View Full Version : Cobra trailer stuck closed
Auxvache
June 9th 18, 12:42 PM
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.
With zero pressure in the struts, you've got to lift the whole weight of the top and it's heavier than hell! I used a block and tackle from the shop roof. The rear handles were strong enough to take the load on a Komet..............Should be OK on a Cobra. Have 2X4's ready to prop it open and be carefull ..............its little more than a gilatine! Three man job, one on the hoist and two placing props.
Let us know how it worked out,
JJ
Guillotine (https://en.wikipedia.org/wiki/Guillotine)
Agree with JJ. (but not with his spell-checker) Use a hoist to lift by the handles, but only far enough to put a couple of blocks under the lid. Then run a strap all the way around the trailer top so when you are raising it and working under it, you aren't relying solely on the handles and their attachment points.
Also, since it is now attached to the roof of your shop, and is at a serious angle, make sure to block the wheels, set the brake and use some method of securing the strap so it cannot try to slide off the end of the trailer top.
Uh.. I forgot the glider is still inside, so JJ is correct- raise the lid all the way, prop it up and hope the handles hold while raising it. No way around it as far as I can see.
son_of_flubber
June 9th 18, 03:44 PM
On Saturday, June 9, 2018 at 10:34:57 AM UTC-4, wrote:
> Uh.. ... hope the handles hold while raising it.
Plastic handles on my Avionics Trailer became brittle and broke after minimal sun exposure.
JS[_5_]
June 9th 18, 04:31 PM
On a Cobra trailer for Duo Discus, I used a hoist in a hangar to the handles then ran a sling under the lid (heavy ratchet straps, could be adjusted to take the weight off the handles) while replacing a dead strut. Yes, heavy and above a height you'd want to lift to.
Requires pulling the wing out to easily access the hardware and remove/replace the strut.
Initially one strut had failed on this trailer, two were purchased but only one replaced. Suggest that if one strut fails, replace both as soon as you can.
This job is easy with the right tools.
The rod ends from the original struts may be needed for the new ones.
Jim
>> With zero pressure in the struts, you've got to lift the whole weight of the top and it's heavier than hell! <<
It may be worse than that: If the piston seals failed but the rod seals held, all the gas pressure is now on the annulus or ring-side of the gas spring thus keeping the rod from extending swiftly. Applying up-pressure on the lid and then waiting may do the trick.
Good luck!
Uli
'AS'
Steve Koerner
June 10th 18, 02:21 AM
Uh-oh, someone doesn't understand how gas springs work:
> It may be worse than that: If the piston seals failed but the rod seals held, all the gas pressure is now on the annulus or ring-side of the gas spring thus keeping the rod from extending swiftly. Applying up-pressure on the lid and then waiting may do the trick.
> Good luck!
> Uli
> 'AS'
On Saturday, June 9, 2018 at 9:21:30 PM UTC-4, Steve Koerner wrote:
> Uh-oh, someone doesn't understand how gas springs work:
I do - I used to design similar devices....
Uli
'AS'
Jim White[_3_]
June 10th 18, 09:25 AM
At 03:22 10 June 2018, AS wrote:
>On Saturday, June 9, 2018 at 9:21:30 PM UTC-4, Steve Koerner wrote:
>> Uh-oh, someone doesn't understand how gas springs work:
>
>I do - I used to design similar devices....
>Uli
>'AS'
>
It might help to unbolt the hinges and lift the lid evenly with two
winches?
On Sunday, June 10, 2018 at 4:30:12 AM UTC-4, Jim White wrote:
> At 03:22 10 June 2018, AS wrote:
> >On Saturday, June 9, 2018 at 9:21:30 PM UTC-4, Steve Koerner wrote:
> >> Uh-oh, someone doesn't understand how gas springs work:
> >
> >I do - I used to design similar devices....
> >Uli
> >'AS'
> >
> It might help to unbolt the hinges and lift the lid evenly with two
> winches?
LAST resort only.
Putting top back on is a very big deal.
UH
Dave Nadler
June 10th 18, 02:16 PM
On Saturday, June 9, 2018 at 5:42:23 AM UTC-6, Auxvache wrote:
> ... So, I'm trolling for ideas...
Sell it as is where is?
Bidding starts at?
Dave Nadler
June 10th 18, 02:20 PM
On Saturday, June 9, 2018 at 5:42:23 AM UTC-6, Auxvache wrote:
> ...So, I'm trolling for ideas...
Seriously, be VERY careful.
Even with an over-head hoist, incrementally support the top with 2x4s
under top's edge-channel to ground, and keep the trailer attached to a car
(with brakes/park set).
Most likely you're going to have to repeat this;
1rst time to get the old stuff out, 2nd time to install new struts.
Careful!
And don't forget to post pictures ;-)
George Haeh
June 10th 18, 04:49 PM
The manufacturer does this all the time. It would be worthwhile contacting them for the correct procedure and equipment.
Steve Koerner
June 10th 18, 05:46 PM
On Saturday, June 9, 2018 at 8:22:46 PM UTC-7, AS wrote:
> On Saturday, June 9, 2018 at 9:21:30 PM UTC-4, Steve Koerner wrote:
> > Uh-oh, someone doesn't understand how gas springs work:
>
> I do - I used to design similar devices....
> Uli
> 'AS'
Well Uli let's talk gas springs for a moment...
If you actually did understand how gas springs work - the everyday gas springs used for car tailgates and glider canopies and Wing Riggers and Cobra trailers - then you would know what a silly suggestion that was. Gas springs already have a controlled leak across the piston. That is what establishes their wonderful damping property. Once stopped, there is no pressure differential across the piston at all. The nominal force produced by a gas spring does not relate to pressure across the area of it's piston (as is the case for a pneumatic drive cylinder); the gas spring force arises from the pressure exerted across the cross-sectional area of the rod itself.
Anyway, don't feel too bad about getting this wrong. I did a google search so that I could provide a link for you. Amazingly, virtually every little story that came up about how gas springs work on the internet was also wrong -- including the wikipedia article on gas springs and a bunch of youtubes on the subject. It wasn't until I downloaded a lengthy manufacturers catalog that I came upon actual sound science that tells the story correctly:
http://pdf.directindustry.com/pdf/dictator-technik/dictator-gas-springs/15657-175377.html
So back to the original poster's issue: You do not need to worry about pressure leaked to the wrong side of the piston and holding the clamshell down.. That is a red herring of the highest order.
Scott Williams
June 10th 18, 07:26 PM
On Saturday, June 9, 2018 at 6:42:23 AM UTC-5, 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.
Why does my cobra top not slowly close? it stays open until I drag it down.
I must admit ignorance of the internal demon that makes it work.
respect,
Scott
Steve Koerner
June 10th 18, 10:26 PM
> Why does my cobra top not slowly close? it stays open until I drag it down.
> I must admit ignorance of the internal demon that makes it work.
> respect,
> Scott
Because it has two gas springs holding it up.
Or are you asking how does a gas spring work? That particular question was just answered and a link provided to a concise explanation with diagrams.
Auxvache
June 10th 18, 10:30 PM
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.
Thanks very much everybody for the helpful replies. FWIW, the top definitely feels actively pulled down, not just mass + gravity. We'll try lifting and blocking it in steps as suggested. And yes photo documentation--probably going to be good for some laughs...
Steve Koerner
June 10th 18, 10:46 PM
On Sunday, June 10, 2018 at 2:30:34 PM UTC-7, Auxvache wrote:
> 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.
>
> Thanks very much everybody for the helpful replies. FWIW, the top definitely feels actively pulled down, not just mass + gravity. We'll try lifting and blocking it in steps as suggested. And yes photo documentation--probably going to be good for some laughs...
Even with no pressure in the cylinders, you'll still have damping force that will be resisting the rate of motion. If you lift slowly enough the damping force approaches zero; then all you'll have is gravity to deal with.
Steve Leonard[_2_]
June 11th 18, 12:36 AM
On Sunday, June 10, 2018 at 8:16:04 AM UTC-5, Dave Nadler wrote:
> On Saturday, June 9, 2018 at 5:42:23 AM UTC-6, Auxvache wrote:
> > ... So, I'm trolling for ideas...
>
> Sell it as is where is?
> Bidding starts at?
I will give a hundred dollars for it.
:-)
Steve Leonard
I have dealt with two Cobra trailers that have had one strut "blow out". one was spectacular sounding. Closed the lid, then PSSSSSSSTTTT! Thought I had pinched and popped a tire! The other died when I was not there, so don't know how quickly it went. Replacing them one at a time on a metal topped trailer for a 15 meter glider just required a prop to hold it open. Glider was out, no big deal. The one on the ASH25 trailer, well, that was another story!
> I have dealt with two Cobra trailers that have had one strut "blow out". one was spectacular sounding. Closed the lid, then PSSSSSSSTTTT! Thought I had pinched and popped a tire! The other died when I was not there, so don't know how quickly it went. Replacing them one at a time on a metal topped trailer for a 15 meter glider just required a prop to hold it open. Glider was out, no big deal. The one on the ASH25 trailer, well, that was another story!
Most Cobra trailers have a hole drilled through one strut (the port strut for my trailer) so that a pin can be inserted to ensure the top stays fully open. I assume the purpose is safety, to prevent the top crashing down if a strut fails.
Charlie Quebec
June 11th 18, 05:14 AM
So younke-tbpooking until you found an answer that agreed with you, then ignored all other evidence?
Don't be an ass, CQ. Steve knows his stuff. Gas springs do not just have pressure on one side of the piston. It is the volume of the rod that compresses the gas as it retracts and takes up more space in the cylinder. The small orifice in the piston that allows gas to pass by the piston also regulates the rate at which the rod extends.
WingRiggers as developed by Steve Koerner and now produced by MM Fabrication use gas springs, so the design and mechanics are well understood by us. Plus, we just had to cut a few apart for fun.
Bob Kuykendall
June 11th 18, 07:25 PM
On Monday, June 11, 2018 at 5:27:01 AM UTC-7, wrote:
> ...Steve knows his stuff...
The guys who designed the 737 rudder PCU knew their stuff.
You know.
These forums are degrading into gutter talk and condescension way too often and way too quickly. People being off topic should be immediately blocked. Those with verbal trigger fingers should chill out.
GROW UP !!!
Bob Kuykendall
June 11th 18, 09:20 PM
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.
Charlie M. (UH & 002 owner/pilot)
June 11th 18, 09:32 PM
In general, yes, forums have some volunteer staff.
RAS seems to be outside this.
I am staff on other forums, good luck on "moderating" on RAS.
Hopefully I don't fall under your negative comments......
Whatever, there IS good info here, just takes some sifting......
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'
On Sunday, June 10, 2018 at 5:46:03 PM UTC-4, Steve Koerner wrote:
> On Sunday, June 10, 2018 at 2:30:34 PM UTC-7, Auxvache wrote:
> > 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.
> >
> > Thanks very much everybody for the helpful replies. FWIW, the top definitely feels actively pulled down, not just mass + gravity. We'll try lifting and blocking it in steps as suggested. And yes photo documentation--probably going to be good for some laughs...
>
> Even with no pressure in the cylinders, you'll still have damping force that will be resisting the rate of motion. If you lift slowly enough the damping force approaches zero; then all you'll have is gravity to deal with.
I like Steve's idea. Maybe use an engine hoist or such on the rear handles with a bit more than top weight( say 200 lb), walk away and see if it starts to creep open.
Another voice hear from.
UH
Andy Blackburn[_3_]
June 12th 18, 12:52 AM
I admit to being a little surprised that the manufacturer of these fine trailers (which I mean sincerely) hasn't made some simple provisions in the design (along with a standard design for a lifting rig using off-the-shelf components) such that mere mortals could open the lid and replace the gas spring. The risk of messing this procedure up is pretty substantial.
Maybe they have such a design and procedure?
Andy Blackburn
9B
Charlie M. (UH & 002 owner/pilot)
June 12th 18, 01:46 AM
So you want to take work away from mechanical types?!?!
LOL........
Yea, this is sorta a "no win" if the struts fail, especially if the glider is still in the box.
But hey, what would MacGuyver do?
And yes, most front hinged trailers have a VERY heavy top without the gas struts helping.
Dave Nadler
June 12th 18, 03:25 AM
On Monday, June 11, 2018 at 5:52:20 PM UTC-6, Andy Blackburn wrote:
> I admit to being a little surprised that the manufacturer...
Consider yourself fortunate to not know how sausages are made...
See ya, Dave
Steve Koerner
June 12th 18, 05:50 AM
> 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.
Charlie Quebec
June 13th 18, 03:14 AM
You are good at ignoring evidence that doesn’t suit your opinion though.
Ben Coleman
June 13th 18, 05:27 AM
I had the strut on my Komet "fail closed" and it took a lot of time and pressure to extend. I assumed the bleed hole/valve was blocked somehow, the end result being the gas trapped on the shaft side of the piston.
Fortunately the other strut was working which made manual recovery possible.
Cheers Ben
Auxvache
June 14th 18, 11:47 AM
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'
Dan Daly[_2_]
June 14th 18, 12:19 PM
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.
<<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
Steve Koerner
June 14th 18, 05:56 PM
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.
Bob Kuykendall
June 14th 18, 07:18 PM
Just curious, did you note whether the orientation was thin rod up or thin rod down?
--Bob K.
Bob Kuykendall
June 14th 18, 07:21 PM
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! :)
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?
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