Multirotors

Pure Electronic Control

There machines work very differently than helicopters, despite the name. These vehicles use brushless motors and pairs of counter-rotating propellers to generate lift. Yaw control is achieved by mounting pairs of counter-rotating props opposite each other, and varying their speed relative to each other: the torque moment of the prop tips while farthest from the center of gravity predominates. Pitch and roll are varying the speed of all the propellers on one side of the vehicle.

Quadrocopters, Hexacopters, and Octocopters are relatively simple & cheap to design and build, because an autopilot performs all control electronically. The only moving parts are the balanced propellers. They are commonly cited as being limited to about 15 minutes of flight with a minimal payload - multicopters are far less efficient than most fixed-wing craft, and they are unable to use fuel engines. Still, they have become very popular as small, stable machines which are easy to launch, cheap to maintain, and simple to utilize for aerial photography or recreational indoor flying.

Common configurations

These use a central lithium polymer battery and stabilization board (with mounted IMU in a core section, with brushless motors and 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 quadrotor, quadrocopter, or quadcopter, with all four props mounted on the ends of four arms extending 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.
• 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.
• Hexa - A hexacopter, with six arms arrayed radially outward from a center point, each sporting a motor
• Y6 - Three arms, with double-stacked (coaxial) propellers mounted on 6 motors.
• Octo - An octocopter which 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 sideways branching structure.
• 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.
• X8 - An octocopter that uses four arms, with motors arranged coaxially pointed up and down
• H8 - An octocopter that uses two strong parallel rails, each containing four rotors, attached to the core at multiple points with weaker rods. Generalizable (less commonly) to H6, H10, or H12 designs
• 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.
• 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.
• Bigger parallel arrays - Arbitrarily large arrays of motors & propellers are economical, if perhaps less efficient than using larger props, and the software problem of controlling them all is a tractable one in most systems. A 12-prop Dodecacopter is a popular choice in this niche, but 18-prop designs and larger have been spotted; These large numbers of PWM-throttle outputs may require more than a typical flight controller board is capable of, so individual motors may be throttled in a dumb parallel fashion, in a hierarchical fashion with daughter microcontrollers, or sometimes even with an entirely separate control solution (as in the case of two quadrotors strapped onto each other, offset in yaw rotation by 45 degrees).

Electromechanical Control

Electronic control is considered elegant because everything can be modified in software, and the only moving parts are the propeller & motor shafts. Additional degrees of

Other Aspects

Drive

Most multirotors are direct-drive brushless outrunners. A few have experimented with a per-rotor geared drive or a belt drive to increase torque & lower RPM, and to use more durable higher-Kv inrunners in a less vulnerable position, like the Parrot AR.Drone and this Chinese police quad.

Endurance

Most multirotors with a modest payload will find 15 minutes to be an achievable target with the right size battery. Longer flights become exponentially harder for a given payload. In theory large, slow rotors are inferior for control, but superior for efficiency. The size of a helicopter's rotor makes it potentially a lot more efficient, but the fact that it has to be variable pitch cuts it back down.

Competitive endurance runs tend to be in the 45-90 minute range.

• How to extend flight time on hexacopter