Hatz Biplane Association

Revisiting the tailwheel leaf spring thing that’s been discussed occasionally here before as I address a shimmy issue…

Seems most people have used an Aeronca Champ spring set, and it appears that’s what’s on my plane. The plans show the side view which looks like the Champ spring but don’t specify the width (the Champ springs are 1½” wide). The Scott 3200 tailwheel on my plane can accept a 1¾” spring, or a 1½” spring with a U-spacer spacer, which is what mine has.

Anyway, not long after I bought the plane I had a shimmy problem, and it was clear that the spring was too flat, causing the wrong pivot angle. I took it off and rebent it to a steeper angle which it improved it somewhat but still gave issues occasionally, especially when flying off pavement and/or with a passenger. It may have relaxed a bit, or probably I should have bent it more, to get the pivot vertical at full gross… I had it vertical with the cockpits empty, looking at things later it seemed to deflect about 2½° with me in the cockpit.

Last night I took it off again and bent it another 4°, roughly. Not easy to bend the leaves and get them all to match and still have the locating bumps line up. Actually it’s not easy to bend them, period. 4° may or may not be enough, so I’m also going to remake the spacer with an angle… 3/16” of taper will give me roughly 5°. The other task for this weekend is to disassemble the tailwheel and thoroughly clean it and regrease it, which by all accounts is something that should be done when you have shimmy problems. Adding a 1/8 or ¼” spacer between the spring and the tailpost is also a suggestion on the American Champion website.

But the real reason for this post is to question whether the Chap spring is appropriate for the Hatz? A Champ has a 1220# gross weight, whereas a CB-1 can gross between 1400-1600#. Mine is 1550 per the W&B (though oddly, the data plate says 1600). Either way, that’s significantly more than 1220. Looking at the ACS catalog, they have a Citabria spring that looks similar, similar dimensions, but is 1¾” wide, which would make it 17% stiffer, all other things being equal (a Citabria grosses at 1650#). Methinks that might be a better choice? Of course the main landing gear location will affect how much weight is actually on the tailwheel. There is also a Scout (2150# gross) spring, which appears to be similar but longer. I’m going to try the things described above first, but if the spring relaxes I may look at a new spring.

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.

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 way 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 similar discussion over on the Biplane forum), 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.

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

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

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

ssearle wrote: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

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.



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.

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.

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

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.