JS3 Glider Tiedowns

This is a set of tiedowns for a Jonkers JS3 18 meter glider. The glider came with no provision for tying the wings down, which you need to do if you want to keep the glider assembled. This design provides a safe way to do that. 

My design goals were:

• Protect the flaperon, even under yawing loads

• Failure mode doesn't leave the glider untied

• Easy to manufacture (3D printed)

• Attaches to the wing at a strong point

• Spreads the load evenly across the airfoil

• Securely holds the strap

The tie-down is a cuff that slides over the wing tip, and down to the first bend, where there is a rib and the spar is thicker. This is about where the wingtip wheel is. I think this location gives a good balance of leverage for holding the wing down, and a strong enough structure in the wing to carry the load. 

The strap runs through a slot inside the cuff. There is no way the strap can slip off the cuff, and it's protected from UV light as a bonus. The strap follows a curved path over the top of the wing, evenly spreading the load over the airfoil. This also has the effect of making the cuff thickest right where the most strength is needed: above the trailing edge. The interior profile of the cuff has a flared section at the trailing edge to give the flaperon some breathing room. It's this cantilevered area that is most likely to fail under load. However, the worst thing that could happen is that the cuff breaks and allows the strap to put a load on the flaperon. As long as the strap itself doesn't fail, the glider will remain tied down. 

In the case of a strong crosswind, a tied down glider will want to yaw into the wind. Tying down the tail helps, but there will inevitably be a load on the upwind wing where the cuff is pulling down on the wing at an angle going forward, if that makes sense. The cuff has to resist crushing the flaperon as it's being pulled forward. That's why I chose a cuff design that goes all the way around the airfoil, and fits tight to the surface. At the trailing edge, the strap pulling forward will force the cuff open, trying to move it to where the airfoil is thicker. The cuff will apply the load to the wing just forward of the flaperon, which prevents the cuff from sliding forward and causing damage. I copied the airfoil of the JS3 wing in that area using a profile gauge, then scanned the gauge and traced it in OnShape. This resulted in a pretty accurate reproduction of the airfoil, and the fit is near perfect. 

I tested the design in two ways. I 3D printed a dummy wing section, and put the cuff over it. The wing section was suspended at the top of a folding ladder, with the strap hanging down under it. I stood on the strap, with my hands on the ladder to steady me. Then I jumped up and down on the strap as hard as I could, using all of my weight. I broke a few prototypes this way, but the final version didn't break. Of course, at my weight, I can't put as much force on the cuff as a real glider wing could, but it was a promising start.  Next, I put the scissor jack from my SUV between the cuff and the strap, and cranked it open. The strap got extremely taut, and it was a bit scary. I stopped when I noticed a little damage to the cuff, and the jack itself was starting to deform.  

I was happy to see that the cuff didn't fail suddenly in an explosion of plastic. It failed gradually, right where I expected it to, at the trailing edge. This is probably due to the selection of filament (PET-CF) and the infill type (gyroid). After this test, I reduced the size of the flare at the trailing edge, to add a little more material there. I won't be retesting, though. For me, this has demonstrated that the tie down is strong enough for what I need it to do. It probably won't shrug off a microburst, but I think this tie down will resist a fairly strong and gusty afternoon breeze.

The strap I chose is from NRS, which is a major brand name in the rafting/kayaking world. I used to have a set that I tied my kayak to my car roof with, and I've never seen a higher quality cam strap. Beware of cheap cam straps. The cams are usually garbage, and will fail way before the webbing does, or they will just let the webbing slip right through under load. The NRS strap is rated for 1,500 lbs, but my guess is that it held more than that in my scissor jack test. It bent the jack, after all. 

You can see where the cam bit into the strap, but it doesn't look compromised. I highly recommend these straps. I bought the 15' by 1 inch straps. You might be able to get away with the 12' ones, but having extra length doesn't hurt in case you want to tie a knot in the end, for extra security. You can always cut it down, too. https://www.nrs.com/nrs-1-hd-tie-down-straps/p4yc 

For those wanting a more compact solution for travel, I also made a version of the cuff that goes over the junction where the winglet and wing meet. They are shorter than the other version, and are probably better than just putting a bare strap over that area. They would prevent the strap from sliding over the flaperon and causing damage. I also came up with a tidy way of wrapping the strap around the cuff that doesn't require pulling it out of its slot. It works on the larger one too. 

The material I chose is PET-CF. It has a nice mix of strength, rigidity, UV resistance, creep resistance, and printability. At $35 a roll, it's a premium filament, but not crazy expensive like carbon fiber nylon. I don't see a need to use exotic materials for this application. If you want to make it stronger, you could simply make it thicker, bigger, etc. Strength to weight ratio is not a concern here. I like the Siraya Tech PET-CF: https://siraya.tech/products/siraya-tech-fibreheart-pet-cf-filament 

Printing a pair of the larger cuffs uses about 2/3 of a roll, so about $23 worth of filament. I also lined the inside of the cuffs with felt tape, to protect the paint. Carbon fiber filled plastic is fairly abrasive, and you wouldn't want it rubbing on your wings. That tape is here: https://a.co/d/0ji3TuVE

The CAD file can be accessed at OnShape. It could be adapted to other gliders by altering the airfoil shape. https://cad.onshape.com/documents/b271d78f7bf840f01442f810/w/16460d42c0b56955aaee7c91/e/d44be6b9e67592d81c5369d1?renderMode=0&uiState=6ae0d4af815ea87e3494e1ae

The model includes designed supports, instead of auto-generated ones. They do the job and remove cleanly.