Hatz Biplane Association

I put the Citabria spring on today. Simple swap, all I had to do is machine 1/8" off the spacer blocks on the rear bolts. The springs are identical except for the width. New, the Citabria spring is bent 45° unloaded. That's about where the existing spring was at the moment, but of course the narrower Champ spring deflects more under load.

Then I went flying, it's a lot better. I can still make it shimmy if I pull the stick all the way back right after touching down on pavement, so "don't do that", but in a normal landing it's fine. Hopefully it'll hold up better.

It does seem like the way to go, at least for the CB-1. It's American Champion part number U3-1543-S. Note that early Citabrias used the narrower Champ spring, that's a different P/N.

Looks like the Luscombe spring has 4 leaves vs. 3 for the Champ and Citabria springs. My W&B says 55# on the tailwheel but that's in level flight attitude, it'll be more in 3 point attitude.

What matters, of course, is the caster angle when loaded; mine looks fine when there's nobody in the rear pit. When I land, the stick is back adding even more load to the tail, I back off on the back pressure and the shimmy reduces or stops. Where it's really a problem is when there's a passenger.

At any rate, I ordered a Citabria spring after verifying the width is the only difference. Should have it next week, so I should have something concrete to report soon.

I took a picture of my Luscombe tail wheel. I don't have shimmy but it looks like I have neutral castor angle.

[attachment=0]20211117_143332.jpg[/attachment]

The springs are 1.25" wide. There are four leafs. The weight on the tail wheel is 47 lbs.

As an aside, I ordered the tire and tube from England. It's a 2.50-3. Not a standard size. It came Royal Mail. Dessers wanted more than the tire and tube cost for shipping. Ordering it from England was a much better deal.

I looked at a CItabria today after flying. It has what looks like the exact same spring on my plane, 1.5" wide and not 1.75" wide as per the ACS catalog. Perhaps older Citabrias used the Champ spring, this is an old derelict deteriorating on a covered tiedown, hasn't moved in years... shame.

There's another newer Citabria in the hangar next to mine, but I just missed the owner today.

[quote="ssearle"]Hello Dana
My experience with the hatz tail spring (piper super cub) I had a positive tail wheel swivel angle resulting too much leverage to the rudder linkage springs and some much shimmy. I rebent the spring to 5 degrees negative tail swivel and everything tamed.
Steve HC0162[/quote]

Mine was OK, for awhile, after I increased the spring arch, but now there's less arch, leading me to conclude the spring isn't strong enough.

There always seems to be confusion on the meaning of "positive" or "negative" caster angle. According to this image, "positive" is what you want; however according to most definitions what they're calling positive is actually negative. Either way, you want what's shown in the "good" image.

[img]http://www.pierceaero.net/techdata/tws.jpg[/img]

What you want, according to American Champion, is for the pivot axis to be vertical at gross weight and full aft CT, so it goes positive (according to the image) or negative (according to other sources) when unloaded.

Hello Dana
My experience with the hatz tail spring (piper super cub) I had a positive tail wheel swivel angle resulting too much leverage to the rudder linkage springs and some much shimmy. I rebent the spring to 5 degrees negative tail swivel and everything tamed.
Steve HC0162

Took some pictures tonight, the spring has definitely relaxed from where I bent it last spring:

[attachment=0]tailwheel angles.JPG[/attachment]

The upper picture is with me standing the wing walk alongside the cockpit, the lower without me, both with a little over half fuel. Before, it was about 0° loaded and something less loaded.

Thus, I conclude it's too weak for the weight it supports. I don't know how much weight is on a Champ tailwheel, but I do know that my Taylorcraft (similar gross to the Champ) was a LOT easier to lift the tail than my Hatz. I think I'll order a CItabria spring, after I verify that there's room for it between the bolts on the fuselage (pretty sure there is).

Thanks Dana,
I’ll jot that down on the plans for when the time comes.
Doug D.

After putting the rebent spring back on the plane the shimmy went away, but there was a bit too much bend, the tailwheel wanted to turn while taxiing or using the towbar. With the tapered shim it was [b]way[/b] too much angle.

7 months later, I'm seeing shimmy again (only on pavement, and I can make it go away by relaxing the back pressure on the stick), but I haven't measured the angle again yet, actually I'd forgotten this discussion (and a [url=https://www.biplaneforum.com/threads/scott-tailwheel-question.20629/#post-249641]similar discussion over on the Biplane forum[/url]), but I definitely should... if the spring has relaxed again that'd be a good sign that I should go with the heavier Citabria spring, and that might be an even better idea for the heavier Classic.

I plan to go flying tomorrow evening, if I remember I'll look at the angle.

Dana,
Is there a follow up to this? I’d be interested in what you found out. Going to be looking for a tail wheel for the classic soon and would be interested in hearing what you found.