Sling Wing - Folding Wing Slingshot Glider

Intro

I love flying wing gliders, but they're very difficult to launch by hand with any significant flight time due to instability at high speeds and typically large aspect ratio. I came across expired US Patent 4836817 for a folding wing glider and have spent the past few weeks designing and testing a printable flying wing based off of that concept. 

The main design complexity is to get the center of gravity, center of lift, and center of drag to work together in both the folded and open configurations to retain good flight and launch characteristics. Secondary complexities were balancing glide performance with roll characteristics and wind tolerance per my wind goal and optimizing printability vs lift/airfoil characteristics. 

Please read through before printing and flying - there's critical information below!

Design Goals (all achieved)

• Printable with no supports on a Mini+ or larger (180 mm wing only)
• Mainly printed parts (low cost)
• Easy assembly
• 80+ ft launch height
• 30+ second flight time
• Hand launch or slingshot launcher with rubber bands or tubing
• Glide ratio 8:1 or better
• Flyable in a light breeze (tolerant of 7+ mph wind speed)

BOM

• Aircraft
  • 1.88" W Tape (clear packing tape ideal, but a surprising variety of tapes including painters tape will work)
  • Wing load rubber bands, #8 or #16 (25 mm x 1.5 mm or 60 mm x 1.5 mm)
• Launcher
  • Launch rubber bands/tubing/elastic (The system will work with many options but the below have been tested. See assembly section for more info).
    • 2x #64-#68 rubber bands (90-140 mm x 6.0 mm)
    • ~400 mm (16 in) long 6 mm OD rubber tubing
  • Flexible string/cord (nylon or cotton ideal)

Print Settings

The Slingshot has internal cuts for added wall lines to improve strength. The SlingWing print is basically a first layer test. Ensure you can lay down a nice first layer! 

I've tried to ensure a wide flight envelope and good performance without needing to use low-weight or foaming filaments, but the following (especially the ones marked with *) are critical print settings that can make or break performance. 

• 0.4 mm nozzle (0.6 mm with 3 wall lines should work, but untested)
• 0.2 mm layer height (tested, <0.2 mm should also work)
• 5 wall lines*
• 0.2 mm first layer* (SlingWings only)
• 10 top and bottom layers* (SlingWings only)

As everything affecting the CG balancing is printed (except the rubber bands), this print works fine with PLA, PETG, ABS, and should work with foaming/LW filaments as long as you can get a good first layer down (though lighter filaments will have lower wind tolerance).

Assembly

Please ignore the dings, dents, and other damage in the pictures. These planes have gone through a lot of flight testing already.

Aircraft:

1. Put a piece of tape about 1 inch wide on the back of one wing. Lay wing flat (tape facing up) and press the other side on. The wings should meet at the bottom and have a gap at the top which will form the dihedral angle. 
    
    
2. Attach two #8 rubber bands or one doubled #16 rubber band to the wing hooks. One #8 rubber band may be used, but for extremely light wind only (<2 mph).
   
3. Give the wing some reflex by adjusting the elevons up equally (helps to use a flat ruler to bend and/or heat them a bit, especially if PETG) and then do some glide testing. You should be able to smoothly glide for 40-80 feet from a gentle throw from ~6 feet high, and the aircraft should be able to recover from a 0 speed drop/stall in <6 feet of height (180 mm wing)
   
4. Test and adjust with a basic glide release (by hand)
   

Launch System:

Note: I've found shorter, stronger elastic is best for this application. Somewhere around 160-200 mm long (relaxed length) the chance of the bands/tubing twisting, pulling to the side, or colliding with itself as it relaxes goes up dramatically (especially for the tubing). This will ruin launches and possibly damage the aircraft.

1. Cut ~10 in (~255 mm) of flexible string/cord and tie it to the StringRings.
    
    
2. A) Rubber bands 
   Loop the rubber bands through the LaunchCatch holes and back into themselves, making sure they're secure and not rolled. 
   
   Stretch and slide the rubber bands through the quick entry cuts in the slingshot handle.
   
   
   B) Tubes
   Cut tubing to length (150 mm recommended) and pull through the holes in the Slingshot and StringRings. Ensure the tubing is not twisted/looped. Insert the TubePlugs to hold the tubing in place.
   
    
   Fold the tubing in through the top clips in the slingshot, if desired

Launching/Flying

This aircraft is capable of flying for a long time and quite far, but it depends on tuned elevons, a good launch, and calm wind (it can fly in wind up to 15 mph, under 7 mph is ideal).

To launch, slide the folded aircraft onto the string, pinch the string together, and pull it over the hooks. Hold at the wing tip as close to the elevons as possible without clamping them, either just ahead or just above. This will align the CG with the launch force and main drag components. Stretch and release.

If you don't hold inline, or if your tubing/rubber bands are twisted or too long and begin to collide with themselves as they relax during launch then your aircraft will spin and/or hit the slingshot, ruining the launch. 

The aircraft should be launched vertically, or slightly angled into the wind (if any) as it must stall for the wings to pop open. If launched in a smooth arc (<~80 deg) the wings are unlikely to open at the apex.

The aircraft can also be launched by hand in an open position by placing one or two fingers above the center (as shown) and gently tossing it. 

Changelog

• 2023-03-22: Updated internal slingshot cuts so default slicer gap closing radius works
• 2023-03-20: Release 2, version 34. Updated launch system with StringRing, changed dihedral to 3°, changed elevons slightly, changed center area to improve weight distribution and increase wind tolerance (tested in 15 mph wind). Launches are much more reliable now as well. Added 230 mm wing design, added print files for MK3S+.
  Big change to launch system: the LaunchCatch could adjust the trajectory of the aircraft during launch as the bands relaxed. I was prototyping a rather complicated hinged print-in-place LaunchCatch that would snap open as the bands relaxed to get out of the way… and then the moment every engineer dreams of - I realized I could do it better with 2 loops and a string :)