NandanPrabu
New member
Tried the model against wind...strong bird. The 70mm EDF arrived today and hopefully in a couple of weeks I will have another Gripen
Foamboard SAAB JAS-39 Gripen EDF (images, plans, build)
- Thread starter Widkin
- Start date
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- edf gripen saab scratchbuild
I managed to build one too. DTFB with paper removed (both sides). AUW is about 960g. It flies well but I had to pull the wing pylons because they were twisting the wing. I also was getting a left roll with no response at high speed (believe it was a twist in the left wing). I need to see if I fixed that and I still need to play with the canard mix. Nice design!
Bayboos
Active member
Hello and welcome.
cosmocop: I had the same problem - strong roll tendency after exceeding certain speed. I even crashed my plane once because of that. I solved it by adding another carbon fiber spar, glued in parallel to the leading edge, just behind the leading flaps, reaching from one inch from the wing tip almost to the fuselage. You can see those new spars at the very beginning of this video:
cosmocop: I had the same problem - strong roll tendency after exceeding certain speed. I even crashed my plane once because of that. I solved it by adding another carbon fiber spar, glued in parallel to the leading edge, just behind the leading flaps, reaching from one inch from the wing tip almost to the fuselage. You can see those new spars at the very beginning of this video:
I may give that a try. I flew it again this last weekend but developed a high speed elevon flutter (only one one side) that forced me to land early. I think it's from a little slop in my control linkages. I didn't notice the roll, but I'm not sure I got up to full speed before the flutter kicked in.
I do think that the leading edge slats may cause some "blanketing" of the elevons at certain speeds and diminishes their response. I'm thinking about cutting the INBD ones and gluing/replacing them straight (leaving just the OUTBD angled down).
I do think that the leading edge slats may cause some "blanketing" of the elevons at certain speeds and diminishes their response. I'm thinking about cutting the INBD ones and gluing/replacing them straight (leaving just the OUTBD angled down).
Bayboos
Active member
If you want the best aileron response and the most lift from the LE flaps, then do the opposite - keep the inboard flaps (not slats) angled down (more wing chord - more lift) and straighten the outboard ones (more effective ailerons further away from the fuselage / axis of rotation - more lever - more response) .
Widkin
Member
True, and a good point. On the other hand, having more camber towards the tip could help to avoid tip stall, something I hoped this bird would have as a benefit from the otherwise (in my opinion) cool looking wing shape.
So perhaps it's a case of reducing at least one of the leading edge flaps, but which one depends on what type of characteristics that is desired?
Or, it's just theory crafting and it won't do anything at all. In any case, I'm enjoying all the reports of how the plane work out, in all the great builds from you all.
NandanPrabu
New member
Not able to do loops in this plane. Goes vertically up and slides back - I have to nose down and take control. Rolls and low passes are easy in this plane. will increasing thrust-weight help (my next version of this Gripen). I have bought a Powerfun 70mm EDF (1.6 kg thrust on 4S battery). But I bought a 4200 mAH 4S lipo battery weighing 420gms...cannot return it
but the overall weight including EDF, ESC, battery, servos, etc will be 1.2kg
but the overall weight including EDF, ESC, battery, servos, etc will be 1.2kg
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Bayboos
Active member
I never had problems with doing loops. Even on 3s battery, with 64mm EDF, it just required building up speed first; but the plane was capable of performing a loop without much problems. Now, since I'm flying on 4s with thrust/weight ratio of more than 1, it's a breeze. I have to admit my Gripen is rather light, but the heavy one should carry more kinetic energy and thus be able to perform more dynamic maneuvers.
Bayboos
Active member
Widkin: I don't believe this plane have any problems with tip stalls. With the sweep angle this big, and cannards angled up and thus stalling first, there is virtually no chance of a wing stall unless performing very sharp pulls with extreme elevator deflections. My own plane does have the wing flat (no leading edge flaps) and I didn't see any tendency to tip stall, both at slow/high AoA fly-bys and high speed sharp turns.
NandanPrabu
New member
Yes...I watched you do that in one of the videos. But, I do the loops quite easily in other models, I guess we made it heavy for 50mm EDF (700gm thrust) in our current build and my various loop attempts with full speed (and against head wind) - still have not done it. Thanks for the note.
Bayboos
Active member
Wait a sec... NandanPrabu didn't you say you scaled the plans down to 75%? And still it weights more than 1kg? WOW, that's a heavy beast! And you are flying it on a 50mm EDF unit! I'm impressed, even more than before! I truly am one of those guys who prefer using wings rather than motor to keep the plane airborne; but it looks like your Gripen is flying more on a power of your sheer will than anything else
Realizing that, I can understand why your plane may not "want" to do a loop. Problem is, your new 70mm EDF unit may not be able to solve it simply because it may not fit inside an airframe so small... But good luck anyway!
Realizing that, I can understand why your plane may not "want" to do a loop. Problem is, your new 70mm EDF unit may not be able to solve it simply because it may not fit inside an airframe so small... But good luck anyway!
NandanPrabu
New member
Wait a sec... NandanPrabu didn't you say you scaled the plans down to 75%? And still it weights more than 1kg? WOW, that's a heavy beast! And you are flying it on a 50mm EDF unit! I'm impressed, even more than before! I truly am one of those guys who prefer using wings rather than motor to keep the plane airborne; but it looks like your Gripen is flying more on a power of your sheer will than anything else
Realizing that, I can understand why your plane may not "want" to do a loop. Problem is, your new 70mm EDF unit may not be able to solve it simply because it may not fit inside an airframe so small... But good luck anyway!
Hi Bayboos. The current build is 75% of plan and uses a 50mm EDF. The overall weight is 630gm. It runs on 3S 2200 mAH battery.
I am planning to build one more Gripen with 70mm EDF (and not put it in the current frame) and I weighed all the components that I bought for this purpose and arrived at 1.2kg for total weight.
Powerfun 70 mm EDF 180 gm
4200 mAH 4S LiPo 420 gm
4 x Servos 50 gm
80A Hobbywing ESC 90 gm
Foam board frame 400 gm (estimate)
Receiver 30 gm
Total 1170 gm
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I'm not aware of any 50mm edf that will put out 700 grams on 3s (that is the thrust I've seen on a 64mm 4s 5 blade). Are those advertised figures or measured?
As for looping, do you have enough elevon deflection for pitch? I had to crank mine up because pitch was sluggish on my initial guess for deflection.
For the leading edge slats - I'm not sure if I'll mess with them or not. I do think having the OUTBD ones down looks cool. I have stalled but it's pretty forgiving (front end drops). I haven't had a wing drop but I haven't played much with slow speed. The 70mm uses up the 2200 4s fairly quickly so you can only do so many things per flight.
Bayboos
Active member
NandanPrabu: yes, that sounds much more realistic. Still on the heavy side, but nothing that wasn't done before. One note about the battery though: it does seem like overkill for this application. I know EDFs are eating trogh the charge like no one's business, but that's the reality of it. Adding larger and heavier battery doesn't always extend the flight time, especially when the airplane is already quite heavy. From my experience, 3000mAh is around a sweet spot: it can give me up to 6 minutes of flight while still allowing for relatively slow and steady flight. Obviously a larger battery should still fly, but it will require more power and more speed to do so, and thus can actually use up the available chargre quicker.
cosmocop: I didn't say the plane don't stall; in fact, that's exactly what happens when "angled up cannards stall first". This is basically a stall configuration for the airframe as a whole; but since the stall begins in front of the CG and close to the fuselage, it's usually gentle and more symmetric. And self-correcting, which is a nice bonus for the inexperienced pilots. This exact property of the airframe is what allows it to avoid "tip stall" condition: an abrupt, asymmetric stall leading to rapid roll, often coupled with a tendency to increase pitch and thus deepen the stall, exceeding the ability to recover without significant altitude loss (or sometimes even at all).
cosmocop: I didn't say the plane don't stall; in fact, that's exactly what happens when "angled up cannards stall first". This is basically a stall configuration for the airframe as a whole; but since the stall begins in front of the CG and close to the fuselage, it's usually gentle and more symmetric. And self-correcting, which is a nice bonus for the inexperienced pilots. This exact property of the airframe is what allows it to avoid "tip stall" condition: an abrupt, asymmetric stall leading to rapid roll, often coupled with a tendency to increase pitch and thus deepen the stall, exceeding the ability to recover without significant altitude loss (or sometimes even at all).
NandanPrabu
New member
cosmocop I did not test the thrust...advertisement. Thanks for the suggestion. I will change the expo and try again today.
Bayboos I should have done my homework before ordering the batteries . I have to live with it for now...cannot return/replace the batteries. Thanks for sharing your experience and helping me understand about choosing the right battery.
I do not understand much of everything else you both speak ...hope to learn as I grow-up.
Bayboos I should have done my homework before ordering the batteries
. I have to live with it for now...cannot return/replace the batteries. Thanks for sharing your experience and helping me understand about choosing the right battery.I do not understand much of everything else you both speak
...hope to learn as I grow-up.
Bayboos
Active member
NandanPrabu: you are at the best age to learn. I did not have such opportunity. My "adventure with aviation" started when I was 16, but no RC planes were involved up until 30... So, if you have any questions, or anything is not 100% clear, please feel free to ask - I'll be more than glad to help. Good luck, and HAVE FUN!
NandanPrabu
New member
Thank you so much for the help...I sure will do.
NandanPrabu
New member
Finally I crashed my model after 23 flights... pilot error. We have started working on the 70mm (110% of the plan). And the 50mm will need some repairs
I wanted to come back to report that cutting and re-gluing the inboard leading edge slats level (instead of down) has given me roll authority at speed. The outboard slats are still angled down (which I think still gives it a "cool" look). It looks like I fixed the high speed (self) rolling tendency as well.
It is a little weird having control authority (roll) decrease with increasing speed but it does help to fly smoother. I'm quite pleased with its flight characteristics now. I have activated canards but they don't seem to make much of a difference in my testing so far. The model has plenty of pitch authority already.
Anyone experiment with cheat holes on this?
It is a little weird having control authority (roll) decrease with increasing speed but it does help to fly smoother. I'm quite pleased with its flight characteristics now. I have activated canards but they don't seem to make much of a difference in my testing so far. The model has plenty of pitch authority already.
Anyone experiment with cheat holes on this?
