Tail Wheel Shimmy

[islandmonkey]

Something that caught my eye earlier today was an advert from Flyboy Accessories.com. I took a look at the RV Rocket Steering Link:

http://www.flyboyaccessories.com/store/index.php?main_page=product_info&cPath=66&products _id=242


and noticed that the castor angle of the red tail wheel (look at the larger picture) looks negative to me. This may be an illusion caused by camera angles and I am not criticising this product, but I want to draw your attention to how bad negative camber angles on a tail wheel can be.

Having owned a Super Cub for a number of years, I went through the stage of suffering severe tail wheel shimmy when landing on tarmac or concrete. Landing on grass was not a problem and as most of my landings were on grass, I did not notice the problem until eventually, I landed on a hard runway. In the summer, if the grass was short and the earth was hard from lack of rain (unusual mostly in the UK and Germany) then I noticed a slight shimmy, which reduced if I took pressure off the tail by putting the stick forward. On hard tarmac or concrete runways, the shimmy was alway there and only reduced slightly by relaxing back pressure on the stick.

The solution and explanation to the problem I found here:

http://www.pierceaero.net/tws.php

http://www.pierceaero.net/techdata/tws.jpg

Take a look at these links to Steve and Gilbert Pierce's website. The explanation is explained eloquently.

I replaced my tail wheel spring and ensured a positive camber angle and Voila! my Cub was no longer a shopping cart with a dickey wheel.

[islandmonkey]

Bumping it up

[Bob Martin]

Anthony,
I read your post and it seems to make sense to me. I quickly went to look at my tail wheel assembly and sure enough it is....as you call it, negative Castor angle. But I don't notice a shimmy in my tail wheel, that I notice anyway. You can bet I will have my wingman watch while we taxi next time to see though. Thanks for the heads up. I'll be interested to see what others think about this
topic.

[rvmills]

Anthony,

That picture on Vince's website showing the Rocket Steering Link on the red TW is actually my tailwheel, after I replaced my original Van's TW with the SEGA TW that Vince sells.

I have not noted any shimmy at all, and I land on concrete/hard surfaced runways most of the time. However, your post did get my attention, so I looked at the links you provided, and then looked at the pics of my TW on Vince's Flyboy Accessories website, and those of Wayne Hadath's TW (who makes the Rocket Steering Link...see it here: http://www.decisionlabs.com/QuickPla.../?OpenDocument)

I wanted to look at both to see if the conversion from the Van's TW to the SEGA TW increased or exacerbated the negative camber. I'm away from home and can't compare the two TWs in person, but in both pics, the negative camber is evident, and I don't think the SEGA TW increases it. It appears to me this is the result of the angle formed by the TW pivot and it's attachment to the spring (the pivot is angled forward at the top). I do not know for sure, but I think that is a fixed (welded) angle (and if that is an incorrect statement, I do hope it gets corrected or modified by a follow-on poster).

All that being said, my TW has been very solid, both with the Van's TW and the SEGA TW. The SEGA TW does seem to steer a bit more easily, and changing to it did lift the tail very slighty, so vis over the nose is very slightly increased. With the Rocket Steering Link, the control arm is notched and a pin protrudes into the notch when the tailwheel is aligned in trail, providing a steerable tailwheel, and good control. Perhaps that also mitigates any shimmy tendency as well.

I hope I read/understood your post and links correctly, and my description above jives with what you are looking at. But bottom line is that so far, FWIW, I've had no shimmying with either the Van's or SEGA TW.

Perhaps Vince or Wayne will see this and chime in with their thoughts, and it'd be interesting to see if those with springs (versus a RS Link), have different experiences.

Cheers,
Bob

[vfrazier]

Sorry for not checking the forums often enough. We were away to Oshkosh, spending the show under a tent in the fly market. Not the best place to watch the show, but a great way to meet people.

We've got well over 250 units in the field now with no known reports of shimmy. Furthermore, regarding tailwheel problems, I believe that if everyone with a Van's type full swivel tailwheel would follow our instructions on locking pin/control arm installation and maintenance, we'd not have any more threads regarding tailwheel unlocking problems!

The photo on question is just a photo. Reality is that there are NO shimmy problems with our tailwheels.