Sunday, August 30, 2015

stabilator cable problems

Well, i’ve had a hell of a time with the stabilator cables, and this is just a status report.  (in other words, not solved yet.)

long story short:, I think the cables are too short.  Posting to VAF resulted in several good suggestions.  Key among them was the observation from an EAA Tech Counselor that I had problems with running the cables in the correct holes last month, and he recommended that I double/triple check that the routing is correct. Unfortunately, I believe that it is.

When both cables are tightened to 45 # (max, per The Book) the top set shows 5 threads (not 3) and the bottom set shows 8 threads.  The threads are 24/in pitch, so the top cable is 4/24” =  0.167” too short and the lower on is 10/24 = 0.417” too short. 

I spent hours (2 or 3) try to ensure that I was looking at the correct cable and not get confused with the running.  I eventually did the following:  the rudder cable was disconnected from the tail region and brought forward so that the protective sleeve was clearly visible.  This served as positive ID of the rudder cable.  BTW, the rudder cable runs from the lowest to the highest hole as it moves aft.  (Just thought you’d want to know.)     Next I detached the upper/right cable from the turnbuckle and pulled it all the way back to the pulley.  I put a metal rod (from surplus piano hinge) through the resulting empty hole to identify it as hole/cable associated with the upper stabilator horn.


IMG 4627

removed upper stabilator cable, lower stabilator cable, rudder cable.


I put the seat ramp back on and reconnected the cables, but they were still too short.  Damn.    If I tighten them up to the specification of “not more than 3 threads showing” I get a tension that is off the scale and results in much too much resistance in pitch.

The reason I’m not really buying the ‘cables are too short, return them for a longer set’ is that it’s both sets of cables.  That certainly suggests that there’s a common (systemic) cause that I’m not seeing.   

I’m also not too interested in replacing the ones in the plane, simply because of the labor involved.  I’d rather use a longer turnbuckle which, conveniently,  happens to exist.  Specifically, the MS21251-B5L turnbuckle is 4.00 in long while the MS21251-B5S (supplied) is 2.25 in.  I think the extra length will address the problem nicely, and without the labor of changing out the cables.  A VAF commentator opined that I may run out of ability to later tighten them when the cables stretch with time, but I don’t think that will be a problem if the -B5L’s are threaded all the way to the midpoint as are the current -B5S’s.

I’m going to call Van’s on Monday and see if they have any other places to look for the mysterious length anomaly and either get their blessing for using the -B5L’s or get them to exchange my current cables for new (presumed longer) ones.  

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