HI,

just wonder if anyone have found a slope free servo ?

Most servos today have more or less slope ; a deadly one for the servos, specially when installed in a "stroker-setup". This tiny initial slope become larger , and soon the pot`s are gone .. unserviceable for F3A.

The only servo i have tried without slope was back in the 80 `s - sanwa contest ( HC & HP ). Mechanically excellent for 500+ flights, and the pot`s became worn in a "normal rate".

HMMM ..... I am a bit suprised that the manufactures still make this kind of servo ; mechanical pot`s and non beweled geartrain all through ... Beside some small improvement it`s the same tech for abouit 30+ years or so ,at least as far i can see ...

In F3A the servo play a major role and we rely on it .. it`s hard enough to struggle with your fingers ...

Kjell Olav

Kjell Olav
Slop-free is hard to find. One issue is that most top-performing servo's today has metal gears. Maybe a good idea for some use, for F3A it's not needed. My experience is first of all to avoid the metal gears. Before the JR DS8401 became available I used the 8411, but replaced the gears with plastic.
One additional reason for faster pot-wear today than with the old Sanwa's (and other non-digitals) is from the fact that the digital servos are much faster to correct errors on the output axle, and are much more precise. The combination of these to factors means that the sero tries constantly to correct for error-input generated by engine vibration. An example : a JR digital updates the output position at around 250 Hz (I have measured it, it's a little higher but I do not remember exactly), the fundamental frequency from the engine at 6000 RPM is 100 Hz. This means that the servo is constantly working to keep the aileron (or whatever) in corretc position by counter-acting the input from the engine vibration. With the old type of servo's the update frequncy is in the 50Hz range, and the servo does not 'understand' that the engine is shaking the surface up and down.
To sum up, there is a price to pay for the general improvement with digitals, even with slop-free gear-box the pot-wear is higher than in the old days.

Ola

Hi Ola !

Like you said; I guess this is the price we have to pay using digitals ...

I have two wishes in the future about this issue :

1: A fully electronic pot

2: A gear boks with beweled geartrain:
An angle between two gears, will make more than just one tooth engage at the same time. More angle, more tooth`s engaged. This will allow a better total tolerance between the gears and a more solid geartrain overall ...

Hmm maybe i must try to pick up my experiment from the past , using a dampening fixture with shock absorbers from model-cars ...

Kjell Olav

Kjell,

On the subject of feedback potentiometers, you may be interested in this earlier thread - http://www.rcuniverse.com/forum/m_36...tm.htm#3665984

I had also thought of using a small damper attached between the adjacent arm of the servo and a fixed hardpoint nearby. If the spring was removed from the damper, it would act as a "first order system" and provide lag without any overshoot. By experimenting with the viscosity of the damper oil and the size of the holes in the damper piston, it should be possible to provide sufficient damping to reduce the wear on the feedback pot without causing any significant increase in the servo response time. There may be a small penalty in terms of weight, but that ought to be acceptable.

If you do get round to trying a damper, let me know how you get on. In the meantime I'll probably keep changing my aileron and elevator feedback pots ever 100 flights or so.

Bob

PS to Electric Flight enthusiasts:
I know that wear on feedback pots is less of a problem with electrics. However just think of all the fun you would be missing with a YS. No substitute for oily hands and the throb of a highly tuned 4-stroke!

Bob,

... I saw your earlier post about this issue ...You are absolutely right !

Fully electronic pot`s are widely used in different application`s.
In my Airline company; technical division, i now and then do some throubleshooting on different avionic system like engine power management systems. These engines are fully electronic controlled, by means there are no mechanical fixtures between the throttel-lever in cockpit and the engine. In this system you find resolver`s (pot`s) , non-mechanical , with impressive accurancy and precision. The technology have been there for many years as you already stated in your post ...
So,until then (if ever ) we have to deal with the servos in the market.

I will continue experimenting with shock absorbes this winter, and hopefully be using them permanently ...
The idea is to attach one end of the damper to the same control Horn/bolt as the pushrod connection, and the other to a mounting bracket attaced to the servo for easy replacement and adjustment.

At least; it`s worth a try ...

Kjell Olav

Hi Ola,

Could you please explain why you think the plastic gear is superior over the metal (for F3A) ?
I thought it is a common practice that metal gear is better in terms of wear and slop

Thanks

Yoav

The new BLS451 brushless motor servos from Futaba are nearly as slop free as I have seen from a Futaba servo. Also I think they are mostly a plastic geartrain. I have them on the ailerons in my Twister and they are working extremely well. Since its electric though I am not sure how the gears will hold up to a glow setup.

sounds interesting ...

Defenetly worth a try on my "banger-powered" Integral ...
Most of the servos i have tried have too short lifetime. Some of them work better than other, and in fact a few keep what they promise, but if i can manage to extend the lifetime even more i will give it a try ...

Kjell Olav

Kjell,

On those electronic pots you refer to I guess you are talking about the Variable Rotating Transformer or RVT, if this is the case, I think we are still far away to have them miniaturized to the point it will be usefull to install in a servo, It will come, but talking about size and weight, potentiometers will still be there for a long while.

Maybe manufacturers should use better potentiometers but then the servo price will be so high that no enough demand will be there for them, all this come in play.

I remember one time I needed to replace a pot in an Jetstar because it had a flat spot, the pot was around $700, first I complainted, then realized it has been working fine for 11.000 Hrs and 30 years (I was younger than the aircraft at that time), expensive, but it does last.

Cheap pots might well be the problem.

Yoav

The metal-gears themselves are more rigid and can survive sudde shocks better (like when hitting the control-surface going out the door or something). But, when the control-surface is shaken by engine vibes, the plastic gears that are a bit softer flexes slightly and dampens the vibes. When metal-gears are slammed together there are no damping, just metal against metal that is not helping to reduce the vibrations. Thus the pot sees more high-frequency vibration that wears it out faster.
Also an issue is that modern high-speed, high gain, high accuracy digital servos often can start oscillations when the pot's gets worn, it seems to me that this is a bigger issue with metal-gears, again bechause there is no damping of the vibes. Oscillations wears the gears and the pots even more and also severly increases the power consumed by the servo.

Hi Bob,

... This is the idea, how i imagine the damper-fixture will be installed. The shock in the picture are just a dummy, and will not be used.

I talked to some "car-guys" and they routed me in to a interesting part of this issue . They showed me some high quality shocks, used for special comp-applications that looks promising for this experiment. Basically, you can adjust the dampening caracteristic without disasammble the shock ,and thereby easier find the best basic setup. (4-step adjustment by rotating 1-4 holes in the piston)Then, if the shock don`t work as you want, you replace internal parts and start all over. In other words, there should be some possebilitys to get this work ...
I do not know the weight of this specific shock`s, but similar construction weight about 10gr. ( spring, preload-bracket removed;not needed).So, if i can get a complete installation down to appr 15 gr pr side, it`s acceptable.

Well, there is no guaranty at all, some experiment`s work`s other don`t ... But anyway, i like to do this, part of the game !

Kjell Olav

Kjell,

You could attach the damper as shown in your photograph.

An alternative way of doing it, would be to fit the servo with a double sided arm instead of a round wheel. The arm on one side of the servo would attach to the control horn in the usual way. The arm on the other side would be attached to the connecting rod of the damper and the body of the damper could then be attached to a suitable hard point.

Both methods should work OK but the latter may give you a little more room to play with.

I like the idea of a high quality damper. Not only would this be easier to adjust but it should also give better repeatability and be less susceptible to leaks.

I know that the damper that YS use for their YS Mount is a Tamiya Part. If you purchase a replacement set of dampers from Tamiya, they come with a choice of 3 different pistons with 1,2 or 3 bleed holes. The Part No is TA 53155 - see attached link - http://www.fusionhobbies.com/PhotoGa...ctCode=TA53155. I would remove the spring from the damper for the reasons stated in my earlier post i.e. to give a closer approximation to a true 'first order system' and minimise the possibilty of oscillations. ( I think these are very unlikely anyway since the basic transfer system of the servo itself is designed to be critically damped without overshoot.)

You can also buy Tamiya damper oil in different viscosities.

At a guess, I would probably start off with the 3-hole piston and a low viscosity oil. This would minimise the inevitable lag that a separate damper would add to the system. You might even find that you had to drill more holes in the damper piston!

Best of luck

Bob

Kjell Olav, I remembered I've seen seen a post in the 3D forum on someone doing the same thing on a "Vision 3d" rudder installation. I made a quick search and found it
http://www.rcuniverse.com/forum/fb.asp?m=2788462

/David

Thanks David,

... seems that the fixture works in that application ... I saw a german pilot in a magazine for many years ago did the same, and he was also happy with it ...
The most dampers have complete spare parts available, besides the hi-state dampers are made of high quality materials for long life and performance.
If this solution end up by replacing seals in the dampers instead of geartrain and pot`s; it`s worth a try...
Hmmm, i wonder if this idea will have some aerodynamic benefit`s also, making a "cleaner" surface by keeping the aileron ( or elev/rudd) in a steady position all time ... ? ...

Kjell Olav

That is exactly is installed in the Bombardier Challenger, and it is called Flutter Damper, has two per surface.

Apereira,

... sure, full scale AC have different fixtures to prevent flutter ...

IF a dampening fixture in our pattern-planes would work , it could be possible to try metal-gear-servo`s again ,without crushing the gear-train and pot`s by a stroker ...

Chad,

Have you compared the new Futaba BLS- servos against the "traditional-digital" nylon-gear servos, how the "softness" feel`s when you try to turn it out of position? ( some kind of "bungee" feeling)

Kjell Olav

Yes, it is actually there to compensate for the in the actuator system.

My point was, it is used for the same reason, so it should work as well in the model industry, it can also be installed separate from the servo, close is enough for it to work.

Regards

Hi Gent`s

... I thought i had to fullfill the idea , and now the damper fixture is installed and tested on my Integral. So far , everything seems promising ...
After some experiment`s and testing , i found the set-up that work, and now it needs more flight time to see how everything hold up over time ...

In the Pictures you can see it installed in the aileron and rudder.

Kjell Olav

Kjell
nice to see some think "new".
how does this infect the speed off the servo, when i buy servoes that is one thing I study in the servo spec.

Hi Rune,

Cold this day`s to fly, but i found my winter-suit and the "statoil-coffee-cup", so actually it is ok , if the wind is not too strong ...

Servos;
I have tuned the dampers so they do not restrict the servo-speed, at least not on ground ... I did some different high-speed / high rate snaps without remarks, still the same feeling , so it seems ok. Several pistons/oil viscosity have been tested to achive the best dampening characteristic without restrict the servo-speed. There will be some friction, but not more than a "tightly hinged ailereon".

This is a type of Damper with large piston area to give a dampening effect with little travel, together with small holes with thin oil to assure good flow at lower speeds, but restrict at higher. The oil ( silicon-based) manage temperature variations very well, and the damper was still working after a longer time in the freezer ...

Well, time will show how everything works ... But i hope they keep my new servos allive longer than my previous ones ; They didn`t last more than 100 flights ...

Kjell Olav

so 100 flights on the Itegral already!!
impressed!!

... only 30 flights ... . But just wait, soon it will !!

I used my 9411SA on the Impact for appr. 70 flights, and then switched them to the Integral.

Those last flight, i sudden heard a strange noise from heaven ; Servos screaming for help!
hmmm , sloppy gearbox, and pot`s worn beyond limit in both ...

The 9411SA is a very presise servo and i really hope they stay tuned ... otherwise i continue the servo-cruisade

Kjell Olav

so ... you then say that its my luck that I didnt order my 9411sa yet?
i have used the futaba 9154 and 9150 both with sloppy gears after just some flights!

... 100 flights is short ...
At the same time i should be careful to judge, i have just tested these 2 in a relatively short period ...

The Aileron on the Integral is big, and the build-in hinge have non-friction; leaving everything to the servo. Maybe i can`t expect anymore from the servo !? Probably; other servos would share the same destiny ...

This is why i hope the dampening fixture will be friends with them, extending the lifetime. The total weight of the dampening fixture is app 20-25 gram ( including bolts, washers, attachment, control-horn etc). This is acceptable together with the 9411SA which will put the hole assy ( damper + servo ) in the "normal" weight range compared to a high performance metal-gear servo. At the same time ; get the benefit of a very presise servo , which the 9411SA is ...

Kjell Olav

I have JR 8411 servos all around in my new 35% Wildhare Extra 300 and all the servos (especially the elevators) have a ton of slop in them. I am almost afraid to fly the thing in fear of getting flutter! I considered replacing all 6 servos with Hitec 5955 but that would be a $500 plus fix. Does anyone have any suggestions for a fix or is there a fix for this? I think this will be my "last roundup" using JR servos. It seems like ever since JR came out with the JR Sport servos a couple years ago, all of their servos have gone down hill. Can anyone offer any help? I called Horizon but the customer rep acted like I was crazy and didn't know that I was talking about. This is the same way they responded a few years ago when the JR Sport servos came on the market. I had some new JR sport servos go south on me and Horizon didn't want to help at all. I think Hitec is going to be my brand of choice from now on out. If a Hitec servo goes out they will fix it with no problem. Customer service means a lot in todays economy, at least it does to me!