Servo chatter & buzz - HK Raven 990 DLG

[Tony Kenny]

Hi All,

My lovely wife bought me a HobbyKing Raven 990 discuss launch glider and I've spent most of the day setting it up. To be fair to HK, I did have to solder new connectors on the servos to cope with my micro rx, but even after that, setup was a bit of a pig.

The aileron servos constantly chatter and buzz when in use and get hot. A little wiggle on the controls or a tap on the ailerons sometimes stops it. But, if I disconnect the control rod completely then I don't get any noise at all. I don't get this from the new servo I put in on the elevator (see below), so I'm wondering if the HK servos are just cheap and nasty and likely to kill the battery, or the model. Or am I missing something?

If it's of interest, the other problems I had with the setup:

1. The elevator servo was too weak to pull the elevator so I've had to install my own and I had to cut the fuz to fit one I had.

2. The control rods for the ailerons were twisting the servo arm so I had to switch the rods to the other side of the arm.

3. The arms do not appear to be in a central point on the servos, I'd had to use different amounts of offset on each to get them somewhere near.

4. The hot glue holding the servos in place doesn't seem to be doing so well, the servos are trying to push their way out in normal use (with deflections set to recommended amounts)

4. Was very difficult to get the ailerons to move equal amounts (probably due to the servo arm positions)

Still, looking forward to enjoying some great flights, even if I do have to replace the servos with ones that will last. Some short chucks in the back garden suggest that it's flying straight, so not all bad

Tony

[Martin Harris - Moderator]

They may well be very cheap/nasty servos - the ones in a PNP Hunter that I bought a few years ago from HK were certainly not up to some of the functions they were installed for and I am now very wary of buying models with pre-installed servos. However, if the elevator servo was incapable of moving the control surface this would suggest either binding linkages or a very poorly servo.

However, could they be digital servos? These will grumble and chatter from small loads - even the weight of a control surface - and this isn't a sign of problems, just the way they work...

[Dave Bran]

Well, you may not be missing something, but we are! You have not explained your set up, radio wise.

While it might be servos it might also be the Rx not pairing its frequency. For example, the FrSky FASST compatibles have a HF mode for high resolution digitals that is entered in a similar manner to binding, and its not at all unusual for people to set them wrongly during bind without realising till the servos keep chattering and then if they don't take the hint, an in flight burn out is quite likely.

There is enough evidence here (especially heating!) that you should NOT proceed in flying it as is!!

[Dave Bran]

Incidentally, I'd not be surprised if all servos now are actually "digital" no matter how labelled/described, its just their quality that varies.

Many years ago I was buying lots of JP micro servos, and one batch had analogue and digital labelling on them inside digital packaging.

When JP investigated from my report back the feedback from China was that they'd gone over to manufacturing digital only and were just using up label stock, so even a servo that was in analogue packaging with analogue label MIGHT be digital!! (And they had told no-one!!!!).

One make I have in the recent past had recommended by a respected seller, bought and had to take out as they were rubbish (two sizes) were Gening, and that had nothing to do with the Rx set up.

[Dave Bran]

Now to Discus Launch gliders (Will he never stop?).

They need good servo resolution, as you need very fine smooth control, a servo that "steps" across its range and exhibits centering and other issues is NO use at ALL and you will not get the best from the plane. (Especially but not exclusively true for small wings like Alula, etc).

I have found that the Ripoff Max (Dymond in USA) range of SD100/SD200 or similar Savox models give the best performance at affordable cost.

[Dave Bran]

Oh, and your list 3 and your second 4 (!!)

It might be giving them credit they do not deserve but one way of ensuring aileron differential is to use the rotational effect of the arm on linear rod movement, and in that case the arms would not be centred, but offset to give differing up and down movement distances.

Coat, get....................................

[Denis Watkins]

As Dave Bran says, the transmitter needs some consideration.

These servos may be doing exactly what the Tx is asking them to do

[Tony Kenny]

Hi All,

Thanks for all the replies and suggestions.

The Tx I use with almost a dozen other models and I don't get this problem with any of them, but, that might be beside the point, maybe?

Why do I think it's an issue? Two things, 1st that the servos are getting hot even to the other side of the wing and 2nd, that tells me that they are drawing current and draining the battery. Combining that with the servos appearing to wiggle lose from their glue, this gives me cause for concern that I may lose the model in flight either due to a failure or loss of power.

This is what I've done to try and resolve the issue.

I've noted that with full deflection of the sticks, the control surface reached full deflection a long time before the stick. Therefore, I've adjusted the max range on the Tx so that max stick movement only gives the manufacturer's recommended control movement. With a small exception being for the flaperons which I've allowed to go as far as the control surface is capable of moving. (That's attached to the throttle stick)

With that set up, I noticed much more fine grained control of the ailerons, but still with the chattering.

If I tap the control surface or even give the stick a little nudge, the chattering often stops, does that tell us anything useful?

Thank you again! Gotta love this forum! Looking forward to DLG flying, the endless hours without running out of power (plenty of 1S Lipos to hand) - and the massive right arm I'm going to get (although I'm already 'inventing' a long stick with remote release to get better launches!)

TK

[Tony Kenny]

Posted by Dave Bran on 27/12/2017 08:17:43:

Oh, and your list 3 and your second 4 (!!)

It might be giving them credit they do not deserve but one way of ensuring aileron differential is to use the rotational effect of the arm on linear rod movement, and in that case the arms would not be centred, but offset to give differing up and down movement distances.

Coat, get....................................

I hadn't thought of that, but, they were different on each aileron and the manufacturer recommends equal deflection of 10mm up/down on each. The position they were in also meant that the arm hit the air frame before the control surface had reached max deflection.

But, saying all these things, some light chucks in the back garden and it seems to fly straight and true, even with the wind swirling around from the house and fences. Shame I'm working this week and can't get out before it gets dark especially with the park covered in soft snow in case of any mishaps!

[Ron Gray]

FWIW, I would firstly try and set/reset the mechanical configuration of the servo connections starting with closest hole in on the servo arm to furthest hole out on the control horn this gives the best servo resolution before having to resort to TX end points. As Dave says, differential can be obtained by having the aileron servo arm pointing slightly forward and not at 90 degrees to the servo. You also mentioned that with the control pushrods disconnected, the noise goes away, this indicates that the control surfaces are ‘fighting’ the servos so double check that they do move freely. Another thing to check is the connection of the pushrods to the control horn and servo arm, is there any binding as this can again ‘fight’ the servo.

If you find that after setting everything up correctly the servos are still getting hot then, personally, I would ditch the servos and replace with new ones, they are not that expensive!

[Tony Kenny]

Ron, thank you! I just assumed the hole they put it in was going to be the right one, expecting the manufacturer to know best. There's only one hole in the control horn, but 3 in the arm, but, won't the servo then experience greater strain with the effectively shorter arm? I also don't think they will have enough movement on them, but I'll try.

I'm still also concerned that the servos try to pull themselves away from the glued surface when moving, suggesting that the glue is insufficient or the servos are experiencing excess load?

Maybe I just need to bite the bullet, remove the hot glue, centre the arms correctly so they are equal and give the right throws without having to set sub-trim and seat them more securely, rather than lose one in flight and lose my new toy! If I have to do that, will be a bit of a disappointment having spent so much on an ARTF. I'm now a bit wary of the HK brand for the future.

[Ron Gray]

If the servos are moving when only applying a slight amount of stick movement then it indicates that the surfaces are not moving freely, either to stiff hinges or binding between the pushrods and horns / servo arms. Double check this by disconnecting the pushrods and move the ailerons by hand. Do they move freely? If they do then reconnect just the control horn end of the pushrod and then push and pull the pushrod (you are acting as the servo), is there any binding / resistance? If there is then you could try slightly bending the pushrod. It could be that the pushrod is not acting in the same plane as the horn so as it moves it binds with the horn, an alternative to bending the pushrod would be to use ball links. Anyway, do the same check at the servo arm end.

I forgot to ask, how are the pushrods connected to the horns / servo arms? Z bends? Clevises?

The bottom line is, if the servos are moving, it is down to Newton’s Law, there must be an opposite force acting on them, which needs to be removed! (Newton’s little known 5th Law!)

[Martin Harris - Moderator]

Posted by Tony Kenny on 26/12/2017 21:00:20:

The aileron servos constantly chatter and buzz when in use and get hot. A little wiggle on the controls or a tap on the ailerons sometimes stops it. But, if I disconnect the control rod completely then I don't get any noise at all. I don't get this from the new servo I put in on the elevator

I've just watched the video on the HK site and there's a caption that states the servos are digital. Out of interest, do the servos get hot at rest or in use and/or with the linkages disconnected? As I understand it, digitals apply higher force (i.e. pull more current) close to the commanded position than analogue ones which progressively reduce loading as they get closer to their goal.

[Bob Cotsford]

If the model uses moulded foam 'living' hinges it may be worth disconnecting each surface and exercising the hinges to loosen them up. The fact that the elevator servo wouldn't move the surface is a bit worrying, is the pushrod free and the linkage able to move with no binding?

[Tony Kenny]

Thanks guys, I'll try some of these things a bit later (busy day with work, and working from home with the model next to me is torture!)

The elevator is 'spring return' and it's quite strong. The's some soft of transparent "tape" as the hinge which pulls the elevator back to a down position, the servo then pulls it up with string and releases. The installed servo couldn't pull it and when I disconnected it, the servo was still not moving as I'd expect, sometimes not at all. That servo is out and will stay out.

Control horn connectors are, sorry not sure what they're called, I'm in software so the 'real world' is a bit unfamiliar. They look like a hex/cylinder with a hole for the pushrod and a screw to keep it in.

Just disconnected one quickly, you're right, the aileron is a lot stiffer than anything else I've had, I'll flex them a bit and see if I can free them up before making any other changes.

[Ron Gray]

Connections like this:

**LINK**

Or this:

**LINK**

[Josip Vrandecic -Mes]

This is my little experience that is indirectly related to this topic. I have a servo tester, which I put together with two power sources (4.8 and 6 V). You can see how much voltage you need, no matter what the sticker writes, the dynamics and speed set in the desired position, how hot / warm, the buzz etc. ... and many other useful informations including ESC testing.

Edited By Josip Vrandecic -Mes on 27/12/2017 13:12:17

[Tony Kenny]

Thanks Josip,

There's no ESC in circuit here, I'm using a Tactic Rx Tr 624 micro rx and a single cell 450mAh Lipo which is in the correct voltage range for both the Rx and the servos. HK recommend a single cell LiPo for this model.

@Ron, the control horn connectors below. One disconnected at present.

[Tony Kenny]

Hi Guys,

Things are going much better now, thank you!

I gave the ailerons a good few bends and also moved the push rods to the bottom hole on the servo arm. There's very little chatter now when in a neutral position and I also got a better range of movement with more granular control. All set up now, waiting for the right combination of weather/work/family to get out and try it!

For the DLG side, I've got a slight up elevator and down flaperon for launch, then into all neutral for the climb. I've got flaperon set up for brakes which I'll also experiment with using a tiny bit for thermal mode. Not expecting many thermals just yet, but hope to get to the hillside soon!.

[Ron Gray]

Excellent sounds like you’ve got it sorted.

[Martin McIntosh]

Got to comment on some of this. Firstly, ditch those screw on connectors and fit good, freely operating ball links. As former club safety officer I banned these from anything other than light indoor models because there is nothing to stop that screw from coming loose in time. I realise that a newby to the hobby would accept these as the norm, which they definitely are not. In rash moments I have bought a few HK ARTF things and the first step is usually to throw away the 50p servos and install proper linkages. A servo will certainly overheat on the rubbish installations they sell you.

Analogue v digital?

An analogue servo is driven by a voltage difference between the input signal and the servo potentiometer position. These were discontinued very many years ago. Digital ones use a full voltage signal sent in pulses to the motor until it reaches position. What they now for some reason call `digital` servos differ in that the pulse is split up into much shorter ones which gives better accuracy and will hold it better against the slipstream. With perfectly free linkages they will often buzz due to the weight of the control surface. Sorry to go off thread a little but I think that needs to be clarified. If your servos are marked `digital` then they should be of the latter type.

[Ron Gray]

@Martin, the use of hi strength thread lock prevents screws coming undone, indeed, the only way they can be undone is with the application of local heat. Obviously I am referring to hi strength, not the lesser strength ones.

[Martin McIntosh]

Hmm, yes, it stops it from becoming undone but the screws eventually bed into the wire and will no longer grip it.

[Tony Kenny]

I'm hoping to try the model later this week or over the weekend. Shortly after I'll look at replacing those connectors.

I also forgot to mention that the push rods had some sort of coating but that was pressed up against that connector and was preventing it from moving, so I had to trim some back (that was one of the original problems). All in all, a lot of little changes had to be made to make this model air worthy. Just wondering if I should feed back to HK or if they'd even be interested.

PS. I am a "noob", only been flying a year, but do fly various different models and have made plenty of repairs and take pride in setting up my models well! Although, I'm pleased to say that despite my numerous crashes, there were never 2 for the same reason

[Ron Gray]

Feedback to HK as its always good to get feedback. However don't expect anything to get done as they source their suppliers from all over and, with a few exception, their relationship with them is purely based on money!

Crashing is all part of the learning curve but as long as you learn from them and don't make the same mistakes again!