Cobra top hinge plate bolt failure

On May 21, 8:30*pm, bildan wrote:
On May 21, 6:25*pm, bumper wrote:

If this problem is primarily affecting Cobra trailers with fiberglass
tops, rather than aluminum, I wonder if the difference in thermal
coefficient of expansion between the two materials might be playing a
roll?

On early Stemme S10-VT motorgliders, the spoiler control rods were
aluminum in an otherwise mostly carbon fiber wing. The spoiler over-
center locks were at the fuselage end. As the temperature dropped with
altitude, the spoilers would first start to come open and then
sometimes even open all the way with no pilot input. Problem was
solved by changing the control rods to carbon fiber.

bumper
zz Minden

Bumper might be on to something.

Polyester reinforced with glass fiber has a linear coefficient of
thermal expansion of 25 (10^-6 m/m K) while structural steel is just
13. *Aluminum is 22.2 which is close enough to steel there may not be
a problem.
Ref:http://www.engineeringtoolbox.com/li...fficients-d_95.....

The fiberglass top is, of course, exposed to the direct sunlight while
the steel trailer frame stays mostly in the shade.

Bill D

Steel trailer frame? Not on mine. I assume Bumper was talking about
relative expansion of the bolts and the clamped parts.

Andy

On May 22, 7:56*am, Andy wrote:
On May 21, 8:30*pm, bildan wrote:









On May 21, 6:25*pm, bumper wrote:

If this problem is primarily affecting Cobra trailers with fiberglass
tops, rather than aluminum, I wonder if the difference in thermal
coefficient of expansion between the two materials might be playing a
roll?

On early Stemme S10-VT motorgliders, the spoiler control rods were
aluminum in an otherwise mostly carbon fiber wing. The spoiler over-
center locks were at the fuselage end. As the temperature dropped with
altitude, the spoilers would first start to come open and then
sometimes even open all the way with no pilot input. Problem was
solved by changing the control rods to carbon fiber.

bumper
zz Minden

Bumper might be on to something.

Polyester reinforced with glass fiber has a linear coefficient of
thermal expansion of 25 (10^-6 m/m K) while structural steel is just
13. *Aluminum is 22.2 which is close enough to steel there may not be
a problem.
Ref:http://www.engineeringtoolbox.com/li...fficients-d_95....

The fiberglass top is, of course, exposed to the direct sunlight while
the steel trailer frame stays mostly in the shade.

Bill D

Steel trailer frame? *Not on mine. *I assume Bumper was talking about
relative expansion of the bolts and the clamped parts.

Andy

I know later Cobra frames are aluminum which is why I listed its
coefficient of expansion. However, I should have said "metal" frame.

I think Bumper was thinking of the top and bottom being firmly
attached to each other at the front and back of the trailer with each
expanding at different rates producing a force on the hinge plates.
Fiberglass tops expand at twice the rate of metal frames. Working the
numbers, it's unlikely the difference could be more than a few mm.

Does opening the top allow you to do individual bolt replacement with
no additional clamping? i.e the struts are fully extended and the
'lift' vector is almost vertical.

KK

3) Do NOT remove these bolts without clamping the
trailer top down and restrained from moving
forward. If you undo the bolts without restraining
the top, even on just one side, even just one bolt,
the gas strut will deliver you a nasty surprise
(as happened to me on the highway to Hobbs).
CAREFUL !


5) I'm designing a clamp to safely hold the top
during replacement of the hinge and bolts...

On May 22, 6:17*pm, Ken K wrote:
Does opening the top allow you to do individual bolt replacement with
no additional clamping? i.e the struts are fully extended and the
'lift' vector is almost vertical.

KK







3) Do NOT remove these bolts without clamping the
trailer top down and restrained from moving
forward. If you undo the bolts without restraining
the top, even on just one side, even just one bolt,
the gas strut will deliver you a nasty surprise
(as happened to me on the highway to Hobbs).
CAREFUL !

5) I'm designing a clamp to safely hold the top
during replacement of the hinge and bolts...

No ! No !!
To clarify #3:
Best to OPEN the top AND THEN clamp the front down...

The "lift vector" is NOT non-existent;
CG of top is AFT of strut attach point and
front of top will pivot around the strut attach
(upwards) and push forward as well...
Unless it is well clamped !
Look at the picture he
http://www.nadler.com/public/2010_Hobbs/2010_Hobbs.html

Hope that helps, From soggy Ridge Soaring,
Best Regards, Dave "YO electric"

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 May 22, 3:17*pm, Ken K wrote:
Does opening the top allow you to do individual bolt replacement with
no additional clamping? i.e the struts are fully extended and the
'lift' vector is almost vertical.

KK







3) Do NOT remove these bolts without clamping the
trailer top down and restrained from moving
forward. If you undo the bolts without restraining
the top, even on just one side, even just one bolt,
the gas strut will deliver you a nasty surprise
(as happened to me on the highway to Hobbs).
CAREFUL !

5) I'm designing a clamp to safely hold the top
during replacement of the hinge and bolts...

What Dave said!

However the clamp does not need to be anything special. I changed my
bolts yesterday using a 2.5 inch C clamp that was quite a bit less
substantial than the one shown in Steve (GW)'s write up.

Trust us - It's far easier to clamp the parts while they are in
alignment than to try to get them back into alignment if you try
without clamping!

Andy (GY)

I have just checked my UK based 15M 1999 Cobra trailer (glass sandwich
top). This has only been towed on reasonably smooth roads (excluding the
odd field) and only very occasionally opened in strong winds .
There is some slight compression to the glass shell at the outer mounting
holes on each handle. When checking the bolt tightness I noticed that the
nuts on the outside mounting bolts (inside the trailer on the hinge plate)
are both not square to the hinge plate as though they were fitted at a
slight angle. (approx 1mm gap is at the top of the bolt ) .
I have vague recollections they had always been like this and assumed at
the time that this was because it was not possible to square everything up
when mounting the bolts at the factory. However, I now wonder if the lid
has moved up slightly inclining the outer bolts.
I “gently” tightened all the bolts a couple of turns (using a small 6”
spanner to limit torque) so it would appear that they were working loose.
I was in a hurry when I did this and seem to remember the extra torque
reduced the gap at the top of the inclined bolts.
Few of questions
- Has anyone else noticed if any of their hinge mounting bolts were at a
slight angle when delivered ?
- Does anyone have a “method” of judging how tight to do these bolts
should be. I am loath to snap them or do more compression damage before I
get round to re-enforcing the handle area and fitting stronger bolts.
- Regarding clamping the hinge to the aluminium cross member. When the
bolts are removed what stops the aluminium cross member from ripping out
of the top shell. I assume it is glued in ?

On May 23, 7:48*am, Ian Reekie wrote:
When the bolts are removed what stops the aluminium cross member from ripping out
of the top shell. I assume it is glued in ?

It is riveted to the top side rails. However one US owner has found
all rivets sheared. All mine seem ok.

In any event you should only change one bolt at a time!

Andy