Control for twin rudders? - RC Groups
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Oct 04, 2012, 12:22 PM
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Rob_P's Avatar

Control for twin rudders?

I just planning out a "prospective" future build at the moment and I'm looking for feedback.

Is there a simple way of controlling two rudders from one servo?

The plane that I have in mind is the KK Invader. I built one as a child and it never did perform well as a glider but I like the shape and think that it might make a nice candidate for light electric power and an RC conversion.

Should I try and mechanically link both surfaces and use a bell crank to push / pull on one of the rudders?
Would it be better to come off both sides of the servo arm with flexible push rods to control horns mounted on each rudder?

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Oct 04, 2012, 02:32 PM
I like real wooden aeroplanes!
Sundancer's Avatar
Pushed my memory button there Rob, the Invader was the very first successful flying model I ever built, mine went rather well about sixty years ago.

It is going to be tricky to drive the twin rudders on a model as small and light as the Invader, especially with the dihedralled tailplane. This latter feature would make it very tricky to use a centre crank and rods (the dihedral is also going to complicate the elevator linkage). I doubt that snakes of any sort would work. I wonder if it might turn OK with just one rudder working? But I'm not sure even how best to drive that. The best answer might be to build it 1.5 or 2 times and use two micro servos in the tailplane.
Oct 04, 2012, 03:47 PM
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owlsabie's Avatar
Would it work to drive the elevators seperately so that do pitch and roll? Like on a vee tail?
Oct 04, 2012, 04:26 PM
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Guizzo 17's Avatar
I use a torsion bar in the rudder hinge and a pull cable that goes to the rudders servo.
see also here.
Oct 04, 2012, 04:28 PM
SB-28 UK Display Pilot
GeeW's Avatar
Pull string to actuate inner rudder only. Small elastic band to centralise rudder when servo centres. So each rudder only works in one direction.....if that makes sense?
I have this on a small canard model (circa 36"span). Works very well, so long as you renew the 'lacky bands regularly.

Oct 05, 2012, 09:59 AM
Registered User
Anyone got a copy of the R6B plan?

Regards Ian.
Oct 05, 2012, 10:19 AM
Registered User
That is going to be tricky. There is usually a lot of lost motion with twin-rudder setups.

I did think that you could hinge the whole tailplane, such that it rotates a few degrees about the normal axis. But, that might complicate the elevator' arrangement ... if you are fitting an elevator, that is.

I like the idea of the single rudder. And, I like the idea of the pull-only system, with a spring return.

If it were mine, I'd plump for the latter, I think. Very light pull-wires could be reasonably routed around the various angles and the whole set-up should be quite positive. I've certainly used flying controls that are spring-biased ... admittedly, on a much larger model ... but they worked really well.

I once knew a bloke who used a similar system to control the throttle on a Tartan Twin. Basically, he had a bit of string which pulled the throttle to OPEN; the thottle returned to CLOSED under its spring pressure. Sounds a bit crap, but it was a really good system, because it was easy to route and it completely isolated the servo from direct engine vibration.
Last edited by bogbeagle; Oct 05, 2012 at 10:25 AM.
Oct 05, 2012, 11:33 AM
Old Timer
California Condor's Avatar
This one uses pull with a torsion spring to return. Rudders move out.
Oct 05, 2012, 12:11 PM
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Rob_P's Avatar
Thank you for the feedback, plenty to think about.
I'm intrigued about treating at as V tail as I'd have to split the elevator anyway. However the standard dihedral of the rear stabiliser is probably not enough.

California, thats a very nice plane, do you have any pics that show the pushrod / control horn set up?
Oct 05, 2012, 12:49 PM
B for Bruce
BMatthews's Avatar
Like you suspect I think that the elevators are simply too shallow an angle for good rudder response. Also you'd have the fixed fins fighting to "correct" for the yaw produced by the ruddervators which would further reduce the yaw effectiveness.

The idea of tensioning the rudders and then using pull only lines is a good idea for a model of this size and flying speed. You want to arrange it so that the spring or rubber tensions are opposing each other so that when the lines come together at the servo or bellcrank arm that they are balancing each other and the servo doesn't see much spring force. Otherwise it'll load up the servo needlessly.
Oct 05, 2012, 02:36 PM
I like real wooden aeroplanes!
Sundancer's Avatar
From experience with a number of "V" tail rudder/elevator gliders I am certain that the angle on the Invader's tail is too shallow to give adequate rudder control. I found that the maximum included angle of the "V" for decent rudder effect is 110 degrees, i.e. each tail half inclined at 35 degrees above the horizontal. Spring centred rudders with pull action certainly sound the best bet on a model of this size.
Oct 05, 2012, 05:14 PM
Registered User
Have a look on
there is the BULLETIN No. 162 of SAM 1788 (Australia) January - February 2010. Go to page 8. Twin Rudder Linkage Systems. There is shown a tricky twin rudderlinkage made by Geoff Dunmore for small models about forty years ago.
Sincerely Yours
Oct 06, 2012, 01:56 AM
I like real wooden aeroplanes!
Sundancer's Avatar
Now that is CLEVER and so neat. Definitely one to file away for future use. Thanks for the lead.
Oct 06, 2012, 02:23 AM
Sic itur ad Astra
sparks59's Avatar
I agree with SD. Geoff Dunmore's method is neat and well thought out. Might even build a twin fin just to have a go!

Oct 07, 2012, 11:27 AM
Bob Imp
Originally Posted by LazyGlider
Have a look on
there is the BULLETIN No. 162 of SAM 1788 (Australia) January - February 2010. Go to page 8. Twin Rudder Linkage Systems. There is shown a tricky twin rudderlinkage made by Geoff Dunmore for small models about forty years ago.
Sincerely Yours
Very clever indeed! Shows there`s always something to learn even after nearly 70 years of aeromodelling!

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