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An answerAn answer to the question http://robotics.stackexchange.com/q/543/37Why are quadcopters more common in robotics than other configurations? said:

You need 4 degrees of freedom to control yaw, pitch, roll and thrust.

Four props is therefore the minimum number of actuators required. Tricoptors require a servo to tilt one or more rotors which is more mechanically complicated.

In a comment, I asked:

How do you get pure yaw motion with a quadcoptor and if that's possible why won't this work with a tricoptor? I don't understand how can you get yaw motion with any system where all rotors are in a plane without first tilting and moving. I would have thought that the main difference between quadcopters and tricoptors would be the kinematic calculations would be more complex.

Another answerAnother answer explained:

you get pure yaw in the following way:

North and South motors rotating the same speed but collectively at a higher (or lower) speed than East and West Motors which are also at the same speed.

This explains why it works with a quadcopter, but doesn't explain why it won't work with a tricopter.

Is it simply the fact that the asymmetry means that you can't imbalance the torque effects to provide yaw movement while still keeping the thrusts balanced to keep pitch and roll constant?

An answer to the question http://robotics.stackexchange.com/q/543/37 said:

You need 4 degrees of freedom to control yaw, pitch, roll and thrust.

Four props is therefore the minimum number of actuators required. Tricoptors require a servo to tilt one or more rotors which is more mechanically complicated.

In a comment, I asked:

How do you get pure yaw motion with a quadcoptor and if that's possible why won't this work with a tricoptor? I don't understand how can you get yaw motion with any system where all rotors are in a plane without first tilting and moving. I would have thought that the main difference between quadcopters and tricoptors would be the kinematic calculations would be more complex.

Another answer explained:

you get pure yaw in the following way:

North and South motors rotating the same speed but collectively at a higher (or lower) speed than East and West Motors which are also at the same speed.

This explains why it works with a quadcopter, but doesn't explain why it won't work with a tricopter.

Is it simply the fact that the asymmetry means that you can't imbalance the torque effects to provide yaw movement while still keeping the thrusts balanced to keep pitch and roll constant?

An answer to the question Why are quadcopters more common in robotics than other configurations? said:

You need 4 degrees of freedom to control yaw, pitch, roll and thrust.

Four props is therefore the minimum number of actuators required. Tricoptors require a servo to tilt one or more rotors which is more mechanically complicated.

In a comment, I asked:

How do you get pure yaw motion with a quadcoptor and if that's possible why won't this work with a tricoptor? I don't understand how can you get yaw motion with any system where all rotors are in a plane without first tilting and moving. I would have thought that the main difference between quadcopters and tricoptors would be the kinematic calculations would be more complex.

Another answer explained:

you get pure yaw in the following way:

North and South motors rotating the same speed but collectively at a higher (or lower) speed than East and West Motors which are also at the same speed.

This explains why it works with a quadcopter, but doesn't explain why it won't work with a tricopter.

Is it simply the fact that the asymmetry means that you can't imbalance the torque effects to provide yaw movement while still keeping the thrusts balanced to keep pitch and roll constant?
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If you can create pure yaw motion with a quadcoptor andthen why won't this work with a tricoptor?
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If you can create pure yaw motion with a quadcoptor and why won't this work with a tricoptor?

An answer to the question http://robotics.stackexchange.com/q/543/37 said:

You need 4 degrees of freedom to control yaw, pitch, roll and thrust.

Four props is therefore the minimum number of actuators required. Tricoptors require a servo to tilt one or more rotors which is more mechanically complicated.

In a comment, I asked:

How do you get pure yaw motion with a quadcoptor and if that's possible why won't this work with a tricoptor? I don't understand how can you get yaw motion with any system where all rotors are in a plane without first tilting and moving. I would have thought that the main difference between quadcopters and tricoptors would be the kinematic calculations would be more complex.

Another answer explained:

you get pure yaw in the following way:

North and South motors rotating the same speed but collectively at a higher (or lower) speed than East and West Motors which are also at the same speed.

This explains why it works with a quadcopter, but doesn't explain why it won't work with a tricopter.

Is it simply the fact that the asymmetry means that you can't imbalance the torque effects to provide yaw movement while still keeping the thrusts balanced to keep pitch and roll constant?