The complete radio control model airplane FAQ.
How do I set up pull-pull controls on my radio control model airplane? I'd like a pull-pull rudder, but can you do a pull pull elevator or pull-pull ailerons?
Here are photo's by Brian
Here are some by Bob Atkins
Photo's by Roger
Photo's sent to Roger
Advantages and descriptions of PULL-PULL controls--taken from numerous posts by Brian Felice on the rec.models.rc.air newsgroup.
There is no fundamental disadvantage to using pull-pulls on any surface.
In fact, there are distinct advantages. The problem seems to be that pushrods are more intuitive and easier to master. Pull-pulls are not difficult but do
require a better understanding of what is needed as well as more planning.
The easiest, and IMO best way to use a pull-pull system on ailerons is to pretend that you won't be using them while building the plane. The one consideration is that there is a fair bit of distance between the servo and aileron; this is usually avoided by those using pushrods due to the rods flexing but it isn't a consideration when using cables. Finish the wing and mount the servo in the well as usual, and mount the aileron control horns as usual (but one on each side, top and bottom). Get a piece of 1/16" brass tubing from the hobby store, sharpen one end from the inside, mount it in a drill or Dremel-type tool (being extremely careful not to let the end 'whip'!!!) and drill a hole about 2" from the aileron, and in-line with the servo arm, toward the lower control horn on the aileron. Leave the tubing in the hole and thread a piece of cable through it inside. Remove the tubing but leave the cable threaded through both holes. Now, take your sheath (I use 1/16" OD X .030" ID Teflon tubing), slide it down the cable and work it through the holes. Glue it in place and trim it off flush with the sheeting.
There must be sufficient room between both ends of the tubing exit and the clevis at each end. About 1 1/2" is required unless you forgo the clevis and adjustment (I do this in tight places on .40 sized planes).
The rest should be self explanatory- thread a piece of wire through a tube and connect to each end using a cable rigging pin and clevis. Adjust for centering and tension. If at any time any part of the system needs servicing or replacement, everything is easily accessible. It's not necessary to use Teflon tube, any small plastic tubing will work. Sullivan sells a pull-pull kit that comes with a length of rigid tubing; this works quite well and is easier to glue (like a bad joke, it's hard to glue the world's most slippery substance).
Tubing is needed anywhere the cable passes through a surface, otherwise it would quickly cut through that surface. The tubing need not extend all the way through to the other side but it's far easier to do this than to use two very short pieces. Besides, it would be almost impossible to thread the cable through a couple of inches of space and then through the opposite guide tube- sort of like threading a needle from a foot away.
Pull-pulls are excellent at preventing flutter. I've seen it cure a nasty
case of flutter several times after all the other 'fixes' failed. Pushrods, no matter how short
or rigid, always have some clearance at the connections. This is how flutter starts--a very small oscillation is allowed by accumulated clearances and tolerances in the
control system. Pull-pulls eliminate this because they are under tension and this
eliminates any free play at the control surface (other than what's in the gear train
of the servo).
Bob Adkins ideas and mine are similar but vary somewhat; he mounts his in the wing sideways and uses Kevlar cable. [If you do use Kevlar, just make sure that there are no wear points that will fray the line and cause it to break. Kevlar has the advantage of being non-conducting, so it can't affect your radio signal.] I mount mine upright and use steel cable. I don't mean to infer that Bob's ideas and methods are wrong, just different. In fact, I believe both systems have their advantages.
I use a pull-<spring return> arrangement for throttle control. It's superior to any other type of linkage I've tried due to -0- play and no 'fore- aft' loading changes. Check any throttle you currently have; move the TX stick one click up and down and note the throttle's movement- there will be some play and the throttle will end up in a different position at the same TX stick position depending on which way the throttle was moved. There is none of this with a spring- cable set up.
The system is set up identically to the way any automotive throttle is- a spring holds the throttle hard to one end and the cable overrides this to either open or close the throttle. I have no doubt that many arguments would ensue should I say which way I set mine up; choose which ever device, cable or spring, you believe to be most reliable to close the throttle and this should be the failure mode. (how's that for skating the issue? <G>) It's easiest to do on a 4 cycle engine; simply use one long bolt for the right-front engine mount. Attach the spring to this via an open loop and the other end to an unused hole in the throttle arm. The cable attaches to another hole in the throttle arm and pulls against the spring to either open or close the throttle (notice the skating again?). A 2 cycle would be set up backwards to this; use a compression spring with the cable through it. The spring pushes against the throttle lever with the cable inside pulling it the other way.
It is often said that this system will cause undue and constant stress on the throttle servo. It would also seem obvious that it would increase current draw on the servo. In practice, this is not true. A particular spring is chosen that will firmly move the throttle but is still light enough not to move the servo at full deflection. When the RX is shut off, the throttle will NOT move due to the servo's internal resistance. While powered, the spring cannot move the servo through the mechanical dis-advantage of the gears. So the end result is.... no effect on the servo or airborne battery. It will take more current to move the servo against the spring but less to move with it.... this is an wash in terms of electrical power used.Brian F.
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