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RV-8 Tailwheel Steering Geometry

RhinoDrvr

Well Known Member
I recently found quite a bit of wear in my Aviation Products tailwheel attach block with 1500 hours on it. This is allowing the fork a fair amount of fore and aft motion that I?d like to stop.

I called Karen and ordered the parts. My airplane currently has a 20 degree attach block installed (see photo attached) but this puts the top of the steering shaft forward of the fork...which I gather is less than ideal. I don?t get any tailwheel shimmy, but I can feel the wheel spinning around the instant I set the tail down until I get some weight on it.

API also offers a 10 degree unit, which would put the steering closer to the correct geometry, but I understand will make steering stiffer...I?m concerned about added wear to the steering system if that is the case.

What angle API blocks are you guys running with your chain setups?

https://photos.app.goo.gl/rzkZ1fGzNB62PX6e9
 
I have an API fork that was made to fit into a standard Van's 'yoke'. (I think we need a better name for that part, like 'attach block').

Works just like a Bell fork.
 
Maybe it?s not spinning all the way around, but there?s some definite feedback in the pedals that it?s not tracking straight at touchdown.

What is the geometry of your setup, Steve? Does the pivot shaft point forward or aft? Most RV?s I?ve seen have the top forward of the bottom, but from what I gather that is incorrect.
 
Maybe it?s not spinning all the way around, but there?s some definite feedback in the pedals that it?s not tracking straight at touchdown.

What is the geometry of your setup, Steve? Does the pivot shaft point forward or aft? Most RV?s I?ve seen have the top forward of the bottom, but from what I gather that is incorrect.

The top is slightly forward of the bottom. Standard Vans attachment block.
 
I did a lot of work on a one-off motorglider that had the vertical pivot shaft of the tailwheel yoke mounted parallel to the aft bulkhead of the fuselage. It is a castoring setup with no steering links. With the tail down on the ground the shaft pointed forward about ten degrees. The wheel wanted to do anything but follow the airplane and, once it spun around and got the tailwheel forward of the post, it sat down an inch or so. That made it very difficult to get it back pointed the right direction. We went through a set of brake pads trying to figure it out before first flight. I finally remounted it to be pretty much exactly vertical with the wheel on the ground and it turned into a pussycat and very well mannered. I get it that the shaft past vertical aft isn't ideal, but whatever you do, don't go past vertical forward trying to fix it.

Ed Holyoke
 
Carl, I'm not convinced this is correct.

Both forward tip and aft tip have positive castor "trail" --wheel contact patch behind steering axis creates a restoring moment when yawed.
In addition to the amount of trail, there is also the effect of potential energy.

With the top of the axis tipped forward, the weight wants to make the wheel point straight aft. Any yaw deflection lifts the tail higher, so there is additional restoring moment trying to straighten the tail.

With the top of the axis tipped aft, the weight wants to make the wheel turn. It is happiest when turned 180 degrees, because that is the lowest height. So this vertical displacement effect is de-stabilizing.

I think Van got it about right - a slight amount of forward tip so the weight effect augments the castor effect.

It has been a long time since I studied castor shimmy, so this might not be correct, but I think shimmy occurs when the natural frequency of the yaw oscillation 'spring' from castor coincides with the natural frequency of the yaw oacillation 'spring' from vertical displacement. They feed each other like a flutter mode.
 
I did a lot of work on a one-off motorglider that had the vertical pivot shaft of the tailwheel yoke mounted parallel to the aft bulkhead of the fuselage. It is a castoring setup with no steering links. With the tail down on the ground the shaft pointed forward about ten degrees. The wheel wanted to do anything but follow the airplane and, once it spun around and got the tailwheel forward of the post, it sat down an inch or so. That made it very difficult to get it back pointed the right direction. We went through a set of brake pads trying to figure it out before first flight. I finally remounted it to be pretty much exactly vertical with the wheel on the ground and it turned into a pussycat and very well mannered. I get it that the shaft past vertical aft isn't ideal, but whatever you do, don't go past vertical forward trying to fix it.

Ed Holyoke

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.
 
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.
 
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,

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.
 
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.
 
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
 
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
 
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.

DADFC0-E8-CD1-A-430-B-8815-57477-A875-F3-E.png

92114-BE9-9901-41-E1-AD43-3-D430-EF57235.png
 
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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.

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!
 
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.
 
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.

Was the 12 ton pipe bender enough, or did you use the 16 ton?
 
Looks like I will need to do this to mine as well. did you guys use a 12 or 16 ton from HF?
 
Did you do this while it was on the aircraft? Is there enough room for the hydraulic pipe bender?

No, using the bender from HF, you definitely have to remove it.

Also, it’s difficult to make sure you get the bend just right so that it’s not cocked off to one side. Unfortunately, I ended up replacing my stinger and starting over.
 
View attachment 13686

This is the ideal angle, obviously not an RV but one of my 'toys' -)
I've read all I can on this recommendation but do we have any videos or math or physics or other data that shows that this is the correct angle? How is it possible that all the shopping cart wheel manufacturers around the world got this wrong?
 
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