Elevator pushrod geometry

[szicree]

In looking at my elevator rigging today I noticed that the elevator horn meets the aft pushrod at quite an oblique angle. On the other hand the forward pushrod meets the control column horn at a right angle. My previous build (RV) had both of these at 90 degrees, which seems to make the most sense. Having one end at a right angle and the other end not is guaranteed to produce different amounts of elevator travel for a given stick displacement depending on which way the stick is moved. Different amounts of movement also means different stick forces.

If what I'm saying is unclear, just picture the control column horn hanging vertically, the elevator horn hanging vertically, and a single pushrod. As the rod moves back and forth, the three members form a parallelogram. If we started with things as on the Kitfox, we don't get the parallelogram and things go kinda wonky.

Furthermore, my manual seems to describe the rigging process as if there is only a front and rear pushrod. Perhaps this was the case on older models, but the SS has a middle rod as well and a total of two idlers. The position of the idlers when the system is neutral is of concern to me in the same way the elevator horn is.

I'd appreciate any thoughts on this topic.

[jtpitkin06]

I think you can relax and not lose sleep on this one.


Yes, the push rod for the elevator angles upward to meet the control horn. That actually improves the elevator feel by giving slightly less leverage for down than up. The practice of having less leverage for nose down control is quite common in production aircraft. The reason is the nose already wants to go down.



When you put in forward stick you are simply removing some tail down force by reducing the load on the stab. Remember the stab is flying at a negative angle of attack.


When you pull back on the stick you add down force but at the same time you really load the horizontal stab with a high negative angle of attack. That results in high stick forces required to pull the nose up.



By having an angled push rod/control horn you can get more leverage for up and a little less leverage for down, resulting in a much more flyable aircraft.


You also will note in the rigging there is more up elevator, 39 degrees, than down. 20 degrees. So the angular displacement you mention is not quite as bad if you measure your push rod angle from the center of elevator travel and not from the take off position.



Run the stab leading edge all the way down to the full flap landing position and align the elevator with the stab. The push rod angle doesn't look so bad with the elements in this position.



In any case, rig it as instructed and sleep well.



Your other question concerns the center elevator push rod. The length is actually set during initial assembly in the fuselage section. The instructions say to screw the rod ends in all the way.



In the elevator rigging section, the manual has you adjust the fore and aft rods. If you find you can't get full up elevator travel, adjust the center rod a bit longer.



John Pitkin

[akflyer]

It also gives more up elevator than down. I think it is around 30 degrees up and 17 degrees down. At least that is what it is on the older models.