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All About Multi-Rotors

multirotor 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[2][3][4][5][6] in which the names quadcopter,hexacopter and octocopter are frequently used to refer to 4-, 6- and 8-propeller 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

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:

  • X4 / 'Quad' - A typical quadrotorquadrocopterquadcopter,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 wood 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 quadrotor 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 & 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 to the core at multiple points. Generalizable (less commonly) to H6, H10, or H12 designs
  • Asymmetric designs - Any of these can be stretched and skewed, possibly with the central core offset, to create a design that offers clearance in the front of the craft for a forward-looking camera un-obstructed by propellers. The center of gravity on these designs must be carefully managed to remain maneuverable.

Electromechanical designs

A much more broad 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.

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.