Got A Screw Loose!

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Daleandee

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Likely true of a lot of us but that ain't where I was going ...

On Thursday, in an effort to escape the doldrums of cabin fever, I went to the airport to put some air under the tires. My brother was there with his youngest grandson. Zachary is seven years old and had been asking about an airplane ride for the last few weeks. The weather, although a little windy, wasn't too bad so after adding his booster seat to the cabin and strapping him in with a video device we headed out to the runway.

Run-up was fine and all went well as I added power and the Corvair drew us briskly down the runway and into the evening air. But something wasn't right. I was having to push the stick too much on climb-out. I checked the trim and it was as it should have been so my mind began pondering what else the cause could be. It was something to be concerned with but nothing to be alarmed over so we did a short hop around the pattern and came back in and landed.

After a check of the trim system I found that the set screw for the cable to trim lever connection was slightly loose and while it would trim nose up (because of the way the piano wire was captured in the connector) it would slip when trying to add nose down trim. After tossing the original hardware and making my own version of what Aircraft Spruce calls a "bug nut" the trim system works as good or better than it ever has.

I wanted to share this as, to be honest, the trim system is something I hardly ever check on preflight or run-up. I do set the trim lever for take-off but don't look to actually see (even if I could) where the elevator trim tab is at.

Something to add to my preflight checklist.

Dale
 

Riggerrob

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May I suggest something used by some aerobatic pilots?
They install plexiglas access panels near many of the control linkages. Plexiglas makes it easier to inspect control bolts during pre-flight inspections.
 

Daleandee

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I believe a checklist should be like fine wine -- it improves with age! Thanks for sharing, and a picture of the "New, Improved" is always nice.
I agree. Making sure all controls are free and correct is part of the pre-flight but from inside the cockpit I cannot physically see the movement of the trim tab ... but I can from outside the cockpit so that will get checked before I enter the airplane. I don't know how many others ever do this and that is part of the reason for my post.

When the airplane was built we used a cable stop from Spruce like this:

1585624427323.png

The replacement was made in the hangar using a AN3 bolt that had to be precisely drilled so that a stop nut could be placed all the way up the threaded end and spaced to still have freedom to rotate freely in the lever while not being sloppy. The drilled hole was to be just beyond the stop nut where the piano cable came through and left enough thread to engage a lock nut. The cable is firmly captured between the stop nut and the lock nut. What we made looks much like this from Spruce:

1585624692357.png

Dunno if this makes it clear ...

Dale
 

wktaylor

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D... I checked several specification for the cotter-pin hole size in 0.190[3/16]-32 threads.

AN3 = D0.070"
NAS464 = D0.070"
NAS6xx3 = D0.075"

When holes are drilled in threads, there is a significant reduction in strength that may/may-not have to be accounted for. Drilling an extra large hole will make matters 'worse' from a cracking perspective. The Part from 'spruce' has a significantly higher strength thru the 'pin-hole'; but it's obvious what looks like a small diameter 'plain screw' [threaded into the end] will not provide much of a grip on the hardened piano wire.

Solving Your problem with [2] thin 'jam' nuts... 1st torqued-against the wire... 2nd torqued against the first nut... which locks the nuts together... may still present a loosening problem since these nuts can only tolerate very low-torque [and the hole in the threads makes this a bit sketchier]…

NOTE.
True setscrews have 'high-hardness' [spring-hardened, CRES or carbon steel] and a special 'point' [cone-point or ring-point, etc] that allows it to embed within the shank of the item being secured; and often have a 'self-locking' plastic pellet or distorted threads to help secure it from loosening... 'all' have an internal recess in the 'head-end' for torque-turning-it into position.

Optionally the part being secured may have a 'flat' machined on the side where a conventional screw [flat-point] is to be installed against... and lock wire or other self-locking feature/mechanical device to secure it.

CAUTION.
In both configurations, I have seen the part being secured by a setscrew-like-device [in this case round and flat wire - solid or wire-rope] crack/fail at the indent/contact mark or the machine step fillet.
 

Daleandee

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Thanks for the detailed reply. I do realize there are concerns with my approach and will monitor it closely. I could use a larger bolt (AN4).

Using a Z-bend in the wire is an option but that approach puts the steel cable through an aluminum lever (a bushing could be used) but Z-bends can be sloppy. There isn't enough room for a clevis attachment.

Perhaps you have a solution in mind?

Dale
 

Dan Thomas

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I'd use at least a 1/4" bolt for that. I did on the Jodel. And rather than pinch the wire against the side of the hole in the bolt, which can nick the wire and cause a failure, I drilled the hole farther away from the bolt head and ran a thin nut (jam nut) down past the hole, put the wire in, and ran the other nut down and adjusted and tightened the two nuts so that the wire is clamped between the nut faces, not the sharp edges of the hole. Much more secure. Carefully countersinking the hole on both sides just a little can reduce the stress riser the sharp hole edges present, too.
 

Daleandee

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Thanks Dan.

I may indeed up size the bolt to an AN4. To be clear (maybe I wasn't before) the wire is captured between two nuts ... one is the stop nut that is put on past the drilled hole up to the end of the shank and the other is a lock nut that is on the other side of the wire and has just enough threads to fully engage. FWIW I did use my deburring tools to try and clean up the small hole that was drilled through the bolt.

Dale
 

Daleandee

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Just wanted to tie up a few loose ends. First ... thanks for the replies and helping me come to a better answer for the concern that I had.

Went back today and with the help of a hangar mate the connection was redone using an AN4 bolt drilled with a hole just large enough to pass the music wire through. The wire was captured between two AN nuts ... one plain jam nut and a lock nut on the outside. Had to remove the lever and up-drill it for the larger AN4 but that wasn't a concern.

I feel much better about it now ...

Dale
 

proppastie

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seems to me the hole not going through the threads is a better solution, stack up of washers each side of the wire then the nut.

Or..... OOPS maybe that will not work because the trim tab has to pivot around the bolt. (I still do not like drilling through the threads)

Where ever Cessna uses these bolts it might be useful to look at that design.

1585791403152.png
 
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Daleandee

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seems to me the hole not going through the threads is a better solution,
I agree with you but as the situation presents itself there isn't a better option. I like Dan's answer and it appears to be well supported from what information I can find. The Aircraft Spruce replacement for the "bug nut" is part # 05-16100 which is a 3/16" (AN3) and it is drilled through the threads. These are used for solid wire cables. BOLT TYPE TERMINAL | Aircraft Spruce

What we did yesterday was to remove the AN3 that I had used and upsized it to AN4 and drilled a hole just large enough for the piano wire to pass through. The hole was also deburred. Inspecting this is now part of the pre-flight but I have confidence that this repair is now properly done.

Thanks for your thoughts.

Dale
 

Dan Thomas

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seems to me the hole not going through the threads is a better solution, stack up of washers each side of the wire then the nut.

Or..... OOPS maybe that will not work because the trim tab has to pivot around the bolt. (I still do not like drilling through the threads)

Where ever Cessna uses these bolts it might be useful to look at that design.

View attachment 95034
Those Cessna bolts are used on the carb heat and mixture and cabin heat or vent cables. Light loads, a bit of slop not a serious deal. With trim tabs, if the bolt or wire breaks, the flutter could destroy the airplane. If there's slop you could get flutter. The extra thread to get jam nuts on both sides of the wire is to prevent nicking or shearing of the wire, and if you thread it just right there is still smooth shoulder in the lever hole. Drilling through the threads is, unfortunately, necessary, and it's the reason we use a larger bolt. Everything on an airplane is a compromise, and you pick the safest compromise.
 

proppastie

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I did Oops it as I thought about it....thanks for reporting these problems and the solutions, we all learn from them.
 

Dan Thomas

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That's the setup that nicks or shears the wire, and the AN3 bolt, being drilled, ends up pretty weak. The wire used for a trim tab is much bigger (or should be) than the wire used for carb heat or whatever, and the resultant hole leaves much too little strength in the bolt. I used .090" wire for my trim tab. Stout stuff. Didn't want it breaking and letting the tab flutter and tear the tail off the airplane.
 

proppastie

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That's the setup that nicks or shears the wire, and the AN3 bolt, being drilled, ends up pretty weak.
That easily could be scaled up from AN3 to AN4 bolt, with a 5/16 tube for a bushing......It is only a concept as to avoid drilling through threads... The minor diameter of 1/4-28 is .206 only slightly bigger than .1875 and stress risers are considered up to 50% (I think) reduction in strength.

I do not understand how the wire captured between two washers is much different than the other design, as regards nicks and shear.

The tab is a slip fit around the bushing. The whole bolt assembly rotates with a .01 gap between the tab and washer.
 
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Dan Thomas

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That easily could be scaled up from AN3 to AN4 bolt, with a 5/16 tube for a bushing......It is only a concept as to avoid drilling through threads... The minor diameter of 1/4-28 is .206 only slightly bigger than .1875 and stress risers are considered up to 50% (I think) reduction in strength.

I do not understand how the wire captured between two washers is much different than the other design, as regards nicks and shear.

The tab is a slip fit around the bushing. The whole bolt assembly rotates with a .01 gap between the tab and washer.
I've seen that used, but the bushing length has to be exactly right so that the hole in the bolt isn't putting any pressure on the wire. If the hole is drilled as small as possible to fit the wire, a few thousandths off on the bushing length will mess it up. Still, it's a better idea if one wants to get the wire as close to the lever as possible to reduce cocking. Thanks for posting it.

Minor area of an AN3 bolt is .0175 square inches. The minor area of an AN4 is .0326 square inches. Big difference, and it's the area that matters since tensile strength is in pounds per square inch.
 

proppastie

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but the bushing length has to be exactly right so that the hole in the bolt isn't putting any pressure on the wire.
Yes..if the bushing is too long in relation to the hole and you will not get the wire through the hole, too short and you could shear the wire....making the bushing a little short and using thin washers or shim stock so it clamps but does not shear is the trick. ....Or good measurement of location of the hole and a lathe to make the bushing. I really like 4130 tube for bushings, I would not use a sinterned bronze bushing (oil-lite) here as they are pretty brittle. I have never used navel bronze in a strength application (ranther than a bushing supported by other material) so I do not know how it would behave. I did once try to cut an oil-lite bushing and it crumbled.
 
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