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  #51  
Old 11-11-2019, 01:54 PM
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airguy airguy is offline
 
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Originally Posted by RandyAB View Post
I’m interested in how one defines “binding”? If an elevator moves freely with no discernible resistance by hand but has JUST enough friction in the system to resist gravity when held at a certain point, is that elevator considered bound?
If you can push it with a finger and it keeps moving when you remove the finger (either way up or down), it's good.
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  #52  
Old 11-11-2019, 04:10 PM
RandyAB RandyAB is offline
 
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Originally Posted by airguy View Post
If you can push it with a finger and it keeps moving when you remove the finger (either way up or down), it's good.
What if it keeps moving but stops before reaching the mechanical limit?
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  #53  
Old 11-11-2019, 04:51 PM
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airguy airguy is offline
 
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What if it keeps moving but stops before reaching the mechanical limit?
That's a judgement call - you want low friction, there is no such thing as no friction. If you have a little binding action there the autopilot servos will be continually bitching about trim as they fight it.

I had mine adjusted perfectly prior to paint - then after I had it painted and reassembled (their A&P, not me), I quickly noticed that the autopilot was not happy and started investigating. Took me a couple hours to get the spacing washers back the way they were prior to it going to the paint shop - they clearly paid no attention to that on disassembly.
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  #54  
Old 11-11-2019, 05:10 PM
RandyAB RandyAB is offline
 
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Originally Posted by airguy View Post
That's a judgement call - you want low friction, there is no such thing as no friction. If you have a little binding action there the autopilot servos will be continually bitching about trim as they fight it.

I had mine adjusted perfectly prior to paint - then after I had it painted and reassembled (their A&P, not me), I quickly noticed that the autopilot was not happy and started investigating. Took me a couple hours to get the spacing washers back the way they were prior to it going to the paint shop - they clearly paid no attention to that on disassembly.
I agree about the judgement call. My setup has no palpable resistance when I manually move it and it will continue to move if I let go under its own momentum but it doesn’t free fall all the way to the low point if i don’t impart some small amount of energy. I am satisfied with this.
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  #55  
Old 11-12-2019, 12:51 PM
rvbuilder2002 rvbuilder2002 is offline
 
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Having a low friction level in the elevator circuit is not of high importance for autopilot operation (though I imagine if it was severe enough it could impact that).

It is because it takes only a small amount of friction to have an influence on stick free pitch stability.

Think of it this way....
The elevator trim tab has a very small amount of influence on the elevator when it is just slightly deflected (as is typical in cruise flight).

It takes only a small amount of friction in the elevator circuit, that when the stick is bumped in the cockpit or the airplane flys through light turbulence bumps that cause a slight displacement, it doesn't return to the exact same position it was in before the event.
When this happens the airplane will no longer be in trim. Each time the elevator gets displaced it may return to a different static position depending on which way it move and how strong the displacement force was.

That is the primary reason that the elevator should be very free. Roll and yaw is important as well but to a much lesser degree than pitch.

I see lots of flying RV's with way more friction in the elevator control circuit than they should have.

BTW, on an RV-6,7,or 9, if the friction is very low the elevators will actually want to move towards the full up position because with the way the control column is hinged, its mass tends to make it sag downward (which moves the elevators towards the up position).
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Last edited by rvbuilder2002 : 11-12-2019 at 12:54 PM.
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  #56  
Old 11-12-2019, 04:52 PM
RandyAB RandyAB is offline
 
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Thanks for the explanation Scott. You would think that the slipstream would overcome some minimal friction to keep the plane at whatever speed it was trimmed for. So, by “very free” does that mean that it always should fall to exactly the same place when not supported? I’m sure that it can be accomplished but I also can’t help but think that once it is joined to the push pull tubes that there would be some degree of friction in that hard linkage that would be magnitudes greater than a minuscule amount of friction at the elevators.
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Last edited by RandyAB : 11-12-2019 at 04:57 PM.
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  #57  
Old 11-12-2019, 11:17 PM
rvbuilder2002 rvbuilder2002 is offline
 
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Quote:
Originally Posted by RandyAB View Post
So, by “very free” does that mean that it always should fall to exactly the same place when not supported?
No

The check that has been described by others in this thread is a good one. Flick the elevator hard and it should stay in motion for a bit (bounce off the travel stop and revers direction).

Friction can be induced at many places in the system. It is often an additive result of a small amount at a few different locations.

One very common source in the side by side RV's is the two bearings that the control column pivots on. The brackets are very stiff and if not shimmed carefully it induces a side load on the bearings which causes system friction.
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  #58  
Old 11-13-2019, 10:09 AM
RandyAB RandyAB is offline
 
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Quote:
Originally Posted by rvbuilder2002 View Post
.........The check that has been described by others in this thread is a good one. Flick the elevator hard and it should stay in motion for a bit (bounce off the travel stop and revers direction).

One very common source in the side by side RV's is the two bearings that the control column pivots on. The brackets are very stiff and if not shimmed carefully it induces a side load on the bearings which causes system friction.
Ok that is helpful. I must have missed that post in this thread.

Regarding the control column, I’ll have to go back in the plans and look. I don’t recall any special shimming being requested aside from the bearings between elevator horns and the bearings the horns articulate with.
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  #59  
Old 11-13-2019, 10:21 AM
PilotjohnS PilotjohnS is offline
 
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When i shimmed up the control column i did itper the instructions before the spar was riveted in. Now a year later i installed for a final time. One side required a “L”
Washer, the other didnt. This will be very hard once the spar is installed.
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