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Need input: Is my Acme Stinger movement excessive?

tedwaltman1

FOUNDER
Delta, CO
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.
 
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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.
 
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?
 
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…
 

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

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
 
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.
 
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.
 
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...e (distance between,be modified in Code setup.

https://www.engineeringexpress.com/wiki/steel-bolt-edge-distance-requirements/
 
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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?
 
"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).

IMG_7906b.jpg

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.

IMG_7910b.jpg
Above picture: Front shock mount

IMG_7909b.jpg
Above picture: Rear shock mount
 

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

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