JohnW

My Feedback: (6)

There is nothing wrong with unequal length arms on a pull-pull system. I do it all the time for the same reasons I don’t match servo arms to control arms on push/pull systems. I.E. reduction.

OK, technically I suppose one does reduce the possibility of flutter on a perfectly matched pull/pull system. However, getting perfect geometry is difficult. Not only must the arms be equal length, the attach point must be exactly in line with the hinge line, and this position must be maintained across temperature/humidity changes, flight conditions, etc. The scenario of having the non-working line go tight is really bad IMO and could introduce flutter. Because of the difficulty of getting perfect geometry, I feel a slight offset such that non-working line goes slack is justified as it ensures you won’t creep toward the bad geometry condition. I don’t mean super sloppy slack, just a little such that at full deflection the non-working line is no longer under any tension.

Think about flutter and how it is caused. If you get flutter when a rudder is under heavy deflection, the problem isn’t with your control system. Something else is messed up, such as hinge line leaks, turbulent flow caused by poor rudder/fin mating, etc. However, at neutral, the control system must be tight because the aero forces on the rudder are fairly equal left/right. At neutral, both lines “work.” This is why a tight center is so critical. If your center is loose, you could easily get flutter. Think about flutter on a push/pull system becasue of sloppy links (same basic idea as pull/pull.) It doesn't occur at full deflection. The sloppy link goes under tension or compression and firms ups. It is when the surface is at neutral that the sloppy link causes the problem.

I’ve set up a lot of planes this way (non-working line goes slack), including 200+ MPH ships and giant scale 3D/IMAC ships. I’ve never had flutter with the non-working line going slack setup. However, I did have a giant scale 3D/IMAC ship that had the “sloppy center” issue due to offset geometry in the “bad” direction. No matter how I tried to tighten the cables, I never could get a crisp center because my geometry was off just a little. I never had flutter, but the plane had tracking (wander) issues in yaw. In that plane, the arms actually were exactly the same length as I used a ball bearing bell crank and matching control horn. The issue was caused by the control horn not being exactly on the hinge line. It was only off by about 1/16” of an inch (on a 35% plane), but boy did it make a big diff. I eventually cut the arm out of the rudder and reattached slightly behind the hinge line so the non-working line went slack and the problem went away.

Cheers