Question about tail tuning...

[K2Freak]

Mine started about how yours sounds but the more I tried to improve it, the more it seemed to go downhill. Feels wrong, hence this thread. It definitely got softer after tonight's rebuild, and I just bought a BK 5005 from the forum. I hope that our problems are common to the MKS, and reflect the servo being at the edge of the performance envelop required.

Hoping for the best...

[Steve Graham]

The MKS HV9780 is a 760 microsecond high voltage brushless titanium gear servo with a .040 sec transit time. I'm doubtful it is at fault but there is always that possibility. I'm powering it from a YGE 90 ESC with the power BEC that delivers 12 amps so I don't believe power to be an issue either.

The BK 5005 is marginally faster and stronger according to the manufactures specs but we all know how accurate those tend to be IMO either servo should function in this heli very well.


Full disclosure: I've had two incidents in probably 50 flights where upon liftoff the heli began spinning hard and had to be shut down. These issues remain undiagnosed but I believe they have something to do with the Spartan Vortex not initializing properly?

[K2Freak]

Mine is an 8910A+ at .05/30 at 5v. About half the strength of yours. Hmm...

[mr008]

Had all kinds of tail problems on an X3 with Vbar and that same mks servo., even had two of them go bad ., After the second tail servo failure I changed to an align ds 525m and problem solved.

[boogie12]

Just curious but have checked your tail servo settings in the setup menu for the tail. I accidentally had mine set to the wrong servo pulse width and frequency and my tail would blow out on my X3 I was using a JR3500G tail servo btw.

[K2Freak]

Set correctly...

[keither1984]

I tested my tail tension with blades of @2800 not so hard to push the tail pushrod , 3000 a little harder and 3400 was quite a bit of tension. As you described earlier it's an elastic type if feel. My tail is not blowing out though. Also I'm running a bk 5005 but just wanted to chime in and let you know that tension is normal I guess. My tail is smooth as can be at a standstill. Hopefully that bk will solve your problem.

[extrapilot]

K2

The reality is, you are fighting lots of variables, and making changes that have potentially negative outcomes (i.e. increased chord/span on the tail blades also increases CRF). Increasing link offset should increase the pitch rate of change, except at the same time, you lose mechanical advantage- which can slow the servo (net performance loss) under load, etc.

The BEC is from my perspective still suspect. It doesn’t matter if you don’t register brownouts; servo power consumption is largely self-limiting, where as the bus voltage droops, the servos’ ability to generate torque decreases (their max power consumption drops), and so the bus voltage stabilizes at some lower value. But what you don’t see is what effect this has on torque and speed at the servo; you don’t have to register brownouts to have insufficient electrical power on the bus to meet the control force demands of the rotors.

There are a few simple tests you can use to remove a slew of variables. One would be to just test run with a larger BEC (borrow a 20A at the field), or run a 2S lipo if your servos support that. If the problem resolves (short of some fine tuning)- you know this is a power/control actuator problem, and not an aerodynamic one. If it remains unchanged, swap the servo (preferably with a more powerful one (keeping the larger power supply in place))- again, just a temporary trial to eliminate that variable.

Also, just please do keep in mind that static friction is not dynamic friction. Sounds obvious, but people get hung up on ‘if it is silky smooth on the bench, then …”. Well, that is not valid. When moving, you don’t have the same type or magnitude of friction. But similarly, you can have new contributions (maybe, for example, you have some drag in the pitch change mechanism, and that at speed, with centrifugal load, is binding up the pitch links from the slider to the grips (usually at the slider). Etc. Sometimes, you benefit from making things a bit loose, and then adjusting them back one at a time to see if anything jumps out as a problem.

You may find at the end of the day this machine with the mains you are flying simply does not have enough tail authority for what you are doing at the headspeed in question. And with that comes other options- but no point throwing out good parts if you have a weakness upstream…

[Atomic Skull]

How much pitch does it have at the extents of the pitch slider travel. If it's too high (enough to stall the blades) it's essentially wasting servo resolution. You really don't need more than 28-30 degrees in either direction.

You always want to keep your servo arm within the +/- 45 degrees from center outside that range the servo resolution becomes increasingly non linear. (the further the arm moves from center the further it must go to to move the pushrod a set amount)

If you are doing a rate mode setup instead of just centering the tail then the values for left right limits will likely be unequal. You can bias the horn slightly in the shorter direction to compensate and keep the horn inside +/- 45 degree deflection. (try it and you'll be amazed at how much this improves tail performance)

[K2Freak]

God I love helifreak. @ep, thanks for reminding me that there is no real vacuum. I.e. can't test any element by itself. Seriously, thank you.

Next bench test is voltage drop with me cranking on the tail servo. I have big becs available in house, but I want to fly the smoother tail rebuild first. If I need to say auf weidersien to yge60, then its Kon'nichiwa hw100.

But first, flight tests tomorrow!

Thanks again, all....

[K2Freak]

@as - post 22. I did have to bias to get it to look right. I'm at 90, 110 now in vbar to have 90* tail servo arm. Your points well taken. Thanks.

[Raptorapture]

I recently had the same phenomenon extrapilot is referring to re marginal BEC. I'm not sure how long it would have taken me to diagnose poor servo performance if the ESC output was not acting up as well.

[K2Freak]

FIXED!!

Abso-freakin-lutely rock-freakin solid!

I couldn't make the tail even budge in pitch pumps, and in climbing flips I may have lost a couple degrees when I really slammed into the pitch. I'm still at defaults on VBar Pro tail settings, so without tuning the tail at all it seems nearly perfect.

This appears to have been based upon the hardware change. One of the times I rebuilt it with the new bearings, something loosened up a little bit. I also changed servos, but I'm not convinced that this was critical.

My schedule and the weather definitely diminished the purity of my tinkering - I won't ever know exactly what the critical change was, but it feels like the bearings. Maybe I'll put the other servo back on one of these days and see if it works just as well. Bet it does.

I always do that - change everything and then wonder what it was that make it work.

Thanks again to everyone that helped me troubleshoot. There's no place like HF!!