Co-axial 8/ flat 8 stability problem

[nickwad]

Instability problem X8

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Hi,
I had a flat octo with a very bendy frame that flew very unstably so converted it to a co-axial 8.
The wobbly problem has been improved only about 5% I have tried gains from 40 to 250 on atti and 100 to 400 on mains , the yaw is currently 125 and the vertical is 140.
It seems there is something miss matched somewhere .
I have treid cable tiring the av 200 to the multi rotor frame , tried caring 2.8 kgs on the camera mount.
But the problem is always there it is not easy to see in the video but it is all the time wobbling about too much to think about videoing with it.
The all up flying weight without camera is 5.1 kgs, I thought it was due to the flexing in the frame but this frame is absolutely solid with aluminum boom clamps on half double skin carbon arms.

Wookong M Latest firmware
6S
Tiger Navigator nm 4014-9 400 kv motors
turnigy plush ESC 40a
DJI 15x 5 carbon props ,also tried 14 x 4.7 APC ( very slightly better)

I have been playing around with multi rotors for 3 years+ now but I am stumped with this problem!
Any Ideas please

 

[nickwad]

http://www.youtube.com/watch?v=19OEjrQmZvk&feature=c4-overview&list=UUUt_ZjaVSbqRPkzgqaD9-tA

 

[nickwad]

Any ideas ?? Every other multi rotor I have flies much better than this !! The problem has all but stayed the same from the flat octo frame to this X 8 frame !

 

[DucktileMedia]

If your frame flexes as a flat octo why would it not flex as an x8? That makes no sense. Your frame is probably flexing at those telescoping joints. I'm surprised it's not worse as now each arm has roughly double the thrust on it. Also it's not necessarily flex that's the problem as it is vibration.

 

[nickwad]

Sorry maybe I did not explain well enough , the arms are not telescoping they are double skin carbon epoxied together held in aluminum clamps. The main flex was in the center plates and the clamp design before. It is VERY ridged now and was not before. All is balanced very well but I will re-balance.
Does anyone know about the Wookong M advanced gains? Do they need altering sometimes? I have read what it says in the manual !, I have lowered the I gain and its helped a bit . It was less windy into the evening though so will re-test in the morning.

 

[kloner]

it has alot of what i used to get at this weight with a similar design. how much does it weigh ready to fly?

I notice the props and the frame rate look out of balance on almost all of them. You want to see each prop speed, slow, yours show all over the place. usually indicates a or a bunch of out of balance props or broken bearings. could be gain related, but usually when your diving into the groin of the beast (changing dji pid) your chasing the wrong tail

the size looks out of whack for the weight (guessing) more so prop length but usually arm length regulates it all and they just have a short look

if you add weight does it get really bad? if you take off weight does it go away?

 

[nickwad]

It weighs 5.1 kgs without camera inc battery, no it does not seem to make much difference with or without weight on the gimbal. The props are balnaced and the motors are new never crashed and all sound and feel smooth.I have played with the gains extensively but figure they are at about the sweet spot as up or down just makes it fly worse!
I dont understand what you mean about props and frame rate are out of balance? do you mean the props are speeding up and slowing in relation to each other shown by the frame rate in the vid ? is this not normal for a multi rotor ?

 

[kloner]

the way yours looks compared to what i know working perfectly looks isn't there.

Are all the props the same size?

that's pretty heavy, what size are the props?

 

[bensid54]

Stupid question so bear with me! Have you rechecked all mounted and rotating parts for slop or tightness, things can get loose and or strip threads without much indication. This sort of thing can cause havoc with machinery.

 

[nickwad]

the way yours looks compared to what i know working perfectly looks isn't there.

Are all the props the same size?

that's pretty heavy, what size are the props?

Thanks for your time ,Yes all the props are 15 x 5 carbon from a non evo S800. as for the weight my S800 evo weighs over 7 kgs fully loaded for flight !

 

[nickwad]

Stupid question so bear with me! Have you rechecked all mounted and rotating parts for slop or tightness, things can get loose and or strip threads without much indication. This sort of thing can cause havoc with machinery.

Thanks, yes I have totally stripped the complete machine and checked all motors bolts props , IMU mounting etc etc

 

[kloner]

what sorts of ranges of wookong gains have you been through?been down to 50? as high as 160?

 

[Bartman]

i'm a little bit late to the conversation but 7kg (just over 15lbs) is a lot for a hexakopter, i'd guess the heli is using most of its lift just to remain in the air and the motors/FC don't have a lot of remaining power with which to maintain stability.

does the heli yaw a lot? what power setting are you at just to maintain hovering flight? are you careful not to descend too quickly for fear you won't have enough power to keep from hitting the ground?

 

[nickwad]

After further testing it seems that it flies perfectly in totally still air , but as soon as there is any wind it can't handle it ? The wobble comes back , obviously some correction is expected in moving air but this is a distinct wobble !


Lowering the I gain helps , but I feel that is just masking the root problem .
Is it possible the turnigy plush 40a esc's don't like the Tiger motors?

 

[Bartman]

when flying at the 7+kg weight, does the heli yaw a lot? what power setting are you at just to maintain hovering flight? are you careful not to descend too quickly for fear you won't have enough power to keep from hitting the ground?

 

[Carapau]

With my octo when I changed from wooden to carbon props and changed the motors I had a bad wobble. Despite balancing the blades meticulously, it seemed that I had changed the resonant frequency of the aircraft and now I was getting bad vibes through to the IMU. So, I changed the mounting pad of the IMU and the aircraft flies perfectly now.

 

[nickwad]

With my octo when I changed from wooden to carbon props and changed the motors I had a bad wobble. Despite balancing the blades meticulously, it seemed that I had changed the resonant frequency of the aircraft and now I was getting bad vibes through to the IMU. So, I changed the mounting pad of the IMU and the aircraft flies perfectly now.

Thanks for that tip I will try doing the same , can you tell me what sort of mounting pad you used ?

 

[Bartman]

 

[DucktileMedia]

You have other problems. You shouldn't need to band-aid your vibration issues by isolating the fc. Try one of kopter worx arm stiffener kits.

 

[Carapau]

If it flies it vibrates and there is only so much you can do. You need to be holistic about this and if you have done all you can do to reduce vibes then there is nothing wrong in changing the mounting pad- look at all solutions. Changing pads in not necessarily a band aid approach at all although IrisAerial does have a point if you have not looked at every other issue. Some pads work well at some frequencies, and other pads at other frequencies or at least this has been my experience with flybarless sensors on my SRs. I have used bog standard double sided 2mm tape and anti vibe pads from Spartan who make Vortex Flybarless controllers. If one of these isn't good the other always has been for me.