Foamboard JAS-39 Gripen EDF Part 1 (model + plans)

2014-02-09 Update 

• I corrected dimensions on detail 56, 68 & 70, see new plans at the end of the article. The details were too small due to incorrect calculation of the folds of the foam. 

2014-01-29 Article updated (scoll down)

• Added weapon hardpoints and wing root covers for more detail
• Updated plans with more notes and details
• Added full-size plans 40x30 inch

Intro

Ever since I saw the awesome JA-37 Viggen made by David Windestål, I've been thinking about making an EDF-jet of my own. However, I did not want to mess around with all those formers and hot-wire cutters, since it seemed so time-consuming and seemed to be more a work of art than easily replicable. I therefore wanted to see if it would be possible to make a good-looking scale jet out of regular Dollartree foamboard instead.

Since I am a Swede too, I wanted it to be a Swedish jet. Since the Viggen was already done, it felt natural to make the JAS-39 Gripen. I also really wanted it to have fully articulating canards, something that the Gripen has but the Viggen does not. 

I recently caught up on the Flitetest podcasts and believe that there might be something like a foamboard EDF jet already in the works. It's going to be very interesting to see how David has solved the design, but until then, here's my idea. I hope someone likes it, and maybe even tries to build it along with me. Tips and suggestions on improvements are naturally very welcome. I should also, for the sake of transparency, note that I am not an experienced pilot, nor an experienced builder, and have only been into RC for the last year or so. There's a not-so-slight chance that this might go very wrong on the maiden, but hey, mistakes brings you experience too, right? 

The model

Pic 1: Render of model. Bluecore nose cone, foamboard body

I guess I should tell you at this point that I work as a design engineer with focus on house construction, and therefore have access some pretty useful 3D CAD-software. I have actually modelled this plane using the same features I would use to create steel plates, which basically involves planar elements with cuts, bevels, bends and chamfers, joined together. It worked perfectly, and made it possible to automatically generate plan drawings from the 3D model. Above is a render of the model. 

Because of the 3D capability, I have started in the opposite end of many other scratchbuilds I've seen, where the plans are published last. I have the complete model and all the plan drawings already available, for download here: PDF plans This also contains a fully interactive 3D PDF, for those that want to check the model out more closely. Please note, the grey pyramid out front of the model is the viewport of the Sony actioncamera, which I placed inside the canopy. 

The drawings are plotted in scale 1:1 on A3, A2 and A1 paperspace, depending on the size of the parts, but you can print the plans on any paper you have available, using Adobe Reader's "tile"-functionality. You just have to tape them together. 

Stats

Since I used a 3D CAD program that can handle materials, I assigned a density for the foamboard and weights for all the components, which means I have a pretty good estimate of the final weight: 

Plane, just foamboard (no glue): 460g
74mm mercury EDF unit: 236g
2x2200mAh 4S LiPo: 2x216g=432g
80A Plush ESC: 100g 

This amounts to 1228g. Add a couple hundred grams extra for cables, receiver, glue etc and I suppose this plane should stay under 1.5kg. Compare that to the static thrust of 20N (2kg) from the Mercury EDF-unit, and we get a thrust to weight ratio of 2:1.5, or 1.33:1. Pretty nice I think. I guess it might be possible to use a smaller battery to get even better ratio with reduced flight time, but that depends on if I need the weight in the front or not. Using a lighter, plastic EDF unit will also enable this to be a really light plane. 

Design

The plane consists of a number of parts of foamboard, illustrated with different colors below. (Please see the full PDF document for plans and numbering for assist in assembly.) The nose was decided to be bluecore foam (depicted in light blue), since the cone can be sanded to a nice shape using a power drill, as demonstrated by David in his Viggen build. I also have a couple of pieces that fit between some foamboard, and I intend to use the foamboard as formers and simply cut off the excess with a hot-wire cutter. I would prefer not to use hot-wire at all, but I don't think I can get a smooth and nice enough shape with just foamboard here, and covering a former with thin paper, like with a canopy, would be too weak. 

 Pic 2: CAD model with different colored parts. 

The curves of the fuselage are made by using multiple scored lines and gently folding it, like the wing folding technique shown in many Flitetest scratch builds. It does actually work very well, as seen on the pictures later. 

Build log so far

I have started assembling the model now, and it works surprisingly well! So far, I have spent perhaps half a day, and most of the model is already done. I have spent far more time honing the 3D model, which took about a weekend of 3D modelling and a couple hours on an evening after work tinkering with the drawings. All in all, a pretty OK work load I think. 

I will put up a build log once I'm done, but for now, here's a couple of pictures as proof that it is indeed possible to build. 

Pic 3. Mockup of parts. Front top fuselage not glued yet, waiting for electronics to arrive. 

Pic 4. Inlet area of the two inlets are about the same as EDF fan area, maybe a little bit larger. I hope it's enough. 

Pic 5. Quite roomy in the pod. Here you see the 4mm carbon fiber rod and steering wheel arm that will attach to a servo and make the canards articulating. 

Pic 6. EDF goes here, inside a foamboard tube. Waiting for EDF to arrive from Hobbyking before I finish this. 

Pic 7. Wingtip

I theorize that using leading edge flaps like the real Gripen, it will create an undercamber effect on the wing tip and prevent tip stalls, much like what was observed in the FT spitfire and FT cruiser. With functional canards, it should be possible to deploy the elevons as flaps to some extent while still maintaining pitch control, which might make for some really interesting maneuverability. 

Summary

This is as far as I've come. I do feel like I want to share the work at this stage, though, since the plans and everything is ready for anyone who cares to join me in this build. I'm currently waiting for components to arrive from Hobbyking China, and hope that they will arrive next week. 

I will create a part 2 in this project once the parts arrive, detailing the build, and then of course a part describing the maiden and (hopefully) successive flights. I do need to wait for some snow to melt, or waterproof the plane before I maiden it. I have just bought some airbrush stuff for that occasion, so well see what happens. 

If you've read all this way, I thank you. This is my first article on Flitetest, and as such I'm sure it needs working on. I humbly appreciate any feedback, be it comments on improvements, tips, questions, or just what you think of the project.

Until next time.

Kind regards,
Niklas

Update 2014-01-29

As requested, I have updated the plans to "full size" with 40x30 inch versions. I also took the time to make some modifications in the plans, such as a better fit of the top and bottom front fuselage parts as well as some more instructions on the drawings.

I have added weapon hardpoints and a cover for the wing root to blend the fuselage with the wing. 

I have also uploaded the CAD model in DWG and DWFX for those that are interrested in those. Was not able to before due to some file restrictions, but placing them in a ZIP worked. 

Here are some new renders for you to enjoy. :)

Pic 8. Render. Note hardpoints and wing root covers

Pic 9. Render. 

Pic 10. Render

Files

See forum thread with latest plans

• Forum thread

Updated plans 2014-02-09

• Full size plans 40x30 inch

Updated plans 2014-01-29

• Full size plans 40x30 inch
• Assembly plans
• 3D models (DWG/DWFX)

Old plans

Links

If you, like me, have trouble finding the type of foam that Flitetest uses over in Europe, I found that it's called "kapaskiva" or "cellpannå" in Sweden. I bought it from Svexo pappersmarknad. It's more of a $8-foamboard than $1 for 700x1000mm, but that's the best I could find.