Topic: It's not RDS but nearly!

I just had to show this linkage/servo system I’ve been working on as it really works great!

It features an external bearing to cope with small horns and small arms to produce an almost clean wing- servo covers are flush and aileron and flap horns are around 1mm or less proud. Tight linkages with big horns doesn’t require as much fuss but small horns and small arms is a much bigger load on servos.

But what I am most pleased about is that it is virtually slop free and the throws I can get are truly excessive!

I’ve pared down the materials, weight and structure to the bare minimum and made it very simple and quick to do.

The downside is that the servo is semi permanently installed. It can be removed (and I have done a few now) but it’s definitely not a field job. But the arm can be replaced with the servo in situ. A cut down Allen key is the only tool needed and this doesn’t necessarily need to be done in the workshop.

What makes the installation so rigid is the fact that the servo and the bearing are bonded to the top and bottom wing skins. A servo mount just bonded to one skin just didn’t produce as good as a result, external bearing or no external bearing. Joe Wurts has been a long time advocate of bonding servos to both skins, he even goes on to say that ribs on either side of the servo well (with the servo attached to one skin) isn’t very effective either and just adds weight for very little gain in rigidity. I’ve been doing that for years but not anymore!

Incidentally, I tested this latest install in stages:
Firstly, I bonded the servo with a 1mm G10 strap with no external bearing. The flap servo has the most load and even with very good double ball raced servos (Sanwa 762) I could see the head rock when moving the flap by hand.
Stage 2, I retro fitted the external bearing but just attached the bearing housing to the top skin. You could clearly see the skin flex and still some head rocking.
Stage 3, I put a thin strap tying the bearing to the bottom skin. This makes all the difference! Head rocking is tiny. In fact the short 2mm pushrods flex looks more of a problem now! The bearing, housing, bush, bolt and strap added just 2g (after taking off the weight of the original servo bolt).

The ‘bits’ you need to source or make to do these external bearings is easy enough with the right tools, patience and ‘trawling’ for parts. Actually installation is very easy and takes very little time. A lot cheaper too than servo mounts which does nothing or very little in improving servo performance.       

Heres the flap throws achieved and another bearing install started using Futaba 3150's, showing some of the parts used.

So how is this better than the system on the Espadita?

Any Reason why you didn't use the RDS Tony?

Espadita system I haven't seen in the flesh yet but from what I read there is some serious engineering involved and not something you could make easily.

I've done 5 RDS installs and repaired 3 (using 3 different RDS systems). It is really difficult to work with and although I don't want to get into a debate about it's relative merits or otherwise lets just say I didn't even bother picking up the RDS system when I collected my xfire2 which now has the external bearing system.

A conventional linkage system is easier to work with and maintain. This is a stronger, lighter version and anyone can do it.  I'd even go further and say there is definitely less slop and more throw than any RDS system I have worked on.   

Great looking system Tony. Thanks for sharing! Enjoy the model 

Z

Interesting Tony, thank you.

You clearly had fitted this system to the Wizard wings you just supplied me, judging by the cutout and the epoxy left from the servo fixing.  Some questions.

Seems you do not use any tape on the servo - glued straight in.  The break-out in my wing is clean.   How?

What material did you use for the bearing mounting block?

How do you cut your 5mm carbon tube spacer so it is square ended?

Bearing sourcing is easy enough for a heli flier/mender    Modelfixings ....... etc.

Jerry

Looks good, seems strange you have not used the RDS, I thought it came supplied with that model.

Scott r   

Looks good, seems strange you have not used the RDS, I thought it came supplied with that model.

Scott r   

See post #4.

Ah I see, just seems a shame you buy a model with RDS then dont fit it, and basically say a normal linkage is better than the supplied RDS,I think these look cool if I wasnt going RDS.

Looks sound as a pound Tony.

For the non-heli flier / non-engineers amongst us, where does one source those bearings? And the carbon spacer is a piece of 5mm o/d 3mm i/d tube? And the thread on the 3mm cheesehead fits the output shaft of the 3150 just right? (standard thread?)

Steve

and basically say a normal linkage is better than the supplied RDS

  The crossfire rds that I had was really only fit for the bin. So normal linkage would defiantly of been the better option.

  Not saying there is anything wrong with the model or the suppliers. But with a model at that Money you would expect to be able to use the supplied hardware. Hopefully its been addressed to the manufactures now.

  The crossfire rds that I had was really only fit for the bin. So normal linkage would defiantly of been the better option.

  Not saying there is anything wrong with the model or the suppliers. But with a model at that Money you would expect to be able to use the supplied hardware. Hopefully its been addressed to the manufactures now.

Well it's gone in the bin. Just for the avoidance of any doubt about the RDS:

It looks like the manufacturer of the aluminium parts of the RDS system has tried to make it from bar which has the same OD as the finished part. The major problem with doing it this way is that it is very hard to find the CENTRE of that bar when machining in the holes etc for the splines to go in and the RDS wipers. This is what the problem was the the RDS parts that Adam saw. Also imho the aluminium used was a little bit too soft.

I now have in manufacture replacement parts in AA6061 which are machined from oversize stock so that all of their centres are common. Whilst I'm at it, I've added in some extra key machined in for the epoxy which holds the wipers and the servo splines. Should be perfect, basically, and come up to the same incredible standard of manufacture of the Crossfire II itself.

Of course it goes without saying that my "proper" parts will come with the model for no extra cost to the customer, so you will be able to use the supplied hardware for a perfect RDS install 

Cheers

Z

Zim

Cant really ask for more than that, good one Zim

Ah I see, just seems a shame you buy a model with RDS then dont fit it, and basically say a normal linkage is better than the supplied RDS,I think these look cool if I wasnt going RDS.

This looks very good, especially with MKS servo. But now to be controversial:it isn't the best way to mount your servo if performance is your aim as it does nothing to reduce skin flex!

The difference in bonding wing servos to both skins is huge. The DS installs in Joe Manor's models uses this method and he uses 'kicked' Gorilla cyano for a permenant install!

I tested loads of ways and in the end laminating epoxy thickened with colloidal silica works best for me. The servo is not prepared. No sanding or cleaning as I want the bond to break if I ever take the servo out. I also use much less epoxy now and just paint the servo with an Epoxy brush- no more. The fact that the servo has a top brace and there is much less skin flex makes it much less likely any flight loads can pop a servo out. Cyano if you want but use the right stuff as too much can buckle your wing skins with the heat it can generate.

As for the bearing mounts I am going to get some CNC'd in G10. The carbon bush or spacer I machined but it actually doesn't need to be that square (as the m3 cheesehead is kept straight by the servo spline).   

I will probably put together a kit of bits sized for popular wing servos to save anyone else the bother of sourcing/making all the components. It wont be much.

It's a simple system and not sexy like RDS but RDS has some performance and serious practical issues- somebody should tell the Emporer he has no clothes.

- somebody should tell the Emporer he has no clothes.

I seem to remember that enginetorque did a while back, but then had to duck quickly..............   

In the interest of thorough testing I removed an aileron servo just to show! Well, actually I confess I stalled a servo last night setting up throws- argh! There's no give in the system- except the servo itself.

The flap arm has also been removed. It’s too much throw for the slope. Smaller will give more torque. I’ll give that a go. 

Go on Tony. Tell them where those quality pushrods come from!

Z

Go on Tony. Tell them where those quality pushrods come from!

Z

Tony's been nicking the kids bike wheel spokes again looks like the kids are walking to school next week!

LOL Rich - not this time...

the main idea of rds is to reduce drag i thought. Of course you could argue that it makes virtually no difference, but cutting a hole in the wing skin is not as good in that regard.

I dunno about rds, i don't know if it is a fad or here to stay, tbh. It's all good fun though.

laminating epoxy thickened with colloidal silica works best for me.

Is that another term for micro balloons?

For a DIY copy of this system I checked the servo horn retaining screws for the HS 85MG servos I have.  They are 2.25mm diameter thread 

Jerry

Tom, that clevice slot was oversized and has a square of clear tape covering most of it. The Vampire linkage system is under the skin (thicker at the sub spar than my Xfire 2). But in any case it’s tiny protrusions and tiny drag penalty and that’s the important point in this install.

Love or hate it, RDS is not a fad. So many manufacturers are committed to it in particular the big servo brands. A clean wing is utopia, it wasn’t until I saw the aforementioned Vampire that I realised that RDS isn’t the only way to achieve it.

But it isn’t just that, I see so many planes through my workshop and you realise that most flyers just live with 2 to 3mm slop (those are the good uns) which if I had never seen the latest crop of DS planes and their amazing linkages I’d be none the wiser.

There are many roads to the same place but definitely the wrong road is the expensive servo mounts, sometimes with external bearings, that you think are easy to fit but actually never turn out that way and although you can unscrew your servo you only ever would if it’s xxxx’d. (Ok maybe you have to change an arm, but wouldn’t it be nice if you didn’t need to unscrew the whole servo out first).

They do nothing for performance but sure look pretty under those carbon servo covers.

Scram colloidal silica is a thickener and doesn't reduce strength as much as microballoons which will still flow out.

If the 85's are for the Wizard I dont think an external bearing is strictly necessary as the horns are so large there is less load on the servo. Tying in the servo will give you the best benefit and to remove arms in situ I have an invaluable cross point driver made at great expense;flattened brass tube with a screwdriver (from £shop) tip soldered on!   

This is all new to me and very interesting/intriguing.
Mike - I thought that for crow braking the more down movement the better?

Flat field spot landing lots of crow may be useful, but on a slope lots can get you into trouble.
I don't build for 90 degree flap, probably closer to 55 degrees, also up going flap (aileron) throw can be significantly less than the aileron throws.
Typically my models only have a small amounts of up flap available.

I have about 70 degrees of flap available but it's on the throttle stick and I don't often use all of it.

Mike’s comments raise some important points regarding small arms and geometry when using an external bearing.

Firstly, you’ll notice I use threaded L bends at the servo end. Physically, with small arms you can’t use a clevice (not at 4.5mm as for my aileron anyway) but the disadvantage is that it puts a side load on the servo and bending a perfect 90 degrees takes some practice. The 2mm pushrod material is quite important for stiffness and taking a nice bend without snapping. I’ve tested loads including various types of bike spokes (which is the joke behind previous posts).

Next, maximising servo travel is limited by the shaft/bush supported by the bearing, so for servos like the Hyperion DS095 that has approx 180degree travel, you wont get that past the bearing! I can get full travel with Sanwa 762 which is approx 110 degrees only by moving the arm one click on from vertical. This means at full down travel the arm is almost horizontal. Because the servo arm is moving in an arc you don’t get a linear response at the surface which is one of the benefits of RDS.

Increasing the gearing (smaller arm height to horn height) does make a big difference in reducing play but of course reduces the amount of throw. (Ever felt how solid the linkages are on a D40 using cheapy Ripmax SD100s? If you haven’t you should!)

So optimising all these variables would mean maximum servo throw with the smallest arm and largest horn for the given travel you need for the surface.

But that isn’t the goal here. The compromise for a near clean wing in my install is that I wanted small horns and that made it very unlikely for a mechanically optimal linkage. That’s the whole point and that’s why an external bearing and real strong servo mounting (and bearing mounting) becomes more important. My post here is to show it can be done easily, with very little weight gain and without the need for expensive servo mounts (which don't really work as well).   

Hi Tony
Thanks for taking the time for posting this insight into your "world".
One question - I have a Pike Brio Extreme with 4 x 5125mgs in the wing, a couple of the servos have quite a bit of play in them, is it worth trying your bearing method to reduce some of the play, or do I really need to replace the offending servos?

Thanks
Alex

Lets get it clear- end bearings limit the amount of side movement on the servo head and will benefit an install with high loads. It should however, help keep down servo wear and keep a tight servo tight longer.

But if it's worn already I'm afraid the benefit is much less and for most folk that's new servos as the hassle of changing gears and cases is rarely worth the cost and effort. (Not for me anyway, maybe a new one where the gears have broken). 

Nothwithstanding Mike's comments about poor installs and geometry but I guess you would have noticed that on your Brio before the 5125's had worn. .

It's pretty obvious to spot what's wrong with the servos though- just waggle the arm!   
 

Tony ,
        Surely a sloppy servo at the Gears would benefit from your Mod , it would hold the horn Tight at 1 end leaving only the gear slop in the servo the other end . Without the Bearing the slop would increase from the gears all through the distance to the horn .
       Hope that Makes sense .
                                 I think if you have 1 mm slop at the horn you probably have 0.25mm slop on the gears . If you hold the horn solid at a bearing , the slop would only be at the gear end , so .25mm .
        I may be wrong mind , but if you bend a drill and rotate it, it will wobble at the tip , if you hold that wobble at the tip the drill would not be able to deflect as much as if it was let free.

                   Just my opinion

               Bob

Bobbr, it depends on how sloppy but if your only getting 1mm play at the horn end and you can see a lot of head play then of course an external bearing will help because it sounds like you’ve got everything else pretty spot on. Most people’s idea of a sloppy servo is much more than 1mm (like 5mm + at the surface) but then it’s usually a lot more going wrong than just head play and gear slop.

Hey,

I'm trying to source allen bolts to replace servo screws on Hyperion DS095 ,which incidentally are interchangeable with Hitec 85's and 125's. Thread size is  2.2mm but 2.1 to 2.3mm will do, Thread length is ideally 14mm but 16mm will do too. I like cheese head but take anything.

Are these servos some non metric size that is messing up my search?

Speak to Ian at Modelfixings.com - he'll sort you out 

Try this ,

   Its a BA thread , maybe 8BA ?
             http://homepages.tesco.net/~A10bsa/bago.htm

              i put the metric up a min ago and deleted it , this is a better size chart for BA screws, oh this will help with ordering from modelfixings


                         Bob

Looks good Tony, but you need to paint that Xfire2, its the same colour as mine....

Hello hello - are you back in country mate??? 

Z

nah mate, back in a few months.....
Missing my flying , you lot will have to give me a round or two start at next comp...

Ah bummer dude - take care... 

Z