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Aug 19, 2019, 12:20 PM
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Markran's Avatar
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Backpack Yak - A Folding Flyer


I am starting to design a plane that can travel in a backpack, carry-on bag or even on a bike with appropriate carrying bag/straps. I've been wanting such a plane since I started this thread requesting one back in 2012. Back then I was just begging manufacturers to make this plane but apparently, that's not going to happen... so I'm going to make it myself. I'll share my progress here and would appreciate your ideas, requirements or advice. I've been a long-time reader of this forum and have done many scratch builds as well as assembling kits like the Twisted Hobbies planes but this will be my first full plane design.

Design Parameters:
* Wingspan: ~28" to ~34"
* Style: 4-channel, 3D-capable park flyer, front-mounted single-engine.
* Flight Time: >5 minutes, probably 2S or 3S with ~8 inch prop.
* Packs relatively flat so all gear to fly fits in a large backpack, airline-legal carry-on bag or bike with appropriate bag/straps. There should be some additional carrying capacity in the backpack, bag or bike leftover for non-plane items. I'm planning to use one of the new game-controller style transmitters like the X-Lite Pro to save space.
* Assembles and disassembles quickly and easily. Does a goal of "From UnZip to Zoom" of about 90 seconds sound reasonable?

Design Objectives:
* It should have enough power to fly comfortably in moderate wind (think sunny beach) and in no wind be acrobatic enough to fly inverted and be capable of basic aerobatics and simple 3D like hover, harrier, etc but advanced 3D is not necessary. It would be great if the handling "flies bigger than it looks" which is why I'm proposing a larger wingspan than the smallest "mini" 3D planes. They can feel kind of twitchy in the air and won't handle even mild lake or beach wind well.
* Materials: For durability and repairability I'm thinking of EPP sheet for the main wing and body. Shaped airfoil, covering or coatings as necessary.
* Main Wing: The main wing can detach, connects to the main body with a clever interlock and can split into two halves.
* Tail Feathers: The tail feathers detach. The control rods need some kind of quick release (I saw a magnetic system once).

Non-Requirements:
Successfully achieving such a usefully cool plane will require some clever trade-offs. To simplify, I'm proposing not focusing on...
* Beginner flyability. I don't think we need dihedral or any other trainer-esque features.
* Construction time, effort and cost are not primary concerns. If 3D printed interconnects or other customized parts are necessary, I'll cover getting them designed and/or fabricated and then share the resulting files here for everyone.
* Landing Gear Optional: This plane is primarily hand-launch, belly land or hand catch.
* The proposed name "Backpack Yak" is merely a fun rhyme, not a design requirement. It doesn't need to look like any particular kind of plane.

Questions:
* Can a plane like this achieve the desired basic aerobatic/3D capabilities if it uses a carbon spar between the back of the main body (aft of the main wing) and the tail feathers or is a foam body in that area necessary for... lift, balance, symmetry? A carbon rod there seems useful to achieve the breakdown shrinkability but not if it nerfs the flight envelope. These kinds of questions are where I worry I'm not knowledgeable enough.
* Most 3D-capable profile foamies seem to have main wings that attach to the middle of the main body rather than the top or bottom. I've always assumed this is a balance requirement to achieve rolls along the main axis of travel. If the main wings could attach at the top or bottom of the main body, that might provide some design flexibility. Thoughts?

I've attached photos showing some of the inspiringly flyable places I ride my bike to daily when I'm home. When I'm not home, I'm often traveling near even better places to fly. I imagine many of you have similar opportunities to fly which are too often wasted. What I've listed above in terms of requirements are really just ideas. I'm flexible on the parameters in order to achieve something fun to fly that's actually with me. A "decent flyer" is better than "no flyer"! It might also be possible to start with a good existing design and then just focus on making it magically shrink and re-expand.

Post if you are interested in following the build of this plane, building it yourself or in collaborating on the design. Or just let me know if you think it's generally a good or bad idea. I think it's an interesting engineering challenge and I'm excited to discover just how much beyond "decent" of a fun flyer a "Backpack Yak" could be. My dream is that we can just unzip a backpack and quickly have a plane in the air that leaves people wondering "Wait... where did that cool plane come from? It couldn't have been from that bag/bike..."
Last edited by Markran; Aug 19, 2019 at 05:06 PM.
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Aug 19, 2019, 03:46 PM
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RNAF's Avatar
Awesome idea! Have a look at the DreamFlight Alula Trek and/ or the Weasel Trek. I owned both, have the Weasel still.
I loved taking a back pack, TX inside, bottle of water and some food and discover the cliffs near Calais, France. Great day out. Fly for 1,5 on the slopes and have some nice treks.
Now I live far away from there and I no slopes near so I sold the Alula. But a motorised one would be great! Or similar..

To describe it, you have a short cf spar in the fuselage. Two plastic tubes, one in each wing. A Phillips screw creates friction for fast and easy mounting. One cf spar in each wing, coming from the very root going to close in the tip. See pics. I think that if you design it where the cf would touch such a tube, you’d have a plenty strong. Main spar. Wings attach with magnets but better with tape. Preserve wing and fuselage surfaces with a wider, well sticking tape. A narrower and less sticky tape helps a lot in a firm wing connection.

For elevator take a look at the Graupner Terry S manual online. Or the Graupner Chip ( BL version ). It’s a smart method, where you can screw the nylon break bolt on/ off easily. It doesn’t have a quick link, just a 90 degree bend in the horn where slight tension keeps it in place.
Last edited by RNAF; Aug 19, 2019 at 03:57 PM.
Aug 20, 2019, 04:52 PM
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Markran's Avatar
Thread OP
Quote:
Originally Posted by RNAF
Awesome idea! Have a look at the DreamFlight Alula Trek and/ or the Weasel Trek. I owned both, have the Weasel still.
I loved taking a back pack, TX inside, bottle of water and some food and discover the cliffs near Calais, France. Great day out. Fly for 1,5 on the slopes and have some nice treks.
Now I live far away from there and I no slopes near so I sold the Alula. But a motorised one would be great! Or similar..

To describe it, you have a short cf spar in the fuselage. Two plastic tubes, one in each wing. A Phillips screw creates friction for fast and easy mounting. One cf spar in each wing, coming from the very root going to close in the tip. See pics. I think that if you design it where the cf would touch such a tube, you’d have a plenty strong. Main spar. Wings attach with magnets but better with tape. Preserve wing and fuselage surfaces with a wider, well sticking tape. A narrower and less sticky tape helps a lot in a firm wing connection.

For elevator take a look at the Graupner Terry S manual online. Or the Graupner Chip ( BL version ). It’s a smart method, where you can screw the nylon break bolt on/ off easily. It doesn’t have a quick link, just a 90 degree bend in the horn where slight tension keeps it in place.
RNAF,

I'm glad you like the idea. I actually already have an Alula and I've always liked the Alula's wing joining mechanism. I've attached a photo of my Alula that shows the mechanism more clearly. I did consider doing a motor mod to the Alula but unfortunately it still wouldn't be the Backpack Yak plane I want because it wouldn't have 3D characteristics.

I found the Terry S manual online but couldn't identify what you are talking about. What does the "nylon break bolt" do? What part of the plane does it attach to?

NEXT STEPS

1. I've ordered some specialized magnets and nesting carbon fiber square tube (see photos) to do some tests of joining mechanism ideas.

2. I've also decided to build up a Twisted Hobbies 3D kit plane in the target size so I can do some experiments on it. I selected the Edge 540 v3 which has a 33" WS. I'm not sure if I should build it stock first and then cut it up to separate the wings and experiment with re-attaching them or if I should mod it while building. Any thoughts?

3. I've also done some research on various sliding tube locking mechanisms (see attached photo). I haven't ordered any of these yet though as I'm still hopeful something simpler and lighter can be sufficient such as a sliding set pin, pressure fit or perhaps an internal elastic band.

4. I've started some design drafts in Sketch Up just to rough in concepts.

5. I'm also thinking about just hacking up some Dollar Tree foam sheets. I still don't know what proportions to target yet so I was just thinking about using them as brainstorming props since I'm more of a spatial thinker.

6. Answering my own question above: Regarding a 3D plane having a wing that attaches to the top of the body, while looking at various planes I came across the new Multiplex Funny Cub and it seems to have excellent 3D ability with a top mounted wing. What I can't tell is whether this will negatively impact sustained inverted flight, which is something I'd like to target with this design.
Last edited by Markran; Aug 20, 2019 at 06:30 PM.
Aug 21, 2019, 03:01 AM
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RNAF's Avatar
There is an aluminium fitting in the elevator that takes the bolt. In the fuselage there is a nut where it screws into.

Top picture shows the captive nut, lower picture shows the fitting in the elevator. You can just slide the elevator under the rudder and screw it in place.
Aug 21, 2019, 06:28 AM
I Look, Listen, and Learn
Timbo383's Avatar
I won't be much help unless I accidentally get an idea but I'm still interested to see the outcome. It's good that you have already given some thoughts and planning on this project and not just jumped into it.
Stating that you don't plan on doing any advanced 3D is good. I don't 3d but know that those guys make their planes as light as they can and adding connection brackets and bracing don't help with making it light. But you should still be good on your basic 3D goal.
I know epp is very flexible but I would think just shoving it in a backpack for several hours before reaching your destination would create some bends that you need to wait a little bit for the foam to regain it's flat and straight memory before flying. Have you considered some kind of box container to put the plane in then put it in the backpack or bag? When you purchase a RTF plane they usually come in a box with thick foam inserts that keeps everything straight and flat. I know...get the plane built first, then worry about that later. But it'll give you something else to think about when you need a break from the build.
Anyways, good luck with this project, I'll be following this one.
Aug 21, 2019, 09:44 AM
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Markran's Avatar
Thread OP
Quote:
Originally Posted by Timbo383
I won't be much help unless I accidentally get an idea but I'm still interested to see the outcome. It's good that you have already given some thoughts and planning on this project and not just jumped into it.
Stating that you don't plan on doing any advanced 3D is good. I don't 3d but know that those guys make their planes as light as they can and adding connection brackets and bracing don't help with making it light. But you should still be good on your basic 3D goal.
I know epp is very flexible but I would think just shoving it in a backpack for several hours before reaching your destination would create some bends that you need to wait a little bit for the foam to regain it's flat and straight memory before flying. Have you considered some kind of box container to put the plane in then put it in the backpack or bag? When you purchase a RTF plane they usually come in a box with thick foam inserts that keeps everything straight and flat. I know...get the plane built first, then worry about that later. But it'll give you something else to think about when you need a break from the build.
Anyways, good luck with this project, I'll be following this one.

Timbo,

You thought is a good one. Thanks for bringing it up! The transport and storage configuration does need to be considered as part of the design rather than an after-thought. Any foam can ding, dent or get surface 'rash' if not protected. I have built quite a few EPP kits as well as scratch built several planes in EPP. It doesn't really have much of a 'memory' like some foams and there are techniques to strengthen it including tapes, coatings and heat-applied films. You're right that 3D planes tend to target very light wing loading. We're going to need to make design trade-offs and I don't see any way around 'spending' some of our weight budget toward the elements that make the plane collapsable. The absolutely lightest construction techniques tend to give up portability and durability.

One approach might be to design the plane to collapse and travel in a planned packing bundle. If the foam elements can sit flat against each other without protrusions, such as control horns sticking into an adjacent foam surface, then the 'bundle' itself could be strengthening and protective. Additional packing materials like cardboard or more foam will add girth, so since the thing is mostly foam to start with, I'm hoping thoughtful planning might allow it to be wrapped in a thin yet protective sheath of something like stretchy nylon that would keep the elements in position and compressed together for strength while preventing surface rash on anything that might rub against it. A stretchy nylon fabric would also allow various control horn-like protrusions to stick out on the outside facing surfaces.
Last edited by Markran; Aug 21, 2019 at 09:50 AM.
Aug 21, 2019, 02:51 PM
I Look, Listen, and Learn
Timbo383's Avatar
Did not think about the flat pieces of foam pressed together giving it plenty of firmness to keep the warping and bending to a minimum. Now if only someone could come up with a solid quick connect control horn idea to solve the horn protrusion problem....and the first one that suggests magnets get shot. . If someone has access to 3d printer I'm thinking somewhere along the lines of those metal two piece twist connect connectors used in cheap wood cabinets and dressers, working right now but I'll try to find one when I get home and post a pic if you don't know.

Edit: Been thinking about that horn idea and I believe there is a very good way of doing that. I'll hand draw it up this evening and post it, someone with cad skills should be able to make it reasonably easy. Visualize a control horn with two prongs at the base and it just pressure fits with a 90 degree twist into the base that is permanently glued into the control surface.
Last edited by Timbo383; Aug 21, 2019 at 03:17 PM.
Aug 21, 2019, 08:15 PM
I Look, Listen, and Learn
Timbo383's Avatar
This is what I was thinking for the quick connect horn, probably too much involved to make it but I believe it could work. I suck at computer work so I pieced my best drawings together to show what I was talking about.
Aug 22, 2019, 05:11 AM
I Look, Listen, and Learn
Timbo383's Avatar
Thought about a simpler screw in type of horn, but all of this is probably pointless if it's all too small for a 3d printer to make.
Aug 22, 2019, 10:04 AM
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Markran's Avatar
Thread OP
Quote:
Originally Posted by RNAF
There is an aluminium fitting in the elevator that takes the bolt. In the fuselage there is a nut where it screws into.

Top picture shows the captive nut, lower picture shows the fitting in the elevator. You can just slide the elevator under the rudder and screw it in place.

RNAF,


Thanks for posting this page. I didn't see it in the manual I looked at and now I get what you're talking about. That is a cool way of connecting. Thanks for the suggestion. Let me know any others you think of.


- Mark
Aug 22, 2019, 10:55 AM
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Markran's Avatar
Thread OP
Quote:
Originally Posted by Timbo383
Visualize a control horn with two prongs at the base and it just pressure fits with a 90 degree twist into the base that is permanently glued into the control surface.

Timbo,


Thanks for those drawings. Neat!

I think the style of pronged twist connector you're describing is generally called a Bayonet Connector. I've attached some pictures I found. I think this could be a good approach, especially the variety called "Quick Release" which just require a quick twist to seat and they won't untwist until pushed in and turned simultaneously. I spent some time browsing and found several interesting variants on Alibaba, including some that have 2 - 12 pin DIN style electrical connectors at the center so servo control could be a part of it. So far, the smallest three-pin version I've found has an outer ring diameter of 19mm which is a bit larger than I think we want as it would be cool to use this kind of mechanism on the end of a carbon fiber tube as the primary spar for the main wings to attach to the body. Pop it on and the servo is connected in the same twist! They make them in lightweight plastic as well as stronger metal versions. The electrical conduits start as small as 24 AWG, so perfect for servo signals.

As for using these on the control horns, I was hoping to design around needing to detach control rods from horns if we can avoid it. If the aileron servos are flat mount and embedded in the wing surface the control horns can just stay attached if they can be oriented in a good way while the pieces are 'bundled' in transport mode. I'm also hoping to keep the tail feather control horns and rods attached. I'm going to try to embed the rudder/elevator control rods inside a square carbon tube and if the tail feathers are hinged properly the left and right sides of the elevator could fold up and make a flat bundle 'sandwich' around the rudder while still attached.

Quote:
Originally Posted by Timbo383
...and the first one that suggests magnets get shot. .
I agree that magnets could be problematic if relied on for structural purposes. I think they could be useful for alignment but prefer mechanical support in addition. There are certain scenarios where a "soft connection" like a magnet could serve another purpose though. If exactly the right strength magnet is used, it can hold securely in any flight scenario but then be designed to intentionally separate in a crash scenario. This could be preferable to a mechanical "hard connection" which could break in a hard enough hit. I've found that in a crash if heavier elements separate 'gracefully' by design it tends to help protect the rest of the plane from greater damage.

I went to the large regional LHS yesterday and spent some time browsing through the building materials pondering and picked up a few elements to play with. One thing I'm also hoping to leverage is the idea of cascading interlocks. For example, perhaps the main wings are locked and cannot untwist until the tail boom is unlocked. Basically, a clever design could use the order of assembly/dissassembly to simplify and save steps. I'd like to keep it as streamlined as possible so it never devolves into feeling like, " Okay, lemme me pull out all these pieces, unwrap them, put them on the ground and then stitch them together one at a time." My dream is that it feels more like "Pop, pop, pivot, twist, snap, done". I realize that may not be entirely possible but starting with that as an aspirational goal will get us closer than not.
Last edited by Markran; Aug 22, 2019 at 11:07 AM.
Aug 22, 2019, 04:17 PM
I Look, Listen, and Learn
Timbo383's Avatar
I like the ideal of a hinged tail feather setup, and since most 3d pilots have their planes more tail heavy the extra weight of the hinge assembly probably won't hurt it.
"Pop, pop, pivot, twist, snap, done".....sounds like my morning wakeup routine.
Aug 22, 2019, 07:12 PM
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Markran's Avatar
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Quote:
Originally Posted by Timbo383
I like the ideal of a hinged tail feather setup, and since most 3d pilots have their planes more tail heavy the extra weight of the hinge assembly probably won't hurt it.
"Pop, pop, pivot, twist, snap, done".....sounds like my morning wakeup routine.

Yes, it would be much simpler to embed the tail feather servos right in the tail but I expect that may put too much weight aft. It's worth trying though just to see. What you said did spark a crazy thought though. I don't know if you've seen some of these new "nano RX" that are coming out but they are getting crazy small. So small it makes me wonder if anyone has ever made a fully stand-alone sub-micro servo. This would include a built-in single channel RX and a very tiny battery with only enough mah for one servo during one flight. That would be weird but maybe somehow oddly useful. No wires running back to a central receiver and ESC. No long control rods. I don't think it's relevant for the Backpack Yak. Just a weird thought.
Aug 22, 2019, 09:09 PM
I Look, Listen, and Learn
Timbo383's Avatar
Interesting thought but I don't think there would be any real weight loss vs the standard setup. Plus you would have to remember to charge the additional battery....what if you forget? Ooops....


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