Logo 550sx tail wag

Hello,

On the first test flight with my new Logo 550sx I have noticed it has a pronounced tail wag in hover. I have disassembled and check the tail rotor, everything appears fine, the thrust bearings are in the right order and greased, and the tails moves smoothly. What else can I check to fix this problem?

I have attached the VBar Neo configuration file for the model.

Many thanks,

Darko

N/a



Edited 1 time(s). Last edit at 10/08/2020 12:26AM by darkoroje.

Hello Darko,

first, give it a couple of flights, so everything can wear in a bit.

Second, according to the setup, the mechanical adjustment can be improved: you have limits of 110/60, which means by comparison, you have to turn your steering wheel in your car 110 % left for a tight circle, but only 60 % right, to get the same effect.

Please check the following: switch off, move the servo arm to 90° to the pushrod, adjust the pushrod so the bell crank at the tail is also at 90°, that's your mechanical center.
Then switch on, and set the servo arm on the servo in a way so it's closest to 90°. Adjust limits. If you get a huge gap, re-set the servo arm on the servo so it's also close to 90°, but on the other side, adjust limits again (will most probably be better, a lesser gap).
Also mind that you do not have to use the full travel on the tail shaft, but only as much as to get like 25-30° of tail pitch, each direction. Too much can cause the tail blades to run into a stall—not in a hover, obviously, but it will also mean you set limits which are not sensible.

You can then use the trim flight to let the VBar find it's center, in a hover, or help in advance by adjusting the trim flight value for the tail in a way so you have approx. 90° angles again as mentioned above.

Some more mechanical things to check/do: check the balance of the tail blades. Pull the blade grips apart (stick a tool through each bolt hole, pull both away from the hub, you will possibly notice that the bearings kind of click into their bearing seats. This will also happen during a couple of flights (wear in).

Lastly, more often than you would think the cause is a stubborn servo. If you have one, try a different one. Futaba servos are still considered universal weapons.

Then, with Pro, you can use the new tail wag suppression parameter, for the tail, which might also help, or you can adjust other expert parameters like I gain, P gain.

Kind regards

Eddi

Born to fly ...
forced to work.

Thank you Eddi. Where in VBar Control can I adjust the trim value for the tail servo, I have not seen that setting anywhere.

Thanks.

Hi,

you can manually adjust the auto trim values, or just use auto trim to do it for you.

Kind regards

Eddi

Born to fly ...
forced to work.

Eddie

I have followed your advice and the tail wag is much improved. However there is still a bit of wag especially in hover.

I have set tail wag suppression to 10 there is no help for it so I am not sure what value to set it to. Also not sure how to change the other parameters like P or I. Is there a guide somewhere to look at?

Finally would a different tail servo make a difference? I have the Savox tail servo that came with the logo 550sx super combo.

Many thanks

Hi,

good to hear.

You can increase tail wag suppression more, to see if or when it starts to make a difference. If it doesn't, it's the wrong tool.

In general, decreasing I (e.g. from 60 down to 40) and/or increasing P (from 80 up to 100) _can help_ but please try it out in a copied bank, so you can switch back to a known good bank should something happen (P oscillation). But that adjusments can only be made per bank, in a sports flying or 3D bank, you will probably want to have different parameters.

You can also try and dial in a tad of D.

Do only one at a time, in the beginning, to see what it does, and be prepared to switch to a 'safe bank'.

As being said, a Futaba tail servo can still be considered a universal weapon, even compared to today's other more exclusive servos.
But that comes at a price, compared to a Savox, and there can be no guarantee, because there are so many variables.

Kind regards

Eddi

Born to fly ...
forced to work.

I had a similar experience when I built my Logo 480 super combo years ago with the included Savox SH-1290MG. It flew fine, but there was a slight, mostly just annoying, tail wag in a hover that I couldn’t get rid of. I switched to an MKS tail servo and it went away. I’m not bashing Savox—I run them on cyclic in all 3 of my Logos, but not on the tail. Just my two cents.

It is flying ok but there is definitely tail wag. Could it be something to do with belt tightness? How tight should the belt be?

Thanks

Hi,

unless you fly hard(est) 3D, the weight of the model with battery installed is OK to adjust the tightness of the belt – just hang the model by the standoff at the rear, with the tail gearbox a bit loose, then tighten it with that tension or maybe a tad more.

Imo and in my experience, that makes not much of a difference, but worth trying.

I've been flying Savox on my 500-600 SEs like forever, I like them, but I'd dare say there are better ones and 'other' ones, and on the other hand, they're quite inexpensive, so I'd expect that more expensive ones should deliver better

A slight wag, and at lower head speeds during takeoff and landing, doesn't really annoy me. The thing's working, and there's nothing bad to be expected, it's just the way it is. But I can understand that it can be annoying, of course.

Kind regards

Eddi

Born to fly ...
forced to work.

Thank you Eddi and Greg for your answers.

The problem is, the tail wag is not just present in hover, but also on tail stops when pirouetting or flying backward either upright or inverted. The tail is just not "locked in" as it is on my other helicopters, for example my Synergy E5 which is also using a Savox servo, and doesn't give me confidence when flying.

I've had a look at various forums e.g. HeliFreak and there is quite a number of posts about Logo tail wag. There are various solutions proposed, from changing the tail servo, thrust bearings, different tail blades, using QuickUk tail etc.

Now I am not the world's greatest heli builder, but I have built quite a few helis over the last 10 years and none of them had this problem. I had a Logo 550 in 2012 which I built according to the manual and it flew great on first try. So I must say I am a bit disappointed that I am having these problems. Don't take it the wrong way, but if it is considered "normal" to have a bit of tail wag with the Savox servo supplied with the super-combo, then a different brand of servo should be included with the kit. I should not be left trying various servos, checking bearings, smoothing the tail shaft or changing settings on Vbar Neo to get the heli flying properly. The default Vbar settings should work. Of course they can later be tuned for better performance if necessary.

Not sure where this leaves me, I am out of ideas on the tail and unfortunately due to the Covid situation it is hard to meet other heli enthusiasts and get advice.

Thanks,

Darko

Hi Darko,

there are different wags: the one in a hover we have covered. If the build and the mechanics are OK, it only leaves the servo, where tail wag suppression might help, or meddling with parameters in one bank for takeoff, hovering, landing, while leaving the ones for aerobatics and 3D at more aggressive settings.

Then, if the tail does odd things in stops or backwards flying, it's something different—trim flight and optimizer flight (given a good mechanical setup in the first place, plus sensible rpm, tail blades suitable for that rpm (e.g. longer ones if you prefer lower rpm), and overall setup (mainly: gain set properly for the rpm).

Since you seem to have updated to Pro in the meantime, I'd recommend tryin tail wag suppression (for the wag in a hover), and do the trim flight and optimizer flight, then adjust the stop gains if necessary, to equalize the stops left/right.
That should already help quite a lot.

Please help with some more info: attach your current setup file so we can see if there's anything which can be improved, and let us know what head speeds you run, in your different banks, so we can also see if the set tail gains are reasonable or unusually low or high.

It's always difficult to diagnose and remedy a tail wag or other phenomenon from remote, this is also why you always read the same things, thrust bearings, blades, rpm, servos, settings, de-burring, smoothing etc.

The Savox servos in general are not bad at all, but obviously they are easier on the budget than e.g. Futaba or MKS, and imo you can not expect the same performance, else the price for top notch servos would not be justifiable. The combos come at a reasonable price, but there's room for improvement, of course. The reasonable price does not mean we include slow sellers or leftovers with it, we have tried what works well and at the same time doesn't over stress the budget, and that's that.

Kind regards

Eddi

Born to fly ...
forced to work.

Thank you Eddi.

I have enclosed the configuration file for the model. I fly three RPM speeds, 1860, 2000 and 2180. Both the main and tail blades are the ones that come with the 550x super combo.

I have gone through the tail setup one more time and everything appears to be mechanically OK, at least as far as I can see. The only thing that might be a problem is that the tail box is not 27mm at the plastic tail box end with the four screws that hold the boom. It is around 26.20mm in that location. Should I use some shims under the four screws, and, if so, what size?

I had the opportunity to fly it today, and perform a trim flight. Unfortunately it was quite windy so not sure if the results of the trim flight are valid. I have used the tail optimizer, as well as experimented with "tail wag supression" setting at various RPM settings.

The tail is better in hover, especially at 1860 and 2000RPM. At 2160RPM (Bank 3) it wags a little. However, if I move the elevator up or down and return the tail to level, the tail will oscillate a little as if hunting for center position. Also when moving the rudder fast to the left or right the releasing it, the tail will oscillate a little. Nothing much, but definitely there: it does not stop crisply.

Best regards,

Darko

Hello Darko,

thank you.

Tail setup: the limits 88-60 look odd. I have a 550 SX here with the same servo, values 98-106 with the ball link on the servo arm in the innermost hole.
This way, I make better use of what the servo is geared for. At the same time, I have no 'ridiculous' tail pitch angles, which would lead to a stall eventually.

The distance between the plates is secondary unless the tail boom was squeezed, or if it was not symmetrical (plates not standing exactly plane parallel), or if there was pressure or tension on the tail shaft or the bearings there. Simple mechanics, two bearing blocks with a shaft through must be plante parallel, and the bearings must not be under constant pressure, else the shaft or bearings will wear early.

The trim flight looks quite off. If the COG and the overall trims are OK, I'd attribute that to the windy conditions.
The optimizer values look reasonable OK. Pend Unt (Tail Wag Suppression) is 0 in all three banks ... did it make a difference at all?.

About Bank 3, imo the overall tail gain is way too high, for a Savox servo, at that rpm.
I found that I can only go to like 55 max, at high rpm, else the tail will over compensate/start hunting or oscillating, during quick load changes or in (fast) forward flight.

80-90 for the lower heads speed, 50-55 for the higher head speed. Fun fact: the tail performance is not bad then, like a BLS 251 running at 90 overall gain (but the fact that the BLS is crisper with almost all respects, including the wallet).

So ... try and see if you can get better limits in the tail setup, still with reasonable pitch angles at the tail blades, change the tail gain to say 80-60-50 or 55 for your three banks, then do the optimizer flight again, and the trim flight, weather permitting.
For the tail, you can then do some more: do full pitch climb outs, see which direction model rotates. Add or subtract collective add if it's letting go or over-compensating. Dial in approx. 1/3 of that value for cyclic add.
Once the trim and optimizer flights are properly done, you can use the stop gains to equal the stop behavior. Mind that there are physical/aerodynamical limits where the tail can not stop any crisper, espeicially with the torque. So it can be necessary to soften the stronger side a bit, to make the stop equal to the softer side.

Getting there, I guess

Cheers

Eddi

Born to fly ...
forced to work.



Edited 4 time(s). Last edit at 10/13/2020 03:19PM by Eddi E. aus G..

Thanks Eddie.

For me the Tail Wag suppression parameter in "Tail Rotor" shows as 29, 30 and 50 for Banks 1, 2 and 3 in VBar Control. Do you not see that?

Thanks.

Hello Darko,

my bad, I was using a wrong software :-/

I can see it now, too, so ignore that please

Cheers

Eddi

Born to fly ...
forced to work.

Eddie,

As regards the tail setup, I am not sure how to get better limits. The tail control rod is already as short as it will get with the ball links screwed in as much as possible. I am already using the innermost (14mm) hole on the servo arm. The only way to change this would be to increase the belt tension by moving the tail box out a bit more, which would allow me to lengthen the tail control rod. But I didn't want to do that, since I didn't want the belt to be too tight.

Am I missing something here? Should I move the servo arm one or two notches to the right?

Thanks,

Darko



Edited 1 time(s). Last edit at 10/13/2020 03:46PM by darkoroje.

Hi Darko,

the limits have nothing to do with the length of the tail boom. The servo arm does a circular motion (center 90° then left/right), the control rod moves forward/back accordingly, the bell crank at the tail redirects, the tail pitch slider on the tail shaft moves left/right).
The farther in on the servo arm the ball link is, the farther the servo has to move for the same control rod movement/tail pitch slider movement.
Then, we want something between 25-30° of (sensible) tail pitch, job done.
The tail shaft allows for more, but that would mean angles where the air stream lifts off the tail blades, causing a stall.
Making the rod or tail boom longer would only shift the center position.

If you move the servo arm a notch left or right of the true center (90° to servo arm/control rod), you also only shift the center position, but that would possibly allow to adjust more equal limits (lesser gap), which imo is better for the overall performance.

If in doubt, please take a couple of photos like here (at the bottom of the page) and post them, so we can see.

Kind regards

Eddi

Born to fly ...
forced to work.

Thanks Eddi.

I have my current limits at 85 right / 60 left. I must admit I don't quite understand why the limits need to be similar on the left and the right side, after all the range of the movement of the servo is the same. and the limiting factor for servo movement is the tail pitch, not the servo range of movement. I also do not understand how you have a larger left/right range on your 550 if you are using the same servo.

I have enclosed three pictures of the tail in the center, left and right positions.

Thanks,

Darko

Hi Darko,

from the pics (w/ blades would have been better) it looks OK to me. The 102 actually on the 550 SX here are too much when it comes to a possible stall, actually, and the maximum left can be mechanically limited, depending on the mounting position of the hub, on the tail shaft (the chamfer on the shaft allows for some leeway).

I mean, if everything works properly, no need to look closer. But I find that, if you run into issues, it's a good time to check everything.
And I found that in many cases, adjusting more precisely helps. What people tend to ignore is that Newton's laws still apply.
In a translation to our tail rotor at hand, you can not accelerate well in 6th gear at low to medium speeds, and you can not run high speeds in 1st gear, you get the best result if you have the proper gear selected. Speaking servo, if you let it breathe (+/– 45° / +/– 100 % servo throw), you can expect the best results. if the servo could do 60/60° you could shift the range without negative effect (I presume ...), so +35°/–55° _with center at –10°_ should bring the same result.

These are theoretical observations, to be honest, but over the years I found that especially with less expensive servos (or even analog servos, back in the day), it's always good to get the mechanical setup right in the first place.
With today's servos, there is more room for sloppiness (no offense please, I more of the shirt-sleeved sort myself, anyway :-) because today's servos have speed and power in abundance (precision / re-centering precision may be a different story, though).

Anyway, I was out of office a couple of days, any news from your end since you posted the pics?

Best,

Eddi

Born to fly ...
forced to work.

I am very happy to see someone discussing this issue here. I have been troubled by "LOGO550 tail wag" for more than a year.
I am going to buy a "Futaba BLS 251" right now! Looking forward to the test results later.

Thank you Eddi and Alanni.

I have found someone else in the club who has a Logo 690 so took the helicopter to him for checking. He could not find anything wrong. I changed the servo to a Align DS825, since Futaba is very difficult to find in the UK. Made no difference. I even bought the aluminium tail rotor clamp (MIK-5128) and installed it, also no difference. I put my old Kontronik HeliJive on, it did improve things somewhat at lower RPM but I had no telemetry so I put the Scorpion ESC back on.

After all the changes the wag is still present, especially at higher RPM (2080), but even at 1780 the wag is there just not so pronounced. It is not just hovering, when flying and just doing a simple circuit the tail will not stand still especially in the wind.

In desperation, I bought a Trex 550x kit, assembled it and moved the Vbar and Scorpion onto it. Flies perfectly and the tail is rock solid. Admitedly it uses the DS820 cyclic servos and MX730 motor, but has the same tail servo.

I would prefer to fly the Logo, it is much lighter and quieter and I always liked how Logos fly but what more can I do? If there are any ideas I am eager to hear them.

Many thanks,

Darko

I might have stumbled onto something. I disassembled the tail once again and noticed that I could not get the tail shaft out of the bearing. After some heating I managed to get the shaft out and I notice that the bearing must have been pressing on the shaft and has carved a trail into the shaft.

Why would this be happening? The tail has been assembled exactly according to instructions.

Thanks,

Darko

Hi,

this could be evidence that the tail gear box is not plane parallel, or that a bearing has too much pressure on it, or is canted, or that the tail shaft has axial play in the assembly.

You can glue the inner ring of the bearing to the shaft with blue loctite (you could even use a bearing glue, but you will probably never get it disassembled again, then, without applying much heat).

Kind regards

Eddi

Born to fly ...
forced to work.