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Rotary drive system

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selwyn smith21/09/2016 09:00:00
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Has anyone used the IRF Machine Works rotary drive system for flying controls? I am upgrading a YT models 108" Stinson Voyager and would like to remove the two elevator servos bolted to the external fuselage. I was thinking of using a rotary drive for the elevators to give a more scale appearance but have concerns over the reliability of the drive given the size of the control surfaces and the available space for servos.

MattyB21/09/2016 12:38:23
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1868 forum posts
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I have not ever owned a model with RDS, but as a glider guider I know lots of people who do and have seen many of these aircraft (from 9oz DLGs to 3m F3X ships) in action. Observations:

  • It clearly can be made to work, but can be fiddly to setup initially to get slop free, accurate surface movement.
  • They are more easily damaged by a bump on landing. This is particularly true of the flap servos on an F3X model which are often extended a long way on landing with crow brakes.
  • If you do have an issue maintenance is far harder on the field than with a traditional linkage, particularly on the slope with cold fingers!
  • Max throws tend to be more limited to ensure accurate operation - should still be fine for a scale model like your Stinson though.
  • I have never seen RDS in an IC powered model. I suspect this is not an accident - I have severe doubts as to how the mechanism would stand up to high levels of vibration, particularly in a model with a large capacity single cylinder petrol engine. Of course your Stinson could be electric, in which case this is much less of a concern.
  • I am not even sure RDS linkages are available in sizes suitable for such a large model as yours (most are designed around premium thin wing servos from the likes of MKS and JR going to pockets mounted in wafer thin hollow moulded aileron and flap servos).

Talk to IRF themselves and see if they have any recommendations, but if you want a less visually intrusive solution I would just move the servos inside the fuselage and run a short, straight wire linkage to a traditional horn on the underside of the surface. Simple, reliable and well proven.

 

Edited By MattyB on 21/09/2016 12:40:20

MattyB21/09/2016 12:44:04
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1868 forum posts
28 photos

Just looked up the model, I see now why you want to do this - that high mounted tail does make the linkage and servo very prominent...

Edited By MattyB on 21/09/2016 12:44:24

Bucksboy21/09/2016 18:15:23
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106 photos

I used it in my 72" Laser powered Tony Nijhuis Spitfire for ailerons and they worked perfectly. As you say, no external linkages so it looked great. I can't remember how much movement I managed to achieve but I never needed much to fly smoothly. The pockets in the aileron were lined with sheet fibreglass to create a smooth friction free surface. I had to get a friend to make the 'top hats' that were bolted to the servo arms. These were turned in aluminium and he made space for two grub screws to hold the angled piece of wire coming out from the servo. The plane still flies and never needs any maintenance for the ailerons, I will use the system again. I've not seen them in other planes at my club, mainly because most don't build anymore and it is more effort.

selwyn smith21/09/2016 18:37:14
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4 forum posts

Thanks everyone, IRF do a 1/8th diameter drive rod in hardened and tempered steel which should be man enough to drive the elevators. I'm not really interested in large control throws as the prototype was not seriously aerobatic and I do like to see scale models flown in a scale manner. I'll email IRF and see what they recommend.

Danny Fenton21/09/2016 19:28:09
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8980 forum posts
3866 photos

You can make your own as I did

the information starts at the bottom of this page, remember you don't need an adaptor to make RDS work

Cheers

Danny

Simon Chaddock21/09/2016 20:33:19
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5306 forum posts
2785 photos

If I were doing it I would mount the elevator horn inside the fin, fixed to the torque tube with a downward link to the servo in the fuselage. I would not be easy to get at once it was done but then I am not afraid to build things in. wink 2

Danny Fenton21/09/2016 21:18:39
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8980 forum posts
3866 photos
This video might help explain how RDS works for those that haven't grasped it yet
Works extremely well and is my favorite way to operate hidden ailerons
Cheers
Danny
MattyB22/09/2016 02:38:51
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1868 forum posts
28 photos

I still have never seen an RDS that didn't develop slop over time. The "Roll Drive" (also known as "Linear Drive System"pioneered by RTG looks much better to me, with a beefy carbon arm that is supported on both sides linked to a tiny connector on one surface of the control surface. Should do much better in terms of reducing slip over time.

Roll drive instructions

Edited By MattyB on 22/09/2016 02:46:12

Danny Fenton22/09/2016 08:57:08
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8980 forum posts
3866 photos

That is interesting Matty, I cannot see how that is going to be any better than a conventional servo arm as it is still relying on a small radius pin at the control surface, or have ai missunderstood how it works? I have not used RDS in competition but I know some serious speed gliders are and hadn't heard of wear creeping in? I guess it depends what you make the pocket out of. I use Kitchen worktop laminate and it has never shown signs of wear.

I know several of my friends in the US are using RDS on very large scale models as it can give a large throw and keeps everything well hidden. The surfaces are also easily detacheable using RDS as they are not physical joined.

There is another method which uses a sliding fulcrum, but that has limits in the wing must be a certain thickness and the throw is very small, so probably only suitable for high performance gliders.

There was also an actuator, something called a SWINGEE which turns a push pull 90 degrees into a twisting load, these do seem to suffer from early life wear and you need to be careful. Though modern units may be much better in this regard.
Cheers
Danny
MattyB22/09/2016 11:30:46
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1868 forum posts
28 photos
Posted by Danny Fenton on 22/09/2016 08:57:08:

That is interesting Matty, I cannot see how that is going to be any better than a conventional servo arm as it is still relying on a small radius pin at the control surface, or have I missunderstood how it works?

No, you're quite correct - essentially it is a additionally supported, streamlined version of a conventional linkage. There is no advantage over a conventional linkage other than it's low profile, and it does need to have a very accurately manufactured pin and connector at the control surface end. However this seems to be easier for some manufacturers to produce consistently than RDS (see below)...

Posted by Danny Fenton on 22/09/2016 08:57:08:

I have not used RDS in competition but I know some serious speed gliders are and hadn't heard of wear creeping in? I guess it depends what you make the pocket out of. I use Kitchen worktop laminate and it has never shown signs of wear.

The guys using them in F3X models are not generally making and installing the RDS any more; it tends to be built in at the factory. I saw a TUD Freestyler (3 or 4, can't remember which) a year or two ago; the owner was very grumpy that having waited in line nearly a year to get it several of the linkages were exhibiting significant slop after only a few flying sessions. One flap surface had 5-7mm! Not what you want from a high cost piece of essentially irreplaceable unobtainium.

Posted by Danny Fenton on 22/09/2016 08:57:08:

I know several of my friends in the US are using RDS on very large scale models as it can give a large throw and keeps everything well hidden. The surfaces are also easily detachable using RDS as they are not physical joined.

Clearly in my first post I was incorrect, and there are people using RDS successfully in large models with IC powerplants; apologies. I would still maintain though that for the vast majority of people RDS will create more pain than it solves because of the extremely high accuracy of manufacture and installation that is required to make it work well for a long time. If you can do this great, but I suspect most of us (myself included) do not have the patience or skill to create a really tight installation.

Danny Fenton22/09/2016 12:04:25
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8980 forum posts
3866 photos

Hi Matty, yes I get your drift, I also would be very cross having waited so long and they wore out that quickly, not good at all. But the concept, properly executed, should be very reliable.

I am not sure that I would agree about them being difficult to make accurately. I use two short lengths cut from the same piano wire as the actuator and use these as spacers to set the pocket gap. the rest apart from the bend being on the hinge line is pretty straightforward.

The biggest issue is the top hat adaptors for the Servos, as you can see i use the std plastic servo discs to make life easier, (negates the adaptor being for a specific servo). However the top hat still needs to be manufactured on a lathe, which rules out many modellers.

Interesting discussion, thanks for joining in. Sorry Selwyn if we have strayed off track

Cheers

Danny

Edited By Danny Fenton on 22/09/2016 12:05:14

MattyB22/09/2016 13:06:49
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1868 forum posts
28 photos
Posted by Danny Fenton on 22/09/2016 12:04:25:

Hi Matty, yes I get your drift, I also would be very cross having waited so long and they wore out that quickly, not good at all. But the concept, properly executed, should be very reliable.

I am not sure that I would agree about them being difficult to make accurately. I use two short lengths cut from the same piano wire as the actuator and use these as spacers to set the pocket gap. the rest apart from the bend being on the hinge line is pretty straightforward.

I think my view may be unfairly coloured by the fact a lot of the installs I have seen (and the only one I have personally worked on) were in the earlier days of RDS on DLGs. Tiny servos, micro linkages and thin control surfaces linkages makes it very difficult to get an accurate operation. I can see that on larger models it might actually be a lot easier to setup.

Posted by Danny Fenton on 22/09/2016 12:04:25:

The biggest issue is the top hat adaptors for the Servos, as you can see i use the std plastic servo discs to make life easier, (negates the adaptor being for a specific servo). However the top hat still needs to be manufactured on a lathe, which rules out many modellers.

I like that method a lot, looks very robust. Not an option for tight installs, but I can see on larger models were weight and space are not at a premium it will be great. Certainly infinitely preferable to getting couplers directly onto tiny servo output gears!

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