The multicopter or multirotor is one of the most popular types of UAV airframe configurations. This configuration uses multiple propellers to provide lift force, instead of using a single lifting rotor like a helicopter. The multicopter is able to control its orientation by adjusting the speed of each of the propellers independently, causing differential thrusts and torques. Multiple types of multicopter exist, and are classified according to the number of propellers they use and how they are arranged. Other variations of the multicopter concept exist, such as those using variable-pitch propellers or ducted fan propulsion units.
The main advantage of the multicopter configuration is that it maintains the vertical takeoff and hover capability of a helicopter, but with the mechanical simplicity of an airplane. The use of multiple propellers eliminates the need of a complex rotor mechanism found in a helicopter. This mechanical simplicity comes at the cost of electronic complexity and cost, as the multicopter requires a flight controller to stabilize the aircraft and calculate appropriate motor speeds according to control inputs. Also, the multicopter requires multiple motors, speed controllers, and a power distribution board. However, recent advances in electronics have made these components smaller and cheaper, and have made the multicopter a viable alternative for general UAV use. This UAV configuration is also less aerodynamically efficient than a helicopter, and generally provides lower flight times and payload capabilities.
Most multirotor UAVs use either 4, 6 or 8 propellers, which can be arranged in various configurations. The diagram below shows the most commonly used configurations. Note that coaxial configurations (where counter rotating propellers are placed one on top of another) can provide smaller vehicles for the same number of propellers. However, coaxial configurations can be significantly less efficient and provide less flight time than non-coaxial configurations.
Other less common multicopter configurations are possible, such as the bicopter, coaxial bicopter, tricopter, or variable pitch quadcopter. The bicopter uses just two propellers which can be independently tilted forwards and backwards to provide pitch and yaw control. However, this configuration tends to be unstable in the pitch axis as it relies on pendulum effect for stability. The coaxial bicopter uses two propellers aligned one in front of another. This configuraiton requires the use of movable vanes to provide control. The tricopter uses just three propellers. Since it has an unequal number of clockwise and counterclockwise propellers, their torques cannot cancel, and the rear propeller must be tilted to one side to cancel the net thrust. This propeller is tilted using a servo actuator to provide yaw control. The variable pitch quadcopter uses four variable pitch propellers that are usually run using a single motor with a belt drive system. The pitch of the propellers is varied to provide control of the aircraft, instead of varying their speeds. This configuration is most useful for acrobatics and racing applications.
This seciton describes how a quadcopter UAV achieves controllable and stable flight. Other multicopter types that use more than four fixed-pitch propellers will operate in analogous ways.
During hover conditions, all the propellers are operated at the same speed and therefore all the propellers produce the same torque and thrust force. Since all the propellers are equally spaced from the center of gravity, the thrust of the propellers produces no net rotating torque on the aircraft. Additionally, the quadcopter uses two clocwise-rotating (CW) propellers and two counterclockwise-rotating (CCW) propellers, so that the propeller torque is cancelled when they are operating at equal speeds, and the aircraft can hover in a stable manner.
Control of the UAV about the roll axis is achieved simply by increasing the speed of the propellers on one side, and decreasing the speed of the propellers on the other side. This has the effect of producing more lift thrust on one side and less on the other, creating a net torque that rotates the aircraft about its roll axis. Since the thrust is decreased on one side by the same amount it is increased on another, the net thrust remains the same. Also, since the left CW and CCW propellers are operating at the same speed, and the right CW and CCW propellers are operating at the same speed, the propeller torques cancel each other. The net effect is a pure rolling torque on the aircraft.
Control of the UAV about the pitch axis is achieved in an manner analogous to its roll control. The front propellers spin at a different speed than the rear propellers, creating a thrust differential that rotates the aircraft around its pitch axis.
Control of the UAV about the yaw axis is achieved by rotating the CW propellers at a different speed than the CCW propellers. The effect is that the clockwise and counterclockwise torques do not cancel completely, and the aircraft has a net torque about its yaw axis. Since the decrease in thrust in one set of propellers cancels the increase in thrust in the other set, the net thrust remains the same.
The main systems of a multicopter UAV are the propulsion system and control system. Electric propulsion systems are used for multicopters, since they provide the required precision, reliability and weight performance. The Propulsion System article provides information on how the propulsion system is set up for a multicopter UAV. The control system is a standard UAV control system, using a flight controller board. See the Control System article for information on how to set up the control system, and the Flight Controller article for information about how to select and set up the flight controller.
A number of factors should be considered when selecting a multirotor product or configuration. The main factor that determines the multicopter performance is the number of propellers. The table below summarizes the effect of the number of propellers in various performance parameters. The flight time is not included on the table, as all configurations usually provide similar flight times of around 10 to 15 minutes with payload. Some optimized designs with low payloads might fly closer to 30 minutes. Multicopters built for endurance have been flown for close to an hour without payload.
The second factor is how the propellers are laid out. The layout of the propellers determines the size of the vehicle, and the space and flexibility it has for placing cameras and other desired payload. Note that coaxial configurations (where counter rotating propellers are placed one on top of another) can provide smaller vehicles for the same number of propellers, but provide significantly less flight time and lifting capability.
Visit the UAV Systems article for a list of the most popular multicopter products on the market.