I'm struggling with 2 different directives: Spec torque for a coarse (edit: fine) thread AN7 in tension is 19.5 to 21.2 ft-lbs (edit: correction 38-41 ft-lbs). However, the recommendation for attaching the tailwheel to a double stack leaf spring is to "not put it on too tight." Those aren't necessarily conflicting recommendations, but as we know the leaf spring's members slide back and forth on each other as the spring is flexed, which is why it's recommended to grease between the springs during assembly. One of the holes in one of the springs is ovalized with this in mind-- to prevent undue shear stress and strain on the bolt.

What I don't get is how it's okay for this sliding/shearing action to take place without some sacrificial shim to take the wear? Does the spring that's in contact with the tailwheel remain fixed while the lower one moves? I realize we're talking millimeters here butt what's the best practice for assembly of the 3200? 20 ft-lbs seems like it would prevent this slip 'n slide altogether.

This is not necessarily a Bearhawk-specific question, it applies to any 3200 assembly.

Although I'm not an expert on the 3200 itself, I believe that it is safe to say that the tail springs themselves were originally considered a wear item. Remember this system of stacked leaf springs was instituted 75 or more years ago, at the same time as standard aircraft equipment included cotton fabric that was expected to be replaced every 3 or 5 years, nitrate dope systems that would need rejeuvenation every few years (and happily incinerate your local Zeppelin), and many other items that were expected to be inspected/repaired/replaced often.

So in my humble opinion it is entirely likely that the tail springs were given a coating of grease to slide better without serious galling on a daily basis, but they were never expected to last for a long time.

A sacrificial shim would not have been used for two reasons... first they figured the springs would not last forever anyway, and second when any sacrificial shim wears thin it would essentially loosen the tailwheel bolt and cause much more serious problems.

Un-requested though it may be, my suggestion is to use a composite mono-leaf tail spring on any experimental installation. You will save several pounds of weight (weight in the worst place on an airplane), never have corrosion issues, etc. Liteflex in Colorado or Flex-Form in SC probably makes something you can cut a piece out of or modify, If you want to get fancy you can have them make a tapered spring, or you can taper one yourself.

19 to 21 ft/lbs is "barely tight" and on a Scott 3200 gonna work loose pretty quick. Where are you getting those numbers for torque?

I replace nuts/bolts every year at annual, and keep torque at 70 ft/lbs.

from SuperCub.org....

"I am curious about what this bolt was torqued to. Strange as it may seem, when a bolt is used in cyclic load like the tailwheel bolt, the higher the pretensioning in the bolt (torque value), up to a point, the less likely it is to fail under an impact load. You therefore would want to pretension the bolt to it's maximum tension allowable.

According to the 43-13 page 7-6 an AN7 bolt used in tension should be torqued to 840 inch-pounds. That is 70 ft-lbs."

http://www.supercub.org/forum/showthrea ... wheel-bolt

Gump

Yes, I was mistaken. I was using the table on 43.13 7-9 and used the coarse thread value (14 tpi) instead of the fine thread (20 tpi.) It shows recommended torque limit though at 37.5-41.6 ft-lbs for use with AN310 . Max allowable torque is show at 70 ft-lbs. I was taught that overtightening bolts usually does more harm than good, and with this "slippage" of the leaf in mind, it seems even more important to moderate the torque.

I had my 170 tailwheel apart to swap the spring once and never really looked for stuff like that. I don't recall it connecting to a double stack of springs though, just a single. Edit: I found a photo and the 170 spring only uses a single spring attaching to the 3200 head, so this issue isn't applicable if you indeed have a 170 spring back there.

That quote from sc.org though is interesting. "Cyclic load" I assume refers to a bolt being loaded cyclically in both tension and shear? Makes sense for the tailwheel bolt. I'm not worried about wearing out the spring, I'm worried about galling and wearing the tailwheel head. And not breaking the bolt prematurely from an undue strain and dropping the rudder on the ground.

My spring looks a lot like the ABI Cub/Husky spring:

It is (very) slightly more complicated than that, but any value between "good and tight" and the max recommended torque will (probably) be fine. Now what does "good and tight" mean?

Fatigue requires strain (displacement) to occur. A typical pre-load is designed to at least meet or exceed the maximum tensile load it will resist. Once the bolt is tightened/stretched, any tension from the load up to the pre-load value is, in a word, invisible to the bolt, and no cyclic tensile loading occurs to contribute to fatigue cycles.

If the pre load is not high enough, then the cyclic loads contribute to a fatigue life, and sometimes more importantly, the clamped pieces are free to move and "liftoff", creating the potential for concentrated and shock loads. A simple moment calculation of the tail wheel saddle joint, with an appropriate safety factor and a torque vs axial load proof table are all that are necessary to design for optimal tensile pre-loads.

The situation is slightly complicated by the fact that it is also a clevis joint due to slippage of the spring. The cyclic shear is unavoidable in the case you describe. Any axial pre-load in a cycled clevis joint reduces its fatigue life (some values called the principal stresses increase, which are a composite description of the stress state). That fatigue life may or may not be important.

Other practical considerations exist as well...anti-lock device effectiveness, etc. Vibration is a biggie...hundreds of G's of acceleration that are easily generated for tiny time periods during random vibration events can eventually work to loosen nuts over time. Cotter pins and castellated nuts can take care of those issues handily.

TL;DR:
If fatigue isn't an issue you are likely to face either from experience or calculation, why not tighten 'er down to the maximum tensile pre-load?

lesuther wrote:TL;DR:
If fatigue isn't an issue you are likely to face either from experience or calculation, why not tighten 'er down to the maximum tensile pre-load?

This is what I'm asking about. Is there any reason to not crank it down in light of the slippage that must occur between springs? We have tons of Maule guys here; probably the most analogous system in terms of weight and design. Ever have any issues resulting from tightening your tailwheel attach bolt to 70 ft-lbs?

Ok looks like I am a bit late to this buuuut I have a spring specifically made for the Bearhawk and the 3200 series Scott / Bushwheel. On the bolt torque You need to crank it down and check it every 10/15 hrs of flight depending on landings and re-tighten as needed. I wouild also recommend the 1/2" bolt (AN8) to keep you from dragging your tail.

Let me know if you need more info.

Wup Winn
541-263-2968
Joseph Or, 97846
http://www.backcountryconnection.com

Zzz wrote:

lesuther wrote:TL;DR:
If fatigue isn't an issue you are likely to face either from experience or calculation, why not tighten 'er down to the maximum tensile pre-load?

This is what I'm asking about. Is there any reason to not crank it down in light of the slippage that must occur between springs? We have tons of Maule guys here; probably the most analogous system in terms of weight and design. Ever have any issues resulting from tightening your tailwheel attach bolt to 70 ft-lbs?

You want that thing tight... it takes a hell of a pounding. Just don't over-torque it, or wet-torque it, as the bolt will fail from low-cycle high-stress fatigue, cracking at the threads inside the nut. But equally, any slack will cause the bolt to feel a lot more stress and strain. This is a bad thing.

Incidentally, I've had advice not to grease that spring. I would be interested to hear more about that.

Hottshot wrote: I wouild also recommend the 1/2" bolt (AN8) to keep you from dragging your tail.

Wup, does that require you to drill out the cast tailwheel head? It's currently bushed (with a very thin bushing) at about 0.44".

Battson wrote:Incidentally, I've had advice not to grease that spring. I would be interested to hear more about that.

Whomever said that probably is thinking that the friction between the springs will resist the sliding and shearing action.

Zzz wrote:
Whomever said that probably is thinking that the friction between the springs will resist the sliding and shearing action.

I use a plain old hose clamp at the aft end of the stack of springs, just forward of the 3200 head. Nothing moves sideways, and the springs flex just fine.

Gump

If I remember right somewhere around 41 is what I use. I'll check tomorrow

Tighten er up till it starts getting easy again, then back it off a quarter turn.

Zzz wrote:

Hottshot wrote: I wouild also recommend the 1/2" bolt (AN8) to keep you from dragging your tail.

Wup, does that require you to drill out the cast tailwheel head? It's currently bushed (with a very thin bushing) at about 0.44".

No, it is already a 1/2" hole just punch out the sleeve and you are good to go. I haven't broken any of the 1/2" bolts

River rat wrote:Tighten er up till it starts getting easy again, then back it off a quarter turn.

That suggests you want to damage the threads, by permanently deforming them, then loosen the nut off to some arbitrary value?

See below - once you go past the little "bump" the metal has been permanently damaged. That increase in strain (reduction in slope) is what makes a bolt easier to tighten once you pass a certain torque.



Z - one other comment I would add: I can't speak for the 3200 tailwheel, but in terms of the BH fuselage Bob has cautioned me against using larger bolts than designed. Basically, he says you just transfer the stress to a different part of the airframe, causing damage elsewhere. The bolts are intended to be somewhat sacrificial to save structural stress elsewhere.

In other words, it's easier to replace a damaged bolt every 200hrs than re-weld the bottom tail-post cluster at 1000hrs. Just repeating what I was told, don't ask me to explain the stress mechanisms at work.

Hottshot wrote:
No, it is already a 1/2" hole just punch out the sleeve and you are good to go. I haven't broken any of the 1/2" bolts

Sounds like a good call. Thanks Wup.

And EZ Flap, I didn't miss your suggestion of a carbon fiber spring. I like the idea, especially the weight savings, but I also don't want to be a test engineer on that one and have to rebuild my rudder... neither of those outfits you suggested had anything purpose built, which means it would be a custom job and new territory for everyone.

Zzz wrote:And EZ Flap, I didn't miss your suggestion of a carbon fiber spring. I like the idea, especially the weight savings, but I also don't want to be a test engineer on that one and have to rebuild my rudder... neither of those outfits you suggested had anything purpose built, which means it would be a custom job and new territory for everyone.

Be careful what you wish for here...

Being exp you can most certainly address cg ahead of time with ease. Not doing so will have you adding weight somewhere to make the CG return to an acceptable position.

I am all about keeping these things light, but light without consideration to CG is an exercise in futility... Almost everyone of these little things we fly loads it's baggage aft. Consequently they are by design, nose heavy when light to leave an acceptable CG envelope for heavy ops. This means when you are light (where you want to be when you are really having fun) you are already nose heavy. Losing weight off the farthest possible arm aft is going to magnify this trait beyond what you'd imagine. I don't know of any really trick cubs out there that haven't had to address this issue by adding weight....

Take care, Rob

The BH 4-place is known to be tail heavy. The one I have flown felt that way on takeoff. Put a baby bushwheel on it and the issue is compounded. We'll see... some things only come out in the wash (or the scales as it were.) Engine and prop are still TBD.

Z,

Thomas Deitrich in Germany makes a composite tailspring, which have some field experience. PM me if you want more info. Price might not be within your budget, though....it's not in mine.

MTV

Rob wrote:
Be careful what you wish for here... ...but light without consideration to CG is an exercise in futility... nose heavy when light to leave an acceptable CG envelope for heavy ops... cubs had to address this issue by adding weight....

Rob's absolutely correct of course. Making something lighter is usually a good thing, but if it causes some other problem then it could become a can of worms. In terms of old traditional aircraft thinking, every pound you can take off the tail allows you to take three or four pounds off the nose. But that is static thinking, based on the assumption that you can remove weight from wherever you have to in order to keep the CG where you want it. As Rob pointed out, that may not always be the case.

Regarding this specific idea of using a lighter tail spring, and then possibly needing to put tail weight back in to compensate for the increased nose-heaviness, there is still a potential benefit to be had. It might be possible to add that tail weight back in with something more useful than just dead weight.

One potentially smart move that you can make as an EXP builder is to build or outfit the airplane so that your emergency kit, your reserve water or MRE's, or your 500 rounds of survival ammunition, can be moved fore and aft quite some distance. Like a long ski tube, or a separate long ski tube in addition.

So when you are flying light or solo, and your airplane would otherwise be nose-heavy, you can balance the airplane using stuff you would already be carrying. Slide the ammo pack or whatever all the way aft in the tube, and you can fly at 75-85% of aft CG (or wherever your airplane flies best). When you load up the plane with the wife and in-laws, then you can move the emergency kit forward so your CG is still safe.

Or, when appropriate, you can REMOVE that 500 rounds of ammo, and save that extra five or ten pounds altogether.

But you might want to save just one of those rounds if it's a long flight with the in-laws