Need input: Is my Acme Stinger movement excessive?

[tedwaltman1]

I’ve had my Acme Stinger now for exactly a year. I have 283 landings on my Acme Stinger, 95%+ of which are three-point. Awesome design.

• After my first flight (5 landings) Feb 2022 with the Acme Stinger I jacked up the tail
• Goal was to insure all nuts/hardware remained in proper orientation, with nothing loosening or needing further attention
• I double-checked all bolt torques were to AC 43.13-1b, page 7-9, standards (link). I noticed that torque recommendations in Acme’s on-line install manual exceed AC 43.13-1b values
• With the tailwheel off the ground, I see system movement:

• Stinger rear orientation view:
• Stinger front orientation view:
• Stinger-mount bolt Left:
• Stinger-mount bolt Right:

Here are my original Feb 2022 & current Feb 2023 Mitutoyo caliper measurements:

• AN6 bolt design specifications (link) indicate a diameter of 0.371” - 0.374”. When mounted, my AN6 bolts were 0.372” pic link
• When installed, my forward A-arm assembly holes were 0.380” yielding a clearance of 0.008”. pic link
• Now, a year & 283 landings later, my forward A-arm assembly holes have elongated to 0.384”, yielding a clearance of 0.012” pic link
• When installed, my rear A-arm assembly holes were 0.379” yielding a clearance of 0.007”. pic link
• Now, my rear A-arm assembly holes have elongated to 0.383”, yielding a clearance of 0.011”. pic link
• When installed, my rear clevis holes were 0.384” yielding a clearance of 0.012”. pic link
• Now, my rear clevis holes have elongated to 0.387”, yielding a clearance of 0.015”. pic link

I spoke to Acme staff after sharing the above video links via email. The response, “… don’t see an issue with the videos you sent.”

Do others have movement in their stinger assembly? Am I nuts to be concerned?

Suggestions? I’m not inclined to redesign their parts in CAD for an interference fit and pay to have custom CNC parts made.

=========
My assembly notes:

Stinger mount: When I Installed my stinger, I had to make a custom shim to properly line up the fuselage with the stinger mount. Your experience may vary. Pic link.

[Scott A]

I was thinking of order the acme - it will be interesting to see what others find. To me that is a lot of slop and would drive me crazy. Seems maybe a hardened bushing is needed, hmm.

[tedwaltman1]

I don't think there is enough edge distance to drill out the holes and add bushings?

[frequent_flyer]

I would have expected the stack of support bracket, spacers, rod end bearings, to have been in compression when the through bolt was tightened and for there to have been zero free play.

If there was any free play at initial assembly it can only get worse as the holes wallow out.

Edit to add - did you consider shimming the stack to zero end float before securing the through bolt?

[cubdriver2]

Maybe zero tolerance bolt?

Glenn

[ak49flyer]

Welcome to the world of moving parts, I guess. Thanks for the detailed report. I’ve not had any experience with acmes, but the T3’s I maintain are sloppy every annual and I’m constantly tightening bolts and monitoring slop wearing the holes larger. I’m supportive of the new tech, but so far it seems far more maintenance intensive than the old leaf springs, and I’m not sure I’m sold…

[skywagon8a]

There appears to be excess clearance between the bolt and the Heim ball. It the bolt undersized?
Also as frequent suggests, the stack should be shimmed to zero play in order to be able to tighten the bolt without stressing the ears.

[TcraftF21]

A proper assembly would have the through bolt tightened to compress the ears and the spacers against the Heim joints so that all/any movement is within the joint. The only real cure is to sleeve the through bolt holes to get rid of any slop. If that bolt was tight originally that would be less of a problem but still not correct. My thoughts on 50 years of engineering.

Jim

[TcraftF21]

On a second look, Acme should have made a custom axle to the correct diameter of the joint and machined the housing to fit. Using a bolt appears to be the cause of the problem.

[jrussl]

You may be able to find some NAS bolts to use. They are made “to size” and not a few thousandths underside.


Sent from my iPhone using SuperCub.Org

[frequent_flyer]

Just one person's opinion -

Shim it, loose assemble, put the weight of the aircraft on the tail wheel, then torque all bolts to Acme spec. Jack the tail and check for free play. I doubt there will be any.

Check again afer a few landings in the rough. It there is still no play then don't worry about until next annual inspection. If there is play then look for a more complicated solution.

If you used Acme specified torque on initial assembly then ask for free replacement of all worn parts.

(Having the weight of the aircraft on the tail when you torque the nuts will ensure all the slack is out of the assembly. Only the weight of the tail wheel when in the air will tend to move it away from that position.)

[Steve Pierce]

My assembly notes:

Stinger mount: When I Installed my stinger, I had to make a custom shim to properly line up the fuselage with the stinger mount. Your experience may vary. Pic link.

Have installed them on FX3s and not had to shim. Will have to check the play on that main bolt next time they are in.

[Oliver]

I’ve seen a similar issue with Acme main gear bolt/heim tolerance, resulting in a “klunking” sensation when taxiing over uneven terrain

[Steve Pierce]

I’ve seen a similar issue with main gear bolt/heim tolerance, resulting in a “klunking” sensation when taxiing over uneven terrain

But it didn't look like any play in the heim to me, just in the frame.

[tedwaltman1]

There is significant movement on both shock-mount bolt assemblies —plus— the main bolt assembly.

Look at the caliper pictures re original and current bolt-assembly clearances.

[DENNY]

That is a fair amount of movement and I suspect will only get worse with use until failure. Normally I would simply say stop tailwheel first landing but I understand with the slats and slow speed possible it works good to do it that way. I am not a big fan of the shocked tail spring assembly of either type mostly because the tend to hurt the AOA on takeoff. They do help protect the airframe from constant tail first landings especially when heavy. I have seen a new tail section broken in just a few years of heavy work by a lodge cub that routinely straightens out tail springs even with a wide tailwheel, so just changing to an Airframes spring may simply move the stress more to the airframe. I suspect that part is powder coated so not to easy to strip and do a close inspection for small cracks. I would consider hand reaming or mill and putting in a steel sleeve then shim as the others recommended and see how it works. If it keeps wallowing out I would go with a leaf spring. It will improve you take off AOA and with a little change in technique you can still get slow ( tailwheel below the mains) but pick it to let the mains absorb all the shock. Expecting small aluminum part to take a high impact beating and not deform might be overly optimistic.
DENNY

[TcraftF21]

Unfortunately the holes in the frame are elongated and must be enlarged for steel bushings. Then a new shaft made out of ground shafting of.375 diameter. Thread both ends 3/8-24 and weld a nut on one end. The other end uses a nut with cotter pin. Using bolts for moving components is not the best since they are all under sized. No ability to grease the moving parts may eventually cause additional problems.

Jim

[frequent_flyer]

I suspect that part is powder coated so not to easy to strip and do a close inspection for small cracks.

My bet is anodized and dyed black.

[Gordon Misch]

Unfortunately the holes in the frame are elongated and must be enlarged for steel bushings. Then a new shaft made out of ground shafting of.375 diameter. Thread both ends 3/8-24 and weld a nut on one end. The other end uses a nut with cotter pin. Using bolts for moving components is not the best since they are all under sized. No ability to grease the moving parts may eventually cause additional problems.

Jim

Agree, except - depending on the alloy for the new shaft, welding a nut on it might not be best. Castle nut on both ends might be preferable.

[frequent_flyer]

Then a new shaft made out of ground shafting of.375 diameter.

I was curious how the bores of these rod end bearings are sized. I found one manufacturer's reference that states 0.3750 -0.0005/+0.0015.

ref - https://us.misumi-ec.com/vona2/detai...t=PageCategory

[Justin Otto]

There appears to be excess clearance between the bolt and the Heim ball. It the bolt undersized?
Also as frequent suggests, the stack should be shimmed to zero play in order to be able to tighten the bolt without stressing the ears.

I’m not familiar with this process, could anyone describe how you would shim to zero play? Thanks

[TcraftF21]

The bolt was loose in the holes in the frame and undersized for the Heim joints. If the diameter of the through bolt would have been .375 and the holes through the housing a nice slip fit, there would have been no problem. Now it needs proper fix.

[tedwaltman1]

Yes, parts are anodized.

Not enough edge distance to add bushings, at least in my version of the parts.

Though I didn’t post videos (yet) there is similar play in the two shock mount assemblies.

As correctly noted above, and as documented in my caliper before and after pictures, the play has already elongated mount holes.

One could add thin shim washers to the Heim joint assembly to reduce or almost eliminate side-to-side movement. But the key issue is the oversized mount hole assemblies which, in my opinion, result in excessive fore-aft movement.

Even with shims, one should not, in my opinion, over tighten the AN6-24 bolt past best-practice industry torque specs. Again, there are two other AN6 bolts which would also, per Acme, require significant over-torquing. And any over-torquing to reduce system movement ultimately also hinders the whole system from operating as designed.

[Gordon Misch]

Not enough edge distance to add bushings, at least in my version of the parts.

I'd think a 1/16" wall bushing of heat treated steel or stainless would be plenty thick. The trick would be getting nice round and axially aligned holes in the frame ears. Bore on a milling machine or line-ream with piloted Critchley reamer. Seems to me the manufacturer should step up.

Edit: Minimum edge distance from center of hole should be about 1.25 - 1.5 diameters. Also, based on the video, it looks like the direction of the load isn't toward an edge. Without having the parts in-hand, bushing looks like it's probably reasonable.

https://www.ideastatica.com/support-...20Code%20setup.

https://www.engineeringexpress.com/w...-requirements/

[40m]

What has Acme said?

[frequent_flyer]

One could add thin shim washers to the Heim joint assembly to reduce or almost eliminate side-to-side movement. But the key issue is the oversized mount hole assemblies which, in my opinion, result in excessive fore-aft movement.

Are you confirming that, when assembled with industry standard torque, there is side play in the rod end bearings and the spacers?

If so, is that play eliminated when Acme specified torque is used?

With the bolt not torqued what size feeler gauge will fit between the mounting ears and the spacers?

[tedwaltman1]

"What has Acme said?"...Their response, direct quote from text message: "...don't see an issue with the videos..."

As seen (hopefully you can see) in this picture, there is NO, ZERO, side-to-side play in the rod-end-bearing, main mount bolt assembly. There is no room for any shims (confirmed this afternoon).



I have not, and will not, over-torque the bolts. As noted above, the AN6 bolts are now torqued to spec per AC-43.13-1b, page 7-9, table 7-1 specifications. Just my opinion, but over-torquing can cause internal component stress (AC43.13-1b, page 6-8, para 6-20). My opinion, but over-torquing will potentially deform the "ears" of the mount assembly. Furthermore, my opinion, but over-torquing will further restrict proper movement of the schock assembly sections, as shown in the below pictures.

Both of the mount holes in the below pictures were originally over-sized by (forward one) 0.008"; now elongated by 0.012"; (rear one) 0.012"; now 0.015". Pictures of caliper measurements are linked in my original post above if you want to verify my math.


Above picture: Front shock mount


Above picture: Rear shock mount

[akwing]

I disagree with acme's statement..that much slop is unacceptable

hard to beat the simplicity and reliability of the standard leaf spring

[Gordon Misch]

that much slop is unacceptable

Agree. Checkout the LC5 fit on this webpage, about halfway down.

https://www.cobanengineering.com/Tol...eranceFits.asp

[Steve Pierce]

I maintain the first production FX3 Carbon Cub which is the local dealer's demo plane with almost 1k rode hard and put up wet hours on it. I have not noticed any issues with the tail stinger on it. If you documented all of this and sent it to Acme and they said that it is all normal I would run it. I would keep watching it and if you start to see an issue bring it up to them. If you have a catastrophic issue as a result of a failed part you were having issues with I would think they would have to make good on it if they are gonna stay in this market and after what they have spent on the shock strut STC I can bet they have every intention to.

[tedwaltman1]

I suspect people don’t see issues because they don’t jack up the tail and hand-exercise the assembly….just my hunch.

I have seen “issues” with all mount holes now exhibiting elongation, as documented in the above caliper pictures.

My first conversation with Acme was largely one sided. My second conversation with another individual at Acme was very cordial, with a good exchange of information. I was promised a set of custom, interference fit mount parts.

Side note: Promising me a one-off set of new parts doesn’t address the potential issues others might encounter.

After being promised new, interference fit parts, I followed up over the course of about 6 weeks with four phone calls to get shipping status. I never received a call back to my last two calls/voice messages. I never received any parts.

[TcraftF21]

It is too late for “interference” parts. The holes in the structure are elongated and therefore the structure must be replaced or repaired and then proper fitted components installed. If left as is, the sloppy conditions will just get worst. PM me if you want to discuss this a little more.

Jim

[Rob]

I spoke to Acme staff after sharing the above video links via email. The response, “… don’t see an issue with the videos you sent.”

Do others have movement in their stinger assembly? Am I nuts to be concerned?

If you lifted up on the tip of your wing and the fork and strut attach fitting had that much play would you call it a day if the strut manufacturer said all is good?

While not an engineer, I agree with Tcraft, the elongation is there, and not going to fix itself, and no matter how much you over tighten it, with the tail loaded, unloaded or standing on your head with washers jammed in between, the elongation is going to continue to get worse until the point of failure. That may take a million more hours, or it may happen next week, that is for folks like Tcraft to determine. As is, either the part should be deemed a consumable item and have a wear limit assigned to it, redesigned, or it will ultimately fail.

If you have a catastrophic issue as a result of a failed part you were having issues with I would think they would have to make good on it

That is a wonderfull notion .... if you dismiss the places this cub has already been, and will continue to go. I was on the ground in a saddle the outfit Ted used to? fly for in the Wrangles serviced to some degree, when one of the original T3's lost half of it stinger in a totally benign landing. The manufacturer stood by it, and 'made it good' (once he got back to the L48 on a pawnee stinger I flew in to him).... I hate to imagine how that would have played out if there wasn't another aircraft on the ground? I'm not being melodramatic, I guess now days most that venture that far out carry a sat phone, but all that country was 'airplaned' for years without the need of a satellite overwatch in the name of experimenting.

I generally think experimental owners need to cut manufacturers more slack than the certified crowd. After all, they are 'experimenting' which infers they will have some experience to gain along the way. But I also happen to think that when an experimental owner brings a valid concern to the table with a manufacturer, the manufacturer should probably be a little (or in this case a lot) more pro active. Because like Steve P alluded to there is a bunch already riding on it. It is my understanding that the T3 stinger (at least on that model) was addressed by a change in the alloy or design of that stinger.

Like the T3 failure, the one coming here is foreseeable, and preventable. permanently repairing what's there, is probably not in the cards, although maintaining (temporary and continuing fixes) what's there until it ultimately 'cycles out' may be.

Relying on 'squeezing' the ears to capture the heim will not solve the problem, and will likely lead to a new problem of compromising the ears. Not squeezing the ears, but simply relying on a press fit of the heims in between them is not going to solve the problem, it is only going to delay it some.
As long as there is any slop anywhere in the assembly (even .0001"), be it between the bolt and the fitting, the bolt and the heim, or the heim and the fitting, the problem will show again. If the slop needs to be there for manufacturing, or design, the only thing that will negate this problem is for the fittings to be of the same hardness as the bolt, and that just slows the process and spreads the wear over two items.

This failure is as easy to predict as the one that took out the original T3 stinger. To be fair, in that case it was partly the owners failure to realize the importance of maintaining an appropriate spring rate to the load so as to not bottom out the spring violently. which in turn tried to make a spring out of the swing arm. In this case, softer aluminum vs hardened steel, slop, and jarring. You don't even need to land tail first for this to happen, the jarring of taxiing will eventually do it, or there would be no need for a shock back there.

I will watch this one with interest. Mostly because I like outfits with good customer service, but also because I like watching problem solvers, and regardless of how big or small anyone perceives this to be, there is a problem, and there is no doubt a solution.

Take care, Rob

[spinner2]

If it were mine I'd open the existing holes and insert a drill bushing. https://www.mscdirect.com/browse/tn?...rsrh=true&rd=k

You need some clearance between the bolt and through-holes to allow movement. I'd think a standard AN bolt would be a good choice. Too tight and you're going to need lubrication and that's going to attract grit.

I don't know the overall width of the ears but you could probably line bore it with a chucking reamer https://www.mscdirect.com/browse/tn/...cking%20reamer
the key being enough flute length to have engagement of the flutes through all three holes at the same time. It might have to be done in steps depending on the wall thickness of the bushing. A thin wall bushing would be best for this reason and also it would leave more of the original 'ear' material.

I made a close fit pin once to take out some slop on a front control stick pivot point. Like was mentioned above I threaded both ends and used two AN310 type nuts. Clearance was about .0005" with a little white grease.

[Rob]

If it were mine I'd open the existing holes and insert a drill bushing. https://www.mscdirect.com/browse/tn?...rsrh=true&rd=k

You need some clearance between the bolt and through-holes to allow movement. I'd think a standard AN bolt would be a good choice. Too tight and you're going to need lubrication and that's going to attract grit.

I don't know the overall width of the ears but you could probably line bore it with a chucking reamer https://www.mscdirect.com/browse/tn/...cking%20reamer
the key being enough flute length to have engagement of the flutes through all three holes at the same time. It might have to be done in steps depending on the wall thickness of the bushing. A thin wall bushing would be best for this reason and also it would leave more of the original 'ear' material.

I made a close fit pin once to take out some slop on a front control stick pivot point. Like was mentioned above I threaded both ends and used two AN310 type nuts. Clearance was about .0005" with a little white grease.

This is most likely how I'd approach this as well, mostly because I am just not the guy who complains about my steak being burned to shoe leather when I ordered it medium rare... *but I do have a memory


Having said all that, we are looking at this through our own eye's and not the eyes of the average consumer pilot / builder. And in your case, those would be the eyes of a world renowned master machinist.

If I were to take a wild @$$ guess, I'd be willing to bet the average builder would stand a better chance at a line dance, than line boring anything.This is not how I perceive Ted's fabrication skills, I'm just saying the average consumer guy... SO what does that guy do? Hire it out? does that sound like a reasonable expectation with so little time / cycles on it? I dunno...

If I were the manufacturer, at a minimum, I'd want to see Ted's assembly. Maybe he got one from a defective run, since some, but not other's have seen this?
I'd also consider bushing them in the manufacturing stage. Then you have the clearance (slop) at the bolt riding on a hardened surface. Maybe those will be the 2.0 models? Maybe Ted's is free of charge since he contributed to the R&D? I dunno?

Take care, Rob

[Steve Pierce]

I think I would message this thread to Matt McSwain who owns Acme and see what he says. He can be found on Facebook very easily. It would be interesting to measure a new one and see how it compares. I have not had any issues with an Alaska Bushwheels tail spring myself. I know a lot of people like the tail suspension but have seen a need for it in the places I go.

[mvivion]

I’m not an expert on the Acme device, and I am most certainly not an engineer, though I can spell it, most days.

But I can say without hesitation that ANY “slop” in a tailwheel attachment can have VERY ugly consequences. Acme should be on this big time. Hopefully they will.

MTV

[Mr. Ed]

I bought a Stinger and then watched Ted’s YouTube video on this issue and talked to him on the phone. My bolt holes in my new, never used Stinger, measured the same as what Ted posted for his new Stinger. So I assume the hole size is consistent and as designed. I don’t know why Acme is resistant to making a simple change to the reamed hole diameter. Easy fix. Maybe it has to do with the STC process being well under way and not wanting to slow the process by making a change. I may bush the holes if Acme doesn’t come up with their own fix. My project isn’t flying yet so no wear on mine.

[tedwaltman1]

Steve, as noted in my original post, in February 2022 staff at Acme viewed the same videos as seen above. Staff at Acme said, “… don’t see an issue with the videos you sent.” If you or someone else doesn't believe me regarding this statement, I have a screen-shot of the text message reply.

My point was, and remains, staff at Acme are fully aware of my hole clearance issues.

When I ordered my stinger, I actually ordered a 2^nd stinger as well for a friend (to save on shipping). I measured each of the mount assembly holes on that 2^nd stinger—the holes exhibited the same clearances as on my stinger. Exactly the same.

I really like Acme's Black-Ops shocks. I think the design of the Acme stinger is awesome. I truly--genuinely--want to see Acme both succeed and to continue to innovate. I expressed my desire to help Acme in my original, February 2022, email to Acme. But--in no uncertain terms--I was told there is no issue. Maybe I am nuts to be concerned—that’s why I asked the question in my original post above.

I’m thankful for at least one other person on this thread having measured and reported on their stinger hole clearances.

As much as I’d like new, properly manufactured, interference fit parts, sending me new parts does nothing to address potential hole clearance issues for anyone else who finds themselves in the same situation.

[tedwaltman1]

Maybe Ted's is free of charge since he contributed to the R&D? I dunno?

Take care, Rob

For the record, I paid full retail price. And as noted above, I bought not one, but two (!!) stinger assemblies [one for a friend to save shipping].