Sunday, May 15, 2022

Release mechanism completed

The release I built months ago is now welded up and the bottom has been filed flat.  I added silicone grease to all the rotating bits and surfaces.  It's more-or-less ready for a final checkout once I'm ready to go launch.

Mounted on the tow vehicle, the emergency release line has an easy run up through the headrests to the passenger seat, for a no-question easy grab.  Having tried this a dozen times at different orientations, it was really consistent.

I left the main bolt slightly loose (it's a lock-nut, so no issue of it falling out), so the mechanism can swivel left/right to follow the direction of the force off runway center-line.  The steel ring itself can pitch down about 45deg and up around about 90deg, again to follow the direction of the towing force.  I'm happy with it.

The trailer is still in the same state of needing final welds, a top deck, rails, and mounts for holding the wings.  It's now nice summer weather, so maybe this can be the final push between work events.

Sunday, January 30, 2022


It's tacked together healthy enough for a test spin around the neighborhood!  It's a bit flexy in torsion and loooong making turns.  Hopefully it'll stiffen up with a deck on top...

I am still not a good welder, but at least it's stuck together.  I struggled a bit with the 1/4" thick webs of the c-channel pieces making a fillet to another 1/4" thick web.  The tungsten tip kept eroding (exploding?) no matter my changes in technique or settings or type of tungsten (tried blue, green, and red).  Perhaps a thicker tungsten would have helped?  The lowest AC balance I can set is 30% cleaning.  I tried frequency of 40Hz, 60Hz, and 80Hz with no real difference between them.

The other thing difficult was the 1/16" 4043 filler rod would melt before I could get it to the weld puddle.  Is 1/16" just too small for the heat that 150-200A throws off?  Or is the eroded tip making too a wide of an arc cone that melts the filler rod too?  I ran about 15-20 LPH on the Argon flow meter.

Some welds are okay.  Some welds are ugly.  A few welds seem nice.  Here's a nice one:

Welding is done for the weekend because the Argon is all gone.  I only have a 60 cu-ft tank and welded for probably 6-8 hours including a bunch of practice.

I need to finish all the places that should have full welds and go back over a few of the early ones that are not very pretty.  The auto-tow mechanism is also still waiting patiently.

Thanks for reading.

Thursday, January 27, 2022


I'm a bad welder.  But I am welding instead of not welding

It does take a whole lot of stuff to do TIG welding: gloves, helmet, welder, argon, regulator, hoses, welding rods, torch, tungsten, foot pedal, ground clamp, wire brush.  Lots of stuff.  At least it's fun.

I did jump to welding some of the trailer parts the same evening after practicing.  Yes, I couldn't wait.  It doesn't look great, but I think it'll hold enough for this purpose.  I'll get better.

The one bummer so far is that my 110VAC connection doesn't let me push more than 120A (on the 200A welder), and I'm needing more amperage for the thick trailer material.  However, I have a sub panel next to my workbench and can work something out for 220VAC over the weekend.  We'll see when I can sneak to Home Depot...

On a side note, the tow release is waiting until I get better before I waste the several hours of hand filing of the base plate by accidentally over-heating or being bad at fillets.  It'll come.

Saturday, January 22, 2022

Tow release is all but welded

The tow release mechanism is finished, shy of welding and a 3/4-inch hole.  I think it looks awesome.  Here are pictures for both latched and unlatched.  The release arm takes somewhere around 3-10lb of pull pressure to activate the release.

The only difficulty was the tolerance stack-up of the thicknesses of the metal made the latch just a bit on the tight side, so I thinned the latch arm and washers by lapping them on 800 grit sandpaper.  This also polished those parts so they slide smoothly.  I think it looks awesome as a result of the polishing.  I will just increase the spacing of the slots if I ever make this again.

The 3/4-inch hole for the bolt to the hitch is larger than any drill bits that I have, so I'm still working on finding a large bit.  But, a quick test with the 3D printed mechanism with a slightly loose locknut suggests it'll be able to rotate in the yaw direction on the hitch.  (Alan, thanks for the heads-up that is definitely needed)

I haven't figured out how to manage the line tension overall.  Placing a strain-gauge on the hitch metal could work!  For the first flights, I'm hoping to get away without knowing or managing the tension directly.

Welding stuff arrives in a few days...  Thursday's high is 29F, so TBD on how quickly I can actually get to trying it all out though.

Tuesday, January 18, 2022

Tow-release from a car hitch

One hobby deserves another ... a TIG welder and necessary accoutrements are on the way to me from somewhere in China (surely!) and from New Jersey (seriously).  It'll be a few weeks for all the stuff to arrive.  And I'll likely have to figure out a 220VAC plug in the garage.  And burn some metal practicing.  Lots of practicing.  But then, I'll be able to start the trailer welding.  Don't take the sarcasm wrong!  This is exactly what the hobby is all about for me: doing something independently because I can, not because it's the quickest (or least expensive).

In the meantime, I started up a small project that will leverage the welder, and clear a nagging infrastructure equipment issue.  For the auto-launching, I really want a release mechanism for the tow vehicle.  The same Prius that tows the trailer can double as the launch vehicle.  The trailer's tow hitch will be pulled out and a new one inserted, which has the release mechanism already attached.

The main two requirements for the release mechanism are:

  1. Allow the line to be pulled in various directions and still transfer the towing force safely.
  2. Allow the driver to pull a release handle that opens the line.

Personally, I've made a few release mechanisms for RC applications that all worked mostly fine.  The ones that didn't work usually jammed because the pull-pin was side-loaded.  This leads me to reconsider those exact mechanisms.

After some looking around online, the Tost release mechanism is standard in nearly every sailplane.  RC hobby aero-tow folks and hang-glider winch-tow folks have different typical release mechanisms, likely owing to their lower load cases.  In-between is something akin to the Schweizer release, with a simple hook on a rotation pin, and a small lever-arm to release the hook end.  I found the Schweizer release is sometimes used on the back of the towplane to release the sailplane or a banner.  Time to copy the good idea...

I started with CAD to draw up my impression of a Schweizer release and printed a few 3D plastic parts.

An assembly with the 3D printed parts is a wonderful way to understand the mechanism and check its functionality.  This always feels like a wasted day as I impatiently watch the printer print, but ideas and tweaks always come from spending the time staring at a single part as it prints.

With a total assembly, I went to the back of the car to see how it all fits.  And yes there is snow on the ground right now.  I'm not 100% sure how the rope will route to the driver quite yet, but I can confidently say this mechanism will release cleanly once the line is pulled.  The large 3/4" bolt will also let the release mechanism rotate if Goat gets off of the runway centerline.  Seems plausible so far.

With all the fit-checking done, I printed 1:1 scale drawings, pasted on 6061-T6 aluminum with 3M Super 77, and started cutting, drilling, and filing.  Two hours later, it's starting to come together. 

It's working great so far.  Once it's all assembled from metal, I'll weld both the left line-guide tab and weld the release housing sides to the bottom plate.

Hopefully my next post will be of burning metal.

Saturday, January 1, 2022

Probably the last trailer work before welding

I had a nice holiday break.  It was mid-60's at times and so it was wonderful to be in the garage.

I drilled small a hole for the tail light wires and routed them through.  It occurred to me at this point that the UMHW plastic spacer between the aluminum frame and the steel bracket would electrically isolate the ground line.  Doh.  I'll either remove the spacer entirely (and add dielectric grease), replace the plastic spacer with a sacrificial aluminum shim, create a jumper, or maybe just see if the stainless steel bolt does fine.  Thank you for the grease suggestions D.

While doing wiring, I did try out the wiring holes through the holes in the cross-members.  Here is a close-up shot of that, and you can also see how the c-channel mitering works.

As cleanup, I made sure that all seven cross-members had the same fit and exact same length (under a 1/16th).  It's overkill for a trailer, but why do something just half-way at this point?

I also cleaned up the 16 wedge shims with a center mark (no pic, sorry) and figured out where to mount them on each cross-member.  This is prepped for welding.

The amber side lights will wait until after welding.  I do have marked out places for 5x each side, thinking ahead that I won't want someone to merge into a dark space behind my car at night.  It should be hard to miss the giant white wings, but I'm not going to count on that.

I think that may be just about it for the trailer prep.  Welding is next.

Tuesday, December 28, 2021

Just a bit more work on the trailer

Trying to check off some more items from the to-do list.

Here are the new 316 stainless bolts with the UHMW spacers and the wedge shims as well.  I may have the wedges tack-welded in place.

I drilled holes for the wiring in the ends of the cross-members.  This is a low-stress location and the nearby welds should be sufficient support.  I was careful to deburr the drilled holes also to prevent from making a stress-riser of a sharp edge or a crack.  That has the added benefit of not chafing on the wires.

Here is a shot of the angle that will be welded to two of the cross-members, in order to support the end of the 4x8 plywood decking sheets butting together.  I'm confident in this supporting the plywood.

It's progressing.  Work remaining prior to welding:

  • Drill holes for wiring to pass through the ladder rungs' webs.  It's much easier now than later.
  • Mount the trailer tail-light brackets with a UMHW spacer for dissimilar metals.
  • Buy & mount the amber side-lights.  This is probably fine after welding though.
  • Make additional wedge shims and position those for welding

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