Cobra top hinge plate bolt failure

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.