RV-8 Tailwheel Steering Geometry

RhinoDrvr · #1 03-23-2020, 12:37 PM

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

smithflys23 · #2 03-23-2020, 12:44 PM

I?m interested to hear the responses you get.
My tail wheel sometimes spins when I set it down.

scsmith · #3 03-23-2020, 12:53 PM

How can it spin around? Isn't it locked with the pin to the steering arm?

scsmith · #4 03-23-2020, 01:59 PM

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.

RhinoDrvr · #5 03-23-2020, 02:16 PM

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.

scsmith · #6 03-23-2020, 02:41 PM

Quote:

Originally Posted by RhinoDrvr

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.

skylor · #7 03-23-2020, 03:08 PM

Quote:

Originally Posted by scsmith

(I think we need a better name for that part, like 'attach block').

I have it on good authority that Van's internally refers to this part as the "po+ pipe"

.

Skylor

Bicyclops · #8 03-23-2020, 04:45 PM

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

scsmith · #9 03-23-2020, 07:22 PM

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 · #10 03-23-2020, 07:26 PM

Quote:

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.