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Jul 21, 2019, 11:20 PM
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Dirty Dee's Avatar
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Build Log

1/20 Scale U-2R (3D Printed)


Decided for my next proect to make a U-2R. I wanted to make one for a while after seeing cetorca's build in this fourm. I will be designing this similar to the 3D Printed A330-300 I recently completed.

Specifications:
Scale: 1/20
Wingspan: 1600mm
Length: 960mm
Wing Area: 23.7 dm^2
Powerplant: XRP 50mm (11 blade) with 4200kV motor


Hoping to keep the build under 1000g with a 3S setup. TWR would be about 0.48 on 3S, and 0.64 with a 4s (1200g AUW). Super Pods will be removeable, and I might even try to make simple retracting gear using mechanical retracts.
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Jul 22, 2019, 09:16 AM
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yes! love it already
Jul 22, 2019, 10:44 PM
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Dirty Dee's Avatar
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Thanks!

Doing a bit of servo placement checks before I get into the design too far. Originally I had NACA0009 for the horizontal and vertical stabilizers, but they were quite thin, and HS-65MGs would be thicker than the stabilzers, and stick out both sides. I bumped them up to NACA0012, and now they are a bit better, and only stick out slightly on one face.

I prefer shorter control linkages for surfaces, but might it be better to mount these in the fuselage and run long pushrods? This method would mean I can take the airfoils down to 0009.

The wings I have an RG15 airfoil with +3 degrees incidence at the root, and -1 at the tips. Since this will be a belly lander, I am thinking of having the control horns on the top surface instead of the bottoms.

Another airfoil I was looking at was the Eppler 205, so if anyone has some insight on these 2 airfoils I'd appreciate any feedback.
Jul 25, 2019, 04:01 AM
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awmeade's Avatar
You'll love the U2R - looks so great in the air, and no vices what so ever flying-wise. Following along
Pic is my 3-metre span slope soarer, which has the ability to take an EDF if I feel like it in the future

Quick edit to add - I'd go with the RG15.
Jul 25, 2019, 02:47 PM
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How many planes do you have lined up or are you just that efficient at making models on SW?! Looks great already!
Jul 25, 2019, 02:54 PM
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Eppler 205 is the one I used on my EL-39 and Melusine glider. Very good allround airfoil.

Have you seen these servos? They are outstanding, and only 9 mm thick. I've used them as steering servo on my monster trucks, and even used them on the flaps of my ig Cessna when I ran out of bigger servos. :P
https://hobbyking.com/en_us/corona-d...0sec-8-5g.html
Jul 25, 2019, 06:11 PM
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Dirty Dee's Avatar
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Quote:
Originally Posted by awmeade
You'll love the U2R - looks so great in the air, and no vices what so ever flying-wise. Following along
Pic is my 3-metre span slope soarer, which has the ability to take an EDF if I feel like it in the future

Quick edit to add - I'd go with the RG15.
I completely forgot you also made a U2R, looks gorgeous! all of your slope planes are amazing


Quote:
Originally Posted by too_oblivious
How many planes do you have lined up or are you just that efficient at making models on SW?! Looks great already!
The base model is the easiest part . I couldnt find much technical drawings of the U2, so I was able to tone down on my OCD and model it more freely. It also helps not having so many features or complex landing gear.


Quote:
Originally Posted by Lynxman
Eppler 205 is the one I used on my EL-39 and Melusine glider. Very good allround airfoil.

Have you seen these servos? They are outstanding, and only 9 mm thick. I've used them as steering servo on my monster trucks, and even used them on the flaps of my ig Cessna when I ran out of bigger servos. :P
https://hobbyking.com/en_us/corona-d...0sec-8-5g.html
I was briefly searching for the airfoil you used on the EL-39 and glider, but for some reason I couldnt find the post. Glad you chimed in.

Ive been spending a fair bit of time trying to learn about different airfoils as well as how to read polar charts. I'd love to try plotting this design out in XFLR5.

Thanks for posting those servos. The hitecs seems to only get as narrow as 11mm, so this one could be a good option. I am leaning towards having the elevator servo in the battery bay and operate a long pushrod, while having the rudder mounted in the tail.


Progress:

I began testing some print option using the horizontal stabilizer. 0.8mm skin thickness with a single 0.8mm printed spar came out to 20g for the piece. I then made a variant with 0.6mm skin thickness, and 0.9mm spar. That piece came out as expected ~25% lighter at 16g.

For the 0.6mm wall thickness I printed at "0.3mm" extrusion width, but still using a 0.4mm nozzle, with 0.15mm layer height.

The 0.4mm "single-wall" print was not successful, and the part was too flimsy. ABS might be too soft, and might need more skin supports like the 3DLabPrint models to work. I do want to try printing at 0.25mm with a 0.5mm wall thickness and see if that might work.
Last edited by Dirty Dee; Jul 26, 2019 at 09:28 AM.
Jul 26, 2019, 07:55 AM
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Quote:
Originally Posted by Dirty Dee

Ive been spending a fair bit of time trying to learn about different airfoils as well as how to read polar charts. I'd love to try plotting this design out in XLFR5.
I have a bit of experience in that field. I helped with the whole stability analysis of my senior design in undergrad, and am currently in a robust stability grad class. I plan on doing the same for the CS-300, because the long term goal is to create a flying test bed for a FC system. If you need any tips or pointers feel free to ask! The U2 is a good place to start with minimal sweep and relatively simple modeling in XFLR5. Thankfully there is PLENTY of documentation to go around too
Jul 26, 2019, 09:31 AM
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Cool, I'll have to pick your brain later then .

I'm trying to recall this from some XFLR5 screenshots I've seen a while back, but cant you use it to get a good approximation for Center of gravity, and an idea of stall speed?
Jul 26, 2019, 09:43 AM
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Yessir, it is able to give an approx of CG, whole plane CP with fuselage effects and run basic CFD off of Xfoil interpolated to a 3D surface. It'll run the whole stability analysis and you can simulate a whole slew of adverse conditions too. It really is a neat piece of software for high level analysis.
Aug 05, 2019, 06:54 PM
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Dirty Dee's Avatar
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finished up some tweaks to the model to make 3D printing easier, and created the intake and exhaust ducts/tubes. Intake area is about 95% FSA, and I tweaked the tail to allow about 95% FSA for the exhaust.

Not sure if the intake shape matters, but right now I modeled it so the intakes converge into a donut shape, trying to keep the intake cross sectional area constant up to the fan. Would it be better if instead it converged into just a open cylinder to the fan face?
Aug 06, 2019, 02:55 AM
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raipe
Aug 06, 2019, 10:15 AM
EDF Jet Jam 2019, June 13-16
eatond's Avatar
Quote:
Would it be better if instead it converged into just a open cylinder to the fan face?
Hi Chris,
I did it like you have on my F-15 and for some reason found I didn't have quite as much power as I thought it should. I ended up making an open space in front of the fan and found the power went up. It's been years ago, but I think Corsair Nut ran into the same issue on one of his builds and resolved it the same way. Don't run the inner walls up to the fan face and just create a cylinder as you suggested.

Dan Eaton
Aug 06, 2019, 10:36 AM
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Dirty Dee's Avatar
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Welcome aboard raipe!

Thanks Dan. Right now the duct ends right at the tip of the spinner, so there is a ~20mm gap. Figure I can add a small NACA inlet near the nose to help draw in air to cool the battery/receiver/servos/etc. in the hatch.
Aug 14, 2019, 09:17 PM
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First wing and aileron test prints. I'm using a 4mm x 2mm carbon tube to help with aligning the pieces together, as well as give the wings some extra strength. This will be permanently glued in to the wing, and there will be a separate 6mm x 4mm carbon spar tube for assembling with the fuselage.

The fuel dump tube and fairing detail did not have any supports when printed, so the finish is not the best on the inboard faces due to overhangs, but that will be addressed when I finalize the design.

Aileron servo pocket needs a bit more wiggle room to install servos easier and not accidentally pinch the wiring. The ailerons also need a printed spar to go through the center section, as they are a bit squishy to the touch.


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