Keep Checking Tail Strut Threads

[Steve W]

I’d be interested in knowing whether the original AN490’s that failed were tig or O&A welded. If tig welded whether the thread portion was stress relieved??

[jaredyates]

OA welded in this case.

[fairchild]

Been rolling this thing around in my head..... gotta think this is vibration induced some how. Either from the air across the tube or some vibration from the horizonal.
seems like it wouldn't be too hard to put a small transducer on the strut and record the vibration present.

A thought-- though this might be some effort---- might be to make a small socket on one end with a completely captured polyurethane core. Like a mini- lord engine mount.
Make the thing like a cup and piston such that if the rubber failed and came out- the unit could not separate. The rubber could be replaced at annual.
I guess it would be good to know if the vibration is coming from the fuselage or horizonal so you would know which end to mount it on. (or the strut its self---)
It might be able to be incorporated into the fixed bracket on the horizonal--- so the strut could be plain with no special ends.
Maybe such a device all ready exists-- like a small damper in a helicopter rotor head.

Though--- I guess until you know if the vibration is longitudinal or transverse- that would change how a damper would look and how it would be located and mounted. Maybe pouring the tube full of visco -elastic rubber might kill the vibrations too (?) Just thinking out loud here-----

[geraldmorrissey]

I remember years ago while recovering a J-3 seeing welding rod bridging between the top of the rudder spar and the fwd edge of the top rudder rib. This to absorb vibration. Don't remember if the horizontal or elevator had them. Might help but I'm not really qualified to comment on a fix. Can't see how it would hurt.
Gerry
Patrol #30

[Battson]

The vibration back there doesn't need fine measurement, the tail plane shakes like crazy during a full power take-off.

Our tail strut broke after a comparable number of flight hours, in flight. Same location for the breakage.

It was not a huge deal, the plane pitched up somewhat sharply and I needed to re-trim. It does load up the flying wires.

[Craig Van Sickle]

I spoke with Bob on the phone for a little while about the struts. He pointed out that there have been several tail strut iterations over the years, and the version that I was using was one of his least favorites. One method is to make a fork right on the end of the streamlined tube. In the initial design of the 4-place the struts didn't have or need adjustment, they were just made this way and drilled to fit. The Patrol, LSA, and Model B have a strut that is fixed at the bottom and threaded at the top.

Since I'm making a new end anyway, he suggested going to a clevis with male threads, welding a round tube into the end of the streamlined strut, and cutting female threads into that tube. For quarter inch threads, this would be a 3/8" OD tube, with enough wall thickness sufficient to create an ID of at most .213, the drill size for the tap. .083 gives an ID of .209, but sometimes we find that the ID of 4130 is not as closely controlled, so .095 is another option.

I couldn't find any aircraft-specific clevis forks with external/male threads (the opposite of AN665), but it looks like there is one at Mcmaster Carr that has 5/16" threads. That means I'll need a 7/16 steel tube. The drill size for a 5/16-24 tap is 17/64 or .2656. 7/16 4130 with .095 wall works out to .248, which should be good.

Setting up to mount my horizontal stab and going back through posts regarding tail wire/strut vibration/cracking. The method you mentioned here makes the most sense to me since the clevis threads haven't been impacted by welding. It also allows for replacement without building a new strut. How has this worked for you Jared?

[jaredyates]

It worked out well until other complications intervened. If I were building another, I'd use the female threads on the strut.

[Craig Van Sickle]

Appreciate the quick response, I'll go that route.

[Nev]

Is this something that could be picked up during a preflight by a close inspection - are there any warning signs ?

[jaredyates]

Is this something that could be picked up during a preflight by a close inspection - are there any warning signs ?

For sure, just give the strut a good tug. If it doesn't come off in your hand, you are good to go.

[Nev]

Apologies Jared - I just read your first post on this topic previous page and see this was how you found the problem.

[Craig Van Sickle]

The LSA and Patrol have a shared lower attach point for the horizontal stab flying wire/strut. (I don't have plans for the larger Bearhawks, but the lower strut and wire attach points appear to be separate.)

It may be wise for LSA/Patrol owners that have encountered intense strut/wire resonance to keep an eye on that lower attach point. An in flight failure could be catastrophic.

On the LSA this attach point is .072" thick and quite narrow where it welds to the longeron. A crack could form adjacent to the weld along the HAZ, particularly on planes TIG welded with no stress relieving. It's obviously plenty strong for normal flight loads, but so are these other items that have failed.


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image_2467.jpg

[Craig Van Sickle]

Looks good. I'm moving the horizontal stab strut on my LSA to the next station forward. It reduces strut length by over 6".

[jaredyates]

Craig you may want to run that by Bob. It's always possible, but it's very rare that he leaves extra weight or material on the table. It may very well be that he considered the location you have in mind but selected the location on the plans for important reasons.

[Craig Van Sickle]

Appreciate the input Jared. For some reason Bob didn’t include a design for the tail strut in the LSA plans.

[spinningwrench]

Certainly looks like vibration induced fatigue failure. The sharp threads make sense as a point of origin. On the Patrol I was concerned with the hammered over bolt used as I termination so I went to a solid tab TIG welded to an AN bolt. Bob said it looked good.

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[Craig Van Sickle]

Nicely done! Do you have a photo of the strut installed?

[zkelley2]

I had this break on me last year after about 150 hours. I had Mark send me a new set. While I was waiting on that, I figured I'd see if I could weld the bolt back together. It held for another 10 hours or so before the new ones showed up. After putting the new one on, I kept the old one, for what I figure is inevitable, so the airplane won't be down.

[Mark Goldberg]

For what its worth - my old BH with 1,300 hours on it had the original struts and never had a problem with them. They were reinforced with a fillet weld at the bend in the streamlined tubing and had the reinforcing washer welded per per Bob's revised drawing. But never failed. The issue back at the tail is vibration like others have said. Mark

[Craig Van Sickle]

Appreciate that Mark. You can see Bob's efforts to engineer parts for the homebuilder all through the plans. Use what is on hand and avoid special fittings if possible.

[Bcone1381]

This inspires my curiosity.
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[whee]

How you you guys have your strut rigged? IE, during preflight is the strut in tension, compression or neutral? I wiggle my h-stab and strut during preflight and the tiny bit of slop I feel in the strut makes me wonder if I should rig it differently. Currently I have it rigged neutral.

[Bentmettle]

Is the clevis oriented so that stab movement puts it in bending? Or does the pinned joint allow the clevis to rotate? looking at the photo in post #24 and it seems to me that the orientation of the pinned joint is "wrong" (but very common) for the load case.