RV-8F

N772RW

Building, testing and operating an experimental aircraft

Elevator Control System

18 Mar 2014

Dad and I spent several days rigging the elevator control system in our RV-8 project, and tried our best to troubleshoot and eliminate any sources of control friction. We mounted the elevator hinges and bearing as recommended on the VAF forum, and removed pretty much all of the pitch friction, to the point where the elevator traveled up and down with very little force on the stick, and would “float” all the way to the stops with just a touch of the controls.

DWG 79 and Rod End Bearings DWG 79 and Rod End Bearings

Lateral movement of the stick, however, presented another problem (similar to the one described by F1 Rocket builder Tony Clinton on VAF). We were getting some binding in the forward and aft rod-end bearings once everything was tightened down.

Aft Control Mount Aft Control Mount

When the top nuts on the rod-end bearings were loose (or if we loosened either forward or aft control mounts) then the stick wpuld “float” left or right easily, but when everything was tightened down, there was considerable lateral friction (wings and ailerons not connected).

First of all, we discovered that if the rod end bearings were not perfectly aligned (i.e. parallel), they would bind on the the steel flanges of the WD-807 Control Column to which they are mounted (see photo below, note marring from the bearing rubbing).

Aft Rod End Bearing Rubbing 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 (very high-tech: we stuck a flat-head screwdriver in the space between the rod-end bearing and the steel flange, so the housing wouldn’t turn when we tightened down the bolt).

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 from the measurements given on DWG 79, but that didn’t seem to completely eliminate the problem.

Troubleshooting

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 below) 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.

Washer Shims on F-849 Forward Control Mount 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, and it’s ugly.

Remaining 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?

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