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

Bayboos

Active member
@Widkin: That's the beauty of creating something new. It almost never comes up perfect for the first time, but then you can work your way towards the perfection and actually seeing the progress in the meantime. That's exactly me: I don't have enough patience to create something 100% new simply because I don't see the process; but I love to work on existing things to change/improve them as much as I can towards a specific goal. And no, Depron is not "the way to go"; it's just a material like any other. It has some advantages and disadvantages, like everything else. In my case it's just a material I know and have easy/cheap(-ish?) access to; but I'd love to work with different types of materials as well.

@Frits: thanks for the reminder. I did re-watch the video. David does in fact use thick paper for the thrust tube; but the tube in FT Viggen is much shorter that in the plane we are working on. I would not trust it in this particular application.

And back to the build/fix log: it turned out that the nozzle needs to be cut open; in fact, I did remove half of it completely to gain enough access to the "internals":

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This allowed not only inserting the tube, but also actually seeing what's going on around the EDF unit itself.

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At this point, the tricky part was to make the tube surround the EDF shroud completely and rest flat against the EDF mounting frame. Since I didn't have access to this spot from the outside, I had to use a trick involving special tool I had from my old career as the actual airplane mechanic: a mirror on the stick.

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Even though I originally cut the tube around 1cm longer than needed, It turned out that after all the adjustments it was exactly the required length, with literally no excess to trim later (and even a tiny bit cut out too deep by accident):

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I also tapered the front end of the tube to make it easier to install it exactly where it's supposed to be:

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After that, I put a solid amount of 15min epoxy on the tapered surface and pushed the tube in place. I also put the entire plane nose down until the resin cured; that way if the excess of the glue would flow anywhere, it will seal the tube-EDF gap even more rather than going in any other direction.

Next step was to glue the half of the nozzle to the tube, to further solidify it's position:

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And after the glue cured, I test-fitted the remaining parts of the nozzle to their place. I have to admit, it looked pretty cool, reminding me of the open engine panels on the real plane (once again, a flashback from my previous career):

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The final step was to put some glue in the right spots, put the parts together and hold them in place long enough until the glue cured.

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Yes, I know, I put a little to much glue in some places; but that can easily be cleaned up after the curing is done (never try to do that DURING the curing process, it will just make the mess much worse) and/or covered with the paint (if/when the plane will actually be painted).

And that's it, the thrust tube is in place, the exhaust diameter is reduced, the plane is put back together and the fingers crossed for the next flight. Also, having the entire week of time before the next weekend, I'll think about some other "fixes" I have to make to complete the plane (closing the cheater holes and re-positioning the canards for higher AoA). Stay tuned :)
 
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Bayboos

Active member
P.S. Improvised static thrust test (holding the plane nose up in my workshop) revealed that the thrust reaches the AUW before I get to 100% power. That doesn't say much, except that... it actually works! Now it's "just" a matter of more detailed tests in the air.
 

Bayboos

Active member
Three news:

1. GOOD: The plane flew, and it definitely have a lot more punch! The thrust exceeds AUW noticeably, making the self-launch easy; climb-out is brilliant and the speed is... well, not astonishing maybe, but definitely way better.

2. BAD: I don't have good enough video to upload it. I'll try to do better next time.

3. UGLY: when I was flying today morning (and alone) I had no problems with the range; but when flying in the evening (in somewhat crowded airspace) the receiver cut out, leaving the plane with no speed, power and control just seconds after the take-off. The plane still behaved ok, gliding to the ground at a little steep angle while making large circles; and hit the ground nose first. The hit was strong enough to break the nose off (and even that only partially!), but nothing else was damaged in any way.

More on that later.
 

Widkin

Member
Glad to hear the performance increased as intended. It's always nice to see theory and practice going hand in hand ;)

Too bad about the break, but sounds like you were lucky under the circumstances. Also, seems like your CG is pretty spot on for it to glide like that, or you have put in a very good failsafe for the elevons. :)
 

Bayboos

Active member
No failsafe on the Tgy 9x :) In case of signal loss, this system drops the thrust (or whatever is connected to CH3) to zero and leaves all other inputs in the last known position. That means the plane is balanced and trimmed pretty well, since the cut-off happened during initial straight climb (full power, all control inputs neutral).

And the "break" was not bad at all; in fact, I did fly it again without any glueing and ended the flight in one piece and gentle, fully controlled landing :) As I said before, the "break" was only partial and everything was still held in place by the combination of the remaining join and the air pressure. I did avoid high G's in this flight, and it turned out ok anyway.

Right now I don't have much time, but I'll post more details (new canards angle, neutral trim elevon deflections, CoG, the extent of the damage, maybe some still photos from the video I have including the impact angle etc.) early next week.
 

Bayboos

Active member
Ok, now I have some time to spare. I'd love to fix what was broken almost a week ago, but before that I need to show you the details - as promised.

New canards angle.

That turned out to be pretty simple. David says (in the Viggen video) that his plane have canards installed at 10 degrees angle. It's not exactly true, but close enough. In fact, the canards on a Viggen are installed parallel to the upper edge of the vertical part of the fuselage. It turned out that the same reference line on the Gripen is very close, so I did not try to re-invent the wheel and did exactly the same thing:

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It may not be obvious from those pictures, especially since there is no edge clearly visible on the outside of my plane, but my canards are glued in as close as I could to the edge of the flat, vertical side of the air inlet. Since the flat parts are 6mm thick on my plane and the curved parts are only 3mm thick, I put the flashlight inside the inlet hoping to make the edge more visible. It's up to you to decide either it worked or not; but I hope you get the idea.

I hope this change will allow stable high alpha flights, just like the Viggen in the FT video; but I didn't have a chance to test it properly. What it definitely changed is...

Neutral elevon deflection.

Right now, the neutral deflection (hands-free straight and level flight) is visible, but so small it's really hard to measure - approx. 1mm on the fuselage side of the elevon's trailing edge.

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With this deflection, the plane flies straight in wide variety of power settings and speeds. And I'd prefer to live it as is, since small positive deflection on the delta wing's trailing edge tends to increase stability and smoothens the stall characteristics. And it works perfectly fine with...

The CoG location.

... right in front of the first "small antenna", approx. 1 1/4" back from the fuselage panels joining line and 2 3/4" back from the "big antenna" leading edge (at it's base). Yes, it is further back than defined in the description of the prototype; but the plane seems to fly ok. I'll leave it there for now.

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(I'm sorry for the angled photo, the plane is hanging perfectly level. And yes, there's a battery inside, just like during the flights)



And finally, we get to

the "crash landing" itself.

This is a still image from the video recording of the moment of impact:

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I know this image is terrible (trust me, the video is even worse), but it clearly shows that the plane hit the ground in approx. 30 degrees nose down and the same bank angle (to the right) attitude. The wingtip and the nose hit the ground at the same time, and the plane was flying slow enough that it stopped immediately. The damage was as follows:

1. the nose cone (held in place by double-sided tape) detached (as it was supposed to);
2. the fuselage's "front wall" broke off from the bottom skin and on one of the sides:

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3. and the entire bottom surface got unbearably dirty (having tons of grass and weeds around, the plane had to hit the only muddy spot available):

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And that's it. I think 15 minutes of cleaning and another 15 of glue application (plus an hour of glue curing) and it will be as good as new, ready for more detailed flight tests (speed envelope, trimming/balance details and high alpha stability).
 

Widkin

Member
Thank you for all the pictures, and the info of trimming and CoG! Very helpful. Fingers crossed for good performance in the next flight, and then maybe a nice paint job to close the deal :)
 

Bayboos

Active member
The rule of causality at work:

CAUSE

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EFFECT

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It is theoretically possible that the antenna broke off during the "landing", but I highly doubt it.

And that would be the end of the EDF Gripen-D Mk.1. Widkin: before I start to build another one (and that may take some unknown amount of time), would you be willing to introduce some changes to the project? It would be highly appreciated.
 

DamoRC

Elite member
Mentor
Ouch! That gotta hurt. Can you replace the nose section or do you have to start from scratch.

DamoRC
 

Bayboos

Active member
Everything from the wing forward is destroyed, and that means cockpit/air intake part of the fuselage (which partially sits between the wings) would need to be replaced completely. The wings are damaged up to 10cm/4" from the leading edge at the root - still repairable, but since it's essential for proper cockpit fitting, it would have to be repaired to "as new" standards.vertical stabilizer is partially broken off, but that's the easiest part to fix. Surprisingly, canards and the plastic parts of the canopy are 100% ok (aside of some tiny scratches and a pile of dirt). All of the electronics are ok (except for the receiver, but that was bad before the flight started).

In other words: well... I'm not sure. The amount of work required to "fix" it is approx. equal to building half of it from scratch plus some to remove the damaged parts from the rest without breaking more of them. That gets pretty close to building completely new plane; so the question is: can I do the new one better? The final decision depends on the answer, but I don't know it yet. I still wait for the Widkin's answer, and if it's positive - the result of the conversation that will follow. That way or another, I will eventually have the Gripen in flying condition; but I can't say either it will have any parts from the old one (except the electronics obviously), and when it will happen. The summer is coming; and I do have some water+air "projects" on my plate that I'd love to test this year.
 
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Widkin

Member
Oh no! That looks very similar to my final flight with my mk I. I feel for you :(

I could introduce some changes. I kind of need to. Just a question of time, and what to do. It would be kind of nice to write up an article when all the final bits and pieces are working.

My thoughts on changes, please add yours:

1. Increase the inlet size some more.
2. Show how and where to place the thrust tube, with drawing
3. Make drawing of the nose cone
4. Update info of correct CG and canard angle

I also have ideas on how to make the wings removable, and making the build process easier, but all of that is kind of pointless if it does not fly good first.
 

Widkin

Member
Missed that it was a second page of posts:

About the repair, I managed to repair a similar damage. But it is as you say, you need to rebuild a lot of parts from scratch, and join with the old. It's not that hard to cut off the front part along the glue joint and reattach a new one, but my main wings and rear fuselage was not damaged. I also had foamboard, which might be easier to fix(?)

Since you have damage also on the wings, it might be too much work to repair vs rebuild from scratch.
 

Bayboos

Active member
I finally gathered enough courage to put it on the bench.

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By removing unrepairable parts bit by bit, I got to the point where some decisions need to be made:

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As you can see, some parts of the cockpit area in front of the joining line are in pretty good shape and can be used as a base to attach new parts. Unfortunately, the left wing was damaged to the point where approx. 4 inches (10cm) of the front part should be removed and replaced. This will be a bit tricky because there aren't many reference points in this area; but It's still doable. Since it took me the longest to build the back of the plane, I'm leaning towards repairing the plane rather than rebuilding it. But don't worry; I will still share some of my thoughts about improving the design sometime during the process.

There is just one tiny problem: the flying season is about to begin, and I really need to find some time to refine my "summertime duo" (FT Storch on floats plus rescue boat) before summer holidays.
 

rogerthat

New member
hi, can you please do a full build log of this im really intrested in building this plane but this forum jumps around too much to follow!
 

Bayboos

Active member
Hello everyone! Did you miss me?

Regardless of the answer, I'm going to bind you anyway.

The Gripen got back on my bench, and I decided to remove some more parts, including some that could be repaired/left in place, but either will make the rebuild easier after being removed, or will just look better brand new. Here's the result:

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I decided to leave some parts on the right side of the cabin (inner fuselage) mostly to avoid cutting off and around a lot of epoxy joins (especially where this part of the plane joins the wing). Because of that, I had to cut the new part of the cabin exactly the same way to match the old one perfectly - otherwise they will not come together straight or even at all.

Fortunately, there is a clever trick to make the cut perfect: cut them together. To do that, I made one full new part, put it against the old one and held in place using some small clamps...

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... and then used the straight edge (held in place by the same clamps) to cut both pieces at the same time. Turned out great, as expected. I did not glue it in place yet.

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After that, I fixed the front piece of the left wing. I couldn't use the same trick here, but the part is not as essential to the overall geometry as the previous one. This time I glued the part in right away, using the same clamps and some scrap pieces of Depron to make sure it's exactly in the same plain as the rest of the wing.

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The last thing I did so far is to cut out the left side of the cabin and the two joiners (front and back) that lay on top of them. I glued the rear joiner (the big one) to the left (uncut) side of the cabin, and then fitted them to the remaining part of the fuselage (plus new part of the left wing). After making sure they will match, I put everything together using some pins to see if everything will fit together and turn out reasonably straight.

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As you can see, it looks pretty well. I'm not entirely sure if it's reasonable to glue those parts together yet; but I will most likely do that before making the outer skin. I still don't know what to do with the remaining part of the right air inlet (the one visible on the first and last photos, at the front end of the right wing) - it is technically fine and would be pretty hard to cut out nicely; but it will make it difficult to fit the new parts around it.

Stay tuned, more on that... sometime later :)
 

Lartz

New member
you guys know that the test pilot for gripen put it in the ground a few times? :D
however it is a great build looks awsome!
i can add i was there the time he crashed it in Stockholm, i was about 200-300meter away, i was young so it could have been 1km even but in my mind it was 2 meters away ;)
 
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Bayboos

Active member
Hello folks!

This week I've managed to make a big progress in the Gripen rebuild project. Let's the photos talk for themselves:

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As you can see, the single biggest and most complex section is done. Only three sections to go (cabin compartment, nose cone and the canopy), each of them more simple than the one before; and the plane will be ready for (yet another) re-maiden. Which may even happen this year...
 

Pieliker96

Elite member
Great work. I'm liking the rounded fuselage and layered construction, it seems like a very interesting and lightweight construction method.