vasek

My Feedback: (4)

hi,
is it geometrically sound to cross the wires on a pull-pull (rudder)?[]

i have several parallel seups, however if crossing the wires is not an issue, it would suite better the fuse for my new plane (assuming the servo arm & the rudder horns have the same width).

thanx for helping,
Vasek

Tall Paul

I cross when I need to get rid of a big change in direction from servo to surface.
With nylon-coated cables I let them rub, with uncoated cables I put a spacer between so they can't rub on each.
What matters is the geometry at the servo horn and the surface horns.
How the cable gets there, just make it a smooth path.

rc-sport

My Feedback: (12)

Ditto what Paul said

vasek

My Feedback: (4)

thanks Paul, will do as you suggest

John Murdoch

My Feedback: (11)

Vasek.. Here's a link with SWB that kind of explains the differences:

http://www.swbmfg.com/rudex.html

John

mulligan

Wouldn't let the cables touch, though- could give you RF trouble. Separate them with a guide if possible.

vasek

My Feedback: (4)

cap-tain thanx for the great link!

george, i will use the inner (yellow) ny-rod tubing as guide/ protection, thanx for the tip.

cheers,
vasek

Lancair-RCU

Just how important is a straight run without the cables touching anything ? Ive used nylon pushrod inners as a guide at the exit holes/slots, the cable will touch the guides.

JoeAirPort

My Feedback: (41)

You can lose some of the servo's torque in friction losses from things rubbing. You use up battery power as well. On my last CAP232 120 I used short pieces of the antenna tubing where the cables exited in the tail on both sides. The cables rub inside the tubes but that's the only way I could do it without cutting huge slots in the fuse. That has worked great for about 75 flights. There are others that route cables inside teflon tubing and they bend quite a bit so they must rub. They work fine from what they say. If I had a 40% Carden, I would make sure nothing rubbed but that's a entirely different level of plane.

Lancair-RCU

Thanks, thats pretty much what Id figured.
With the power of todays servos, losses to friction will be minimal. We're losing more power by using larger than normal control horns on our servo's. Im not sure where I read it, but you loose a LOT of potential torque the further from the servo centre you get. I endeavouring to use standard servo horn spacing but will probably need to go a size up.

Red B.

Lancair-RCU wrote:
The friction losses are independent of the servo "power", its a property of the control linkage setup. It's correct that the hight torque of modern digital servos cause friction losses to be of less importance. There are other problems associated with friction in the control system. With high friction systems servo current draw will increase and also it will be more difficult to achieve proper centering of the control surfaces.

You do NOT loose torque or power by using a longer servo arm. The torque of the servo is independent of the length of the servo arm. Torque is force times lever arm. Using a longer servo arm will cause the available force to decrease. This is compensated for by using a longer control horn that will decrease the force necessary to deflect the control surface.

The good thing about long servo arms and control horns is that any slop in the linkage between the servo arm and control horn will be of less consequence. IMHO, in order to achieve a control system that works with as little slop as possible one should use maximum servo throw, the longest servo arm available and then choose a control horn that is short (or rather long enough) to achieve the desired control surface throw.

/Red B.

Chevelle

My Feedback: (2)

One way to prevent the cables from rubbing is to mount one cable on the top side of the servo arm and the other on the bottom side. That would offset the cables enough to prevent them from touching.

JoeAirPort

My Feedback: (41)

You are correct that using very long servo arms to get 3D thows gobbles up servo torque. But that is not friction, it's bad mechanical advantage. After learning a hard lesson early this spring I will unlikely use the outer most hole in the servo arm and the inner most hole on the control horns anymore. It may give you monster throws, but it will likely strip your servo unless it is grossly over sized. It's all about trade offs. And the trade off to huge throws is not having as much of the servo's torque working the control surface. If this confuses anyone, I'd say always go 1:1 as far as the distances from the hole to the axis of rotation on both sides of the control rod. That way if you have a 100 oz servo, the control surface will see the entire 100 ounces (less a little bit of friction).

As far as crossing over pull pull cables, it makes one of the cables go slack when you run the control surface to one end or the other. I did this on my CAP232 120 and it really works fine. the slack is not an issue. There are ways to fix this with crossing cables but I really didn't feel like taking it any further (it flies fine).

OldRCer

When I crossed my pull-pull wires I put a 1/4 ' riser block under one end of servo to keep wires from touching.