2026 Speed Challenge - Telnar1236

[telnar1236]

This is my thread for the 2026 speed challenge. I'll likely be building two planes for it, the Super Duper Sabre design I've been working on optimizing in the background for some time and an as yet nameless design which is intended to run a 90mm fan on 8s, or maybe even 12s.
First is the Super Duper Sabre. It will be a 3D printed 6s 80mm jet. The eventual goal is to fly it on a 9000 mAh Li-Ion battery which should give it 7-10 minute flight times, but for the speed runs I'll probably use a high voltage 4000-5000 mAh Li-Po. Based on CFD and motor performance calibrations I did with a 50mm scaled down version, I'm predicting a top speed in the 140-160 mph range. Probably won't win, but the goal with this one is to have a very high performance plane that is also really nice to fly.

The other, and by far the more competitive, design doesn't have a name yet (suggestions anyone?). It's intended to fly on an 8s 90mm power system, or possibly even 12s depending on how crazy I feel. My predictions for top speed are in the 190-220 mph range with an 8s system but those could be wildly off since I have no calibration data from a plane with performance anywhere close to that. In contrast to the Super Duper Sabre, this design is stripped down and fully optimized for speed alone. It will only have two control surfaces (elevons on the horizontal stabilizer) and will take off from a dolly which it will leave behind on the ground. This saves the weight and complexity of having to have retracts and allows neater ducting and crucially a much thinner wing.

Finally, I guess you could call these the 1.5th and 2.5th entries in the competition. (not actually entries but more prototypes). Both jets will have baby siblings which I use to test the aerodynamics before going all out with the big version. The Super Duper Sabre has a scaled down 50mm version which I flew last year and a related 70mm design called the Ghost that I finished building in February but have yet to fly which tested construction techniques and some of the hardware like the gear struts.

And the 90mm jet will get its own smaller prototype too, probably another 50mm design.

 

[telnar1236]

Reserved for Super Duper Sabre

 

[telnar1236]

Reserved for 90mm Jet (or whatever entry two ends up being)

 

[telnar1236]

Finally had the chance to fly the Ghost (prototype for the super duper sabre) this weekend and it firmly confirms the aerodynamic decisions I made, but as intended, also revealed an issue I'll need to fix for the super duper sabre.

First the good, and there's a lot of it. The big thing with these laminar flow airfoil designs is speed and the Ghost is fast - I think my 110 mph prediction was almost certainly too slow, although I haven't had the chance to clock it with GPS yet - if I had to guess it's better than 120 and the first thing the people watching me fly said about it was to remark that it was fast. It also slows down great. The large wing and wide span flaps meant it could come near a stop with some head wind. The flaps did introduce a nose down moment, but that was easily trimmed out with a 10% mix with the elevator. And it's just nice to fly - the Sabre planform reliably results in a great flying airplane, and the stiffness and precision of a 3D printed design means it's rock solid in the air while remaining quite maneuverable, and despite its larger size for a 70mm EDF, it climbs like a rocket. On top of all that, it gets very long flight times - I flew it for 5:10 of useful throttle with mixed throttle flying and some speed runs and gentle aerobatics and landed with 34% capacity remaining with a 4000 mAh pack. Overall, I think this plane might meet the minimum goals I originally set out in my experimental jets thread if I flew it with a 5000 mAh pack of 130 mph and 7 minutes of time on throttle, although only barely.

Now the bad. There were two primary issues, and a more minor one, although one may or may not be an issue with the plane. The biggest is that the low-profile main gear struts which are a shared design with the super duper sabre are not strong enough. The first flight I had a nearly flawless touchdown and they held up ok, but the second flight I caught a gust of wind and bounced and one of the main struts broke about halfway down and I needed to do a go around and a gear up landing. I'm going to replace the CF-ABS struts with PA612-CF struts which should be roughly twice as strong which should hopefully fix the issue. The next biggest was that on the go around for the gear up landing, on the downwind leg, the plane suddenly had a moment where it really wanted to fall out of the sky. I was able to recover and land mostly safely, but it was touch and go for a moment there. That said, I could have just caught some really nasty wind shear - it was quite windy - or the gear breaking could have damaged something prior to the belly landing and I have no way of separating out the damage from sliding through the grass from anything before that. And finally, the most minor issue is that stalls are a bit squirrelly. Like the 50mm Super Duper Sabre, it gives you plenty of warning, but I didn't feel confident pushing it through the beginning of the stall and seeing what would happen, especially as windy as it was. My solution to this is simply going to be not to stall it.

I'll try to get video soon, but for now it's full steam ahead on the Super Duper Sabre. All the issues with the Ghost are minor and easily corrected or won't affect the Super Duper Sabre at all and the performance of the laminar flow airfoil on a bigger plane was frankly stunningly good. This was also the first flight of a design that fully incorporated the inlet and nozzle design changes from my static testing and the power system thrust and efficiency improvements worked better than I ever expected.

 

[telnar1236]

30 mph gusts this weekend, so no good chance to fly the Ghost and get video or a GPS speed but I've started printing the speed planes.

Due to its much greater simplicity, the 90mm jet is coming together faster. To try and stay closer to the spirit of this competition, I'm using normal PLA+ instead of my more usual advanced filaments (and to make up for the crazy filament blend in the Super Duper Sabre). This plane is designed to go very fast a few times, so I don't really need to design it to stand up to being left in my car repeatedly or worry as much about maintainability.

In the contrast, the Super Duper Sabre is designed to use the capabilities of 3D printing to the greatest extent possible to keep the weight down and give the best performance and durability. The majority of the plane will be printed in CF-ABS for stiffness and precision, with ABS, two types of CF nylon, normal nylon, and TPU making up the balance. While it should still be manageable by the average hobbyist printer, it will be several steps up in the complexity and printing performance required.

And finally, the 90mm jet has a color scheme now. The red and white should hopefully be very visible, and by using white for the most highly stressed parts, I should keep the PLA as cool as possible in the summer sun, just to reduce the risk of any catastrophic failures.

 

[telnar1236]

More good and bad news on the Ghost. Good news is the top speed is 125 mph, although that is with a bit of a tailwind, and the main gear didn't break in the same way this time. Bad news is the gear broke in a different way and the plane flipped and got a bit beat up.
Last time, the struts broke in shear, but this time the layers separated around the pin from the retract unit. I'll need to do some thinking on how to fix this since I'm pretty near the highest performance material I can print already and I can't add much material to the strut and keep it able to fit in the Super Duper Sabre or the Ghost

 

[telnar1236]

But at this point, I need to switch to building the Super Duper Sabre. The Ghost has mostly served its purpose for the moment and the clock is ticking, so any improvements to the gear struts will be made in the new plane and I'll finish the Ghost after I've built my entry in this competition

 

[telnar1236]

More of the fuselage of the 90mm jet and it now has a name - the Demon. The Super Duper Sabre started off as the Wraith, and I have the Ghost now, so it continues the theme. Hopefully the name doesn't predict the flight characteristics though.

Instead of building a scaled down version, I'm thinking I'm going to just make the power system swappable and start off with a 4s 70mm EDF before working up to the 8s 90mm power system this will eventually fly with. The design is so simple and bare bones, and it's all PLA, so there's not much reason to build it smaller since the assembly time won't be reduced at all and a 50mm version wouldn't actually be all that much reduced in size.

I thought the size comparison to my 90mm F-106 was a bit fun. The F-106 flies on a 6s 90mm setup with over 1 kg less thrust than the Demon will have but is substantially bigger. And here it is next to my 64mm 4S sport jet which is already pretty fast, being able to hit over 100 mph. It's only a bit bigger (although it has a decent bit more wing area to keep stall speed under control) but will weigh about twice as much and have almost 5x the thrust.

 

[telnar1236]

And some more nose for the Super Duper Sabre as well. It's always really nice printing with CF ABS compared to normal ABS because you don't really need to worry about warping at all.

Also got a timelapse of the print this time:

And finally, I have repaired the Ghost again despite originally planning not to. I still need it to test the next gear design and it's much faster to fix the Ghost than it will be to fix the Super Duper Sabre if it gets damaged. The new landing gear has a separate loop of material around top of the strut to help avoid layer separation but I'm a bit worried that I'm weakening the strut in bending and shear to improve the layer strength so I might just get a different type of failure again. But the CF nylon is more than twice as strong in the in-plane directions and I'm not removing all that much material and will still have the loop bonded in place with CA so I'm hoping it strikes the right balance.

 

[telnar1236]

Got the rest of the wings for the Super Duper Sabre printed out

I'm going for a 2 spar structure same as on the Demon since I don't necessarily trust the 3D printed mounts to resist the moments generated by a 45-degree swept wing at 150 mph. The front spar is entirely there to keep the wing from twisting out of place.

And incidentally the wing spars on the Demon for comparison. The Demon's straight wings should generate less of a moment, but it's also potentially a lot faster so it needs a ton of strength.

And since I didn't put it in my previous post, a size comparison between the Super Duper Sabre and Demon.

They have the same wing area and aspect ratio and are also about the same length since the Demon is actually derived from an earlier draft version of the Super Duper Sabre that I decided not to go with due to its limited internal volume despite it being a lower drag setup. I just moved the inlets backwards to reduce the length of the EDF ducting, got rid of the canopy, and made some changes to the wing and tail since I no longer needed to fit retracts. And of course redesigned it to use a 90mm EDF.

 

[telnar1236]

Figured out why the Ghost suddenly decided it didn't like steering straight. It turns out the wheels were melting, fusing to the axles and then locking up completely. They must have already been near that temperature and as the weather has gotten warmer, suddenly they went over it and suddenly the airplane wanted to skid sideways across the runway instead of tracking down. Fortunately an easy fix by changing the GF-PETG to nylon.

 

[LitterBug]

Figured out why the Ghost suddenly decided it didn't like steering straight. It turns out the wheels were melting, fusing to the axles and then locking up completely. They must have already been near that temperature and as the weather has gotten warmer, suddenly they went over it and suddenly the airplane wanted to skid sideways across the runway instead of tracking down. Fortunately an easy fix by changing the GF-PETG to nylon.

Maybe throw some graphite on the axle too....

 

[telnar1236]

Maybe throw some graphite on the axle too....

Probably a good idea

 

[telnar1236]

Some more progress on the CAD

I had to reroute the ducting a bit to make sure there was space for the main gear and wing spars. A smooth duct capable of providing clean air to the fan with minimal drag is absolutely critical to this design's performance so having the spars run through the duct wasn't really an option.

Also got the 17g metal gear servos that will drive the elevons on both the Super Duper Sabre and the Demon so now I can start figuring out how to package them as cleanly as possible

 

[telnar1236]

Got the first two parts of the core fuselage printed. With them present, you can really start to see how the wings fit onto the rest of the airframe.

I realized that the forward of the two parts wasn't quite stiff enough in a couple of directions so I hot glued some wooden skewers in to make it stronger. Normally I'd say that's good enough for v1 since I typically anticipate needing to replace parts the first few times I take a plane this complicated up, but I'm actually reprinting it for this one, both because I have so much analysis and testing of prototypes and because of the amount of time it will take to get everything painted before flying it (I do not want to try flying an all-black plane at 150 mph so I'm also planning to take the unusual for me step of doing most of painting before the maiden flight). So the wooden skewer won't actually make it into the flying plane.

I also redesigned the main gear yet again, this time to improve the torsional stiffness (bet you never thought a speed plane design would spend this long on the landing gear). I was doing some testing and realized that under certain conditions the wheel could twist about 15 degrees away from straight which probably contributed a lot to the friction and breakage I kept getting. I think this is probably due to the limitations on the tolerances I can achieve with my printer. There needs to be a 0.25mm gap with the strut to get something that behaves like a line fit off the printer, but a lot of that 0.25 mm extra dimension comes from roughness that wears away and leaves too much space.

So now there is a "key" on the lower strut that slots into the upper strut to keep it from twisting as easily.

 

[telnar1236]

Ok, got the core fuselage assembled correctly this time with the improved structure and no wooden skewers required.

I've also continued to make minor adjustments to the design based on what I learned with the Ghost. This time, it's the addition of a bit of extra length to the tail to improve the streamlining around the nozzle.

This reduces the width of the ring around the nozzle from 4 mm down to 1.2 mm. I don't expect there to be a huge impact, but my philosophy towards streamlining this plane is every little bit counts.

 

[MadMonkey]

Looking good... I have a feeling this one's gonna be speedy!

 

[telnar1236]

The duct and thrust tube are now fully designed. I was debating if I wanted to include a tail cone for the motor because it might mess with the cooling of the motor, but based on the results of the testing and CFD I did late last year, the benefits are just too great to ignore. We're looking at a 10-15% improvement in efficiency which is pretty significant.

I'm also designing an airfoiled fairing for the motor wires. I don't have CFD or testing on this, but I did look at the impact of an airfoil in the inlet ducting for the Ghost and it has almost no penalty, where I suspect the less aerodynamic wires might be stealing a couple percent thrust.

I'm still working on modeling the last section of the core fuselage since it's the most complicated part as it needs to hold the flap mechanism, part of the ESC cooling duct, connection points to the EDF fuselage, wire pass through from the wings, one of the mounting points for the wing, and mounting points for the EDF housing, as well as access to all of these things so I can install and maintain them.

 

[telnar1236]

And the main gear finally works properly - no falling breaking off the retract unit, no wheel melting, no skidding sideways, and no snapping on less-than-optimal landings. I printed the new gear struts using PA612-CF15 carbon fiber nylon, the wheels out of normal PA6 nylon, and the tires out of 85A TPU. I also added an extra washer on the outside of the wheel to hopefully reduce the friction from the wheel rubbing against the head of the screw I'm using as an axle.

And I followed Litterbug's suggestion and added graphite to the inside of the wheel, the screw, and the two faces of the wheel contacting the washers.

 

[L Edge]

The duct and thrust tube are now fully designed. I was debating if I wanted to include a tail cone for the motor because it might mess with the cooling of the motor, but based on the results of the testing and CFD I did late last year, the benefits are just too great to ignore. We're looking at a 10-15% improvement in efficiency which is pretty significant.
View attachment 258136
I'm also designing an airfoiled fairing for the motor wires. I don't have CFD or testing on this, but I did look at the impact of an airfoil in the inlet ducting for the Ghost and it has almost no penalty, where I suspect the less aerodynamic wires might be stealing a couple percent thrust.
View attachment 258137
I'm still working on modeling the last section of the core fuselage since it's the most complicated part as it needs to hold the flap mechanism, part of the ESC cooling duct, connection points to the EDF fuselage, wire pass through from the wings, one of the mounting points for the wing, and mounting points for the EDF housing, as well as access to all of these things so I can install and maintain them.

Now that you found a increase for the tail cone, I am going to suggest you look at putting at least 4 stator vanes for a better increase of least 6-10% thrust gain. This allows the flow to changing rotational into linear axial flow for better thrust. You should use a NACA 006 airfoil(taper trailing edge to a point) and stay between 5 to 10 degrees for incidence angle. Over ten degrees will reduce top speed.

So here is tail cone with the built into 4 stator vanes that gave the 6 bladed EDF plenty of power

In design, one other factor that I played with is reducing the tip gap between the blades and outer wall. I used Scotch magic tape (2 layers of tape .010-.015) and it increases the internal pressure of the duct as well as reducing spillage back into the entrance of the EDF so the thrust increases 4-6% more.
Imagined 3d printing a thin ring and putting that in between the blades and housing. Remember that the blades lengthen when spinning, so leave some small gap(less than 1 % of the blade tip chord) otherwise you will loose 3% thrust due to interaction between rotor tip and duct wall.