Keep Checking Tail Strut Threads

[jaredyates]

We had some family friends in town that wanted to go for a ride on Wednesday. As usual during the preflight, I gave a tug to the horizontal stabilizer strut. As not usual, it separated from the clevis and we didn't get to fly. Looking at the cross section, I'd say the crack has been forming over time, and the gray portion is what I broke when I pulled on it. I suspect it would have lasted a while longer before breaking in service, at a most inconvenient outpost I'm sure. At least this way it is broken at home.

As we have known for a long time, don't forget to keep an eye on these points.

[rodsmith]

Good catch, will definitely have the struts and wires on my preflight check list.

[mswain]

Yikes. Glad you found that on the ground at home.

[Aero_tango]

Thanks for sharing...how many hours on this struts.
Michel.

[jaredyates]

350 hobbs, 285 tach, plus or minus.

[Mark Goldberg]

This is the second instance of the AN490 failing. The first occurrence Bob thought was due to the builder's struts being too short - and the end that broke had much more threads showing on that modified bottom end of the strut. There is a lot of vibration in the tail of these planes. That is the cause according to Bob. Certainly needs to be checked on preflight. Mark

[Ray Strickland]

Looks to me like there should be an automatic replacement at every annual in addition to the above mentioned preflight check.

[svyolo]

Jared;
I saw your video with the tufts on the tail showing the tail stalling. Any chance you could remount the camera and have a look at the lower strut in flight. If it is vibrating maybe a solution can be found. Very low hours for what could have been a catastrophic even.

[jaredyates]

I don't think losing the strut in flight would be a big deal. As I understand it, Bob added the struts after flying the original prototype for a while without them. I was more concerned about it coming loose a long way from home and not wanting to be tempted to depart without it.

[Gerhard Rieger]

This is my solution, Tail struts from the supper bellanca adapted for the Bearhawk

[jaredyates]

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.

[spinningwrench]

Aircraft hardware has a better fit than typical hardware store nuts and bolts. You want to have a. Class 3A (male) and Class 3B (female) threads for whatever hardware you use. You can order the taps from McMaster to make these threads if you are cutting your own. A good fit really cuts down on stress concentrations.

[whee]

Good catch Jared! What is your planned fix; replace with like assembly or build a new strut IAW the plans since your tail position is known? If mine ever fails I'll build new struts IAW the plans.

EDIT: I see you posted while I was relying. Thanks for reporting back Bobs suggestion.

Looks to me like there should be an automatic replacement at every annual in addition to the above mentioned preflight check.

This would require the replacement of the entire strut every annual which isn't very practical. With only two reported failure I'm not sure this it that big of an issue.

Jared;
I saw your video with the tufts on the tail showing the tail stalling. Any chance you could remount the camera and have a look at the lower strut in flight. If it is vibrating maybe a solution can be found. Very low hours for what could have been a catastrophic even.

I don't think it would have been all that big of deal if it had failed in flight. Bob flew the prototype for a reasonably significant amount of time before adding the struts (it was 20 or 40hrs, its been a few years since I talked to him about it so I can't recall exactly).

[Ray Strickland]

So, is the threaded rod welded to the strut? (Plans builder here.) If so, I understand why it is not practical to replace it often.

[jaredyates]

I think our replies crossed paths, see the post above for my plan. I'm hoping I've got some 7/16 .095 tubing in my collection but won't know for sure until I'm back home to check. Nothing worse than ordering a foot of tubing!

[jim.mclaughlin924]

It seems logical to me that vibration must be involved since the design is stronger than needed for the loads it is designed for. I experience a significant flutter of the aft lift struts on my Rans s-7, later kits used a larger strut to solve the problem. I added a strip of turbulator tape on the top and bottom and which completely solved the problem. I may go ahead and do that on my Patrol, I do not see a down side. The tape is available at sites that sell glider stuff.

[svyolo]

Jim;
I was thinking about the turbulence thing, but the problem is, you have to know you have a vibration problem first. If you don't have one, no problem. If you don't have vibration, then add something, you might CREATE a vibration problem.

I think part of my test program will be mounting a GoPro in a bunch of different locations. That way hopefully I can see any harmonics that might cause problems, like this, and also the control cables that Battson had wear excessively in a short time.

I am glad to hear these struts are "extra".

[LSA052]

I think adding it to the preflight check list is in order.
Rather than automatic replacement at every annual, and based on 285 tachometer time at failure,
I suggest doing a dye repentant test every 100 hr or at conditional inspection (annual)

[tailwind]

Have not seen any evidence of a problem with my own plane. In any case it is not good to find broken parts. Problems reflect on all experimental aircraft. I would hope that we would do our best to deal with maintenance issues.
We talked about an internal thread in the streamlined tube and male on the Clevis. I am not a designer but hear is a possible option.
Turnnuckle fork AN161