RV-8 Tailwheel Steering Geometry

[Bicyclops]

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

[RV8JD]

This has been posted before:

http://mybearhawk.com/flying/shimmy.html

"According to my research, the pivot point of the tailwheel assembly (the king pin) should be vertical, 90 degrees to the ground when the aircraft is fully loaded."

And from Vince Frazier (Flyboy Accessories):

http://www.vansairforce.com/communit...32&postcount=7

"RV tailwheels often (always?) are leaning forward a bit. Van may have made a boo-boo on the original design that has been carried forward ever since. It isn't optimal."

[scsmith]

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.

[scsmith]

Originally Posted by Bicyclops

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.

[RV8JD]

Originally Posted by scsmith

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.

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. Posts #5,13, 15, and 21 in that thread summarizes my experience and 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.

[scsmith]

Originally Posted by RV8JD

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.

[RV8JD]

Originally Posted by scsmith

Usually (not always) some damping is good. (would have thought the pneumatic tire would help?).

I agree that damping would be helpful, but the pneumatic tailwheel tire has reduced lateral tire stiffness (compared to the solid tire), which I believe is detrimental (at least in my case).

Originally Posted by scsmith

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.

That makes perfect sense, as freeplay in the system is usually detrimental to stability.

[RhinoDrvr]

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?

[scsmith]

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.

[RhinoDrvr]

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.