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  #1  
Old 03-17-2014, 09:39 PM
aeropunk aeropunk is offline
 
Join Date: Aug 2008
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Posts: 82
Default Troubleshooting Control System Friction

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.

Lateral movement 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... :/
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  #2  
Old 03-17-2014, 10:07 PM
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R. Daniels R. Daniels is offline
 
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Default binding torque tube

Brian, my '8 has been flying since 2000. It appears Van's has made a change in the design of the rear support P/N F837-1. I would suggest you shim the aft support either in the front or rear two bolt holes. You should be able to achieve a finer adjustment that way vs. adding washers to the front support. Sure the F837-1 will be tilted slightly but the rod end bearing will allow for that tilt. Using thin washers you should be able to make very fine adjustments in the distance between the front and rear rod ends until the distance matches the torque tube length.
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  #3  
Old 03-17-2014, 10:11 PM
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AlexPeterson AlexPeterson is offline
 
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Default

Very clear post - nice work! If your washers do the trick, build a plate from .032" or whatever you need to shim that mount out as the washers are doing. This shim should be as big as the bracket you are mounting over it.

As for going insane, well, that was maybe established when you embarked on this journey!
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  #4  
Old 03-18-2014, 04:10 AM
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jthocker jthocker is offline
 
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Default

Quote:
Originally Posted by R. Daniels View Post
Brian, my '8 has been flying since 2000. It appears Van's has made a change in the design of the rear support P/N F837-1. I would suggest you shim the aft support either in the front or rear two bolt holes. You should be able to achieve a finer adjustment that way vs. adding washers to the front support. Sure the F837-1 will be tilted slightly but the rod end bearing will allow for that tilt. Using thin washers you should be able to make very fine adjustments in the distance between the front and rear rod ends until the distance matches the torque tube length.
I ran into the same problem, and shimmed under the F837-1 mount with thin washers.
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  #5  
Old 03-18-2014, 07:54 AM
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Toobuilder Toobuilder is offline
 
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Default

Ran into the same thing on my -8. It looks like we have a case of production tolerance stack up. (Assuming Van has any actual production tolerances in his weldments) The center to center distance of the balls defined by the control torque tube ears does not match the reality of the holes drilled in the fwd and aft mounts. I'd space the fwd mount out from the spar, but use some aluminum sheet stock rather than washers. Make it the same size as the area contacting the spar. Or make a new mount with the hole the correct dimension fwd.
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  #6  
Old 03-18-2014, 07:54 AM
brad walton brad walton is offline
 
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Default Same problem

Like R. Daniels, my rear support is also of the old design. I had binding also and corrected it by putting washers under the predecessor to your 837-1. Can't recall for sure, but I think I shimmed up the two aft bolt holes of the four which connect it to floor support angles.
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  #7  
Old 03-19-2014, 08:20 AM
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Jeff A Jeff A is offline
 
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Default Forward mount

Installed per the plans, my rod end bearings contacted the control column throughout it's entire range of motion.

I fabricated a new F-849 Forward Control Mount quite easily. If I recall, the hole had to be moved forward nearly 1/4".
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  #8  
Old 03-19-2014, 01:32 PM
Al RV8 Al RV8 is offline
 
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Default

Same problem. I had tried to fix this as best as possible and gave up. I read this thread this morning and ran down to the basement and put washers under the F837-1 at the rear locations. The WD-807 now swings freely. It took 3 thick washers on each side.

Thanks Jon!!!!!

VAF rocks!!!
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  #9  
Old 03-19-2014, 01:53 PM
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akarmy akarmy is offline
 
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so what's really going on with this? Is it that we have to make the front bracket and the tolerances are not tight with how the angle is drilled by hand? Because mine seems to swing free just fine with no changes at all when I bolted everything in to test the control hookup. Am I just lucky? Maybe the new kits have changes that solve this? Hum.
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  #10  
Old 03-19-2014, 02:02 PM
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rvbuilder2002 rvbuilder2002 is offline
 
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Quote:
Originally Posted by Toobuilder View Post
Ran into the same thing on my -8. It looks like we have a case of production tolerance stack up. (Assuming Van has any actual production tolerances in his weldments)
It only takes a very small side load on a rod end bearing for it to develop some friction. This is often the cause of control system friction in an RV (hinge points for elevators, rudder, etc.).
It is pretty much impossible to manufacture a welded assembly like this and hold a tight tolerance.
Even with brackets in question held in a fixture, once welded and cooled, it is not uncommon that the dimension between the legs will have drifted from what was designed.
It could be designed to be machined after welding, but the expense can not be justified, compared to adding a shim or washer where appropriate. In this case a thin shim behind the fwd. bracket would be my choice.
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