Saab 105 64mm 3D Printed EDF Jet

[telnar1236]

The entire fuselage is now printed. Everything fits together pretty well, and it seems to print pretty easily too so far.

Unfortunately, I'm going to need to reprint the core fuselage and tail fuselage. With how wide the fuselage is, the duct needs to curve quite sharply to fit a single EDF and I was somewhat worried about how it would impact the thrust, so I decided to measure the thrust from both a 70mm and 64mm EDF installed vs. uninstalled. In the open air the 70mm EDF got 1500 g of thrust and the 64mm EDF got 1200 g, but as soon as I put them into the plane both only produced about 550 g of thrust which was a problem. I was able to track the problem down to two areas of the ducting.

In the nozzles I added in some vanes (in red) since I was worried the air would come out somewhat sideways and hurt the thrust. I was about 50-50 on if they would help, and they very much did not. My first step was to remove them as much as possible and the thrust immediately jumped up to 650 g which still wasn't good but was definitely an improvement. Then, to see if the problem was more in the inlets or the nozzles, I ran the 70mm EDF without the core fuselage in place and the thrust jumped up to 1350 g which is much more acceptable.

So, I think my next steps are fairly clear. On the tail fuselage, I need to fully delete the vanes since they clearly hurt, and I suspect that even after I cut them out the remnants I couldn't get with a pair of scissors were still killing some of the power. On the core fuselage, I need to make the inlet ducting much smoother and straighter. I hadn't actually looked at the cross-section here before today, and it's pretty clear that the angle of the inlet ducting is far too steep now that I do. In order to do that, I'll probably need to shift the EDF unit back which will also require a redesign of the mount, and I think to make triply sure the fan gets enough air, I'll also add a cheater inlet since the inlet will still need to be fairly steep.

 

[FlyerInStyle]

No worries, I wouldn't be posting this stuff online if I didn't want to be asked questions about it.

PETG would probably work, but it's heavier than either PLA or ABS and despite its reputation, not as strong as PLA. If printing in PETG it will be about 200g heavier than ABS and about 70g heavier than PLA, so especially if you end up using a weaker 70mm fan with a big 3300 mAh pack, you could end up with something quite underpowered and heavy. Pretty much regardless of the material, if you crash, you'll end up replacing parts though. Really the only reason I see to use it is if you're worried about temperature resistance but don't have the enclosure set up to print in ABS.

The plane is a mix of areas requiring infill and not. Most of it is fully modeled, but there are some locations, mostly around bits that are more there for aesthetics, where infill is used. The majority of the parts use 0.4 - 0.8mm walls and 5% infill, but some more highly loaded bits like the landing gear use 1.6 - 2.0mm walls and 40% infill. When I release the files, there will be printing and assembly directions like with my F-104 with the required wall thickness and infill per part.

Thanks, pretty happy with how it's turning out so far. The battery is under the canopy and a bit back.
View attachment 252521

I don't have the servos modeled in yet, but it will use 7 of the common 9g servos most people have, 2 for ailerons, 1 for elevator, 1 for rudder, 2 for flaps, and 1 for nose gear steering. They'll be about here in removable pockets for easy replacement if needed. They're located as close as possible to the control surfaces to keep the length of the linkages reasonable.
View attachment 252522

Thanks for the clarification! I am thinking maybe I'll get one of those classic tent enclosures and use either abs or try some asa aero. As for servos, that's quite a lot, why not link nose gear and rudder? Also, I just had the idea, the Saab 105 had reverse thrust, maybe if you feel like exploring something new it might be fun to add. Not in the regular way of reversing the motor but in the way to was originally. (I believe it blocked it right after the exhaust port) May be a fun challenge for 2 extra servos.

 

[telnar1236]

Thanks for the clarification! I am thinking maybe I'll get one of those classic tent enclosures and use either abs or try some asa aero. As for servos, that's quite a lot, why not link nose gear and rudder? Also, I just had the idea, the Saab 105 had reverse thrust, maybe if you feel like exploring something new it might be fun to add. Not in the regular way of reversing the motor but in the way to was originally. (I believe it blocked it right after the exhaust port) May be a fun challenge for 2 extra servos.

My experience with using more servos vs. linking multiple control surfaces together has generally been that the extra mechanical complexity is not worth the slight reduction in weight and price. Probably still the most complicated design I've ever created was a 6 channel LW-PLA F-104 with a 50mm fan that flew on 3-4s. To save weight, it only had one servo per channel, even for the flaps/leading edge flaps and ailerons.

And it flew fairly well until it crashed since it was wrapped in aluminum foil which blocked the radio signal on occasion. To this day, I think it's the best-looking airframe I've ever created.

But the big lesson I took away from it was that having extra servos is worth the extra weight. The inside was a spider's web of control rods and the servo driven retracts for the main gear never quite worked right because of that decision (one of the reasons I didn't rebuild it after the crash). And the assembly process was a huge nightmare with getting everything aligned and installed correctly.

 

[telnar1236]

This is what I've arrived at for the new duct design. Overall, it's much smoother and should be a big improvement.

The new tail fuselage without the vanes is printed and with the same setup as the version with the vanes cut out, I got 1270 g of thrust on the stand with the inlets removed. I'm not sure if this is because the best configuration would have partial vanes or due to something else since these measurements aren't trying to be super precise.

 

[FlyerInStyle]

My experience with using more servos vs. linking multiple control surfaces together has generally been that the extra mechanical complexity is not worth the slight reduction in weight and price. Probably still the most complicated design I've ever created was a 6 channel LW-PLA F-104 with a 50mm fan that flew on 3-4s. To save weight, it only had one servo per channel, even for the flaps/leading edge flaps and ailerons.
View attachment 252563
And it flew fairly well until it crashed since it was wrapped in aluminum foil which blocked the radio signal on occasion. To this day, I think it's the best-looking airframe I've ever created.

But the big lesson I took away from it was that having extra servos is worth the extra weight. The inside was a spider's web of control rods and the servo driven retracts for the main gear never quite worked right because of that decision (one of the reasons I didn't rebuild it after the crash). And the assembly process was a huge nightmare with getting everything aligned and installed correctly.
View attachment 252564

That makes sense. The ducts look pretty good. What do you think fo the reverse thrust? Also why the steerable nose wheel? Is it because the control surfaces gets no airflow at such low speeds so it's needed or more of a commodity? I see more edfs having it than props

 

[telnar1236]

That makes sense. The ducts look pretty good. What do you think fo the reverse thrust? Also why the steerable nose wheel? Is it because the control surfaces gets no airflow at such low speeds so it's needed or more of a commodity? I see more edfs having it than props

Reverse thrust might be a bit too complicated to fit on this plane, especially using a mechanical system instead of the ESC. You'd probably need to go up to a 90mm size jet or so to get that kind of detail without some weird looking fairings for the mechanism. There's not a ton of space around the nozzles because of the duct so I probably couldn't fit in a servo too easily either. That said, it shouldn't really need it - with the flaps it should slow down quite nicely and worst case, with a reversing ESC it should stop quite quickly.
I think the big reasons you get steerable nose gear more on jets are that they almost exclusively have tricycle gear and have longer takeoff runs requiring more of a runway. A lot of prop designs are tail draggers, so you can stick the tail wheel on the rudder with extra linkages and call it a day a lot of the time. I can't think of an rc plane, prop or otherwise, that's larger than a UMX with tricycle gear where the nose wheel is not steerable, though I'm sure they're around. I also think they're more important on jets due to the longer takeoff run and higher stall speed. With a prop plane, you rarely need more than about 30 ft or runway to get off the ground, even if you might want to use more so you have a prettier takeoff, where with an EDF you might very well need 200+ ft which means you need to steer the plane more on the ground to stay on the runway. And, like you said, you have no airflow over the tail till you're up to speed, so a steerable wheel, there's no control over the first part of the takeoff run.
I don't yet know if this plane will be able to be hand launched. It's a bit of an awkward shape to hold, so it may need a second person at the least. If it can be hand launched, both the main gear and nose gear are unnecessary. If it cannot be hand launched, however, it will require a paved or similar runway, so I personally would not want to try and fly without steering. If you have a dry lakebed or salt flat or similar, it's probably possible, but also probably not a great idea.

 

[telnar1236]

While I'm reprinting the fuselage, I've also been using my other printer to make some of the other parts that weren't affected by the change, so I now have the landing gear and wheel hubs printed. Both will be able to use normal PLA without any trouble, but for my version, I used carbon fiber PLA for the gear struts and glass fiber PETG for the wheel hubs. In my experience, carbon fiber PLA resists creep a bit better from repeated landings, so I've used it on most of the fixed landing gear I've built on my simpler planes. For the hubs, however, this is the first time I've used GF-PETG. From some reading I did, it should be quite a bit more wear resistant than PLA and only outperformed by nylon and TPU for the easier to print hobby filaments, but just based on the feeling of the hubs I'm skeptical, so it will be a case of waiting to see how they perform.

Each wheel will consist of a hard plastic hub and a soft TPU tire. With the slightly concave surface on the hub, the tire is mostly retained by its shape, although it also needs a bit of CA to help keep it aligned.

 

[telnar1236]

Got the fuselage fully reprinted. It doesn't really look any different from the original since most of the changes are internal. You can somewhat see how the skin of the wing roots didn't buckle because of the addition of another rib, however.

The wing structure is also now designed so I should be good to start printing the wings.

Finally, I ran some CFD on the airframe, mostly in the interest of understanding hand launch capability. With the flaps, the landing speed should be about 28-29 mph and the stall speed a bit lower than that at around 26 mph. This is slightly better than the 50mm jet that inspired this project which I'm fairly sure is hand launchable although I haven't tested that yet. The stall should also be very gentle due to the washout in the wing and the wing fences which is again beneficial for hand launching.

My two big concerns, therefore, are thrust and where the person throwing it can hold it. Even with the improved ducting and the cheater inlet, it probably won't be all that efficient since it needs to bend quite a bit due to the location of the inlets and nozzles. I'm guessing it will have enough thrust with the 70mm 4s fan I have, but maybe not with my 64mm fan or a weaker 70mm fan. The fuselage is also just big enough to be awkward to hold. I have larger hands, and I can hold it like I would for a toss, but I'd be worried about losing my grip with the combination of the EDF thrust and needing to propel it up to speed. One possible solution here is that I might design an optional grip that can replace the main landing gear.

 

[telnar1236]

And now it's got wings and landing gear.

Both are of a fairly conventional design both in terms of structure and form. The wheels are certainly the best I've designed so far in terms of ease of assembly and I think the balance between keeping everything aligned and making it easy to get the tire over the hub is about perfect now.

 

[telnar1236]

It's been a moment since I got distracted by trying to build a plane for the recent speed competition in about a week start to finish, but I'm now continuing to build this plane as my current main project. I got the vertical stabilizer printed as well as the ailerons and rudder (not in the picture).

 

[bisco]

looking good!

 

[telnar1236]

Visually, the plane is now mostly complete. I got all the horizontal stabilizer, all the control surfaces, and the nose gear mount printed.

Currently, every part of this plane is printed in some form of heat-resistant filament (mostly ABS, but some GF PETG and TPU in the wheels). The only remaining parts to print are the servo mounts and EDF mount/internal duct. At this point I'm building up a bit of a backlog of planes to test, between my speed plane, this one, and the final set of upgrades for my 50mm jet trainer, but this is the predicted weather for the next week, so it might be a bit before I get to.

 

[FlyerInStyle]

Visually, the plane is now mostly complete. I got all the horizontal stabilizer, all the control surfaces, and the nose gear mount printed.
View attachment 252955
Currently, every part of this plane is printed in some form of heat-resistant filament (mostly ABS, but some GF PETG and TPU in the wheels). The only remaining parts to print are the servo mounts and EDF mount/internal duct. At this point I'm building up a bit of a backlog of planes to test, between my speed plane, this one, and the final set of upgrades for my 50mm jet trainer, but this is the predicted weather for the next week, so it might be a bit before I get to.
View attachment 252956

How is PETG-GF? I am considering acquiring some but have no idea how it prints. how is it on weight and strength? Good luck on the maiden when you get to it! I am really excited to hopefully build one myself

 

[telnar1236]

How is PETG-GF? I am considering acquiring some but have no idea how it prints. how is it on weight and strength? Good luck on the maiden when you get to it! I am really excited to hopefully build one myself

I'm guessing there will be something of a range based on the quantity and type of glass fibers they use, but the stuff I'm using seems good so far. Weight-wise, it seems identical to normal PETG which is pretty typical for composite filaments so too heavy to be ideal for most 3D printed planes. I haven't tested it on strength, but in my experience, claims for PETG strength tend to be a bit overstated, and in most applications, I've found PLA or ABS holds up better. The glass fiber will probably make it more brittle without a significant improvement in strength. I'm currently testing it as a material for the wheels to see how well it holds up there. Supposedly, it's fairly wear-resistant, second only to nylon which is a big pain to print. That said, the GF PETG still requires a fairly capable printer since it's more abrasive than even carbon fiber and prints at 250 C which means you really should have an all-metal hot end and hardened nozzle in order to print it for any significant time.

Thanks, I'm hoping to get enough of a break in the weather this weekend, but we'll see if that works out.

 

[bisco]

just beautiful, well done!

 

[bozman]

The entire fuselage is now printed. Everything fits together pretty well, and it seems to print pretty easily too so far.
View attachment 252536
Unfortunately, I'm going to need to reprint the core fuselage and tail fuselage. With how wide the fuselage is, the duct needs to curve quite sharply to fit a single EDF and I was somewhat worried about how it would impact the thrust, so I decided to measure the thrust from both a 70mm and 64mm EDF installed vs. uninstalled. In the open air the 70mm EDF got 1500 g of thrust and the 64mm EDF got 1200 g, but as soon as I put them into the plane both only produced about 550 g of thrust which was a problem. I was able to track the problem down to two areas of the ducting.
View attachment 252537
In the nozzles I added in some vanes (in red) since I was worried the air would come out somewhat sideways and hurt the thrust. I was about 50-50 on if they would help, and they very much did not. My first step was to remove them as much as possible and the thrust immediately jumped up to 650 g which still wasn't good but was definitely an improvement. Then, to see if the problem was more in the inlets or the nozzles, I ran the 70mm EDF without the core fuselage in place and the thrust jumped up to 1350 g which is much more acceptable.
View attachment 252538
So, I think my next steps are fairly clear. On the tail fuselage, I need to fully delete the vanes since they clearly hurt, and I suspect that even after I cut them out the remnants I couldn't get with a pair of scissors were still killing some of the power. On the core fuselage, I need to make the inlet ducting much smoother and straighter. I hadn't actually looked at the cross-section here before today, and it's pretty clear that the angle of the inlet ducting is far too steep now that I do. In order to do that, I'll probably need to shift the EDF unit back which will also require a redesign of the mount, and I think to make triply sure the fan gets enough air, I'll also add a cheater inlet since the inlet will still need to be fairly steep.

Maybe consider adding a cheater vent/hole in front of the fan?

 

[telnar1236]

Maybe consider adding a cheater vent/hole in front of the fan?

Already done

 

[telnar1236]

It flies! No video yet, but I should be able to get some in the next week or so hopefully. Overall, it flies decently well - it's quite floaty even though it feels like a bit of a brick when it's on the ground and it's fairly stable in the air. It was very windy today, so I didn't try to do too much with it and couldn't get too good of a sense of its flight characteristics with how much it was being tossed around. It will tip stall, which isn't too surprising, but overall, it seems pretty trainer-y as best I can tell.

I performed the maiden flight with a 70mm fan and a 3000 mAh pack since it's a struggle to balance it with a 2200 mAh pack. With the wind and it being a maiden flight I didn't push it too hard to try and get a sense of vertical or top speed, but I suspect neither is great. It has more than enough power to fly well, but I think this is one of those planes that hits its tops speed around 2/3 throttle and won't go much faster after that because of aerodynamics (it is certainly not the sleekest thing out there). Now, on to the good - I set my timer for 3 minutes and came down with 56% remaining. Since the airframe balances fine with a 4000 mAh pack, I suspect 7-8 minute flight times will be possible. With its overall stability and how floaty it feels, I think this might be a good design for me to modify for FPV.

I want to get a couple more flights in to make sure I'm not missing anything bad, and I think I would like to design working suspension on the main gear, but the design is pretty much ready to release right now, so all I really need to do is write some instructions. I'll probably post it as a resource in the next couple of weeks.

 

[FlyerInStyle]

It flies! No video yet, but I should be able to get some in the next week or so hopefully. Overall, it flies decently well - it's quite floaty even though it feels like a bit of a brick when it's on the ground and it's fairly stable in the air. It was very windy today, so I didn't try to do too much with it and couldn't get too good of a sense of its flight characteristics with how much it was being tossed around. It will tip stall, which isn't too surprising, but overall, it seems pretty trainer-y as best I can tell.

I performed the maiden flight with a 70mm fan and a 3000 mAh pack since it's a struggle to balance it with a 2200 mAh pack. With the wind and it being a maiden flight I didn't push it too hard to try and get a sense of vertical or top speed, but I suspect neither is great. It has more than enough power to fly well, but I think this is one of those planes that hits its tops speed around 2/3 throttle and won't go much faster after that because of aerodynamics (it is certainly not the sleekest thing out there). Now, on to the good - I set my timer for 3 minutes and came down with 56% remaining. Since the airframe balances fine with a 4000 mAh pack, I suspect 7-8 minute flight times will be possible. With its overall stability and how floaty it feels, I think this might be a good design for me to modify for FPV.

I want to get a couple more flights in to make sure I'm not missing anything bad, and I think I would like to design working suspension on the main gear, but the design is pretty much ready to release right now, so all I really need to do is write some instructions. I'll probably post it as a resource in the next couple of weeks.

That's great! Can I get the design files by any chance or could you post them as a resource? I really want to build this!

 

[telnar1236]

That's great! Can I get the design files by any chance or could you post them as a resource? I really want to build this!

I'll be posting them as a resource in a week or two, once I have the chance to get a few more flights in and make sure there's nothing seriously wrong hiding in the design as well as write up some proper instructions.

If you're in a hurry to build it, I can post the STLs here now, very much in a beta state, since I know they work well enough to fly, but I'd recommend waiting till I put together directions and post the resource since currently I have flown this plane precisely once and there are still some minor refinements I'd like to make, mostly to reduce weight a bit.