From RC Helicopter Wiki
Coaxial rotor RC helicopters are generally small electric helicopters which are very well suited to those pilots just starting out with RC helicopters. Lacking the traditional tail rotor system used by most helicopters to counteract the torque reaction of the main rotors, the Blade CX2 uses an arrangement known as twin coaxial main rotors.
This setup employs two main rotor assemblies stacked atop one another, sharing a common rotational axis (hence "coaxial"). The rotors spin in opposite directions (one clockwise, the other counter-clockwise) and thereby cancel out the normally generated torque, and thus eliminating the need for a tail rotor.
The "co-ax", as it's generally known, employs dual electric motors (one for each rotor) and a fairly sophisticated electronic control system. It's within the electronics that the helicopter's rotation is stabilized, and the main motors are synchronized in order to maintain the excellent stability these helicopters are known for.
By varying the speed of one of the main motors, it's attached rotor can be made to spin faster or slower than its counterpart. This speed variation changes the torque balance in such a way as to induce a small, yet controlled bit of motion on the helicopter's yaw axis. A basic gyro is also incorporated into the electronics package on a coax helicopter, which lets the electronic speed controllers know to vary the rotor speed to keep the nose of the helicopter pointed in a certain direction.
The upper rotor is generally responsible for yaw control, while the lower rotor is linked to cyclic control. Most coaxial RC helicopters are fixed pitch, meaning that they ascend or descend vertically based on the speed of the main rotors, and their cyclic pitch control (in the elevator and aileron axes) is affected by tilting only the lower rotor assembly. Through a few small linkages and a rudimentary swashplate, the lower rotor of a coaxial RC helicopter can be tilted in whichever manner necessary to cause the helicopter to move in the desired direction.
Since the coax helicopters are generally fixed pitch (yes, there are a few exceptions to this), and since only one of its two rotors are affecting directional change on the airframe, coaxial helicopters are generally slower and less nimble ( = more stable) than helicopters with a single main rotor. They are also not capable of 3D flight as collective pitch helicopters are. It's this slowness and inherent stability that makes coax helicopters appropriate for beginners (and a LOT of fun for those who've been flying a while) and also makes them very popular. They can be useful training aids insofar as they will help teach a beginning pilot the necessary skills to maintain orientation, even if they will not help much with other aspects.
Some popular RC coaxial helicopters are the E-Flite Blade CX and CX2, the new E-flite Blade mCX, the Walkera Dragonfly (and many other models from Walkera), and the Co-Co Lama. Most of these helicopters are very small and lightweight which means that they're better suited for indoor-only flight.
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