Multirotor

Multirotor
e-volo, the first manned electronic multirotor

A multirotor^[2] or multicopter is a rotorcraft with more than two rotors. Multirotors often use fixed-pitch blades, whose rotor pitch does not vary as the blades rotate; control of vehicle motion is achieved by varying the relative speed of each rotor to change the thrust and torque produced by each.

Due to their ease of both construction and control, multirotor aircraft are frequently used in model and radio control aircraft projects^{[3][4][5][6][7][8]} in which the names quadcopter, hexacopter and octocopter are frequently used to refer to 4-, 6- and 8-rotor helicopters, respectively.

Radio controlled multirotors are increasingly used as a low-budget option to create aerial photography and videos of sites and buildings.

Electronically controlled configurations

Hexacopter.^[1]

These use a central lithium polymer battery and 'flight controller' or stabilization board (containing an IMU, mounted in a core/hub section), and brushless motors & propellers mounted on nacelles extending outwards. The props are fixed-pitch, and the motors are mounted rigidly to the structure - all control is done in software throttling the motors differentially, necessitating a very rapid feedback loop.

Electronic multirotors come in a number of different configurations:^{[citation needed]}

• X4 / 'Quad' - A typical quadcopter, quadrocopter, quad-helicopter or just quad, with all props mounted on the ends of arms arrayed radially outward from a central hub, pulling upwards at opposite ends of the craft; May be switched between 'X' or '+' configuration (with one arm leading 'forward') in software.
• Tricopter / Y - Two props on arms pointing to the sides or slightly forwards, and one arm backwards. The rear motor can be tilted laterally via a servo to provide yaw control.
• Y4 - Arrayed like a tricopter without the servo, this uses two normal props in front on separate arms, and two coaxial ones in the rear mounted to one arm.
• H4 / H-Quad - A quad with a long, flat bar for a chassis, and the props mounted on two cross members bolted to the ends. Tends to fly in 'I' configuration for ease of camera mounting.
• V4 / V-Tail Quad - a quadcopter with the front props on normal long booms, and the rear props located in close proximity, tilted at an angle from vertical. This should give lower efficiency and flight times, but better orientation visibility and potentially better stability.
• 'Hexa'- A typical hexacopter, or just hexa, with six arms arrayed radially outward from a center point.
• Y6 - A type of hexacopter that can be made more compact for the amount of lift, but is less efficient, with three arms arrayed radially outward from a center point, and one motor mounted at the end of each arm pointing up, and one pointing down.
• 'Octo' - A typical octocopter, or just octo, that follows the pattern of one motor per arm arrayed radially. Common on 'heavy lift' designs that re-use parts from smaller part inventories. May have independent radial arms or a branching structure.
• X8 - An octocopter that uses four arms, with motors arranged coaxially pointed up and down.
• H8 - An octocopter that uses two parallel rails, each containing four rotors attached at various points. H6, H10, or H12 designs are similar, if less common.
• Asymmetric designs - Any of the above designs can be stretched and skewed, often with the goal of providing an unobstructed view to forward-looking cameras. The center of gravity in such designs must be carefully managed to preserve maneuverability.

Flight Control

Flight control in electronically controlled multicopters is achieved using a minimum of four control channels. One channel is usually labeled throttle and increases or decreases power to all motors evenly. This causes the aircraft to ascend or descend. The other three channels, labeled aileron, elevator, and rudder, control the roll, pitch, and yaw axes respectively. These three control inputs work by causing a change in aircraft attitude (tilt or direction). For example in an X4 configuration, forward tilt or pitch is controlled by increasing the speed of the two rear motors while decreasing the speed of the two front motors. Left or right tilt or roll is controlled by differences in the speed of the two right motors vs. the two left motors. The flight direction or yaw is controlled by changing the relative speeds of adjacent, counter rotating motors.

Electromechanical designs

A much broader field with a variety of mechanisms of action. These designs are often built as experiments at the amateur hobbyist level, but with the exception of the tricopter have not yet attained the popularity of the pure electronic designs.^{[citation needed]}

Variable pitch

These models utilize the same type of variable pitch rotor and swashplate as a helicopter, but (usually) use it by applying cyclic differentially to non-coaxial propellers. This allows both very agile control, as demonstrated by MIT's ACL, and the potential to replace individual electric motors with belt-driven props hooked to a central internal combustion engine. Variable pitch is a rare option present in a few custom builds.

Servo thrust vectoring

These models, such as the bicopter, the tricopter, and some VTOL gliding craft like the IAI Panther, utilize both differential thrust as well as at least one motor which is mounted on a servo, free to change its orientation. The tricopter, and to a lesser degree the bicopter, are extremely popular alternatives to electronic multirotors which operate on pure throttle control.

Flap thrust vectoring

Wherever it is possible to rotate a motor/prop, it is also possible to redirect its flow using control vanes in the propeller downwash. Not a common solution on commercial models, but present in a few custom builds.

Examples

• Cierva Air Horse - a British three rotor helicopter, three rotors were used to give a large lift without compromising rotor strength.^{[citation needed]}
• e-volo - a German prototype electric multicopter with 16 rotors, the first electric multicopter in the world to achieve manned flight.^[9][10] The large number of low-cost motors make it economical, quiet and provide redundancy with ability to maintain control with up to four failed motors.^[11]

References

Wikimedia Commons has media related to Rotorcraft.