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