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