Cobra top hinge plate bolt failure

I have 3 Wing Riggers that I need to get shipped out today, but as
soon as I get a chance I intend to investigate changing the bolts to
the next larger size, 3/8" grade 8. Of course that will mean drilling
the holes larger and it will also mean that the head will not be
neatly counterbored into the handle. If that plan works out, I will
post back here.

GW

On May 18, 9:44*am, Steve Koerner wrote:
I have 3 Wing Riggers that I need to get shipped out today, but as
soon as I get a chance I intend to investigate changing the bolts to
the next larger size, 3/8" grade 8. *Of course that will mean drilling
the holes larger and it will also mean that the head will not be
neatly counterbored into the handle. *If that plan works out, I will
post back here.

GW

I'd be interested to hear how that works out. There is a least one
spacer block inside the frame extrusion and it looks like it would
have to be extracted, drilled, and replaced. There may be two each
side. Given the limit access to the extrusion opening it could be a
bit awkward to work the inner blocks.

If I was going to larger bolts I think I'd relocate the handles and
keep them on 5/16 socket cap bolts. They wouldn't need the spacer
blocks as long as a moderate torque was used.


Andy (GY)

On May 18, 11:44*am, Steve Koerner wrote:
I have 3 Wing Riggers that I need to get shipped out today, but as
soon as I get a chance I intend to investigate changing the bolts to
the next larger size, 3/8" grade 8. *Of course that will mean drilling
the holes larger and it will also mean that the head will not be
neatly counterbored into the handle. *If that plan works out, I will
post back here.

GW

Why not use the proper metric size?

Peter

Like Eric’s, my trailer has “bridged” its way around the west, at
least until I got bigger tires and wheels (Eric, get bigger tires and
wheels). And like Eric, I have the more flexible aluminum top and
tight hinge bolts.
Based on the table Bill provided the thermal expansion of aluminum is
2.5% higher than glass reinforced polyester. assuming the these 30
foot trailers were assembled at 80 F, left outside on a 0 F night and
soon after dawn the top was 20 F hotter than the floor due to radiant
heating; the fiberglass top would be about 0.095” longer than the
bottom. I don’t know if the hinge and latch pins can take up that much
play. This might explain the crushed fiberglass under the handles.
Looking at a steel bolt going through 2 inches of aluminum on the
frame, under the same 80 F to 0 F conditions, the bolt would only
tighten by .001”; certainly within the elastic range of the the parts
and at loads too low to damage anything.
Another possibility is crosswind loads on the open trailer. Of course
you usually park into the wind for assembly or disassembly, but dust
devils are not too uncommon in Arizona this time of year. The air
springs won’t provide much lateral support when extended. The cross
wind loads, including those on the tail doghouse which act over the
full length of the trailer, would create a substantial load couple
across the 4-foot hinge spacing at the front of the trailer. The bolts
on the downwind side would be in tension with the outside bolt seeing
the highest load, perhaps enough to crush the fiberglass under the
handle. This would cause this bolt to lose its clamping force and
allowing the hinge to pivot around the inner bolt (if that is what you
saw).
In either case, the stronger bolts and the baseplate Steve put under
the handle should solve the problem.
Mike Koerner

Thanks Mike. I think the open trailer in the wind theory is very
good. I know that Andy and I have had to disassemble in the wind many
times and there is a tremendous lever arm working on the hinge plates
in that situation as you point out. That could be what has fatigued
the bolts in tension as well as caused the slight compression to the
fiberglass and the base of the handles. In my case the broken bolt
was discovered after a long drive back from Moriarity where my last
disassembly at Moriarity was in pretty strong wind and the trailer
was, in fact, not lined up to the wind as it should have been. So, it
fits with the patients history.

Steve Koerner (GW)

On May 22, 10:11*pm, Steve Koerner wrote:
Thanks Mike. *I think the open trailer in the wind theory is very
good. *I know that Andy and I have had to disassemble in the wind many
times and there is a tremendous lever arm working on the hinge plates
in that situation as you point out. *That could be what has fatigued
the bolts in tension as well as caused the slight compression to the
fiberglass and the base of the handles. *In my case the broken bolt
was discovered after a long drive back from Moriarity where my last
disassembly at Moriarity was in pretty strong wind and the trailer
was, in fact, not lined up to the wind as it should have been. *So, it
fits with the patients history.

Steve Koerner (GW)

What's more, the trailer was facing north in a west wind and it was
the left side bolt that popped. That's the side that would have been
under tension. I guess I should read my own Wing Rigger FAQ document
which clearly states that the trailer should be pointed into the
wind. I had asked where to assemble and I put the trailer in the
particular spot that was recommended off the edge of the apron. I
should have moved it when it was disassembly time and the wind was
stronger but I didn't need to bother with the Wing Rigger since my son
was standing by to assist. The wind was not so strong that we
couldn't safely rotate the wing to slide into the trailer -- maybe
around 10 or 12 knots.

I think the bolt had already fatigued and that was its final straw.
The fiberglass compression and handle damage was actually the same on
both sides.

GW

On 5/22/2011 7:54 PM, MKoerner wrote:
Like Eric’s, my trailer has “bridged” its way around the west, at
least until I got bigger tires and wheels (Eric, get bigger tires and
wheels).

That would make the rear of the trailer too high for convenient rigging
with the trailer attached, something I commonly do when traveling with
the motorhome (the trailer's primary motive force). Removing it to rig
is a nuisance, especially since I need to reconnect it after rigging, so
my wife can retrieve or follow me, as needed.

A Cobra trailer dealer told me they are designed to be supported
entirely by the tail and the tongue without damage, so I don't think the
bridging causes the trailer any problems. I do have to replace the
aluminum tail skids every 100,000 miles.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "Transponders in Sailplanes - Feb/2010" also ADS-B, PCAS, Flarm
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you need to know tinyurl.com/yfs7tnz