Tricopter updates

Having established that I could build and fly a tri it was time to update to decent quality components.

I bought carbon fibre arms, power distribution body (with onboard BEC) and tilt mechanism with Blue Bird digital servo from rcexplorer.se 

Prices are very reasonable for these excellent components and postage is only US$7 from Sweden, better than most Australian outlets.

David's tricopter parts are almost works of art, especially the 10mm square woven carbon fibre arms.

From HobbyKing I bought Hextronic DT750 motors and some Turnigy Slow Fly 10x4.5 props.


Hextronic DT750 motors

I need to share my experiences with these DT750 motors. They are well suited to tricopters but do have some design faults which need to be dealt with first.

ESC wire support
The 3 wires are not flexible, just continuations of the single core windings, and will break with bending and vibrations. To prevent this they must be supported around the base with big blob of epoxy glue. I used 5min Araldite and it worked well.

Motor shaft "pull-up"
The shaft is about 40mm x 4mm and threaded. In theory you don't need a vibration inducing prop adapter, just a nyloc nut. BUT unfortunately the shaft is not rigidly fixed to the motor bell. Tightening the nut enough to stop the prop slipping pulls the shaft up through the motor bell. This increases the bearing friction making the motor heat up and draw more current. The shaft is only held in the bell by one tiny grub screw, which is not enough to resist the "pull-up" from the prop nut.

I had some nasty crashes caused by a slipping tail prop because I couldn't tighten it down enough. Broke an arm and ruined an expensive digital servo.

The solution is to put a nut onto the shaft right down to the unthreaded part, then a washer, prop, washer and nyloc nut. That way the prop doesn't push down on the motor bell. Unfortunately I had already cut the shaft too short before I discovered this problem so have had to resort to bulky 4mm prop adapters.

Quest for the best propeller
Turnigy Slow Fly 10x4.5 props are OK but very thin and flexible.

10x5 Gemfan Carbon nylon Graupner copies from Banggood are much stiffer but less efficient giving shorter flight times.

Emax 10x4.5 props from Banggood are a bit stiffer than the Turnigys and may be the perfect choice. Testing continues.

Tricopter build

Here's a quick and dirty tricopter build mostly using materials from my local hardware store.

It covers the basics of design and assembly. The tri flies OK but performance would be improved with more balanced motors and prop adapters and some P and I adjustments.

I haven't covered KK2 setup and tilt mechanism construction but David Windestal's has...

KK2 setup
Tricopter V2.5 build (including tilt mechanism) 

Tricopter

Thanks to great build videos by David Windestal I have recently become interested in tricopters. 

Until now I have not bothered with multi rotors at all and still prefer wings and airfoils to motors. But it looked easy enough and searching through my spares box found most of the required electronics. I had three 2822 1450kV motors and three 25A Plush ESCs so only needed to buy a flight control board and some counter rotating props.

Why a tri and not a quad? 

Well every man and his dog has a quad which makes tricopters cooler.

Tricopters have a tilting tail rotor for steering, which gives more "plane-like" flight characteristics. Also there are only 3 motors / ESCs rather than 4 so they are cheaper.

KK2 flight control board

The brains of a multi rotor is the flight control board. I chose the simplest board, the KK2 (A$26 from Hobby King).

The KK2 takes your Aileron, Elevator, Throttle and Rudder commands (or stick movements) and translates them into motor speed changes and tilt servo movements.  

The KK2 has 2 flight modes: Manual, where the tri will stay at the flight angle you command until you make a correction. Self-level, where the tri will return to level when the sticks are centred. The KK2 also supports the Super Simple Gimbal which is really simple and really works, kind of like steady-cam for an onboard camera.

KK2, Rx and servo power supplies
KK2 board on a tricopter requires two power connections, one to power the board and the receiver, and the other to power the tail servo. They need to be separate for it all to work smoothly. Something about gyros and servos and power loops can cause jittering and instability apparently.

The ESC from motor 1 (front left) plugs into the M1 pins on the KK2 (top right) and provides 5V to power the board and receiver. Do not be tempted to plug 12V in anywhere or you will have to order a new KK2 board (yes I did). There are battery monitoring pins but make sure you know what you are doing before connecting, or just dont use them.

On tricopters a second BEC must be connected to M2 pins (or M3...M8) to provide 5V to the servo. You can use the built-in BEC from the ESC on motor 2 if it is rated high enough. Otherwise add an external BEC.

More than one "switching" style BECs should not be connected on these pins at the same time. Linear BECs are OK. If the ESCs on both motor 2 and 3 have switching BECs remove the red servo wire from one of them to disconnect it's BEC.

On a Quad you can use one normal ESC connected to M1 and three OPTO ESCs because there are no servos needing power. OPTO ESCs do not have an onboard BEC.

Firmware upgrades

There is a lot of chat on RCgroups about which firmware is best for the KK2 and the ESCs on a multi rotor. Apparently Stevis for the KK2 gives more options and better performance than the stock firmware, and SimonK firmware for the ESCs gives smoother and more responsive performance. To start off I used the stock firmware on everything just to get a feel for flying multi rotors, and it all worked perfectly well.

I have since flashed the KK2 with Steveis V1.9 using a $5 USBasp programmer from ebay, and bought Afro ESCs pre flashed with SimonK. Performance is smoother and more responsive.

Here are some early videos. I'll add a build video and onboard footage soon. 

At the moment I'm spending time learning to fly better and optimising the tricopter to reduce vibrations. Stay tuned.

Testing and maiden flights

Arm repair time-lapse

SPECS

Motors - Turnigy 2822/14 1450kV

ESCs  - Turnigy Plush 25A (stock firmware) x 3

Updated to Afro 30A (simonK firmware) One with BEC to power the board and 2 OPTO (no BEC) plus a Plush 18A ESC (which has a linear BEC) just to power the servos

FC board - KK2.1.5 stock firmware
Updated to steveis V1.9 firmware

Props - weak flexy green 8 x 4.5" Hobby King / Gemfan
Updated to much stiffer Gemfan 8045C carbon nylon CW/CCW pairs from Banggood