Cobra top hinge plate bolt failure

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)

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

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...ents-d_95.html

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

Bill D

I have an older (early 1992) Cobra with a fiberglass top. I haven't
spent the past 19 years on the road but the trailer doesn't live in a
hangar, either. I just checked visually and all four bolts are intact
and tight, the hinge plate is tight up against the aluminum cross
member, and there is no sign of movement or looseness or distortion
anywhere. I'll remove the bolts one by one to inspect but I'm
wondering, as at least one other person did, if the cause was simply a
bad batch of bolts, or perhaps improper tightening at the time of
manufacture. For sure, once there's looseness anywhere in the bolted
system, the likelihood of failure rises significantly.

Chip Bearden
ASW 24 "JB"
USA

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.