Hidden control horns: The RDS (rotary drive system) - RC Groups
Thread Tools
Dec 05, 2005, 09:02 AM
FLYER spelled I-squared-R
fliir's Avatar

Hidden control horns: The RDS (rotary drive system)

I am building/converting the Top Flite DC-3. The feature of this kit that I disliked the most (from a scale standpoint) was the exposed control horns on the flaps and ailerons.

The flaps were solved fairly easily as they are bottom hinged, but the ailerons were more difficult. I needed something for a (relatively) thin wing and center-hinged ailerons.

I was pleasantly surprised to find the Rotary Driver System (RDS). This is used with some regularity by the high performance glider builders who have thin wings and need low drag. It seems to be a great solution for general and scale use, but I can find relatively few references to it on scale forums (both here and at other sites).

Here is a thread that links to most of the sites with RDS info:


I selected the 3/32 IRF machineworks shafts and pockets, with the Kimbrough adapters. The entire setup, shipped, was $28. You can build your own and the only components that must be purchased are the Kimbrough adapters at $5 for a set of (2).

My IRF parts arrived over the weekend. Shipping was very prompt and everything is well made. I will post pictures of the installation with HS-81 servos as I proceed.

Last edited by fliir; Dec 05, 2005 at 09:11 AM.
Sign up now
to remove ads between posts
Dec 05, 2005, 06:09 PM
Registered User
747drvr's Avatar
I use that system and like it . It's now going in my 1/5 scale DeHavilland Vampire.

Dec 05, 2005, 08:40 PM
Suspended Account
i'm really intrested in this system. although i will need a smaller dia. rod (1/16) for my application. do you think this would work with a smaller servo (i.e. HS55)?

post lots of build picks!!
Dec 06, 2005, 07:20 PM
FLYER spelled I-squared-R
fliir's Avatar
I know it will work with the HS81 size (standard). There are also instructions on the Genie site (file#6) for making a custom fit adapter to fit ANY servo.

For those waiting, I am out of town on business for the next (3) days, so I won't be able to post any photos until the weekend.
Dec 20, 2005, 11:20 PM
FLYER spelled I-squared-R
fliir's Avatar
Here are the components. The Kimbrough adapters are all that you really need to purchase for "light duty" applications, but I purchased shafts and pockets from:


Here is the shaft and pocket. This is the 3/32 size, but Walt makes the sets in 1/16, 3/32 and 1/8.
Dec 20, 2005, 11:24 PM
FLYER spelled I-squared-R
fliir's Avatar
Here is the Kimbrough adapter on an HS-81. The Kimbrough's come with (5) adapters to fit most common servo splines. A pair cost $5
Dec 20, 2005, 11:30 PM
FLYER spelled I-squared-R
fliir's Avatar
To install, first install the pocket in the control surface. I centered the pocket on the hinge line, but this is NOT a requirement. In fact, I have found that the RDS method allows for quite a bit of adjustability. It is not necessary to hold great precision in either the vertical or horizontal axis.
Dec 20, 2005, 11:34 PM
FLYER spelled I-squared-R
fliir's Avatar
Next step is to install the servo. After the servo has been mounted (targeted loosely at the center of the trailing edge), align the shaft and drill a hole through the trailing edge for the shaft. The hole should be enlarged vertically, as the shaft is intended to have play in the vertical plane. By contrast, the shaft is to be supported on both sides.

The shaft is held in the Kimbrough adapter by two hex-head setscrews. I shortened the supplied shafts and ground a new flat for the setscrew. Walt told me in later communication that he will supply the shorter shafts premade upon request.

One note, the original RDS concept mounted the shaft 45 degrees to the trailing edge. The 90 degree mount that I have chosen is apparently a newer application.
Dec 20, 2005, 11:44 PM
FLYER spelled I-squared-R
fliir's Avatar
With the 32 degree shaft (angle of the bend in the shaft), the pocket is WAY oversize, so left-to-right alignment is a non-issue, perhaps 1/4 to 3/8 of an inch of clearance. Here is the net result - an aileron without any visible means of support and plenty of travel:
Dec 20, 2005, 11:50 PM
FLYER spelled I-squared-R
fliir's Avatar
Finally, since there is no connector, the pocket can slide off the shaft for removal. If hinge-pin style hinges were used, the aileron could be removed at any time for maintenance. I am using CA hinges, so this is not a benefit for me on this project.

Here is another view of the completed installation:
Dec 20, 2005, 11:52 PM
FLYER spelled I-squared-R
fliir's Avatar
The biggest surprise was how un-critical the installation was. There were no "messy" alignment issues. It just slid right together.

The IRD pockets are exceptionally nice. The spacers between the two sides are (milled?) micarta, so they should have excellent humidity and temperature stability.
Dec 20, 2005, 11:57 PM
Got H2? Fuel Cell R/C Car
Keith43221's Avatar
now those are pretty darn cool! Learn something new everyday, I do.
Dec 21, 2005, 11:32 AM
Balsa is love....
glue_sniffer's Avatar
Wow! That is a fantastic way of doing it, so clean, and amazing at how un-critical the installation is. Thanks for sharing.

Dec 21, 2005, 12:00 PM
Registered User
AmpAce's Avatar
David, great thread!

I've seen some sketchy information about a system similar to this in the past, and have always wondered about it's effectiveness. It does look good, although I have to wonder about long term use and consequent wear. Also seems like it might have a tendency to hang up if any moisture got into the pocket in cold wet weather.

Another question, how tightly does the system hold the control surface in position? Is it tight enough to prevent any surface flutter on high performance models?

Certainly looks promising where an out-of-sight contol linkage is needed.

Hope you carry through with a long term evaluation of this system when you get it flying.

Thanks for sharing your experiences by way of this well documented and photographed thread!

Dec 21, 2005, 12:18 PM
FLYER spelled I-squared-R
fliir's Avatar

This technology comes from high performance gliders. It doesn't seem to be widely known in scale circles. The well known (Dr.) Mark Dreyla has designed a specialized servo mounting system for use with RDS. Hotliner pilots report no problems with flutter. It has LESS backlash than a good ball-link system. Follow the links in Post #1, especially File 6 on the Genie site.

With regard to moisture, it would require getting liquid into the pocket and then having it refreeze. Pretty hard to do, as there is no heat in the area (no motor, battery, etc.). If it is cold enough to freeze, how do you get the water into the pocket in liquid form?


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Sold RDS Rotary Drive System slozuke Aircraft - General - Miscellaneous (FS/W) 0 Dec 16, 2006 11:19 AM
Rotary Drive System MikeHLG Hand Launch 6 Nov 24, 2004 06:54 PM
Hidden control horns & linkages lv2soar Scale Kit/Scratch Built 4 Jul 11, 2004 06:13 AM
Rotary Drive System yflint Scale Sailplanes 11 Nov 04, 2003 02:30 PM
RDS (rotary driver system) and large scale models. Aero-BearAZ Scale Kit/Scratch Built 1 May 28, 2001 12:00 AM