by Bill Gaylord » Sat Mar 19, 2016 1:49 pm
I haven't used sheathing for probably 8 years, as it's dead weight. I've used small sections in wings as aileron cable guides, but there's no need to guide the entire length. In larger models the weight isn't an issue, but it really counts with these small models. A pushrod support every 3" should be fine. For this model I'd probably only use one at most, midway between the servos and the tail surfaces. You may be able to get them to buckle if you load them by hand, but the forces in flight are much less on these small models. I either add a small ear along a former to guide the pushrod, or for rudder/elevator pushrods it's also convenient to add a light balsa plate, say 1/16"x1/8" across the fuselage, with holes in it for the 2 pushrods. I quit using light ply for the task also, as it's unnecessary weight, except for very small 1/32" ply ears glued along fuse formers, but not all the way across. A thin sheet plastic ear with a hole in it for to guide the pushrod, glued along a former, is more than ample. Clear sheet plastic has also been my choice for control horns, in more recent times. It's light, free, glues well with a light scuffing, can easily have any hole size bored out in it, and had no grain that can split. I'll probably never use light ply again for control horns. I think builders just tend to gravitate toward wanting to use wood, which is why so many use light ply. The sheet plastic can also be doubled, where you need more strength, such as cable ailerons where the cable exits at a bit of an angle, and will try to "flex" the control horn.
For this model, I don't think I used any pushrod support, with .020" wire. If you can get the tail pushrods to run straight with no flexing where they exit the fuse and connect to the control horns, then they're obviously less prone to having a "default" direction that they're always trying to buckle into. I could probably get them to buckle if I moved the rudder or elevator by hand, but there's no issue in flight.
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