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  #1  
Old 04-05-2009, 08:11 PM
Bob Axsom Bob Axsom is offline
 
Join Date: Mar 2005
Posts: 5,685
Default Possible conversion roll to Yaw trim?

When I built our RV-6A I bought and installed the simple little roll trim accessory kit. It works fine but I keep my fuel burn balanced within 1/2 hour and roll trim is NEVER a problem - I don't even think about it. The little black handle just sits there waiting to add one more task to the removal of the seat pans. Rudder trim does not exist (although I read today in this Forum that it is an option on RV-8s) but it is largely satisfied by the wedge on the left side of the rudder. The amount of right rudder varies quite a bit between takeoff/climb, normal cruise and racing and I would like to fine tune the ball to center all the time. I have been calmly simmering this problem on the back burner for years and this possible solution just bubbled up to the surface. Has anyone disconnected the aileron link and connected it to the rudder cables via cables and pulleys behind the cover panels in the baggage area? If so what were your results?

Bob Axsom
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  #2  
Old 04-05-2009, 08:28 PM
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Ironflight Ironflight is offline
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Join Date: Jan 2005
Location: Dayton, NV
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Default

Bob,

I think I've seen some posts on rudder trim that were pretty simple spring biases on the cables, but not in the baggage area. I even seem to remember some pictures.

Louise's -6 (Mikey) has three-axis trim that is pretty neat - it was built with a servo and a tab in the left aileron and the rudder, in addition to the servo and standard tab in the elevator. Three-axis electric trim, and plenty powerful, without any spring forces on the controls. We might try to replicate it on the -3.

Paul
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Paul F. Dye
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RV-8 - N188PD - "Valkyrie"
RV-6 (By Marriage) - N164MS - "Mikey"
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  #3  
Old 04-05-2009, 09:55 PM
roee roee is offline
 
Join Date: Feb 2006
Location: San Diego, CA, U.S.A.
Posts: 770
Lightbulb Some ideas...

Haven't done it yet, but ideas for rudder trim are also simmering in my head.

Normally I'd favor aerodynamic trim, i.e. a trim tab. But in the case of the rudder, I'm actually gravitating toward a spring bias system as a way to kill two birds with one stone: 1) adjustable trim capability, and 2) retention -- preventing the rudder cables from ever going slack and potentially snagging on part of the airframe.

I haven't worked out a detailed design yet, but my thinking is along the following lines. Attach an extension spring to each of the lower outboard corners of the rudder pedals (i.e. near where the rudder cables connect) and attach the other end of each spring to a fixed structure forward of the pedal (i.e. near the firewall). So that by itself accomplishes the retention function by pulling both pedals forward with equal force, but not yet adjustable trim.

Now, to add the adjustable trim capability, attach the forward end of one of the two springs to a MAC servo instead of to a fixed structure. The servo should be mounted such that the travel is fore-aft, with the center of travel at the same location (mirror image) as the fixed structure on the other side. So with the servo at center of travel, the spring forces on the two pedals are equal. With the servo away from center, we introduce a bias in one direction.

In its simplest form as described above, this design is quite simple and requires minimal hardware. If there isn't enough room though between the pedals and the firewall, then a pulley system might be necessary (and the servo mounted elsewhere), but I would avoid that mess if at all possible.

Note also that by having one spring fixed and the other on a servo, the rudder pedal tension will be stronger with the trim adjusted to one direction than the other. But with good design (spring selection, geometry) this difference can be kept to a minimum, and I suspect that it will not be noticeable, especially given that yaw characteristics and rudder use in these airplanes is inherently asymmetric in the first place. If one really wanted to make it symmetric still, then one could use two servos (and some electronics to keep them in sync) or one servo and a pulley system, but again I would avoid such complexities if they are not absolutely essential.
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  #4  
Old 04-06-2009, 01:17 AM
Bob Axsom Bob Axsom is offline
 
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Posts: 5,685
Default More thoughts

I have seen a drawing from Van I believe that has the attachment on the fwd side of the outboard peddles with pulleys and springs and a central bias mechanism. After sleeping on my earlier proposal I realize the attachment point may have to be farther back in the aft fuselage to have a small angle and an unobstructed range of motion. The beauty of converting the existing roll trim system is it is already in the airplane and it is not being used. I saw the bungie cord method in the bay outboard of the passenger seat for right rudder only and that is clever but not the way I want to go. I also do not want to add trim servos, etc. just something that I can use to dial in what I need without adding drag - I am looking for speed as well as the trim function. More thinking required.

Bob Axsom

Last edited by Bob Axsom : 04-06-2009 at 03:45 AM. Reason: Typos & cleanup
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  #5  
Old 04-06-2009, 01:39 AM
PCHunt PCHunt is offline
 
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Location: San Diego, CA
Posts: 1,838
Default Thoughts

It would seem to me that if someone were designing a spring-based rudder trim, (or aileron or elevator trim), it would be important to have long springs that had very gradual changes in force as they changed length. And always have two springs opposing each other at neutral.

That way the springs would have minimal force against pilot input on the rudder (aileron, elevator).

All things considered, aerodynamic trim tabs seem superior, although more difficult to build.
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  #6  
Old 04-06-2009, 09:30 AM
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osxuser osxuser is offline
 
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Location: Pasadena CA
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I messed around with doing a Bias spring setup on a customers -6, and in the end decided:

1. It wouldn't be hard to make something that works

2. It would be exceeding difficult to make something that works well, and is still simple.
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  #7  
Old 04-06-2009, 10:13 AM
Bob Axsom Bob Axsom is offline
 
Join Date: Mar 2005
Posts: 5,685
Default

Quote:
Originally Posted by osxuser View Post
I messed around with doing a Bias spring setup on a customers -6, and in the end decided:

1. It wouldn't be hard to make something that works

2. It would be exceeding difficult to make something that works well, and is still simple.
Ah yes, #2 is the rub. I'm sure there is an elegent solution and I will develop my version but it is not urgent nor easy so the pot simmers.

Bob Axsom
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  #8  
Old 04-06-2009, 11:07 AM
CNEJR CNEJR is offline
 
Join Date: Jan 2007
Location: Conroe, Texas
Posts: 517
Default Been there, done that.

I developed a rudder trim system on my 6 based on spring bias. However the best & most economical one can be found on ML Blueskunk site. For $85.oo machining on it is beautiful & simple. Works well.
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  #9  
Old 04-06-2009, 11:19 AM
DBone DBone is offline
 
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Location: McKinney, TX (T31)
Posts: 209
Default chuck beat me to it

Chuck just told me about the site this past weekend. Here's the link: http://www.mlblueskunk.com/Rudder_Trim_System.html
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  #10  
Old 04-06-2009, 07:21 PM
Bob Axsom Bob Axsom is offline
 
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Posts: 5,685
Default Man! That is the sweetest piece of work I've seen

Man! That is the sweetest piece of work I've seen in a while. I'm buying it. Thanks for the revelation.

Bob Axsom
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