There are different ways that coaxial RC helicopters can be designed. The main rotors of a RC helicopter can be mounted in two ways. In conventional helicopters, a main rotor is mounted on the top of the fuselage, and a smaller tail rotor is added to the end of the fuselage. The tail rotor is used to control the yaw, and every other aspect of flight is controlled by changing the pitch of the main rotor blades. It’s also necessary to have a tail rotor so that the torque from the main rotors can be canceled out. Torque is created when the main rotor spins. For every action, there is an equal and opposite reaction, so the nose turns in the opposite direction.

Coaxial Helicopters - How Do They Work

Besides using the tail rotor to control the yaw, two main rotors can be mounted on the helicopter, and spun at different rates. The difference in thrust causes the helicopters nose to turn. This duel rotor design is used in several full scale helicopters. The CH-47 Chinook is a good example of this duel rotor design, and is frequently used as a military transport helicopter because of its large lifting capacity. RC Helis use the same principle, but most of the time each rotor is mounted on the same axial, hence the term “coaxial”.

Twin rotors that spin in opposite directions

The rotors of a coaxial helicopter need to spin in opposite directions to cancel out the gyroscopic force. On some helicopters, both rotors are mounted on top of each other The main axial of a coaxial helicopter is actually two separate axils. One axial is mounted inside the other. Each sub axial is connected to a gear at the base of the main shaft, and each gear is connected to an electric motor. These two motors operate independently of each other, so the speed of each rotor can be changed.

Since the rotors need to spin in different directions, the top and bottom rotor blades are curved in different directions. This is because each rotor needs to be traveling into the air flow in order to work. The bottom rotor spins in a clockwise direction, and the top rotor spins in a counterclockwise direction.

Two Channel Coaxial RC Helis

It is possible to build a coaxial helicopter so that it only needs two main rotors, and eliminate the tail rotor entirely. Tilting the main rotors forward slightly, results in a constant forward movement. These helicopters are great for beginners due to their simple controls.

Three Channel Coaxial RC Helis

In some coaxial helicopters, a tail rotor is still added. The tail rotor is oriented in the same direction as the main rotors. The forward and reverse rate of the helicopter can be changed, because this system allows pitch control. You can fly forwards, backwards and also hover with this kind of helicopter.

Four Channel Coaxial RC Helis

Even though the coaxial design is most frequently used in small, fixed pitch RC helicopters, it can also be used for models with cyclic collective pitch. The only difference between these models and the conventional designs is their lack of a tail rotor. As with the models above, yaw is controlled by changing the rate at which each rotor spins, but the rotors can change their pitch. This allows the helicopter all four degrees of freedom: pitch, bank, throttle, and yaw. The Esky Lama V4 is good example of the four channel coaxial design.

How The Coaxial Design Benefits RC Helicopters

• Yaw can be controlled by changing the speed of the main rotors, so the pitch can be controlled with the tail rotor.
• The main rotors spin in opposite directions, which gives the helicopter stability.
• Coaxial RC helicopters are a good way to learn how to fly, because they are very stable.

An RC helicopter using a contra-rotating propulsion system will have two rotors spinning in opposite directions. This allows the torque generated by the rotor blades to be canceled out, and removes the need for a tail rotor to keep the RC helicopter from spinning. Without a tail rotor, the rc helicopter’s tail boom can be eliminated, saving weight.

One possible contra-rotating layout has one rotor blade mounted at each end of a long fuselage, as it is for the full scale Boeing CH-47 Chinook, and is known as a tandem rotor. This concept is explained well by the Tandem Rotor article on Wikipedia. Alternatively, both contra-rotating rotors can be placed on the same shaft, resulting in a coaxial design (sharing the same axis). Another good Wikipedia article on Coaxial Rotors explains the theory behind this.

Coaxial rc helicopters are very stable and easy to fly. The Bladerunner series of indoor rc helicopters made by Interactive Toy Concepts, and the ESky Lama V3 and Lama II V4 electric RC helicopters are examples of designs using co-axial rotor blades.