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[UserError]

Hello,
I'm a bit stuck trying to configure the (twin) rudders on my RTR Severn conversion. The twin part is not a problem here.

I generally understand leverage/throw  but seem to be unable to apply this to the setup:  It appears that movement is always greater in one direction.  I've tried adjusting the arm on the servo (putting it off centre), or the arm length etc but don't seem to be abled to get a smooth setup.

I've attached a pic of the rudder arms and where the servo is located (could be moved,  want to keep it central though).

Also a pic of what, to me, should work.  The servo arm in neutral is parallell to the rudder arms (which are more parallell than shown) and the link rod is pretty much 90 degrees to the arms.  This setup still seems to favour one side more than the other - unless this is an illusion, or the small inaccuracies cause it?

Are there any fundamental principles which apply to linkage geometry?

Cheers
Jonathan
PS, Stan:  I can't do the setup you sketched as my rudder arms aren't at 90 degrees

[Peter Fitness]

Jonathan, here's a (very fuzzy) photo of the twin rudder set up on my Grand Banks cruiser. As you can see, there is a direct link from each side of the servo arm to each rudder. It's a very simple and positive arrangement.

Peter.

[dreadnought72]

It's late, so I may be wrong, but I think you're a bit doomed. 

Here's why

The above is a Flash simulation of linkages arranged the way you have them.

The top arm is the servo. The links are all the same length. The bottom arm the rudder. I'm keeping the servo throw between +/- 35 degrees or else the links snap!
The rudder throw varies between 31.3 and -42.7. It's not equal and (I think) never can be with this arrangement.

Plus, the limits (where links and arms are almost parallel) is woefully inaccurate. Peter's system (which I'm sure you could adapt) seems more mechanically sound.

Andy

[wartsilaone]

Here is the arrangement I went with. I am sure you could do something like this. It's easy and it works.

[tt1]

That's looks a very neat and practical method Ali, not seen that before, thanks for sharing.
           Regards, Tony.   

[malcolmfrary]

The servo arm, rudder arm, the linkage and a line between the servo and rudder pivot points should form a parallelogram for equal throw.  If anything crosses over, unequal throw will result.
In Peter's case, the unequal throw on each rudder is compensated for by the opposite inequality on the other rudder, so that's OK.
In wartsilaone's case, there is a matching pair of parallelograms.
In the layout shown in the second photo, just move the servo between the rudders (as wartsilaone) and keep the connection in the place as shown.

[nick_75au]

Move the link bar to the other side of the arms,same side as the servo link, the servo can still be in the end hole, the link arm goes into the next set of holes in.( if that makes sense?)

Nick

[Marc St Hilaire]

The servo arm, rudder arm, the linkage and a line between the servo and rudder pivot points should form a parallelogram for equal throw.  If anything crosses over, unequal throw will result.
In Peter's case, the unequal throw on each rudder is compensated for by the opposite inequality on the other rudder, so that's OK.
In wartsilaone's case, there is a matching pair of parallelograms.
In the layout shown in the second photo, just move the servo between the rudders (as wartsilaone) and keep the connection in the place as shown.

Dead right!

I had great fun with setting up my HDML rudders. In the end I opted for the rudder arms (tillers) facing for'd and angled in slightly. This gave a different amount of rudder throw in the turn, the outer rudder getting more angle. This works in the same way as a car's steering arms, taking account of the greater and lesser radius distances in the turn - if you see what I mean. This should give less drag and be more efficient, in theory, but I never worked out whether it was an advantage or not. But it looked dead sexy.  

[UserError]

Hi,
Sorry about the delay in replying: we've just moved out of our house so been a bit distracted from the important things...
A lot to investigate here  - many thanks.
Dreadnought's animation is spot on, this is exactly what is happening.  Will have to reposition the servo as per one of teh above arrangements.
Once again, thank you for your time to share your experience.
J