JACKKNIFE SWING-WING BUILD THREAD

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Resurrected an old BAR project back from when I was a newbie: a swing-wing, my very first scratch-built glider. If ignorance is bliss, then it’s true I really didn’t know what I was in for. I just jumped right into the build enthusiastically without much more than a sketch and a mind sim. 😬

The one major class of variable geometry glider I’ve always wanted to scratch build is a swing-wing. At the time, building a glider that required moving parts might have been an overly ambitious goal, since I'd never tackled the mechanics of it despite its relative simplicity. So I built one, and there was no problem with it gliding nicely in hand tosses. No, the problem was the squirrely launch where it did two wide, 360° corkscrews on the way up. Then there was the problem with the burn strings not completely releasing the wings no matter how strong the elastic was.

So now with a few years more experience and improved building skills, I decided to resurrect my former shelf-queen and dissect what went wrong.

1) Because there was a minute amount of play between the wings and the wing holder panels, it may have contributed to the less-than-straight launch trajectory.

2) Also the angle of attack of both wing holder panels was slightly different on each, about a quarter degree negative on one and the other maybe a half degree positive. Didn’t notice it at the time, and I think it happened when I sanded in the dihedral. Guess some or all of those things may have something to do with the less-than-stellar launch behavior.

So this time around I decided to really simplify things.
  • First, go with a dihedral-less swing-wing. Entirely workable based on recent experience with other dihedral-less scratch-built projects. Then there were the production kits like the Edmonds Aerospace Twinsee, Ecee, CiCi, etc. which were designed without dihedral to make his kits easy to build even for kids. Estes also had a line of dihedral-less swing wings like the Crusader and Tomcat, as well as their current Gryphon.
    So this will necessitate a swept-wing design for this project.

  • Attaching the wings and support plates flat should take a lot of the alignment problems out of the equation.

  • I’m also going to mount the burn thread wing hold-backs closer to the forward end. Routing the thread near the aft end caused them to tangle and not release the wings fully or consistently.
Small changes, but I trust they'll make all the difference in reliability and workability.

Sketch of what I’m planning:

Jacknife sketch.jpg
 
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Started in on the forward fuselage. The rest of it back to the stab will be a carbon rod. The hole up front is where the deployment elastic will be threaded.

forward fuselage.JPG
 
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Fabricating wing holder panels. Wing stops are hard balsa sandwiched between 1/32” ply.

wing holder panels.JPG


Went old school. Wing pivots are simply 3/16” dowels. Plastic straws with the perfect inside diameter serve as bearings.

plastic straw bearing-dowel pivot .JPG


Plastic bearing installed and trimmed flush with the surface of the wing holder panels.

flush trimming plastic bearing.JPG


To be continued...
 
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The wing root edges where they will contact the stops receive basswood inserts to reinforce this area.

basswood reinforcement to root edg.JPG


Wing assembly in launch position:

pivot launch position.JPG


Wing assembly in glide position:

pivot glide position.JPG


Dry fit of the dowel pivot pin and “lock washer” assembly (tight fitting 1/64” ply squares).

dry fit dowel pivot - lock washer.JPG
 
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Anchor pin for elastic installed. An offset guide pin was added to get a better deployment angle. This way the wings are initially pulled outward instead of just pulling them forward (which by sad experience I know can keep them locked in their launch position).

elastic anchor and offset guidepin.JPG

deployment angle.jpg
 

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Resurrected an old BAR project back from when I was a newbie: a swing-wing, my very first scratch-built glider. If ignorance is bliss, then it’s true I really didn’t know what I was in for. I just jumped right into the build enthusiastically without much more than a sketch and a mind sim. 😬

The one major class of variable geometry glider I’ve always wanted to scratch build is a swing-wing. At the time, building a glider that required moving parts might have been an overly ambitious goal, since I'd never tackled the mechanics of it despite its relative simplicity. So I built one, and there was no problem with it gliding nicely in hand tosses. No, the problem was the squirrely launch where it did two wide, 360° corkscrews on the way up. Then there was the problem with the burn strings not completely releasing the wings no matter how strong the elastic was.

So now with a few years more experience and improved building skills, I decided to resurrect my former shelf-queen and dissect what went wrong.

1) Because there was a minute amount of play between the wings and the wing holder panels, it may have contributed to the less-than-straight launch trajectory.

2) Also the angle of attack of both wing holder panels was slightly different on each, about a quarter degree negative on one and the other maybe a half degree positive. Didn’t notice it at the time, and I think it happened when I sanded in the dihedral. Guess some or all of those things may have something to do with the less-than-stellar launch behavior.

So this time around I decided to really simplify things.
  • First, go with a dihedral-less swing-wing. Entirely workable based on recent experience with other dihedral-less scratch-built projects. Then there were the production kits like the Edmonds Aerospace Twinsee, Ecee, CiCi, etc. which were designed without dihedral to make his kits easy to build even for kids. Estes also had a line of dihedral-less swing wings like the Crusader and Tomcat, as well as their current Gryphon.
    So this will necessitate a swept-wing design for this project.

  • Attaching the wings and support plates flat should take a lot of the alignment problems out of the equation.

  • I’m also going to mount the burn thread wing hold-backs closer to the forward end. Routing the thread near the aft end caused them to tangle and not release the wings fully or consistently.
Small changes, but I trust they'll make all the difference in reliability and workability.

Sketch of what I’m planning:

View attachment 487085
Is this Swing Wing similar to the Tomcat Swing-Wing Glider?
 
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Is this Swing Wing similar to the Tomcat Swing-Wing Glider?
IIRC the Tomcat was based on a BT-60 main body and used an ejection-activated valve to release the wings. My swing wing is more old-school basic and uses a burn string to keep the wings folded.

The Tomcat was cleverly engineered IMHO, and I'm real tempted to build one of the repro clone kits that MoreRockets sells.
 

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IIRC the Tomcat was based on a BT-60 main body and used an ejection-activated valve to release the wings. My swing wing is more old-school basic and uses a burn string to keep the wings folded.

The Tomcat was cleverly engineered IMHO, and I'm real tempted to build one of the repro clone kits that MoreRockets sells.
I believe jh aerospace has some real nice swing wing type of rocket gliders, etc.
www.jhaerospace.com
 
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I believe jh aerospace has some real nice swing wing type of rocket gliders, etc.
www.jhaerospace.com
Agreed. I have some of his swing wings and they're great. The guy's an engineer so the wing retention and deployment design are really efficient and unlike what's come before (it doesn't use a burn thread to keep the wings in launch position for example).

What I'm working on is something that can use an 18mm motor and is larger, so it's easier to track and recover. I'm sure it won't have the duration of jhaerospace's gliders, but I'm okay with anything that can stay airborne for 1 - 2 minutes that I can keep eyes on or fly in a smallish field and not have it thermal away.
 
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Finished up the pylon and motor pod and attached it today. The hole for the elastic in the front part of the fuselage was reinforced with a launch lug which was then trimmed flush.

That’s about all the time I’ll have to devote to the build today, too many other chores have my name written on it that are nagging at me to get done.

pylon-pod attachment.JPG

To be continued...
 
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The offset elastic guide pin needed something (a cap) to keep the elastic from slipping off. Fortunately the perfect solution presented itself in the form of an igniter plug with its stem removed. I pushed a sewing needle through it as an anchor, and cut off all but about 1/4”…

elastic guide pin  cap-1.JPG


…then tapped a hole in the small dowel that served as the offset guide pin.

elastic guide pin-tapped.JPG


Needle was cemented in and works like a charm.

elastic guide cap installed.JPG
 
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Almost done. Just need some heat protection on the wing holder panels to shield it from the motor exhaust plume. The pylon/fuse joint also needs a bit of reinforcement, so I’ll add a strip of 1/64” ply there.

Jacknife-1.JPG

Jacknife-2.JPG

Looking forward to its initial flight. That’s going to have to wait till the winds settle down some. According to the weather forecasts, that won’t be for several weeks unfortunately.

Overall, I’m pretty satisfied with the way it turned out. Since it’s more of an experimental build, I’m really more interested in seeing if it’ll work or not, instead of how long it stays aloft. It either works or it doesn’t, and I’m good either way because it will have satisfied my curiosity.

To be continued...
 
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LAUNCH REPORT

Test launched it today…or tried to. Didn’t go well — hung on the rod just long enough to crisp the wing support plate, then shook itself loose…only to CATO about 20’ up. 😱

Guess it was a bad motor. Or…it could’ve been my fault because the motor fit too tight (I wrapped a small amount of tape around it to keep it from falling out every time I attached the clips. It was meant to spit the motor immediately. If it didn’t, I suppose the few microseconds of pressure buildup from the ejection charge could’ve blown things apart)? I’ll never know for sure, I never found the motor casing after searching something like a 50’ circle around the launch pad. I never found the nose cone or portions of the fuselage either for that matter.

I’m going to salvage as much as I can. I’m hoping I can free the wing hinge/support plate and wings from what’s left of the fuselage, since it’s the most difficult to replicate part. I also need to patch the stab (which probably got damaged when the glider pinwheeled down horizontally after the “event”).

To be continued...

CATO.JPG

tail damage.JPG
 
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Repairs will probably take longer than expected. I had to dismantle the whole forward part of the glider. Fortunately I was able to rescue the wing holder panels. The burned area under the aluminum tape was more extensive than it appeared before I stripped it off. Scraped away the charcoaled areas and filled it with CWF and balsa dust, then doped it over.

dismantled.JPG
 
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Everything still seems to be in working order as far as the hinges.

IMG_7247.JPG


Since I couldn’t remove the part of the old fuse sandwiched between the wing holder panels, I fabricated a replacement with that portion cut out so I can just cement it under and around it.

Wing holder dismantled.JPG

fuse-new.jpg


Decided to just replace the damaged stab with a new one. Repairing it was just taking too much time and effort. Fortunately I was able to salvage the rudder and reuse it.

tail repl.JPG
 
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Yup. I put the CG exactly where the instructions say it should be, but it stalled so bad that it literally dropped straight down the last 10' and wiped out the motor pod. There are no misalignments, no unintended decalage in the tail or wings...I haven't been able to diagnose why it doesn't work. It flies fine with trimming tosses, so at this point, I don't have a clue. I added a chunk of nose weight but haven't worked up the nerve to launch it again.
 
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Have you ever built and launched any of the QCR Rocket Glider Kits?
Just found and downloaded the plans/templates for the QCR Neverloop IV D Boost Glider. If you're interested in the plans I can send them to you. Online searches have not turned up any other QCR gliders or rockets, unfortunately.
 

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Yup. I put the CG exactly where the instructions say it should be, but it stalled so bad that it literally dropped straight down the last 10' and wiped out the motor pod. There are no misalignments, no unintended decalage in the tail or wings...I haven't been able to diagnose why it doesn't work. It flies fine with trimming tosses, so at this point, I don't have a clue. I added a chunk of nose weight but haven't worked up the nerve to launch it again.
I’ve seen two or three videos on YouTube of the HDI2004 Swinger and the launches and glide recoveries looked really nice.
 
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