When first getting in to the hobby there are alot of new terms that you may not be familiar with.
Here is a simple explaination for all these terms.
Coaxial helicopters have two sets rotor blades on the same shaft spinning in opposite directions.
The torque generated by the blades spinning at the exact same speed in opposite directions keeps the helicopter from spinning without the use of a tail rotor.
Out of all RC helicopter designs coaxials are the easiest to hover and fly but have limited forward speed.
Coaxials are extremely stable and can be flown indoors even in tight spaces. Most coaxials on the market are 3 or 4 channel models.
How do 3 channel coaxial helicopters work?
The 3 channels means that there are 3 channels on the remote that control the motion of the helicopter.
On a 3 channel helicopter the 3 channels would control the following: up/down(Channel 1), turn left/right(channel 2), and forward/back(channel 3).
The forward and backwards motion on a 3 channel is usually controlled by a small tail propeller that spins clockwise or counter-clockwise to slightly push or pull the helicopter forwards and backwards.
The hovering is very stable but forward and reverse speeds are very limited.
How do 4 channel coaxial helicopters work?
The 4 channels means that there are 4 channels on the remote that control the motion of the helicopter.
On a 4 channel the 4 channels would control the following: up/down(channel 1), turn left/right(channel 2), forward/back(channel 3), and left/right lean aka aileron(channel 4).
The 4 channel helicopter is different from a 3 channel in that a tail rotor is not used to control the forward and backwards movement. Instead, small servos(motors) are used to control the angle of the helicopter's bottom rotors.
Tilting the rotors forward and backwards creates the forward and backwards motions.
In the same fashion when the rotors are tilted left or right the helicopter leans left and right and is also known as the aileron. The aileron can be combined with the forward motion to move the helicopter diagonally left or right.
The forward and backward motion on on a 4 channel is much more pronounced and gives the helicopter much more agility over a 3 channel.