A few things to think about when comparing a pull-pull system to a pushrod system:
1. The details of the installation may make all the difference. It might work in some situations and in others might be a step backwards.
2. Cables will not like the relatively sharp bends that might be desired for a compact installation. (Like a mid-span flap actuator.) A prior posted comment about needing a straight run supports this. Cable manufacturer’s list the minimum bend radius for reasonable life. The larger the diameter of the cable the larger the pulley radius needed. Cables with smaller strands but higher strand counts bend more easily but the strands are damaged more easily. The same tradeoff we see with flying wires. Compare solids to 1x7, to 1x19 in the same nominal diameter.
3. You have to have a guaranteed way to keep cables on a pulley or you are going to have a fouling issue. The more guides and guards you install (that actually do something) the more friction you will have in the system. In all the full scale systems I have seen cables always have more friction than suspended pushrods that use bearings.
4. The structural/weight advantage of a pull-pull system is nebulous. To avoid “soft” controls you will need to preload the cables. That preload—however much you decide you need in order to avoid soft controls—is reacted 100% of the time thru the structure. Therefore, as one example, your aft fuselage will have compressive forces in it even in storage unless you “unrig” it after flying and before you hang it on the wall. By doing that, you risk losing your trim. So you will need a reliable way to get that back each time, or a setup that tolerates the preload for the life of the model. Springs in series with the cable, or a spring-mounted crank are some traditional solutions. But the stiffness of the spring needs to be high enough that the applied flight loads don’t overcome it. Or else you get soft controls at peak loads. The structure spacing out the controls (ie. the length of the aft fuselage) has to be even higher stiffness.
5. Loads on some components will be higher. If it took x-pounds of pull on the elevator horn with a traditional system, it will take x+preload with the pull-pull system. So there’s going to need to be a sweet spot for the tension adjustment. Looser is bad; tighter is broken.
6. The more work you put into a wire/cable system to eliminate stretch, the more your primary structure is going to have to give to prevent overloading something. In full-scale light planes, the rudder is often actuated by cables. These sometimes have preload springs, and sometimes the preload is supplied by the pilot’s legs.
7. I have not seen a full-scale aerobatic plane that used cable controls for anything but the rudder. That may have changed with more modern aircraft? A ball bearing supported lightweight pushrod gives great control stiffness with little drag.
8. I wonder how old-school conventional wisdom would reconcile this: “We can’t use R/C clevises on control line planes because our control loads are higher,” vs. “they are using pull-pull systems on a lot of R/C planes….”
I suspect that a pull-pull system on a control line plane will have some teething problems. If a simple, reliable system was demonstrated, I might reconsider building the great Panther sport-carrier design by Joe Demarco as published in Model Aviation. The high tail and the resulting push-pull cable seemed problematic....
Topic: Pull-pull control surfaces. (Read 2550 times)