COBRA Hydraulic Jack

On Saturday, November 21, 2020 at 8:16:14 PM UTC-8, Steve Leonard wrote:
I think Cobra has now changed to a vertical acting jack like what John has. On at least some trailers.

Steve Leonard
I once built a dual scissors jack for my DG400 using two electric car jacks similar to this one:
https://www.amazon.com/Trihelper-Car...ag=googhydr-20
I replaced their cheap controller with heavy-duty (10A min) switches. The neat part was that I had one jack on a reversing switch. This allowed me to tilt the fuselage which is very useful in getting the correct dihedral angle during assembly. I powered the jacks from the glider's battery.

Tom

On Saturday, November 21, 2020 at 11:16:14 PM UTC-5, Steve Leonard wrote:
I think Cobra has now changed to a vertical acting jack like what John has. On at least some trailers.

Responses/comments from Jay Campbell re my questions:

On photo 0992 and others, there's a return spring that's not connected to anything. Ignore it; it's a relic of a failed experiment. Also note the tubing spacer that slides over the bolts at the ends of the X to keep the arms aligned. In photo 0995, nuts on the inner sides of the arms do the same thing on the rolling ends of the X.

On photo 0993, there's a retaining strap/angle on the lower channel. Not visible is a similar retaining strap on the underside of the upper channel. These keep the mechanism locked together even when picking it up by the upper channel to slide back into the trailer.

The jack is retained at the top by a 1/4-20 bolt (drilled and tapped into the top of the jack post). At the bottom, the two outer clips retain it. This allows the jack to be removed/replaced (note the two jacks--one a spare--in the BOM) when/if it fails even if the fuselage is resting in the cradle (not that this would ever happen).

The range of motion is limited by the compressed/extended height of the jack and appears to be less than on my factory double-scissors jack (factory hydraulic jack may be different). Jay says it's workable but something to keep in mind. I already do that--I don't lower my scissors jack completely so sometimes roll the mainwheel up on a piece of 2x6 to get it in the cradle if the tail is a little high. In the fully extended position, you may have to adjust wingstand height to align the main pins on uneven ground when solo rigging/derigging, as I do. This will also depend on the glider. Those with a lot of ground clearance won't need this (e.g., IIRC, the Slingsby Vega). Those with a lot less clearance will (e.g., older LS3s with weak rubber shock mounts)

Make certain there's room for the jack on the left side in the stowed-in-trailer position. Clearances could be tight so Jay pop-riveted two small pieces of channel to the fold-down door of the trailer to prevent the assembly from sliding into the wing.

Jay mentioned that several other examples of his design are in use by other pilots. Perhaps they could comment on their experiences. Also, if Cobra has moved to a vertical jack design, could someone post photos of that? Thanks.

Finally, a general question for those with hydraulic jacks: I solo rig exclusively so frequently use the fuselage jack to raise/lower the fuselage to align the main pin bores. How difficult is it to make fine adjustments when opening the valve to lower the hydraulic jack? Do you often end up overshooting and then having to raise the jack back up into the right position? In the past, some Komet trailers used a similar design incorporating a worm-drive-Acme-thread jack, similar to what some vehicles use for a spare tire change. This would involve a lot of cranking vs. a few strokes of the jack handle but might be better for fine adjustments in the "down" direction. Thoughts?

Chip Bearden
JB

If it were me, I'd be painting a whole lot of exposed metal on that DIY scissor jack with Plasti-Dip or some other similar rubberized paint. That and some serious shaping of edges, etc.
Craig

I build one with Jay's jacks with his help about 10 years ago.

It works great and is very robust.

Richard
www.craggyaero.com

I built one of Jay's jacks many years ago with his parts list, pictures and prints.

It works great and is very robust.

Richard
www.craggyaero.com

On Sunday, November 22, 2020 at 9:01:16 AM UTC-8, Richard Pfiffner wrote:
I built one of Jay's jacks many years ago with his parts list, pictures and prints.

It works great and is very robust.

Richard
www.craggyaero.com

If you need more clearance in the lowered position a double-acting bottle jack is the answer:
https://www.northerntool.com/shop/to...E&gclsrc=aw.ds

Tom

On Sunday, November 22, 2020 at 6:08:23 PM UTC-5, 2G wrote:
On Sunday, November 22, 2020 at 9:01:16 AM UTC-8, Richard Pfiffner wrote:
I built one of Jay's jacks many years ago with his parts list, pictures and prints.

It works great and is very robust.

Richard
www.craggyaero.com
If you need more clearance in the lowered position a double-acting bottle jack is the answer:
https://www.northerntool.com/shop/to...E&gclsrc=aw.ds

Tom

Just to keep the nomenclature straight: the link shows a two-stage telescopic cylinder and it is still only single acting, i.e. one can power it up but either a spring inside or the load will have to bring it back down. This is NOT a double acting cylinder which could be extended and retracted hydraulically.
This one will most likely only lift the designated 2t on the first stage, which is still way more than needed in this application here.

Uli
'AS'

On Sunday, November 22, 2020 at 7:09:45 PM UTC-8, AS wrote:
On Sunday, November 22, 2020 at 6:08:23 PM UTC-5, 2G wrote:
On Sunday, November 22, 2020 at 9:01:16 AM UTC-8, Richard Pfiffner wrote:
I built one of Jay's jacks many years ago with his parts list, pictures and prints.

It works great and is very robust.

Richard
www.craggyaero.com
If you need more clearance in the lowered position a double-acting bottle jack is the answer:
https://www.northerntool.com/shop/to...E&gclsrc=aw.ds

Tom
Just to keep the nomenclature straight: the link shows a two-stage telescopic cylinder and it is still only single acting, i.e. one can power it up but either a spring inside or the load will have to bring it back down. This is NOT a double acting cylinder which could be extended and retracted hydraulically.
This one will most likely only lift the designated 2t on the first stage, which is still way more than needed in this application here.

Uli
'AS'
The lift height you will need depends upon what is needed to extend the gear fully and still clear the ground. This is glider dependent, but will likely be more than what the first stage can produce. And, more importantly, it has a lower profile to allow the fuselage dolly to be well below the glider's nose. This is critical for positioning the glider to be jacked up. Again, this depends upon the individual glider's ground clearance.

Tom

Which small floor jack at Harbor Freight? There are a whole bunch of them. I would love to replace the bottle on mine.

On Tuesday, November 24, 2020 at 12:00:24 AM UTC-6, wrote:
Which small floor jack at Harbor Freight? There are a whole bunch of them. I would love to replace the bottle on mine.

I believe that it is the $30 cheapest 2 Ton floor jack that I used the bottle from. Lots of spare parts now left over in the junk bin.
https://www.harborfreight.com/2-ton-...ack-64874.html

I have seen the same bottle in cheap jacks at Walmart and other big box stores.

That being said it is always best to take your old bottle with you, plus a ruler, to compare old with new before purchase.

My $0.02,
John (OHM)