How does a tricopter work

Principle
A multicopter without a controller is practillay not flyable.

The system of a multicopter is not stable in real life, so a user would have to use all his concentration on trying to hold the copter stable and hovering. One will not be able to fly it while trying to stabilize it.

Gyroscopes are required to keep the system stable. The gyros are measuring the movement of the tricopter in three directions yaw, nick and roll. The controller compares the sensors measuring with the users commands and compensates the errors. So if you want the tricopter to be hover and the roll gyro detects rolling to the left, the controller powers up the left motor a little while braking the right motor to compensate the unwanted movement. This is happening all the time, just watch a Video of a hovering tricopter to see the everchanging speed of each rotor. It is like trying to balance a plate on a stick.

The same principle is used for the nick function.

The yawing of a tricopter is a special case. If you are using a rotor, it produces torque that tries to spin your flying object into the opposite direction of the rotor. A helicopter is using his tail rotor to compensate this direction. A quadrocopter, hexacopter, octocopter an all copters with an even amount of rotors are using the fact that two rotors in the same plane, that are turning in opposite directions with the same speed, are neutralizing their torque.

A tricopter has two rotors that are spinning in opposite directions but there is a rotor left who produces torque that tries to spin the tricopter. Because of this, tricopters are usually using a rear servo to tilt the rear rotor to compensate this torque in the same manner a helicopter tail rotor does. The advantage of this solution is more yawing agility in comparison to multicopters with even amounts of rotors.

Advantages of a tricopter:

 * three points in space define a plane: The tricopter is very agile
 * yaw Servo makes yawing faster
 * three motors/esc are cheaper
 * easy to bring cause of colapsing arms
 * power consumption
 * small(depends micro -> full size)

Disadvantages of a tricopter:

 * no redundancy: If you loose a motor you will crash instantly
 * its not a good build for someone who is starting
 * more rotors -> more stability

Conclusion
A tricopter is good for getting into the multicopter scene and it is quite fun to fly them but if you are into aerial photgraphy you should consider using a hexa or octocopter.

The comparison of the above facts lead to another interesting solution: The Y6 Copter
 * three points in space
 * no servo necessary
 * small (depends micro-> full size)
 * quasi reduntant (you won't crash but you need to compensate the spinning manually if you loose motors)