Foamboard SAAB JAS-39 Gripen EDF (images, plans, build)

[Bayboos]

Anyone have an idea what's on those pictures?



Yeah, that one was pretty obvious Interesting thing: it's way smaller than I thought. Every single part fits my 18" x 24" cutting mat (two of them barely, but still).



I still don't have any thrust source for this beauty (any opinions about HK 10 Blade High-Performance 70mm EDF Ducted Fan Unit? Any motor suggestions?), but I don't expect to need one very soon anyway. It will be build using 3mm/6mm depron (no alternatives available here) with some FCS ("Fancy Carbon Stiffeners") here and there. This should be really fun...

 

[Widkin]

Oh, exciting!

Regarding size, I just realized I might not have been clear about the scaling up 10%: the plans are still in the original size (about scale 1:11), so the PDF plans need to be scaled up 10% to get the 1:10 scale. Use the scale ruler down to the left of the plans. Really sorry if that was not clear, the whole project is a work in progress. I will update my other post.

However, if you build it in depron, it should be a lot lighter than my version, so maybe you don't need the extra lift, and are better helped by the reduced drag.

Let me know if you have any questions regarding assembling it. It is a bit tricky and far from perfect, which is why we're still in the scratchbuilding forum and not the article section I guess. I really look forward to feedback from you guys.

 

[Bayboos]

Oh boy...

Well, I didn't start cutting the foam yet; so nothing is lost. On the other hand, the current size of those plans is just a tiny bit ABOVE anything I can have printed out here. This certainly can be improved by better layout. Especially removing unneeded commentary, like material used etc. (repeated near each part while it could be added only when "different from anything else") should help a lot.

Please tell me, what do you expect the total weight of this plane should be? I never build any EDF plane (or any jet-like plane); but judging by the overall size and my past experience I'm aiming somewhere between 800g and 1kg (28-35oz). This plane will definitely be faster than any of my previous ones; but I don't expect it to require a lot of speed to stay airborne.

I did notice some significant differences between v2 plans and the described build process; but I do not worry about that - I happend to live in a country where figuring out impossible solutions is a subject of national proud (and also I have a lot of technical background, including full-scale planes).

 

[Widkin]

You're absolutely right, the plans can be improved a bit, and I'll try to compact things for the final version.

The thing is, everyone have different printing capabilities, so it's tricky to decide on size. After some input from users in my original article, I decided to make the plans in a size that could fit on two Dollartree foam boards, which are 20x30inch (my plans are 40x30). There are some tips in the comments of the article on how to print tiled prints that you could tape together. Adobe reader has a function for this, but it unfortunately splits more parts than necessary.

Personally I just print using the Bluebeam PDF viewer, and select each parts manually with a "print viewport", one for each A3 paper, which I can print at work. For the bigger details, I need to print a several viewports and tape them together. It takes a few minutes, but really is not very hard to do. I do the same with the FT plans.

Edit: I'm not at all sure of the weight for a depron build, as I have no experience with depron. My first version had a flying weight of about 1400g, 460g of which was the foamboard. You version should be a lighter, depending of components of course.

But let's not guess - we're technicians after all. I'll check the total volume of the material when I get home. With that, you could calculate your weight if you know the density of depron.

 

[Bayboos]

That's true. The thing is, current plans are just few cm bigger than A0 format. It's the biggest defined format in metric units; meaning: it's the bigges readily available printing format for a reasonable price. I can print the plans without margins/border lines, and I'm fine. Judging by what I saw, it should be possible to fit 110% size plans on the same format; which would be close to perfection (fits both A0 and 40x30) - I don't think you need anything more

Edit: my experience tells me that - unless you are very careful - the foam takes apprx. half of the weight of the "empty shell" airframe Fortunatelly right now I'm gaining experience with some very light planes (the Twirl you see on my pictures weights around 200g AUW; currently working on FT 3D depron profile, aiming for 200g AUW as well). If I'll be able to build the Gripen to less than 500g for the airframe alone, I will land close to the upper designed limit (100-150g for the propulsion, 150-200g for the battery, approx 100g for "everything else").

 

[Widkin]

Good idea, I'll probably stick to A0 for the next update.

I checked my CAD model and it seems to contain about 1.13 m2 foamboard when built in scale 1:10. My model weighs in at 1117g with everything except battery, nose cone, wing root covers and hardpoints, just checked. My EDF unit is 236g and the 100A Plush ESC is 100g.

According to the CAD model, the foam weighs 634g, which, if I remember from the build, is about right. That leaves about 150g for glue, cables, receiver, servos etc, which seems reasonable and in line with your reasoning.

I also need my 335g 3300mAh 4S up in the nose to get a (hopefully) good CG, which sits at the black tape line just behind the vertical "antenna". I hope it is enough, otherwise I must move the EDF even more forward...



Weight: 1117g. CG with battery at the front black tape line.


Inside nose. Lots of space inside.


Version 1 in the air.


Version 1 in the air


Render of V2

 

[MrHydrodynamic]

Tack för uppdateringen, måste nästan springa ned till Arkitektkopia och skriva ut min egen!

Thank you for the update, nice work! Wish I could contribute more!

 

[Widkin]

I gave it a try yesterday evening, but was not able to get her airborne by hand launching. I had no one to help throw it, so I just belly-landed a few meters further ahead and vacuum cleaned the field from grass and straw.

I'm thinking of putting wheels on it, or some kind of grip in the bottom to help launch it by hand. Maybe one of the belly hardpoints could be useful for this?

Anyway, a vertical test by holding the nose showed that my setup has about 1:1 power to weight ratio, maybe a little more with 100% full batteries, so that should be enough to fly her OK I hope. It's a bit better than version 1, which probably had too small inlets to get good air flow.

I'll keep you posted on the progress.

 

[Frits37]

Hi Widkin,

Any progress lately? I'm very interested!

 

[Widkin]

Hi Widkin,

Any progress lately? I'm very interested!

Hello,

I do not have that much to update, personally. I tried flying with the original edf unit, but it did not take ait from a hand launch. Too little lift, and too much drag of the upscaled airframe. I did however rip the edf out and put in a mercury alloy edf unit, with a 10 blade metal fan, 74mm. But I still only get about 1:1 static thrust, so I'm a bit stumped on the power and have not really done any more. Too much other stuff to do for me at the moment anyway, I don't have time to fly.

There is however a guy that did build a Gripen from my plans, and it flies nicely. I guess that the original scale works better than my upscale version, because he used the original one.

I'll try to get him here to post some images and feedback.

So, the bottom line is that she flies, and flies good. I just have not had the energy or skills to get there myself. Really, I'm more engineer than pilot anyway

 

[Widkin]

Some photos

Hi again,

I was sent some photos from a friend who built the Gripen. Original scale, so about 1:11.

According to him, it flies stable and is easy to control. He used the canards fixed in a flat or slight upwards angle - not sure how much test is done with using canards for elevation control, but there are several flights done by now.

Enjoy the pics. All cred to Peter.

 

[Widkin]

Here's some videos from the same guy.

 

[Bayboos]

That looks really nice! A bit slow, but very stable. Do you know the power setup and/or the weights (airframe only/AUW)? It would be really helpful for me. Thanks in advance.

 

[Peter C]

Hallå Bayboos and thanks!

I have 70mm fan and a 3000mAh battery setup. The same as used in the Viggen. it is a bit heavy, thats because it is a bit slow. But if you build it with lighter foam and with a smaller battery it should go better. This is the first flying so you can make fine adjustments on it. I will get back to you about the weighs , but about 1kg with battery .

 

[Bayboos]

Hello Peter.

70mm and 3000mAh is stated in the video description (along with 4s). What is more important it the motor kv (that would allow me to estimate the static thrust) and the plane's weight (for determining estimated performance envelope).

I already have 70mm fan that was originally a spare part for Durafly™ Zephyr V-70 - an EDF glider that weights 1050g~1150g (exacly what I'm aiming for with the Gripen) powered by 3s lipo. Judging by the available videos, the Zephyr can fly really fast with this setup and the fan is providing noticeably more than 1:1 thrust to weight ratio.

I'd love to know more details about your setup to figure out what I can expect from my own when it's completed. Thanks a lot in advance; good luck and have fun!

 

[Peter C]

Here you got all the details about the setup http://graysonhobby.com/ft-viggen-70mm-edf-power-package-ultimate.html

I think it is kv 3200

I hope this information is helpful!?

 

[Bayboos]

Yes, thank you.

Looks like both setups are quite similar; with the Grayson one giving slightly more thrust from higher voltage battery. That makes the weight crucial for the performance comparison. The battery I'm going to use is Graphene 3s 2200mAh 65c, which weights approx. 50g less than yours, and the motor/fan saves additional 15g. Those savings are small, so I put all my hopes with the lighter airframe.

Thanks again for all the info; good luck and have fun!

 

[Bayboos]

It starts to resemble something familiar...

... yet still nothing familiar enough. I had to make some tough choices due to different building material and some missing info on the plans; but I believe everything will be fine. And the plane will fly. At least once.

 

[Widkin]

Exciting! Good luck

What material are you using, depron?

 

[Bayboos]

Yes, 3mm and 6mm thick. Plus come carbon obviously, and perhaps some balsa/ply here and there (still thinking about not having to glue the EDF unit in...). I will most likely shorten the "triangle" part of the "cabin" to make room for "cheater holes" at the bottom - the EDF unit I have eats a lot of air, I'm not sure the narrow air duct (especially at the inlet area) will be able to provide enough at low speeds.

There are still some mysteries to solve, but I have at least a couple of next steps figured out.