh LiPos, a FatShark transmitter (run off it's own battery) and some custom LED strips to aid orientation. The weight of the unit is about 7.3kg.

Does anyone have any suggestions please? I'm not sure where to go from here to correct it (and I think I got out of this one pretty lightly) and I don't want to fly it again until I have some idea as to what has gone wrong?

Steve Q.

 

[Bugilt]

I would expect the weight and lipos to be the issues. I've only see a loss of power like yours when the lipo was bad.

Can you tell us a bit about your lipos? C rating, age, cycles, brand, and anything else relevant.

 

[steveq]

Hey Bugilt,

Thanks for your help.

My batteries are a Gens Ace 6s 5000mAh 22.2v 45c 6S1P high discharge LiPo battery. Sorry I though they were 5500mAh.

They are virtually brand new and wouldn't have been cycled 10 times yet. They get put on a store cycle through the week and charged up when I need them on the weekends.

I wonder if the extra weight and my orientation LEDs are causing the drama?

Perhaps I should invest in a couple of 6600mAh LiPos?

Steve

 

[Carapau]

I think this one is quite easy. With the extra weight, and quite a bit extra, you have not adjusted your gains- or if you have, not properly. You can hear the motors oscillating as such suggesting the gains are too high. This will only add to the vibrations in the airframe and as we know with the S800, when the vibes get up, the aircraft comes down. When ever adding new weight you need to back the gains right off and then build them up to the correct point. Another tip, which is one most of us tend to learn the hard way, is that when you hear or notice anything wrong, you need to land immediately and investigate rather than trying to brazen it out. So, for the next flight, drop the gains to something like 150. Yes this is low and the aircraft may have loose handling but you will be able to up the gains to the best spot hopefully without risk of a repeat of the flight in the video. Best of luck and hope you get airborne successfully asap.

 

[steveq]

Thanks for your advice Carapau! I'll do exactly as you have suggested. I think I'm very lucky to have come out of this pretty much unscathed!

I have since realised that the gains in the back of the manual allow for a weight of 6.1kg and does not allow for the retractable legs. So my extra weight is coming from the legs, as my minor additions would be lucky to be 200g if that.

I'll pull the gains back to 150 and try again, and this time, not try and fight it, land it straight away if there are issues.

Thanks so much, I'll post my results!

Steve Q.

 

[hjls3]

added weight caused vibrations to get to the IMU is my guess.

 

[steveq]

With what I am reading I agree hjls3. I'll drop the gains back, and see what happens, then I'll consider upgrading to the new IMU and mount it on the new plate at the rear.

 

[boranup]

Steve I think DJI released new gain settings for the Evo with Zenmuse which may help also - http://download.dji-innovations.com/downloads/wkm/en/WooKong-M_Quick_Start_Guide_v1.04_en.pdf bottom of page 6

 

[steveq]

Thanks boranup... it's closer to my weight that's for sure.

So after another test, here are the gain settings I used (based on someone else's set up with a similar rig):
170 170 140 150
140 140

The test was far more successful (but still not without problems)... only a small amount of oscillation every now and again, far more stable in the air.

It seems to be able to hold GPS position much better and responded ok to flying, although nothing like the unit did prior to the gimbal/cam/legs etc.
It's hard to tell, but I think I saw a fair bit of vibration.

What it did do though that scared me was start losing altitude, I corrected accordingly but pretty much had to use full stick to recover... just in time.
After that I landed and came home.

What causes it to lose altitude like that?

Perhaps moving the IMU will fix this?

I just found this:

Dear DJI Dealers,

After continued and extensive testing, DJI has found that if a S800 is heavily loaded (for example above 7kg), then in some flight situations the WooKong M IMU may have difficulty maintaining a fixed altitude .

DJI have specially developed a new Wookong-M IMU with an improved hardware and algorithm for this particular situation and the IMU also has a metal case, this new version is better suited to S800 setups weighing more than 7kg. The new IMU will be released in early April.

Should I keep playing with the gain?

Thanks heaps for eveyone's help

Steve Q.

 

[Carapau]

Yes, do tweek the gains. In order to set the gains do not go just off a predefined set of values or what someone else has used (they are only useful to see if you are in the right ball park). Each aircraft is different so set them up yourself. Don't be afraid of doing this. Set a morning or a day apart to go and dial in your gains. It can be a real pain in the backside to do but it is vital in order to get the aircraft flying at its optimum. One motor oscillating is enough to cause some pretty bad vibes. One other thing to perhaps look at as a short term fix, is the way in which the IMU is mounted to the frame. What tape are you using? Some tapes are better than others at absorbing vibrations. I use just some Ripmax 2mm double sided tape and it seems to do a pretty good job.

 

[steveq]

Thanks Carapau,

I need to have a better understanding of how the gains work. Looks like I have some reading to do.
Do you know of anything to help me in this area please?

Steve Q

 

[FerdinandK]

Have you activated the "voltage monitor", and if so, what are your settings there?

best regards
Ferdinand

 

[steveq]

Hey FerdinandK,

I do have voltage monitor activated my first level of protection is 21.8v and the second level 20.0v

Here is a short video of the sudden altitude loss I experienced. Sorry about the shakiness in the footage, my 13 year old step-son is shooting it for me.
I lowered the legs in case I couldn't save it (to protect the gimbal from taking the full brunt of a potential impact) and gave it full throttle up. Just caught it in time. Whew!

With everything I have read and thanks to MombasaFlash's help I will be ordering the new IMU v2 tomorrow and mounting it on the rear tray, as it fixes the sudden loss of altitude issue.

Then it's "just" gain tweaking to smooth out the motors.

Nearly there I think. For those interested, here is a shot of my fully built S800:

View attachment 12845

Once again I'll post the results for others who may suffer from this issue.

Thanks all,

Steve Q.

 

[steveq]

Hey All,

Today I replaced the IMU v1 with an IMU v2 and mounted it out of the central hub, on the supplied plate near the battery.

I kept my gains at:
170 170 140 150
140 140

​Took it for a test flight with a great result.

The "descent of doom" has disappeared altogether, and in light but blustery conditions it held GPS lock very well. It also seemed responsive to the controls.
The more I flew it, the more confident I became.

I'm pretty confident to say that the problems have been resolved. I got 13 mins flying time (until auto land on low battery) of 2 x 6s 5000mAh batteries. I've just picked up 2 x 6600mAh batteries, although it adds and extra 400g to my already 7.3kg, I'm interested to see how much more flying time I get.

Thanks for eveyone's help and suggestions!

In saying this, setting the gains seems to be a "hit and miss" approach, surely there is some sort of procedure you can follow to optimise gain setting on individual machines (or maybe I just haven't found it yet)?

Thanks again,

Steve Q.

 

[MombasaFlash]

I replied (rather belatedly, sorry) to your latest PM. Please verify your GPS sensor location values. On the photo you sent (with the IMU.v2 on the damper tray) your GPS 'x' value is completely wrong. The screenshot shows a value of -20 and, looking at the photo of the setup where the GPS mast is about 2cms forward of centre, this value should be more like +2.

Your IMU location values look about right.

 

[cycoptic]

guess who quarts? hehehehe.
post the quarry shoot

 

[steveq]

Doh... cycoptic... no idea! Reveal yoursef! lol

For those interested here is a shoot I did in a quarry. It's my first attempt at shooting using my DJI-S800:

 

[ProPilotWannaBe]

Doh... cycoptic... no idea! Reveal yoursef! lol
For those interested here is a shoot I did in a quarry. It's my first attempt at shooting using my DJI-S800:

Nice job Steve! I love the color range from orange browns to deep green with some blue and white sprinkled in for good measure. Wish I has as colorful scenery remotely near to where I am. Here it's just farmland unless I put in some decent driving time to go somewhere.

 

[steveq]

Thanks ProPilotWannaBe, yeah I'm lucky I have a few nice locations within about a 30min drive from my house. Wet and windy here at the moment. Not good flying weather. I can't wait to go out and do some more!

 

[deluge2]

Doh... cycoptic... no idea! Reveal yoursef! lol

For those interested here is a shoot I did in a quarry. It's my first attempt at shooting using my DJI-S800:

Wow, great video both technically and aesthetically! Post more after the weather clears up.

One question, when you write: "I got 13 mins flying time (until auto land on low battery)", do you really mean auto land, as in fail safe or RTH? That might be flying too close to 'the edge'...

Steve