Each time I see this picture it makes me smile!
Foamboard SAAB JAS-39 Gripen EDF (images, plans, build)
- Thread starter Widkin
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- edf gripen saab scratchbuild
Each time I see this picture it makes me smile!
Widkin
Member
Wow, with such a low weight, this looks promising!
Stange that the thrust tube was too short in circumference, though. But good that you can solve it. I'm impressed but the patience and skills you show. I mostly built mine I two sittings, so I'm not sure depron is the material for me, although it looks really tempting.
Stange that the thrust tube was too short in circumference, though. But good that you can solve it. I'm impressed but the patience and skills you show. I mostly built mine I two sittings, so I'm not sure depron is the material for me, although it looks really tempting.
Bayboos
Active member
Thank you. You know, it would take me much less time if 1) I'd have lower number of kids , and 2) I would not have to convert the plans.
Building with Depron foam (or in a matter of facts any non-covered foam) is only a little bit more time consuming than paper covered foam, provided that the plans are already designed for this material. Non-covered foams are generally less stiff and thus require some additional support here and there; the tricky part is to figure out where to add this support and how much of it without making it too heavy.
And here's the problem I have right now: the cockpit does not fit at all. As I can see from the available pictures, the cockpit should extend the flat part of the upper fuselage in straight line (to properly accommodate the canopy); the one I have does not do that. In fact, it rises quite a bit; it looks almost like the cockpit skin (as cut out from the plans) is... too long - exactly the opposite than the thrust tube. But that's nothing unexpected: the more curved lines the part have, the more modifications it requires when using different material. The cockpit part is the worst of all; and will definitely take a lot of time to make it right.
, and 2) I would not have to convert the plans. Building with Depron foam (or in a matter of facts any non-covered foam) is only a little bit more time consuming than paper covered foam, provided that the plans are already designed for this material. Non-covered foams are generally less stiff and thus require some additional support here and there; the tricky part is to figure out where to add this support and how much of it without making it too heavy.
And here's the problem I have right now: the cockpit does not fit at all. As I can see from the available pictures, the cockpit should extend the flat part of the upper fuselage in straight line (to properly accommodate the canopy); the one I have does not do that. In fact, it rises quite a bit; it looks almost like the cockpit skin (as cut out from the plans) is... too long - exactly the opposite than the thrust tube. But that's nothing unexpected: the more curved lines the part have, the more modifications it requires when using different material. The cockpit part is the worst of all; and will definitely take a lot of time to make it right.
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Bayboos
Active member
It definitely does have the look...
I had to improvise with the cockpit a bit to make it work better with slight changes in dimensions...
It's still not done; I'll have to trim the sides a little to better fit the canopy lines, but that will be done with the completed canopy in place. And as a nice side effect ("happy accidentTM"?) the top part of the nose is more flat - just like in the original.
And here's the comparison between the first cockpit I built, with the front bulkhead exactly from the plans (top) and the final one (bottom).
Originally, I thought about building a simple nose cone from one sheet of rolled-over depron; but now it looks so good that I think I'll have to go with full-scale approach, carving the nice curves out of solid block of foam. It's a lot of work; but the plane looks so great that I would hate myself for spoiling it with any simplifications.
And here's the serious question for those who did manage to fly this beauty: I'm going for fixed canards (like FT Viggen), but since the original plans include moving canards only, there is no indication of the neutral canard position. Can anyone provide any info (foto/video/drawing/description) about the most neutral position of canards when they are NOT used as control surfaces? I believe a slight positive angle (relative to the wing) would be the best option (again: similar to TF Viggen), but I would love to see/hear any input from the actual flight trials. Thanks in advance.
I had to improvise with the cockpit a bit to make it work better with slight changes in dimensions...
It's still not done; I'll have to trim the sides a little to better fit the canopy lines, but that will be done with the completed canopy in place. And as a nice side effect ("happy accidentTM"?) the top part of the nose is more flat - just like in the original.
And here's the comparison between the first cockpit I built, with the front bulkhead exactly from the plans (top) and the final one (bottom).
Originally, I thought about building a simple nose cone from one sheet of rolled-over depron; but now it looks so good that I think I'll have to go with full-scale approach, carving the nice curves out of solid block of foam. It's a lot of work; but the plane looks so great that I would hate myself for spoiling it with any simplifications.
And here's the serious question for those who did manage to fly this beauty: I'm going for fixed canards (like FT Viggen), but since the original plans include moving canards only, there is no indication of the neutral canard position. Can anyone provide any info (foto/video/drawing/description) about the most neutral position of canards when they are NOT used as control surfaces? I believe a slight positive angle (relative to the wing) would be the best option (again: similar to TF Viggen), but I would love to see/hear any input from the actual flight trials. Thanks in advance.
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I am following you progress with much interest: great job! To your question: the canards are part of a dynamic system and hence have no neutral position in flight. For the looks you could take the taxi position as a reference. I would't know which position would be optimal in flight.
Bayboos
Active member
Thanks Frits for your reply. I know how the canards are supposed to work in the real plane; but we are not building the real planes. My question was about the neutral position for this particular RC plane; that means a position that will allow it to fly straight and level with no elevator input even with changing power setting and speed.That's why my question was directed towards the folks who already built and flown the plane, since they are the only ones in the world who can know the answer
And another update from the building front: not a perfect fit yet; but going in the right direction:
This bottle is a little damaged and will not end up on the final product; but is good enough to create a template. The final canopy will be cut from a brand new bottle, and hopefully will be much closer to the original, "two cones" Gripen canopy/windshield.
And another update from the building front: not a perfect fit yet; but going in the right direction:
This bottle is a little damaged and will not end up on the final product; but is good enough to create a template. The final canopy will be cut from a brand new bottle, and hopefully will be much closer to the original, "two cones" Gripen canopy/windshield.
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Widkin
Member
Taking great shape!
About the canards: I did not use them fixed, but had the zero input position be slightly positive. Not sure about the angle, though. My flight also was very short, so my input is mostly theoretical.
If I remember, Peter, who put up some videos, had them in a zero degree position, but said he was going to try a bit positive later.
I think the absolute best thing you can do is to use the exact same angle as the FT Viggen. The planes are pretty similar, and the FT team have tried and tested the concept a lot more in the air than anyone has tested this RC Gripen.
About the canards: I did not use them fixed, but had the zero input position be slightly positive. Not sure about the angle, though. My flight also was very short, so my input is mostly theoretical.
If I remember, Peter, who put up some videos, had them in a zero degree position, but said he was going to try a bit positive later.
I think the absolute best thing you can do is to use the exact same angle as the FT Viggen. The planes are pretty similar, and the FT team have tried and tested the concept a lot more in the air than anyone has tested this RC Gripen.
Bayboos
Active member
Definitely not done yet, but I couldn't wait any longer. It flies!
Olny two flights were made, and the overall outcome is "modeartelly positive". Conclusions so far:
1) even with AUW ~700g (yes!), it's still underpowered;
2) recommended CoG seems ok for the first flight, but I believe it can be moved back a little later;
3) eats trough the batteries in a ridiculous pace;
4) does not care about the wind very much.
Still, I'd like to complete the trials in a bit more bearable weather.
Olny two flights were made, and the overall outcome is "modeartelly positive". Conclusions so far:
1) even with AUW ~700g (yes!), it's still underpowered;
2) recommended CoG seems ok for the first flight, but I believe it can be moved back a little later;
3) eats trough the batteries in a ridiculous pace;
4) does not care about the wind very much.
Still, I'd like to complete the trials in a bit more bearable weather.
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Widkin
Member
You're brave, doing the maiden in that weather, and with such a light model to boot! Congrats on a successful flight, its great to see that it worked!
Too bad it still needs more power, though. I guess the scale look is just not working too great, with the small inlets. Also, the length of it probably kills some power, just compare to the shorter Viggen.
Any ideas you have for upping the effectiveness are welcome, it'll be interesting to follow your further testing.
Too bad it still needs more power, though. I guess the scale look is just not working too great, with the small inlets. Also, the length of it probably kills some power, just compare to the shorter Viggen.
Any ideas you have for upping the effectiveness are welcome, it'll be interesting to follow your further testing.
Bayboos
Active member
Small inlets are not a major issue in this particular case since there are still large holes at the bottom of the air duct (visible on the video, especially during low/slow passes). I'm afraid that the main cause is the cheap-ish EDF unit. Even though it came from relatively fast and much heavier plane (Durafly Zephyr V70, AUW ~1100g), it may be too weak for a plane with much greater surface area (both total, and wing only), even if it's lighter.
But there's one more possibility, and that's the internal shape of the exhaust. Currently it's just an outside skin plus some former rings; of which one in particular is pretty "high" and may cause significant waste of thrust (acting similar to "muzzle brake", just in the opposite direction and with no projectile). Some changes in the CoG and position of the canards may help a little, too - right now the plane requires noticeable up elevator deflection to fly straight an level (canards are at 0deg, but can be adjusted).
All this needs some more workshop time (including actually completing the airframe) and more flight time; but the time is one of those few resources I don't have a lot of.
P.S. There are some reports of the EDF/motor unit being capable of handling 4s. I'd prefer to check all the aerodynamic solutions first, but it's good to know there are "some power options" available as well.
But there's one more possibility, and that's the internal shape of the exhaust. Currently it's just an outside skin plus some former rings; of which one in particular is pretty "high" and may cause significant waste of thrust (acting similar to "muzzle brake", just in the opposite direction and with no projectile). Some changes in the CoG and position of the canards may help a little, too - right now the plane requires noticeable up elevator deflection to fly straight an level (canards are at 0deg, but can be adjusted).
All this needs some more workshop time (including actually completing the airframe) and more flight time; but the time is one of those few resources I don't have a lot of.
P.S. There are some reports of the EDF/motor unit being capable of handling 4s. I'd prefer to check all the aerodynamic solutions first, but it's good to know there are "some power options" available as well.
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Bayboos
Active member
P.S 2 As I think of it right now, the aforementioned "additional inlet holes" can also have a negative impact. They obviously help when the plane flies slow/very slow; but they may also reduce the thrust at higher speeds simply because such holes completely remove the "ram air effect" - an effect of increased inlet pressure due to ram air forced into the inlets by the plane's speed.
I also found an example of an airplane of similar size and weight: HobbyKing BAE Hawk - Red Arrow 70mm EDF. This plane have very similar inlet and exhaust cross sections; yet it apparently flies much faster and at lower currents than my Gripen (even on 3s). There is definitely something that needs to be improved; it's "just" a matter of finding that thing and actually improving it. And that's what I'm known (in some circles) to be pretty good at, so I'm still hoping for the best.
I also found an example of an airplane of similar size and weight: HobbyKing BAE Hawk - Red Arrow 70mm EDF. This plane have very similar inlet and exhaust cross sections; yet it apparently flies much faster and at lower currents than my Gripen (even on 3s). There is definitely something that needs to be improved; it's "just" a matter of finding that thing and actually improving it. And that's what I'm known (in some circles) to be pretty good at, so I'm still hoping for the best.
Bayboos
Active member
Workshop time does include research time. Since there is a little to no chance of flying this weekend, I did some googling for existing info about EDF efficiency. And I have to admit: there isn't much of it. But I got a bit lucky. I found a couple of threads on the rcpowers.com forum:
Getting the ducting right for an EDF
An EDF Thrust tube (Tutorial)
Both threads are roughly lead by the same users and revolving around the same test setup. To summarize, one of the users conducted a series of trials on the thrust tube geometry using "cheap 70mm EDF from HobbyKing" - exactly what I'm dealing with The final conclusions were:
"The EDF's Thrust Tube needs between around 5 to 20% diameter reduction at its exit. The air should exit the thrust tube at a higher velocity than it entered the system at. If you reduce the tubes exit area by 20%, you will have 20% higher air velocity at the system exit. It actually will be a little less than that because of back pressure and friction. In a real world application, F1 tested a 70mm ducted fan. On this system, He proved the thrust tube exit size should be between 59mm to 62mm for the best performance. That is a 5% to 16% exit size reduction. At 59mm (16%), it will give you the best speed, and at 62mm (5%), it will give the most power. With exit sizes smaller than 58mm, you will have a very little thrust velocity speed increase, but the system will have a massive power loss. With exit sizes larger than 62mm, the system will have less power and speed."
Now, remember the former rings I mentioned before? One of them had the internal diameter of approx. 68mm - slightly smaller than the EDF casing - but the tube it was installed in is actually much larger. In fact, the lowest diameter of existing exhaust tract is at the nozzle; and it's still larger than the EDF. Judging by the article quoted above, I was loosing A LOT of the thrust just by those two "design errors" alone.
The former ring is already gone, that was an easy and quick fix. Unfortunately I did not have a chance to fly the plane after the fix and I really want to introduce one change at a time; but I can prepare a thrust tube that will actually follow "the guidelines of proper thrust tube design" and will be easy to install between flights. Looks like a very interesting round of tests in coming my way; I just hope the weather will cooperate.
At this point I don't worry too much about the shape of inlet duct; but I will close the "additional inlets" ("cheater holes") later just for the look alone. I just need to make sure the plane will still be flyable with those holes closed; or to figure out how to introduce new ones without compromising the external appearance.
Getting the ducting right for an EDF
An EDF Thrust tube (Tutorial)
Both threads are roughly lead by the same users and revolving around the same test setup. To summarize, one of the users conducted a series of trials on the thrust tube geometry using "cheap 70mm EDF from HobbyKing" - exactly what I'm dealing with
The final conclusions were:"The EDF's Thrust Tube needs between around 5 to 20% diameter reduction at its exit. The air should exit the thrust tube at a higher velocity than it entered the system at. If you reduce the tubes exit area by 20%, you will have 20% higher air velocity at the system exit. It actually will be a little less than that because of back pressure and friction. In a real world application, F1 tested a 70mm ducted fan. On this system, He proved the thrust tube exit size should be between 59mm to 62mm for the best performance. That is a 5% to 16% exit size reduction. At 59mm (16%), it will give you the best speed, and at 62mm (5%), it will give the most power. With exit sizes smaller than 58mm, you will have a very little thrust velocity speed increase, but the system will have a massive power loss. With exit sizes larger than 62mm, the system will have less power and speed."
Now, remember the former rings I mentioned before? One of them had the internal diameter of approx. 68mm - slightly smaller than the EDF casing - but the tube it was installed in is actually much larger. In fact, the lowest diameter of existing exhaust tract is at the nozzle; and it's still larger than the EDF. Judging by the article quoted above, I was loosing A LOT of the thrust just by those two "design errors" alone.
The former ring is already gone, that was an easy and quick fix. Unfortunately I did not have a chance to fly the plane after the fix and I really want to introduce one change at a time; but I can prepare a thrust tube that will actually follow "the guidelines of proper thrust tube design" and will be easy to install between flights. Looks like a very interesting round of tests in coming my way; I just hope the weather will cooperate.
At this point I don't worry too much about the shape of inlet duct; but I will close the "additional inlets" ("cheater holes") later just for the look alone. I just need to make sure the plane will still be flyable with those holes closed; or to figure out how to introduce new ones without compromising the external appearance.
Widkin
Member
Oh, if you have not had a thrust tube, I understand why you loose so much power. The inner rings are indeed sized to allow the placement of a thrust tube of a size that should allow max thrust, i.e. only slightly smaller at the exhaust. It seems we have read the same RC group threads.
I have had a version of the plane with a cheap HK EDF, 70mm. Running on 4S 3300mAh Turnigy nanotech. If I remember, it pulled 60-70A at max, and could lift much of the plane vertically, while holding the nose, but not fully. It weighted in at around 1.4kg, so it had at least more than 1kg of thrust. So you should be able to get enough thrust for your much lighter version
I have had a version of the plane with a cheap HK EDF, 70mm. Running on 4S 3300mAh Turnigy nanotech. If I remember, it pulled 60-70A at max, and could lift much of the plane vertically, while holding the nose, but not fully. It weighted in at around 1.4kg, so it had at least more than 1kg of thrust. So you should be able to get enough thrust for your much lighter version
Bayboos
Active member
There is no thrust tube on the plans; I didn't notice it on any of the photos either. I'm working with all I have. Can you post a photo of your tube, and how it fits with the nozzle?
I didn't find the info on RC groups; it's from RCpowers forum - a site selling plans for prop'n'slot jets.
I'm not going to use 4s, at least not until there is a hope to make the plane fly "nice" on 3s only. I don't actually need 1kg of static thrust (although it would be nice...); what I need is at least 700g of thrust that does not drop dramatically as soon as the plane gains some speed. During the two short flights I made with this plane, it was perfectly capable of flying slow at high AoA (and not even using full power). But when I went WOT and the plane started to pick up speed, it behaved like flying into a thick soup, with little to no thrust left to gain any more speed or altitude.
I didn't find the info on RC groups; it's from RCpowers forum - a site selling plans for prop'n'slot jets.
I'm not going to use 4s, at least not until there is a hope to make the plane fly "nice" on 3s only. I don't actually need 1kg of static thrust (although it would be nice...); what I need is at least 700g of thrust that does not drop dramatically as soon as the plane gains some speed. During the two short flights I made with this plane, it was perfectly capable of flying slow at high AoA (and not even using full power). But when I went WOT and the plane started to pick up speed, it behaved like flying into a thick soup, with little to no thrust left to gain any more speed or altitude.
Widkin
Member
Hmm, you're right, the tube is not on the plans. The idea was is to use something thinner than foamboard, like folded cardboard, or really thick paper. I used photo paper for home printing, A3 size. Still, it rattled a lot, maybe due to my imperfect folding and taping. You may be better off with a depron tube, folded to the right size for your EDF. The rings need to be adjusted to fit.
The joining of the exhaust was simpel with a thin tube of paper, since it did not add and to the diameter. If you have a thicker tube, you can join the nozzle part a bit from the edge of the tube (letting the tube stick out a cm or so) and then grind the whole thing to a tapered point. Hope you get the description, I do not have a picture of it.
The joining of the exhaust was simpel with a thin tube of paper, since it did not add and to the diameter. If you have a thicker tube, you can join the nozzle part a bit from the edge of the tube (letting the tube stick out a cm or so) and then grind the whole thing to a tapered point. Hope you get the description, I do not have a picture of it.
Bayboos
Active member
Well, all the elements on the plans have the material specified - it doesn't make sense when all of them are made out of the same material; but it may be useful in cases like this. Also, I would not recommend making anything around the exhaust from such a flimsy material like paper (even if it's photo paper). It needs to be rigid and stiff enough to avoid any vibrations., otherwise it can do more harm than good.
Do you remember that I "had" to make the tail sections larger to fit the main pat of the fuselage? I choose to do that by inserting a rectangular piece of foam. It did make the fitting better, but it also increased the diameter of the nozzle (to more than 75mm). I spent the last two days modifying this part to decrease the diameter to more acceptable range:
Right now the internal diameter of the nozzle is almost exactly 70mm; and after inserting the tube (made out of 3mm Depron) it will drop further down, to approx. 64mm. That will give 17% reduction in the exhaust area (9% in diameter) compared to the EDF unit itself; which I believe is somewhere in the vicinity of the sweet spot
Unfortunately the weather forecast is worse and worse every day, with low clouds, rain/snow, temperatures balancing around freezing point and the wind gusts above 50km/h (30mph). Looks like I have at least one week more before next flight trails. And that in turn means I will drop the "one change at a time" idea and introduce as many of them as I can. And, as usual, hope for the best
P.S. And right now I realized that I probably messed up the order of actions. Since the front part of the exhaust tube will have larger diameter than the back, it will be difficult to slide it inside trough the reduced nozzle. I should probably leave the nozzle "open" and glue it together after the tube would be in place. But that's the beauty of "rapid prototyping": sometimes I tend to be too rapid...
Do you remember that I "had" to make the tail sections larger to fit the main pat of the fuselage? I choose to do that by inserting a rectangular piece of foam. It did make the fitting better, but it also increased the diameter of the nozzle (to more than 75mm). I spent the last two days modifying this part to decrease the diameter to more acceptable range:
Right now the internal diameter of the nozzle is almost exactly 70mm; and after inserting the tube (made out of 3mm Depron) it will drop further down, to approx. 64mm. That will give 17% reduction in the exhaust area (9% in diameter) compared to the EDF unit itself; which I believe is somewhere in the vicinity of the sweet spot
Unfortunately the weather forecast is worse and worse every day, with low clouds, rain/snow, temperatures balancing around freezing point and the wind gusts above 50km/h (30mph). Looks like I have at least one week more before next flight trails. And that in turn means I will drop the "one change at a time" idea and introduce as many of them as I can. And, as usual, hope for the best
P.S. And right now I realized that I probably messed up the order of actions. Since the front part of the exhaust tube will have larger diameter than the back, it will be difficult to slide it inside trough the reduced nozzle. I should probably leave the nozzle "open" and glue it together after the tube would be in place. But that's the beauty of "rapid prototyping": sometimes I tend to be too rapid...
Widkin
Member
I never got around to completely finishing the plans, mostly because I did not find the performance om my own prototype to be that great. Seems like depron is the way to go, so I appreciate that you are doing this and share the progress.
Glad to hear you have a plan to fix the exhaust. Fingers crossed that it works out, and that the weather clears up soon.
Glad to hear you have a plan to fix the exhaust. Fingers crossed that it works out, and that the weather clears up soon.
