Cobra trailer stuck closed

On Monday, June 11, 2018 at 4:20:38 PM UTC-4, Bob Kuykendall wrote:
On Monday, June 11, 2018 at 11:54:14 AM UTC-7, wrote:
Irrelevant.

Fully relevant. Engineers were surprised to discover that under certain conditions of wear and abuse, the PCU would produce outputs that were exactly the opposite of those commensurate with the input conditions.

I've disassembled three or four different types of gas spring (each of my gliders has two), and I'm pretty sure that there are possible failure modes in which the unit resists opening instead of assisting it. They're probably not common failures, but I'd bet solidly against impossible.

Bob K.

Just catching up with this thread now and finding the very belittling response by Steve to my suggestion deeply insulting! When I said that I understand the inner workings of a gas spring and that I designed similar devices, than that is fact based. We carefully took apart several different gas springs and then designed a gigantic version of it - the type that holds down the tag-axle of a concrete mixer truck! It had some other functionalities built in but I will not get into that.
Steve is correct in describing the type of spring he took apart and that the kind of behavior as described by the original poster with those types is unlikely but is that the only type? How does he know that this is the type used in that particular trailer?
As other posters mentioned above, the sniping and 'I know it all and the rest of you are stupid for suggesting otherwise' is highly unprofessional and discouraging towards posting anything. The purpose of this and similar fora is to exchange ideas and help each other out in solving issues we have with our equipment; not to pile-drive others.
N'uff said!
Auxvache, good luck in getting the trailer opened up again and please keep us posted as to what you find out.
Uli
'AS'

Just catching up with this thread now and finding the very belittling response by Steve to my suggestion deeply insulting!

Well Uli, I'm sorry you were insulted by me. As an engineer, I'm definitely not good at dancing around an issue and trying to make people feel good that are 100% wrong on technical matters. But aside from being sorry about hurting your feelings I also think you brought it upon yourself by doubling down and citing your credentials instead of getting to work on google and figuring out how the damn things work when I first rang the alarm bell for you.

Anyway, I'll buy you a beer when we meet someday.

You are good at ignoring evidence that doesn’t suit your opinion though.

On Monday, June 11, 2018 at 9:50:11 PM UTC-7, Steve Koerner wrote:

I'm sorry you were insulted by me.

Passive voice FTW!

On Saturday, June 9, 2018 at 7:42:23 AM UTC-4, Auxvache wrote:
Anyone have any experience with the gas strut failing in the compressed position? My 2002 fiberglass top didn't quite open fully on its own in March, so I ordered replacement struts. Unfortunately, the next time out the top only opened a few feet, then progressively less as I tried to lower and then raise it. Now it is stuck closed and will not open even a crack despite two people hauling on it. Did I mention the glider is still inside? I can't get to the strut from the front door with the fuse and wing in the way.

So, I'm trolling for ideas on how to force the top open without damaging something. Thanks in advance.

Dissuaded from plan A (stuffing the trailer with helium balloons) and plan B (a small explosive charge), I was lucky enough to find a friend with a hoist mounted to the ceiling of his hangar.

We used a 2x4 and small straps to distribute the lifting force on the handles. Hoisted it enough to get the top propped, then disconnected from the handles, moved the trailer back a little, and put a long 2x4 across the inside, hooked the ends with straps to the hoist, and lifted some more. (FWIW, there's not much room behind the rudder as the top gets higher.) The stuck strut put up a commendable fight, and after three feet or so there was a pronounced tilt toward the affected side.

Maybe we should have waited longer to see if it would gradually hiss open, but instead I crawled inside and unbolted the strut from its top attachment.. None of the awful things I was picturing happened, and the strut stayed compressed. Removing it completely, we were interested to find that the whole cylinder had expanded enough to make getting it out of its square sleeve difficult. It eventually accepted defeat and getting the new one in was easy enough.

Thank you again all for your helpful suggestions! Happy flying.

Thanks for the update, Auxvache! Glad you didn't have to resort to plan 'B'.

.... and the strut stayed compressed.
Steve - insert the sound the little birdie makes, when it comes out of the clock, here! ;-)
Uli
'AS'

On Thursday, June 14, 2018 at 7:03:27 AM UTC-4, AS wrote:
Thanks for the update, Auxvache! Glad you didn't have to resort to plan 'B'.

.... and the strut stayed compressed.
Steve - insert the sound the little birdie makes, when it comes out of the clock, here! ;-)
Uli
'AS'

If you took pictures, that would be a good article for Soaring.

On Thursday, June 14, 2018 at 4:03:27 AM UTC-7, AS wrote:
Thanks for the update, Auxvache! Glad you didn't have to resort to plan 'B'.

.... and the strut stayed compressed.
Steve - insert the sound the little birdie makes, when it comes out of the clock, here! ;-)
Uli
'AS'

Umm Uli, are you tripling down on your theory that gas leaked across the piston to cause the original problem even though it has been clearly demonstrated to you that such could not possibly be the case since gas springs have an intentional leak across their piston to establish the damping rate?

By far, the normal way for a gas spring to fail is simply for the nitrogen to leak out the stem seal. This apparently wasn't a normal failure since the OP says the tube is distorted. With mechanical stuff there's always an infinity of ways things can break. Yet knowing how stuff works is almost always helpful. It does allow you to scratch off dumb suggestions from people who allege credentials but don't really know what they're talking about. I hope I'm not hurting your feelings again.

Steve, in the catalogue of the same firm (Dictator) you mention, there is at least one gas spring type that works thanks to the difference between outside air pressure and inside nitrogen pressure. Granted, it is a pulling spring, while all others are the pushing kind that work as you describe it. It simply means there are different systems possible.


One of my club's trailers also had a strange gas spring failure once, luckily on an outside system: the spring gradually refused to open completely, after some time it opened only 8 inches or so. It felt as if it was blocked by some mechanical lock. But it still could be compressed for the remaining stem length, so it wasn't a blocked leak hole nor loss of nitrogen pressure. Any ideas on what went wrong?

we were interested to find that the whole cylinder had expanded enough to make getting it out of its square sleeve difficult.

Any ideas what caused this? Rust? Gradual deformation over time (I haven't seen that before)?

I dealt with a few strut failures on an older Komet trailer where the gas springs were mounted externally and exposed to road grit. But the gas springs in my 26-year-old Cobra are still going strong, in part--I suspect--because they're mounted in the correct orientation and are protected inside the square tubing, which is inside the trailer. But I've never actually inspected them.

Chip Bearden