I found myself in both camps, I moved a ZMR build over to a EMAX full carbon frame for my learner FPV rig. This left me with a spare ZMR frame, so I got to thinking ... which is always dangerous.
I love tricopters and after unsuccessfully reducing the size of my Titan tricopter I decided to try and make a tricopter out of a ZMR frame. This is the process I went through.
The finished version:1
Heres a quick video of the finished version:1 flying
The first thoughts was to use a 10x10 carbon arm that was left over from the Titan trim, But after some mockups I decided against it.
First mockup with the 10x10 carbon boom, rejected.
Testing the layout options. Trying to fit the servo in and spacing.
Design re-think, Doh, why not just use a ZMR arm. So after a little fiddling I found that the camera plate slots on the ZMR frame are almost identical to the ZMR arm holes. All I had to do was use a 3mm drill to widen the gap to allow a 3mm bolt through.
I also used some washers to provide some extra strength so the bolt wouldn't pull through the plate in a hard crash.
I used 10mm standoffs like I did for my other ZMR builds to secure to the bottom plate. It was surprising strong. I used the outer arm holes to provide some more rigidity. This did make the distance to the tail motor mount a little longer, 20mm compared to the front motor distance. But I think it may help in-fact, I hope it will give the tail more leverage and drive.
Ok, on to the big challenge the tilt mechanism.
Firstly a huge shout out to David Windestal, Captain Tricopter !!. He designed and made public under CC Licensing an awesome 3D printable tilt mechanism.
I started with the v1 of these and started play around with it on a ZMR arm.
After a little playing, trimming it become clear that it was going to be really easy to zip tie it to the arm, even with the larger servo it fitted nicely between the tilt and top plate. The spacing also provided the tilt a lot of protection.
I designed a 3D fully integrated plate that had the alignment all done, I had a 3D file of a ZMR arm, I then just joined Davids bottom tilt design at the required height and alignment so that the zip tie groves lined up. I also spent some time making it so that the server would just slot in. I had spare servo that I could test with, it was not ideal. But it was free and on hand. ( I late changed it for a smaller one)
First design printed out.
First design Tilt and servo all buttoned up.
Test fitted to the mocked up frame.
Design re-visit
So after the first design you always find issues and better ways of doing things. So I found that if I used a DYS 1806 that with the prop was very close to the large servo, there wasn't enough clearance.
I plan to start with the DYS1806 2400kv, 5045 props, then move up to the EMAX2204 , 6045 props. I needed this to be strong. To help both of these issues I decided to increase the thickness of the motor mount plate area. I expect to crash, so I NEED strength / durability.
The other issue I ran into was that my 3D prints were not producing good strong splines in the motor tilt portion. I also wanted the option to try and use different servos so didn't want to be limited to the spline that was part of Davids design.
So to allow the use of servo horns I just altered the design to have a recess. This allowed for servo horns of various sizes from different servo's to be easily installed.
The other change was to split the bottom plate so that I could swap out servos / servo types without having to remove the main tilt assembly.
Servo horn screwed in, it only requires one screw.
All zip tied on to the ZMR frame
The design allows for 3 zip ties to hold it on. 2 using the ZMR arm holes and one extra at the arm base.
I also swapped to a smaller servo, its on the best. I had these left over from a HobbyKing Trifeca tricopter build. Its a Turnigy TGY-9018MG. Its smaller than the one I started with, time will tell if is a durable servo for this sized model. I had to then design a new servo mount plate. It has a cut out slot that orientates the servo and stops and movement backward.
Now I can easily swap servos out if required. Just design a aft bottom plate.
Now with this design I can center the motor tilt and slide the servo onto the servo horn and zip tie it down. The ZMR arm angle really helps hold it place. Since the arm narrow outward once you zip tie the servo on any force applied to pull the servo back actually tightens the zip ties.
Full complete frame mock up. Very happy with how the tail worked out.
Electronics
First build
DYS1806 2400kv Motors
Rotorgeeks 12 amp ESC
Naze32 Acro / Baseflight
5045 Props
D4RII / Telemetry
I wanted to try and keep the design to only use the ZMR bottom plates, that meant I had to try and fit everything between the plates. This meant that I had to do a full soldered wire harness and not use a PDB.
I used rotorgeek 12amp ESC that I got second hand. The front ESCs are forward in the frame to allow room for the NAZE in the center.
The front arms are only held in with 2 bolts / stand offs not the usual 4. Time will tell if that impacts durability. I gave me the required room to fit the ESCs in.
Tight fit in the nose. I also used one of my ZMR 3D printed LED bars. Got to have some bling.
The routing of the motor wires.
The rear motor ESC is mounted off to one side. This will allow room for the D4RII to be mounted on the other side.
Top plate on, making sure everything fits in the 13mm gap.
Doh, I forgot the batt lead. Had to de-solder and add. ( It was late and cold in the garage)
Onto the NAZE and D4R. Its a tight fit but very doable. The NAZE uses the ZMR FC mount points and I used nylon bolts/nuts. The D4RII was held with a little 3M double sided tape and a zip tie.
Everything was powered on and double checked.
Top plate added for the final time. I am really happy with the over outcome from a re-purposing and looks point of view.
I added as many standoff as I could to try and help with rigidity. Any stand off that was close to wiring was cover in heat-shrink. The Standoff that were on on the arms required 3mm spacers. 8045 prop spacers are perfect for this.
Time to load the firmware and tune.
UPDATE :1
Planning ahead I have ordered a tilt mechanism from Davids site and also one of the recommended digital servos. But so far after one flight the current setup is holding up well.
I will just need to design a server plate for the new servo. This will also allow me to run the custom Cleanflight firmware that adds much better performance for tricopters.
3D Files all on http://www.thingiverse.com/
For a TGY-9018MG servo setup
ZMR250 Arm protectors