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  #11  
Old 03-23-2020, 08:45 PM
scsmith scsmith is offline
 
Join Date: Jan 2008
Location: Ashland, OR
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Quote:
Originally Posted by RV8JD View Post
Yes, shimmy is an instability phenomenon which involves an adverse coupling of modes, like flutter is. And like flutter, not always intuitive. My shimmy experience is limited. See this article beginning with the paragraph "There are many types of tail wheels out there."

I think Van's design is adequate for Van's standard setup. But it may not have much shimmy margin for modifications or changes that deviate from the basic design and geometry, or for wear and the resulting freeplay that occurs.

Although tailwheel shimmy events on RVs are very few and far between, they seem to occur with non-Van's parts. I had two bonafide shimmy events with the Condor2 fork, hub, and 8" pneumatic tailwheel tire, which I documented in this thread. I have since gone back to the standard JDAir full bearing TW setup with the 6" tire. Post #21 in that thread summarizes my thoughts (I never did go back and try the steering arm as I suggested in that post). That may have worked, based on other's experience).

And "Airzen" had a couple of shimmy events documented here.
Usually (not always) some damping is good. (would have thought the pneumatic tire would help?). Stiction is bad (if that is a word).

Another thing I have found on a couple of RVs that were squirrely was that the attachment block was loose on the spring rod with play in the two bolts. Replacing those with taper pins tightened everything up and solved the issue in each case.
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  #12  
Old 03-23-2020, 10:04 PM
RhinoDrvr RhinoDrvr is online now
 
Join Date: Jun 2014
Location: Lemoore (Fresno), CA
Posts: 157
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Hmmm,

Well, Karen at API is going to send me a 10 and a 20 degree block. I may just pay for both and do a little experimentation to see which is the better setup.

Even better, is there anybody out there running a 10 degree API attach block on their RV-8?
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Evan Levesque
RV-8 N88MJ (Built by Michael Robbins)
Lemoore, CA

Last edited by RhinoDrvr : 03-23-2020 at 10:09 PM.
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  #13  
Old 03-24-2020, 12:13 AM
scsmith scsmith is offline
 
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Location: Ashland, OR
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Evan,

As you experiment back and forth with the different angles, please be absolutely sure that the attachment block is not able to twist within bolt clearance, allowing the wheel to rock to the left or right.

I recommend using taper pins rather than bolts to insure that it can not rock back and forth.
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Steve Smith
Aeronautical Engineer
RV-8 N825RV
IO-360 A1A
WW 200RV
"The Magic Carpet" Flying since Sept. 2009
Hobbs 635
LS6-15/18W sailplane SOLD
bought my old LS6-A back!!
VAF donation Dec 2020
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  #14  
Old 03-24-2020, 12:25 AM
RhinoDrvr RhinoDrvr is online now
 
Join Date: Jun 2014
Location: Lemoore (Fresno), CA
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Steve,

I just rebuilt the assembly last annual, with oversize bolts to ensure a tight fit. I will do as you suggest though; last thing I want is to go off roading as a part of this experiment.
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RV-8 N88MJ (Built by Michael Robbins)
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  #15  
Old 03-24-2020, 06:20 PM
Bicyclops Bicyclops is offline
 
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Location: LA, California
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Quote:
Originally Posted by scsmith View Post
Ed, what you describe sounds like the lower end of the pivot axis is forward of the upper end of the axis. So can you clarify what you mean by 'shaft pointed forward'? It seems to me that if the upper end of the shaft is forward, then the weight effect will help keep the wheel straight.
Yeah Steve,

That's exactly right. The top of the shaft was aft of the bottom end when on the ground. Sorry if I spoke confusingly. You're right, if the upper end of the shaft is forward the wheel will tend to want to follow the plane which is a good thing, though some report shimmy. It was originally mounted parallel to the aft fuselage bulkhead which would be perpendicular to the ground in level flight, but with the tail on the ground the bottom end of the shaft wound up well forward of the top end. Taxiing was a real struggle. The tailwheel kept trying to get in front of the pivot and it would take a bunch of power and braking on one side to get it pointed the right way again. You had to get it to lift the whole aft fuselage as it came around. We stopped the plane and actually picked up the tail so we could straighten it out a few times before we concluded we had to fix it somehow. As I said, once we got the shaft vertical as it sat on the ground, it tracked perfectly and was the easiest thing in the world to taxi with no tendency to shimmy at any speed.

If I had to pick between the 2 pictures posted by RV8JD, I'd take the #1 picture labeled bad tailwheel geometry. The #2 picture labeled good tailwheel geometry is exactly what worked very poorly for us.

Ed Holyoke
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  #16  
Old 03-24-2020, 06:57 PM
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azrv6 azrv6 is offline
 
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Evan, I have been using the 20 degree API tailwheel on my RV6 for 24 years now with no problems. As you know, routine disassembly, cleaning and greasing is important. I also change the tire out about every 2 or 3 years and put the higher quality bearings in the new tires when I get them. Dave
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  #17  
Old 03-24-2020, 07:09 PM
titanhank titanhank is offline
 
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Location: Friendswood, Tx
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I have the jd air tailwheel. It had negative camber and was hard to steer. I pulled the tailwheel stinger, put a 9deg bend in it and stood the tailwheel pivot axle up straight. It is like power steering.


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Last edited by titanhank : 03-24-2020 at 07:59 PM.
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  #18  
Old 03-24-2020, 07:31 PM
jrs14855 jrs14855 is offline
 
Join Date: Apr 2008
Location: Lake Havasu City AZ
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Default Tailwheel

Post 23 is how the pivot point should be and the simplest way to correct improper angle.
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  #19  
Old 03-25-2020, 02:58 PM
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BlakeFrazier BlakeFrazier is offline
 
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Location: Evansville, IN
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Quote:
Originally Posted by titanhank View Post
I have the jd air tailwheel. It had negative camber and was hard to steer. I pulled the tailwheel stinger, put a 9deg bend in it and stood the tailwheel pivot axle up straight. It is like power steering.
Just one quick note for clarity... that is actually one of our Screaming Eagle tailwheels, slightly different from the JDAir fork.

It's fairly common for these Van's springs to take a bit of a bend over time, as many folks here can attest after the tailwheel strikes the fiberglass on the bottom of the rudder. This is the first one I've seen bent the opposite direction... if anyone else is considering doing this, just make sure you do it safely! I imagine you'll be putting quite a bit of pressure into a pretty stout spring, so you had better make absolutely sure it doesn't break free in the process!
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  #20  
Old 03-25-2020, 08:25 PM
titanhank titanhank is offline
 
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My stinger was laser straight before bending. The 9deg bend was with a harbor freight pipe bender with the smallest die installed. I bent it cold. I used the tailwheel hanging weight on the end to ensure it was plumb. Overbent to 12 degs with spring back to the desired 9 degs. Worked like a champ.
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