3D Printed MIG-21 - RC Groups
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Nov 16, 2015, 02:19 PM
German Engineering.......
HugePanic's Avatar
Discussion

3D Printed MIG-21


Hi,

I am currently developing a 3d printed MIG-21, and I will use this thread to write a build log.


Since i found a real-life MIG-21 in my area, and after too many DCS MIG-21 nights i had to build one of these.

I reallyreally whant to build a scale model with retractable landing gears, flaps and so on...

After my first 3d printed plane (PC-6) i found that more thinking and developing prior to printing is better to avoid problem.

So I decided to build a flying prototype first.
  • Scale 1:10
  • 70mm EDF
  • 4s3000mAh Lipo
  • 80A ESC
  • Elevatro and Aileron only
  • Catapult launche, belly landing
  • no bells & whistles

I have found a basic 3d model on grabcad, that I am using as master. Anyhow, right now I have spend at least 15h alone fixing and modifying the master-geometry, to have all the parts surfaces.

I am using a MH51 airfoil for wing end elevator. The rudder uses a symetrised version of that same airfoil.

I have bought some aileron servos (towerpro MG92B) and one larger elevator servo (Dymon DS-2100MG).

The Elevator and aileron design is about 80% done, first samples are printed to evaluate the print parameters and for hinge testing. All the printing will be done with ABS on my RostockMax V2 printer.

I have an EDF unit (Wemotec Mini Fan EVO with Hacker A30-14M-DF and a ESC on order. Both should arrive this week.

So next week i hope to start modeling the fuselage with all the attachments for servos, find, rudder, wing, canopy, launch-hook,.....
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Nov 16, 2015, 02:32 PM
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superevan's Avatar
Very cool. What's your target weight?
Nov 16, 2015, 04:00 PM
German Engineering.......
HugePanic's Avatar
good point... it was hoping for 2kg flight ready.

Right now i estimate the following after doing a quick slicing with my std. parameters:

Weight [g]
EDF 57
Motor 110
Lipo (4s3000) 350
RC-stuff 150
HTP (2x) 50
Wing (2x) 430
Fuse 1050
VTP 105
aileron 2x 6

Sum 2308g

I hope to find 300g in the fuselage to remove due to non-structural parts.
I also hope to not spend 300g in unneccessary stuff like wing-spars, wires, control rodīs...
Nov 16, 2015, 05:38 PM
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superevan's Avatar
It's hard to tell but are you designing the support structures in your wings yourself or are you just telling the slicer a % of infill? I only ask because you might be able to save some wing weight if you optimize what supports get printed. But then again, if you are, never mind
Nov 16, 2015, 11:57 PM
German Engineering.......
HugePanic's Avatar
Right now I plan to use the %-infill for most of the parts, but I will do more studies for each part.

I have found out at the pc-6 project that a 5-8% lines infill is pretty hard to beat. I am using only one perimeter globally. locally I can increase the % and perimeter lines.

Sometimes even 0% infill with 2perimeter is more heavy than 1perimeter and 5% infill.

on the mig-21 I will have a large battery cover, that will be just 2ply an thick.
in theory the whole front is not load bearing.
in reality I have to mount the launch hook there an I need the area as landing gear.

Can you share more of your experiences, I would be glad to see more about that.
Nov 17, 2015, 02:43 AM
Registered User
I think you could save a lot of weight by designing the supports yourself, like this guy: https://www.rcgroups.com/forums/show....php?t=2387111
I tried making a wing this way yesterday, but my slicer refuses to print the outside shell in one go. Instead I end up with lots of ridges on the outside where the internal supports are...
Did you try using the shell itself as a hinge? You could make a very smooth wing that way.
Nov 17, 2015, 05:38 AM
German Engineering.......
HugePanic's Avatar
I have done some life hinge test's.
The only suitable material for this was garden trimmer line "nylon". ABS and PET are way to brittle. Since the plane will be scal"ish", I am ok with hinges...

About infill:
I have tried that. the problem is that all slicer's I have tested only create walls with minimum thickness equals the nozzle diameter.
If you have less infill, you also have to add material for all hard-points like servo and hinge brackets.

If I ask the slicer to make the infill, it will make walls with only 50% of that normal weight. the "lines" infill brings one line every second layer.


About slicer:
The only way i have found to create thin structures is to add thickness above the nozzle diameter. So for a 0.5mm nozzle you have to design about 0.6mm thick wallīs. A different approach is to set you nozzle diameter to 0.4999999 mm. This will also help creating 0.5mm walls. You can also check the accuracay settings of you STL-file creation. I use 0.01mm accuracy, that works well. I have heard from problems with less accurate settings.

I have yet to ask the spitfire rebuilder/printer what the actual wing density is.
Last edited by HugePanic; Nov 17, 2015 at 06:03 AM.
Nov 17, 2015, 08:09 AM
Registered User
Slic3r can do thinner walls, change "extrusion width" under the advanced settings.
Nov 17, 2015, 08:35 AM
Registered User
I think you do not need so much material in the middle of the wing. One spar and some stringers under the surface should be enough, which would save a lot of weight. I attached my first test piece, imagine that with a spar. I will try to make a small plane for a 35mm EDF this way..
Last edited by RaoulDuke; Nov 17, 2015 at 08:40 AM. Reason: double post
Nov 17, 2015, 09:18 AM
German Engineering.......
HugePanic's Avatar
Quote:
Originally Posted by RaoulDuke
Slic3r can do thinner walls, change "extrusion width" under the advanced settings.
can you do this for internal structure only?

0.5mm (1 perimeter) skin is pretty flexible if not supported every 10-15mm.
Nov 17, 2015, 09:24 AM
Registered User
You can change external perimeters, internal perimeters and infill. Did you look at the STL from my last post? The ribbing could stiffen the skin enough that you would only need a few spars for a rigid wing. You could even make the spar solid and use Slic3rs 3D honeycomb as infill.
Nov 17, 2015, 09:43 AM
German Engineering.......
HugePanic's Avatar
yes, I have checked your design.

I have a nice picture for you in the attachment.

In the end I had lot's of warping (basically the profile got thinner) and the weight was more than the infill-style. I printed my "stringer" design with 2perimeter=1.0mm.
Nov 17, 2015, 11:57 AM
Registered User
That looks very clean, I think it would work with crossing stringers and one perimeter. Here is my test piece, no spar so far but the shell feels very strong. The surface looks bad though, because the makerbot slicer refuses to print the outside perimeter in one loop.
Last edited by RaoulDuke; Nov 17, 2015 at 11:58 AM. Reason: ribs vs. stringers
Nov 17, 2015, 02:15 PM
German Engineering.......
HugePanic's Avatar
what is the size of your segment? my profile is about 20cm long. The red stringer test sample feels too weak for me. maybe with comlete spars (connection of upper and lower profile side) I can get the needed stiffnes.

what material did you use?

maybe you can alter your print order after slicing in the gcode. you can write excel macros to shuffle code.
Nov 17, 2015, 02:58 PM
Registered User
Itīs about 50mm long with 80mm chord. Printed with PLA at 0.2mm layers height with a 0.4mm nozzle.
I think moving the stringers slightly away from the shell might do the trick. They will stilll stick to the shell, but the slicer should treat them as independent parts. I will try another piece with a spar and aileron hinge tomorrow...


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