The complete radio control model airplane FAQ.
The object of using pull-pull controls is superior control and the elimination of several nasty tendencies of typical pushrod controls. Done in a reasonable fashion, its as easy to use pull-pull systems as it is to use pushrods, and has several advantages including light weight, high strength, more attractive appearance (or a less objectionable appearance if you prefer), and far superior ability to resist flutter.
Most of the pictures to follow are not out of focus but rather odd
looking because of the clear coating on the plane.
This is a typical servo set-up when using pull-pulls. The cable is wrapped
through a rigging adapter (standard
DuBro hobby product), and crimped back onto itself. I have never had a crimp fail.
This view is taken from the bottom-center of the plane.
After leaving the servo, the cables must exit the fuselage. This is the easiest way I know of to do it there are Teflon tubes inserted and glued through the bottom support rails of the fuselage. This method of passing the cable through a surface is used throughout the plane.
Finally, the cables attach to standard control horns through standard clevises (clevi?) using standard 2-56 rigging connectors.
There is Ackerman used on all control surfaces and it is just barely
visible in this picture. Notice how the clevis attachment point is very
slightly behind the hinge line? This is positive Ackerman, and quite necessary
for happy pull-pulls and a happy pull-pull owner. It assures that the cables
will become slack as the rudder is moved away from the neutral, or center position.
This is the complete rudder pull-pull ass'y.
As this page is being written to explain how to use pull-pulls, and
aileron installation seems to cause the most confusion, lets take a look
how to do this simply and easily.
The trick is..... there is no trick! Mount the aileron servos exactly as you would if using a pushrod. Then drill a hole at a shallow angle right through the wing, preferably up against a rib, and insert a piece of guide tubing. Once the plane is complete and covered, install the servo, fish the wire through the tubing (easy to do if its steel cable), attach both ends and viola!
The bottom view of the aileron connection. Note the Teflon tube that
transverses the wing up against the rib.
This method works on all surfaces for pull-pull use. The elevator pull-pulls look like this....
The same technique is used here. Two elevator servos, each controlling
one elevator half. The cables make an
effective twist on the way to the elevator horns. Again, a piece of Teflon tubing is used as a guide through the
wood. Ackerman is used here too as it is on all control surfaces.
So now you must be thinking.... Gallagher, is it just that easy?
Yes, its just that easy. The throttle is a bit
different though not any harder, but it does require a slightly different approach.
A standard throttle assy with pull spring return for an actuator. The spring is connected to the throttle arm, as is the cable from the servo. The spring resists the cable but only enough to provide sufficient tension. The spring WILL NOT move the servo when the power (RX) is turned off. This is a fairly critical factor in successful usage of a spring return throttle.
The throttle can be set up so that the spring either opens or closes it. Simply rotating the throttle arm around will acomplish this change. Choose whichever one (cable or spring) you believe will either not fail, or fail last and you will be as close as possible to a fail-safe system. The use of two or more springs would further increase reliability but is overkill in my opinion.
And after all these pull-pulls are assembled, it turns into a Wedge....
Any questions or comments to-