Check out the whole "T" copter family:

• TX8 Octocopter
• T6 Hexacopter
• T4 Quadcopter (10" props)
• T4 Quadcopter Mini 315 (7-8" props)
• T4 Quadcopter Mini 250 (5" props)
• Tubular Crossfire 2 quadcopter
  I've added a post on the forums over at DIY Drones for discussing and improving these designs.
  Also, check out the T4 build overview video.

Features:

• completely 3D printable (without support)
• simple assembly
• includes a single motor arm or optionally use the coaxial arms from the T6 and create an X8!!
• strong braced tube section arms with plenty of room to conceal motor wiring
• designed for a single 3S or 4S battery located right in the center of rotation/thrust with room for up to a 6000mah 3S or 4200mah 4S battery (155 x 50 x 30mm capacity).
• easy cable routing with beveled cable "tunnels" (and even a few cable tie mounts)
• plenty of space to mount ESCs - concealed but still well ventilated for cooling - perfect for a 4-in-1 ESC too.
• top plate is designed for the new Pixhawk flight controller from 3D Robotics - but is also fine with APM, KK2 and others
• mounting holes available on the "nose" that can be used to attach things like the included Go Pro vibration mount.
• RF "invisible" frame - as opposed to carbon fiber or aluminium
• All-up-weight 1,011 grams (excluding battery of your choice - including 3DR 880KV motors, 4-in-1 ESC, Pixhawk with GPS and telemetry). Still flies great when loaded with another 1kg of batteries and camera gear (see video).
• Sketchup file included so you can make changes to suit yourself

Background

I've been experimenting to find a strong "tubular" profile that could be printed reliably. The angled sections in these designs are 35 degrees to vertical - so well within the typical 45 degree capability of most printers. They arms only present about 11mm of flat surface to minimize obstruction to airflow.
The rest of the design are a collection of ideas that have accumulated over the past year building various quadcopters and Y6 configurations (3D printed, aluminium and bought frames).

Videos

• Flight video with Tarot Gimbal and Go Pro attached
• Flight video with GoPro mounted on top - no gimbal
• Tubular arm versus "conventional" arm design

History

3 August 2014
Version 1.05
Just a small change to add slots to the arm motor mount holes so that 4 bolts can be used (in a 16-19mm pattern) regardless of which "corner" of the motor the wires exit.
1 April 2014
Added a T4 univeral camera plate accessory as a separate thing.

22 March 2014
Version 1.04.1

• No changes to any existing parts
• Added a vibration reducing mount for a GoPro Hero 3 - for folks who don't want to set up a full camera gimbal. I used these dampeners because I had them lying around. They removed all vibration "jello" but may be a bit too soft for aggressive flying. I think these ones would be a better option. Anyway, I've included several STLs with different hole sizes which will hopefully suit most dampeners out there (5mm, 7mm, 8.5mm). Flight video is above.
  15 March 2014
  Version 1.04
• about 12mm narrower "waist" for a small reduction in plastic/weight
• additional cable tunnels at the front of the flight controller
• slots in the side and rear of body.stl provide another option to cable tie accessories (eg transmitters with aerials pointing down)
• small notches out of the bottom tray provide an outlet for cables to run under each arm (eg for lighting strips)
  The following files have been updated:
• T4Body.stl
• T4TopPlate.stl
• T4BottomTray.stl
  I'm also adding the component weights and print time in the instructions.

8 March 2014
I flew a few different batteries today to compare. This is with my 880kv 3DR motors which is a bit over-weight as it is version 1.00 and has most of the power components from my T6 inside. I'm guessing a 1.02+ version with something like the 3DR 850kv motors would weigh at least 100 grams less with longer flight time.
The results:

• 3S 3500mah = 10 minutes (only used 2472 mah - old battery)
• 3S 6000mah = 18 minutes (used 5676 mah)
• 4S 4200mah = 16.5 minutes (used 4027 mah)
  Version 1.03: A small update that adds two more holes to the body and top plate near the front cable tunnel. This provides a triangle of fixing points on the "nose" which could be used for "accessories" such as a simple camera mount.

7 March 2014
CRASHED HARD!! But this frame handled it well (see photo above). I printed off another arm after work, reassembled and had time for a quick test flight - all back to normal. Replacing the arm was about a 10 minute job.
5 March 2014
I just noticed a hole missing at the front of the top plate. It's not required (or any real issue if missing) so I've just updated T4TopPlate.stl and the Sketchup file without changing version numbers.
4 March 2014
Version 1.02: I've spent another full day drawing on this thing. The biggest parts are now 199mm long and 140mm wide. Hopefully that will squeeze onto a few more print beds out there. The arm will also fit onto that sized bed if you rotate it a bit.
I've started to version the Sketchup files and left older ones in the files if anyone prefers an earlier design.
3 March 2014
Version 1.01: As requested I've put this thing on a diet. It now only requires a 140mm wide (and 220mm long) build platform so it should fit easily on a Replicator 2.
Also I've updated T4Arm.stl adding a small hole to allow a cable tie to be used for strain relief where the wires exit the motor.