V120D02S vertical bobbing problem

[foxycub]

Hi. I recently got a V120D02S. I managed to set it up without problems and it seems to be all aligned (swash level, blades tracking and level at 0 pitch etc...), but I seem to have a strange bobbing issue when hovering. The heli simply won't stay in one place (vertically), it is constantly moving up and down by a few inches.

The vertical bobbing doesn't seem to be period (e.g. moving with a specific frequency), it is more random in nature. I'm flying inside, so it isn't wind/turbulence.

Given that everything seems aligned and tight I was wondered if it could be coming from the gryo. Before taking it all apart and messing with the stock gryo settings I thought I'd ask here in case this behaviour is normal for fly-bar-less helis. Any help would be really, really appreciated. I last flew routinely about 13-14 years ago so I'm a little rusty and out-of-my-depth (back then it was a Raptor 30... before all this LiPo tech made electric so accessible!).

[foxycub]

Further to my previous post, I have read in some other threads that the gyro is susceptible to vibrations. I do notice that the tail boom on my heli seems to be vibrating somewhat (although I don't know if this is normal). It seems plausible that this is causing a problem for the gyro, which is then leading to the bobbing phenomenon.

With this in mind, I completely dismantled the tail section, removing the shaft and checking all cone gears. I couldn't see any problems, everything seems to be in good condition and well connected. I also removed and re-attached the head, also setting it back up from scratch. Despite these efforts it doesn't seem to have made any difference. The bobbing is still there and the tail is still vibrating slightly.

I'm starting to run out of ideas. I've checked the collar... is there something I could be missing??

[Murph_d]

I'm no expert, but I've been flying my v120do2s for a couple of years. Some of my thoughts for what they're worth.

First of all, a vibrating tail boom is not normal, and every modern fbl gyro is more or less susceptible to vibrations. In my experience, that susceptibility is demonstrated catastrophically, such as a rrod ( right roll of death). Up to that point, the fbl gyro handles moderate vibes extremely well.

Are you using the small bushing in the tail boom? For me, things work better without it. If you disassemble the tail again, replace the bearings.

Logically the bobbing (especially random) would indicate a fluctuation in the pitch of the main blades, most likely due to something broken or loose.The primary culprit is of course that main shaft collar, which you've checked. . Make doubly sure that there is no up and down play in the main shaft. Check the main blade grips for cracks, and the ball ends on all the linkages. They break rather easily. Make sure the feathering shaft is straight.

Sometimes an issue can be very hard to track down. It's probably a sneaky mechanical issue, but it you can positively rule that out, the fbl unit has to be the culprit.

I never liked the 6 axis gyro, and I switched it out for an older 3 axis unit. I also broke down and upgraded to 2s. For what it's worth, mine flies smooth, stable and precise. It will do moderate 3D for 5 minutes ( in 2s configutation). In small flying spaces, I love this bird.

Be persistent. Good Luck

[foxycub]

Hi Murph_D. Thanks for your reply. It's been a while since I created this thread but wanted to post again in case other people are experiencing the same issue. I ended up totally rebuilding my V120D02S a couple of times and checking each component for bends/breaks/fractures but could never track down the source of the tail vibrations and that's where I think the bobbing issue is originating from. That being said, the bobbing problem is less obvious when flying outside so I'm left wondering if turbulence (when flying in confined spaces) might also be a contributing factor.

[onepiece]

I also have random "pitch pumps" in my V120D02S...especially noticeable when I'm trying to hover motionless. Almost like a radio glitch. Each vertical pitch pump starts and finishes in under a second but the bird can travel vertically +/- 6 feet during an episode! The pitch pumps happen irregularly, but I get at least 2 for every minute of hovering. Motor speed doesn't sound like its increasing or decreasing...so it remains a mystery to me. Since I had the time and $, I replaced the motor and ESC. No significant change in this random behavior.

For fun, I took off the canopy, held the bird by the skids in my hand and spooled up to about take off throttle speed. Sure enough, I saw the forward servo (elevator servo?) making random, glitchy motions. I did feel some vibration, but under these 'hand held' conditions, I can't say the vibrations were excessive. Blades were tracking perfectly. If "just" the elevator was glitchy...why didn't the bird pitch FORWARD instead of vertically??? I've no idea.

Like foxycub, outdoors its less noticeable because I normally fly 2 'crash recovery heights' altitude or better. Like foxycub, sure wish I could get rid of the unwanted random (and scary) pitch pumps.