Long post -- beware!
Dad and I are rigging the elevator control system in our RV-8 project, and we are trying to troubleshoot and eliminate any sources of control friction. We've mounted the elevator hinges and bearing as recommended by other threads
on this forum, and removed pretty much all of the pitch friction, to the point where the elevator travels up and down with very little force on the stick, and will "float" all the way to the stops with just a touch of the controls.
of the stick, however, is presenting a problem (similar to the one described by F1 Rocket builder Tony Clinton in this thread
). For reference, we're working off of DWG 79 "Control System," and we seem to be getting some binding in the forward and aft rod-end bearings once everything is tightened down (pictured below and circled in red -- click photos to enlarge).
Photo #1: DWG 79 and Rod End Bearings
Photo #2: Bearing-to-Control-Mount Measurements
Photo #3: Aft Control Mount
When the top nuts on the rod-end bearings are loose (or if we loosen either forward or aft control mounts) then the stick will "float" left or right easily, but when everything is tightened down, there is considerable lateral friction (wings and ailerons not connected).
First of all, we discovered that if the rod end bearings are not perfectly aligned (i.e. parallel), they will bind on the the steel flanges of the WD-807 Control Column to which they are mounted (see photo #4, note marring from the bearing rubbing).
Photo #4: Aft Rod End Bearing Rubbing
So we devised a way to firmly tighten both the jam nut and the primary nut on the fore and aft rod end bearings without twisting the bearing housing to one side or the other.
But we still had binding once everything was tightened down -- unless we loosened up one of the Control Mounts, and then -voila!- the friction was gone. We verified that we had the correct fore-and-aft position for the WD-807 (see photo #2) from the measurements given on DWG 79, but that didn't seem to completely eliminate the problem.
Okay, so what is going on here?
Our current theory is that the length of the WD-807 is (obviously) fixed, and the distance between the welded rod-end bearing attach flanges on the WD-807 is (obviously) fixed, but the distance between our rod-end bearing bolt holes (in the forward and aft Control Mounts) is just a bit too short
. And when we tighten everything down, there's enough compression in the WD-807 to deform the flanges enough to cause binding and friction.
Now we are experimenting with washer shims behind the Forward Control Mount (see photo #4) to move that mounting hole slightly forward. It seems to be working so far, and we may need to fabricate a new F-849 Forward Mount.
Photo #4: Washer Shims on F-849 Forward Control Mount
Finally, here is a video of everything in motion -- you won't really see any friction, but it's there. Anybody see something we're missing?
Video #1: Control System Friction
(click to play)
So here are the Big Questions:
Is the control system really this finicky? Are the tolerances really this close? If any of these bolts start to loosen and move slightly while in flight, will the controls start binding up? Why wasn't this designed to be more user friendly? What are we missing here? Most importantly: Are we going insane?
Thanks in advance to anyone nuts enough to tackle this one... :/