A Tale of Two Tail Struts
Source: 2019 Q1 Beartracks, Jared Yates
It was the best of times, it was the worst of times. We had some visitors in town that wanted to go for an airplane ride, so we drove out to the airport and started the usual preparations. Bearhawk folks have known for some time that the airfoil-shaped front support struts for the horizontal stabilizer are a candidate for extra preflight scrutiny. As usual, in my preflight walkaround I grabbed the strut and gave it a tug, but not as usual, the bottom end came off in my hand. The friends were understanding about not getting to go for a ride, or at least they said they were, so I removed the upper bolt still holding the strut on, and went back home.


When the guests were gone I was able to study the broken AN490, which you can see above. This cross section shows that a crack had been developing across the part for some time, with the dull gray part being the section that was holding together at the time of my inspection. Would it have been safe to fly if I had neglected to break it? Probably so. It is said that Bob flew the prototype Bearhawk without the struts before adding them later in testing. It probably would not have broken yet, but it was going to happen sooner or later. At least this was a time of minimal inconvenience.
A few years ago I had contemplated stripping the covering off of the horizontal stabilizer to add wooden strips and give the ribs an airfoil cross section, also adding provisions for an electric trim servo. That project never made it to the top of the to-do list, but since that mod leads to a reduction in the incidence angle, it also requires longer tail struts. This meant I had a pair of spare AN490s on hand, so my plan to fix this was to weld a new one in place of the broken one and carry on.
But as things tend to go, fixing one thing seldom means merely fixing one thing. In recent years we noticed that in cruise flight, the elevator was displaced in a stick-forward position from neutral, based on looking out the window at the counterbalance. This meant we needed less incidence already, even without switching to the profiled rib shapes. If we were going to be making new tail struts, we might as well account for this. I mentioned the part failure to Bob on the phone, and he had some input. First, he reminded me that the tail strut we were using didn’t match his original design. The original does not allow for any adjustment, it’s just made in place to fit. In the early days of the factory, the tail struts were made with welded-in AN490s, which paired with the AN665 female-threaded clevis. The thread size is 1/4×28. One of the problems with this arrangement is that in the welding process, the protective cad plating layer is burned off of the AN490. Threads are extreme stress risers, and this combined with inadequate corrosion protection is not a recipe for success.
Bob had several ideas for an improvement, and here are the two that I liked the best. First, he suggested welding a tube onto the end of the strut, cutting female threads into that tube, and swapping the AN665 out for a clevis with male threads. He also suggested considering a 5/16” thread size, which would be much more durable. With this arrangement, the exposed threads would still have their protective plating, and the cross section area of the clevis inside the threaded area would be much larger.
The first step was to have the parts manager get to work sourcing the supplies, starting with a male threaded clevis with 5/16×24 threads. I couldn’t find one in the aviation catalog, but McMaster Carr carries one as part number 4749T11, $6.05 each. Next we needed a foot of 4130 tubing, 7/16x.095. A few inches would have been fine too, but there wasn’t any in the scrap pile and the minimum order is a foot. The tap and drill were already on hand.
Before starting, I measured the original strut length (36-5/8) to make it easier to guess the new length. Next I cut out the old AN490 with my new portable bandsaw mounted in its Swag Offroad benchtop table (thanks for the tip, Rob Caldwell), and was surprised to find water running out of the tube. The bottom end of the strut is sealed, but apparently it was possible for water to run down the leading edge of the horizontal stabilizer, then between the stabilizer and the strut, then between the layers of the smashed strut end. This would motivate me to weld closed, then re-drill the strut top hole, sealing the struts against future leaking.


Once the slots were cut, I drilled just outboard of the end of the AN490, which liberated the old fitting.

A rotary file made easy work of smoothing the remains. The 7/16 tube fit nicely in that slot, needing only prep for welding, and trimming to length once the welding was done.

Cutting the threads into the tube was a slow process of back and forth every quarter turn. If I had any doubts about how much work the tap was doing, the hot temperature of the tube reminded me.

Next, I needed to reduce the incidence angle by shortening the spacers in the horizontal stabilizer mounting structure. Rather than shorten the old spacers, I made new ones at half length. A folded wire tie held them nicely for painting. In the final installation, I installed a jam nut to keep tension on the threads of the clevis, and I ended up shortening the 7/16 tube by about an inch compared to the image above. In retrospect I should have shortened it before cutting the threads, since that would have saved half an hour of tapping. The end result seems very durable, and initial flight tests after the adjustment suggest that if anything, it may not be enough correction, but at least it is a step in the right direction. I’ll need to fly with a heavier load and further aft CG to see where the elevator falls, but it is currently in trail with the stabilizer at casual local flying weights.
A little more room here would have been helpful. Original spacers are 1/2″

Original spacer

New Spacers Cut

Wire tie holds the new spacers for paint